HomeMy WebLinkAboutNC0004961_1. RBSS CAP Part 2_Report_FINAL_20160212F)l
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Site Location:
NPDES Permit No.
Permittee and Current
Property Owner:
Consultant Information
Report Date:
Riverbend Steam Station
175 Steam Plant Rd
Mount Holly, NC 28120
NC0004961
Duke Energy Carolinas, LLC
526 South Church St
Charlotte, NC 28202
704.382.3853
HDR Engineering, Inc. of the Carolinas
440 South Church St, Suite 900
Charlotte, NC 28202
704.338.6700
February 12, 2016
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Contents
ExecutiveSummary.........................................................................................................................
1 Introduction.............................................................................................................................
1.1
Regulatory Background................................................................................................
1.2
Report Organization.....................................................................................................
2 Summary
of Previous and Current Studies............................................................................
2.1
Comprehensive Site Assessment................................................................................
2.1.1 Identification of COls.......................................................................................
2.1.2 Soil Delineation...............................................................................................
2.1.3 Groundwater Delineation................................................................................
2.2
Corrective Action Plan Part 1.......................................................................................
2.2.1 Proposed Provisional Background Concentrations for Soil and Groundwater
2.2.2 COI Occurrence and Distribution....................................................................
2.3
Round 2 Sampling........................................................................................................
2.3.1 Groundwater....................................................................................................
2.3.2 Round 1 and Round 2 Source Area and Groundwater Data Comparison......
2.3.3 Surface Water and Areas of Wetness.............................................................
2.4
Round 3 and Round 4 Background Well Sampling......................................................
2.5
Well Abandonment.......................................................................................................
3 Site Conceptual Model...........................................................................................................
3.1
Identification of Potential Contaminants.......................................................................
3.2
Identification and Characterization of Source Contaminants .......................................
3.3
Delineation of Potential Migration Pathways through Environmental Media ...............
3.3.1 Soil...................................................................................................................
3.3.2 Groundwater....................................................................................................
3.3.3 Surface Water and Sediment..........................................................................
3.4
Establishment of Background Areas............................................................................
3.5
Environmental Receptor Identification and Discussion ................................................
3.6
Determination of System Boundaries...........................................................................
3.7
Site Geochemistry and Influence on COls...................................................................
4 Modeling.................................................................................................................................
4.1
Groundwater Model Refinement..................................................................................
4.1.1 Flow Model Refinements.................................................................................
4.1.2 Fate and Transport Model Refinements..........................................................
4.1.3 Summary of Modeled Scenarios.....................................................................
4.1.4 Model Assumptions and Limitations................................................................
4.1.5 Modeled Scenario Results..............................................................................
4.2
Surface Water Model Refinement................................................................................
4.2.1 Methodology....................................................................................................
4.2.2 Results............................................................................................................
4.3
Geochemical Modeling.................................................................................................
4.3.1 Objective.........................................................................................................
4.3.2 Methodology....................................................................................................
4.3.3 Assumptions....................................................................................................
4.3.4 Results............................................................................................................
4.4
Refined Site Conceptual Model....................................................................................
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
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RiskAssessment..........................................................................
5.1 Step 1: Conceptual Site Model ...........................................
5.2 Step 2: Risk -Based Screening ...........................................
5.3 Step 3: Human Health Risk Assessment ...........................
5.4 Step 4: Ecological Risk Assessment ..................................
Alternative Methods for Achieving Restoration ............................
6.1 Corrective Action Decision Process ...................................
6.1.1 Evaluation Criteria .................................................
6.1.2 COls Requiring Corrective Action .........................
6.1.3 Potential Exposure Routes and Receptors ...........
6.2 Alternative Evaluation Criteria ............................................
6.2.1 Effectiveness.........................................................
6.2.2 Implementability/Feasibility...................................
6.2.3 Environmental Sustainability.................................
6.2.4 Cost.......................................................................
6.2.5 Stakeholder Input ..................................................
6.3 Remedial Alternatives to Achieve Regulatory Standards..
6.3.1 Groundwater Remediation Alternatives ................
6.3.2 Monitored Natural Attenuation Applicability to Site
6.3.3 Site -Specific Alternatives Analysis ........................
6.3.4 Site -Specific Recommended Approach ................
Selected Corrective Action(s).......................................................
7.1 Selected Remedial Alternative for Corrective Action .........
7.2 Conceptual Design.............................................................
7.2.1 Source Removal — Excavation ..............................
7.2.2 Monitored Natural Attenuation ..............................
Recommended Interim Activities ..................................................
8.1 Well Installation..................................................................
8.2 Additional Groundwater Sampling and Analyses ...............
Interim and Effectiveness Monitoring Plans .................................
9.1 Interim Monitoring Plan ......................................................
9.1.1 Data Quality Objectives .........................................
9.1.2 Sampling Requirements ........................................
9.1.3 Reporting...............................................................
9.2 Effectiveness Monitoring Plan ............................................
9.2.1 Data Quality Objectives .........................................
9.2.2 Sampling Requirements ........................................
9.2.3 Reporting...............................................................
9.3 Sampling and Analysis.......................................................
9.3.1 Monitoring Well Measurements and Inspection ....
9.3.2 Sample Collection .................................................
9.3.3 Quality Assurance/Quality Control ........................
Implementation Cost and Schedule .............................................
10.1 Implementation Cost..........................................................
10.2 Implementation Schedule ...................................................
References...................................................................................
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Riverbend Steam Station Ash Basin
Tables
2-1 Summary of Horizontal Hydraulic Gradient Calculations
2-2 Comparison of 0.45 Micron and 0.1 Micron Filter Sample Results - Groundwater
2-3 Ash Porewater Analytical Results — Round 1 and Round 3
2-4 Ash Basin Water Analytical Results — Round 1 and Round 3
2-5 Groundwater Results within Waste Boundary
2-6 Background Groundwater Analytical Results — Rounds 1, 2, 3, and 4
2-7 Groundwater Outside the Waste Boundary Analytical Results — Round 1 and Round 2
2-8 Constituents of Interest Evaluation
2-9 Surface Water and Areas of Wetness Sample Analytical Results — Round 1 and Round 2
4-1 Summary of Modeled COI Results at the Compliance Boundary*
4-2 Mountain Island Lake Surface Water Concentrations*
4-3 East Basin Surface Water Concentrations*
9-1 Interim Monitoring Plan Sample Locations
9-2 Sampling Parameters and Analytical Methods
10-1 Estimated Capital and Annual Costs for Corrective Action - MNA
* Table is presented in the text of this CAP Part 2 Report; all other tables are attached separately
Figures
2-1 Groundwater, Surface Water, and Area of Wetness Sampling Locations Map
2-2 Potentiometric Surface Map — Shallow Flow Layer
2-3 Potentiometric Surface Map — Deep Flow Layer
2-4 Potentiometric Surface Map — Bedrock Flow Layer
3-1 Site Conceptual Model — 3D Representation
3-2 Site Conceptual Model Cross Sectional
3-3 Receptor Map
3-4 Site Vicinity Map
3-5 Water Supply Intake Locations
8-1 Additional Assessment Wells
Appendices
A CSA Supplement 1
B Groundwater Flow and Transport Model
C Addendum to Soil Sorption Evaluation
D Surface Water Mixing Model Approach
E Geochemical Modeling Report
F Baseline Human Health and Ecological Risk Assessment
G Evaluation of Potential Groundwater Remedial Alternatives
H Monitored Natural Attenuation Technical Memorandum
Note this hard copy includes the report portion of appendices only. Complete appendices with all
attachments are provided on the accompanying CAP Part 2 CD.
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Acronyms and Abbreviations
pg/L
micrograms per liter
213 Standards
North Carolina Surface Water Standards as Specified in T15 NCAC 02B .0211 and
.0216 (amended effective January 2015)
2L Standards
North Carolina groundwater Standards as Specified in T15A NCAC 02L Standards
AOW
area of wetness
BERA
baseline ecological risk assessment
BG
background
BR
bedrock
CAMA
North Carolina Coal Ash Management Act of 2014
CAP
corrective action plan
CCR
coal combustion residuals
COI
constituent of interest
COPC
constituent of potential concern
CSA
comprehensive site assessment
CSM
Conceptual Site Model
D
deep
DO
dissolved oxygen
DQO
data quality objective
Duke Energy
Duke Energy Carolinas, LLC
EPC
exposure point concentration
HAO
hydrous aluminum oxide
HFO
hydrous ferric oxide
HQ
hazard quotient
HSL
health screening level
IMAC
interim maximum allowable concentration
Kd
sorption coefficient
MNA
monitored natural attenuation
MW
monitoring well
NCAC
North Carolina Administrative Code
NC PSRGs
North Carolina Preliminary Soil Remediation Goals
NCDENR
North Carolina Department of Environment and Natural Resources
NCDEQ
North Carolina Department of Environmental Quality
NCDHHS
North Carolina Department of Health and Human Services
NPDES
National Pollutant Discharge Elimination System
NTU
Nephelometric Turbidity Unit
POG
protection of groundwater
PPBC
proposed provisional background concentrations
RBC
risk -based concentrations
RBSS
Riverbend Steam Station
RMS
root mean squared
S
shallow
SCM
site conceptual model
SU
standard units
TDS
total dissolved solids
USEPA
U.S. Environmental Protection Agency
Work Plan
Groundwater Assessment Work Plan
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Acknowledgments
HDR Engineering, Inc. of the Carolinas would like to express its appreciation to Duke Energy
Carolinas, LLC for its guidance and interim report reviews, and to the parties listed below for
their assistance with data analysis, report preparation, quality reviews, and overall development
of this corrective action plan.
• The University of North Carolina at Charlotte — Groundwater Modeling and Soil
Sorption Analysis
• Electric Power Research Institute — Groundwater Flow and Transport Model Third -
Party Peer Review
• Geochemical, LLC — Monitored Natural Attenuation Evaluation and Soil Sorption
Analysis
• CH2M Hill, Inc. — Remedial Alternative Analysis
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Executive Summary
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of coal
combustion residuals (CCR) surface impoundments in North Carolina to conduct groundwater
monitoring, assessment, and remedial activities, if necessary. A Groundwater Assessment Work
Plan (Work Plan) for the Riverbend Steam Station (RBSS) was submitted to the North Carolina
Department of Environment and Natural Resources (NCDENR') on September 25, 2014, and
subsequently revised on December 30, 2014. The revised Work Plan was conditionally
approved by NCDENR on February 19, 2015. A comprehensive site assessment (CSA) was
performed to collect information necessary to evaluate the horizontal and vertical extent of
impacts to soil and groundwater attributable to CCR source area(s), identify potential receptors,
and screen for potential risks to those receptors. The RBSS CSA Report was submitted to
NCDENR on August 18, 2015 (HDR 2015a).
Subsequent to submittal of the CSA, CAMA requires submittal of a corrective action plan (CAP)
for each regulated facility no later than 180 days after submittal of the CSA. Duke Energy
Carolinas, LLC (Duke Energy) and the North Carolina Department of Environmental Quality
(NCDEQ) mutually agreed to a two-part CAP submittal, with Part 1 being submitted within 90
days of submittal of the CSA and Part 2 being submitted no later than 180 days after submittal
of the CSA. The RBSS CAP Part 1 was submitted to NCDEQ on November 16, 2015.
Based on the "Coal Combustion Residual Impoundment Risk Classifications" report published
by NCDEQ in January 2016, CAMA has identified RBSS as a high priority site. A public meeting
regarding the proposed risk classifications is scheduled for March 1, 2016. NCDEQ will release
final risk classifications after review of public comments.
Duke Energy owns and formerly operated the RBSS, located on the Catawba River/Mountain
Island Lake in Gaston County near Mount Holly, North Carolina. RBSS began operation as a
coal-fired generating station in 1929 and was retired from service in April 2013.
Decommissioning of RBSS is ongoing. After 1957, following installation of precipitators and a
wet sluicing system, CCR generated at the site was disposed in the station's ash basin located
adjacent to the station and Mountain Island Lake. Discharge from the ash basin is permitted
under the National Pollutant Discharge Elimination System (NPDES) Permit NC0004961.
Groundwater at this site flows to the north/northwest from the Primary and Secondary Cells of
the ash basin, ash storage area, and cinder storage area toward Mountain Island Lake. The
groundwater flow direction is away from the direction of the nearest public or private water
supply wells. The Catawba River/Mountain Island Lake serves as the primary hydrologic
discharge feature for groundwater within the shallow, deep, and bedrock layers at the site. No
drinking water wells are located within the downgradient Compliance Boundary of RBSS.
' Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Based on results of the CSA, concentrations of constituents of interest (COIs)2 attributable to
the CCR source areas at the RBSS site are beneath the ash basin, ash storage area, and west
of the cinder storage area. COI transport from the source areas is generally in a northerly
direction towards the Catawba River/Mountain Island Lake. COls in groundwater that are
attributable to ash handling at the RBSS site are antimony, arsenic, boron, chromium, cobalt,
hexavalent chromium, iron, manganese, sulfate, total dissolved solids (TDS), thallium, and
vanadium. Chromium, cobalt, iron, manganese, and vanadium were found to be naturally
occurring constituents in groundwater across the site. Further sampling and analysis are
necessary to determine if COI exceedances are the result of source -related impacts or are from
naturally occurring conditions (as discussed in Section 2).
The refined groundwater model predicts that several COls exceed regulatory standards at the
Compliance Boundary as discussed in Section 4.1.5; however, based on results of the
groundwater to surface water modeling, no water quality standards or criteria are exceeded at
the edge of the mixing zones in Mountain Island Lake and the East Basin.
A human health and ecological risk assessment was conducted as part of this CAP. The
ecological risk assessment indicates that potential risks are above risk targets for several
constituent of potential concern for some water -dependent mammals and birds. Additional data
and further refined assessment are needed to address uncertainties associated with the
evaluation of these scenarios including the occurrence of these ecological receptors in the
areas adjacent to the ash basins, delineation of source -related and background COls, and
refinement of the exposure and toxicity assumptions used in the ecological risk characterization.
The human health risk assessment indicates that potential risks are above risk targets for the
recreational fisher and subsistence fisher from ingestion of fish caught near the site. Similar to
the ecological risk assessment, additional data regarding site -specific conditions, delineation of
source -related and background COls to the risk assessment results, and the evaluation of the
exposure parameters and fish ingestion models used in the risk assessment are needed to
address these results.
Duke Energy is actively excavating ash at the RBSS site. Excavated ash will be used
beneficially off -site or will be relocated to a new off -site lined landfill.
An evaluation of site conditions, consituents, and a review of alternative methods for restoring
groundwater quality found that, in conjunction with source removal at the RBSS site, monitored
natural attenuation (MNA) is recommended as corrective action for groundwater impacts
beneath the site. An interim monitoring plan has been developed to provide baseline seasonal
analytical data for the RBSS site and will be implemented with sampling activities planned for
the first two quarters of 2016. Interim monitoring results will be used to evaluate compliance and
may be used, as needed, to refine the groundwater fate and transport, groundwater to surface
2 If a constituent concentration exceeded the North Carolina Groundwater Quality Standards as specified in T15A
NCAC .0202L (2L Standards), Interim Maximum Allowable Concentration (IMAC), North Carolina Preliminary Soil
Remediation Goals for Protection of Groundwater (NC PSRGs for POG), North Carolina Department of Health and
Human Services Health Screening Level (NCDHHS HSL), North Carolina Surface Water Quality Standards as
specified in T15 NCAC 02B .0211 and .0216 (amended effective January 2015) (213 Standards), or U.S.
Environmental Protection Agency National Recommended Water Quality Criteria, it has been designated as a
"constituent of interest".
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
water interaction, and geochemical models. The monitoring results will also be used to confirm
natural attenuation continues to occur and remains an effective corrective action for the RBSS
site. If MNA is deemed insufficient for restoration of groundwater quality, other alternatives
discussed in Section 6.3.3 will be evaluated and, if warranted, implemented to augment MNA.
The performance of these remedial alternatives will continue to be monitored and evaluated to
determine if modifications to the measures are required.
Per CAMA, "The Groundwater Corrective Action Plan shall provide for the restoration of
groundwater in conformance with the requirements of Subchapter L of Chapter 2 of Title 15A of
the North Carolina Administrative Code." This CAP meets the requirements of 15A NCAC 02L
.0106 and the requirements of the referenced section of CAMA.
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
1 Introduction
Duke Energy Carolinas, LLC (Duke Energy) owns and formerly operated the Riverbend Steam
Station (RBSS) located adjacent to the Mountain Island Lake portion of the Catawba River
(Mountain Island Lake) near Mount Holly, Gaston County, North Carolina. RBSS began
operation as a coal-fired generating station in 1929 and was retired from service in April 2013.
Decommissioning of RBSS is ongoing. From 1929 to 1957, coal combustion residuals (CCR)
from RBSS's coal combustion process were dredged from the primary basin to the ash storage
area, where it then decanted back to the primary pond area, leaving behind ash. Following
installation of precipitators and a wet sluicing system in 1957, CCR was disposed in the station's
ash basin located adjacent to the station and Mountain Island Lake. Discharge from the ash
basin is currently permitted under North Carolina Department of Environment Quality (NCDEQ)3
Division of Water Resources under the National Pollutant Discharge Elimination System
(NPDES) Permit NC0004961.
1.1 Regulatory Background
The North Carolina Coal Ash Management Act of 2014 (CAMA) directs owners of CCR surface
impoundments in North Carolina to conduct groundwater monitoring, assessment, and remedial
activities, if necessary. A Groundwater Assessment Work Plan (Work Plan) for RBSS was
submitted to NCDENR on September 25, 2014, followed by a revised Work Plan on December
30, 2014. The revised Work Plan was conditionally approved by NCDENR on February 19,
2015. A comprehensive site assessment (CSA) was performed to collect information necessary
to evaluate the horizontal and vertical extent of impacts to soil and groundwater attributable to
CCR source area(s), identify potential receptors, and screen for potential risks to those
receptors. The RBSS CSA Report was submitted to NCDENR on August 18, 2015 (HDR
2015a).
CAMA Section §130A-309.209(b) requires implementation of corrective action for the
restoration of groundwater quality in accordance with Subchapter L of Chapter 2 of Title 15A of
the North Carolina Administrative Code (T15A NCAC 02L) and requires the submittal of a
corrective action plan (CAP) for each regulated facility no later than 180 days after submittal of
the CSA. Duke Energy and NCDEQ mutually agreed to a two-part CAP submittal, with Part 1
being submitted within 90 days of submittal of the CSA and Part 2 being submitted no later than
180 days after submittal of the CSA.
The RBSS CAP Part 1 Report (HDR 2015b) was submitted to NCDEQ on November 16, 2015
and consisted of the following:
• background information
• a brief summary of the CSA findings
• a brief description of the site geology and hydrogeology
• a summary of the previously completed receptor survey
• a summary of constituent of interest (COI) exceedance and distribution
3 Prior to September 18, 2015, the NCDEQ was referred to as the North Carolina Department of Environment and
Natural Resources (NCDENR). Both naming conventions are used in this report, as appropriate.
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
• development of proposed provision background concentrations (PPBCs) for soil and
groundwater
• a detailed description of the site conceptual model (SCM)
• results of the groundwater flow and fate and transport model
• results of the groundwater to surface water interaction model
The purpose of this CAP Part 2 is to provide the following:
• a description of exceedances of groundwater quality standards, of surface water quality
standards, and of sample results greater than interim maximum allowable concentrations
(IMACs) and North Carolina Department of Health and Human Services (NCDHHS)
health screening levels (HSLs)
• a refined SCM
• revised groundwater flow and fate and transport model results
• revised groundwater to surface water model results
• site geochemical model results
• findings of the risk assessment
• evaluation of methods for achieving groundwater quality restoration
• conceptual plan(s) for recommended proposed corrective action(s)
• a schedule for implementation of the proposed corrective action(s)
• a plan for monitoring and reporting on the effectiveness of the proposed corrective action
The information provided in the combined CAP Part 1 and CAP Part 2 meets the requirements
of regulation 15A NCAC 02L .0106 (f) for corrective action.
As required by CAMA, Duke Energy plans to excavate the primary source, which is the coal ash
contained in the ash basin (Primary and Secondary cells). In addition, Duke Energy is removing
the ash stored in the ash storage area and cinder storage area. Excavated and removed
material will either be used beneficially off -site or will be relocated to a new off -site lined landfill.
Regulation 15A NCAC 02L .0106 (f)(4) requires that the secondary sources, which would be
potential continuing sources of possible pollutants to groundwater, be addressed in the CAP. At
the RBSS site, the soil located below the ash basin could be considered as a potential
secondary source. Preliminary information to date indicates that the thickness of soil impacted
by ash would generally be limited to the depth near the ash soil interface. As discussed with
NCDEQ, after excavation, soils left on -site will be sampled and analyzed, and the analytical
results will be incorporated into the groundwater contaminant fate and transport models. If this
evaluation indicates that modification to the proposed CAP is required, Duke Energy will
prepare and submit a revised CAP.
Based on the NCDEQ January 2016 Report, "Coal Combustion Residual Impoundment Risk
Classifications, CAMA has identified RBSS as a high priority site. A public meeting regarding
the proposed risk classifications is scheduled for March 1, 2016. NCDEQ will release final risk
classifications after review of public comments. However, regardless of the remaining public
meetings, RBSS is considered "High Priority" by CAMA and therefore excavation of the ash
management area is required.
Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
Report Organization
The information identified above has been organized in this CAP Part 2 Report as follows:
• Section 1 provides an introduction to the RBSS site and the intent of corrective action
under CAMA.
• Section 2 provides a summary of the CSA and CAP Part 1 reports and comparison of
Round 1 and Round 2 groundwater, surface water, and area of wetness (AOW)
sampling results and a summary of Round 3 and Round 4 background well sampling
results.
• Section 3 discusses the SCM and site geochemical controls on contaminant mobility.
• Section 4 discusses the purpose, methodologies, and results of refined groundwater,
groundwater to surface water interaction, and geochemical modeling. Refinement of the
SCM following evaluation of the model results is also discussed in this section.
• Section 5 summarizes the human health and ecological risk assessments.
• Section 6 presents an evaluation of remedial alternatives to achieve groundwater
restoration.
• Section 7 provides a concept -level discussion and plans for recommended corrective
action(s).
• Section 8 discusses recommended interim activities to be initiated in 2016.
• Section 9 provides a plan for interim and effectiveness groundwater monitoring.
• Section 10 provides a schedule and cost opinion for CAP implementation and post -CAP
monitoring.
Applicable tables, figures, and appendices with supporting documents are included with this
report.
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
2 Summary of Previous and Current Studies
This section presents a summary of previous and current studies including the following:
• Summary of CSA;
• Summary of CAP Part 1;
• Presentation of Round 2 groundwater sampling results and data comparison to Round 1
sampling; and
• Presentation of Round 3 and 4 background well sampling results.
Round 1 sampling data were previously provided in the CSA Report. Subsequent sampling
rounds occurred after the CSA submittal and are presented in this CAP Part 2 Report.
2.1 Comprehensive Site Assessment
The purpose of the RBSS CSA was to collect information necessary to characterize the extent
of impacts resulting from historical production and storage of coal ash, evaluate the chemical
and physical characteristics of the contaminants, investigate the geology and hydrogeology of
the site including factors relating to contaminant transport, and examine risk to potential
receptors and exposure pathways. The following assessment activities were included as part of
the CSA.
• Completion of soil borings and installation of groundwater monitoring wells to facilitate
collection and analysis of chemical, physical, and hydrogeological parameters of
subsurface materials encountered within and beyond the waste boundary and
Compliance Boundary4.
• Evaluation of laboratory analytical data to supplement the SCM.
• Update of the receptor survey previously completed in September 2014 (updated
November 2014).
• Completion of a screening -level risk assessment.
Note that subsequent to submittal of the CSA Report, additional evaluation of the initial round of
sampling results has been conducted. Responses to NCDEQ comments and additional
information in response to the exceptions identified in the CSA Report are provided in
Appendix A.
2.1.1 Identification of COIs
If a constituent concentration exceeded the North Carolina Groundwater Quality Standards, as
specified in 15A NCAC .0202L (2L Standards), the IMACs', NCDHHS HSLs (hexavalent
4 Per 15A NCAC 02L .0102, "Compliance Boundary" means a boundary around a disposal system at and beyond
which groundwater quality standards may not be exceeded and only applies to facilities which have received a
permit issued under the authority of G.S. 143-215.1 or G.S. 130A.
5 Appendix #1 of 15A NCAC Subchapter 02L Classifications and Water Quality Standards Applicable to The
Groundwaters of North Carolina, lists Interim Maximum Allowable Concentrations (IMACs). The IMACs were issued
in 2010 and 2011; however, NCDENR has not established a 2L Standard for these constituents as described in 15A
NCAC 02L.0202(c). For this reason, IMACs noted in this report are for reference only.
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
chromium only), North Carolina Preliminary Soil Remediation Goals (NC PSRGs) for Protection
of Groundwater (POG), North Carolina Surface Water Quality Standards as specified in T15
NCAC 02B .0211 and .0216 (amended effective January 2015) for Class WS-IV waters (2B
Standards), or U.S. Environmental Protection Agency (USEPA) National Water Quality Criteria,
it was designated as a COI. The following constituents were reported as COls in the RBSS site
CSA:
• Soil: arsenic, boron, cobalt, iron, manganese, nickel, selenium, and vanadium (CSA
Table 8-4).
• Groundwater: antimony, arsenic, boron, chromium6, cobalt, iron, manganese, sulfate,
total dissolved solids (TDS), thallium, and vanadium (CSA Table 10-8).
• Surface water: aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese,
selenium, thallium, vanadium, and zinc (CSA Table 9-1).
2.1.2 Soil Delineation
Horizontal and vertical delineation of source -related soil impacts was presented in the CSA
Report. Where soil impacts were identified beneath the primary and secondary ash basins, the
ash storage area, and the cinder storage area the vertical extent of impacts beneath the ash/soil
interface is generally limited to the upper soil samples collected beneath the ash.
2.1.3 Groundwater Delineation
Groundwater impacts at the site attributable to ash handling and storage was delineated during
the CSA activities with the following areas requiring refinement:
• Horizontal and vertical extent to the west of the ash and cinder storage areas
• Horizontal and vertical extent outside the northeast boundary of the ash basin
Additional groundwater monitoring wells are in the process of being installed to delineate these
areas. Based on the results of the CSA, eight additional assessment monitoring wells, one
replacement well, and six new background monitoring wells are currently being installed. The
additional background wells are located hydraulically upgradient of the ash management area.
Data obtained from these wells will be used to increase the understanding of background
conditions at RBSS and determine naturally occurring concentrations of COls. Results of the
additional assessment well installation and sampling will be submitted to NCDEQ under
separate cover.
2.2 Corrective Action Plan Part 1
The purpose of CAP Part 1 was to summarize the CSA findings, evaluate background
conditions by calculating PPBCs, evaluate exceedences per sample with regard to PPBCs,
develop a refined SCM, and present preliminary results of results of the groundwater flow and
transport model and groundwater to surface water model.
6 Unless otherwise noted, references to chromium in this document indicate total chromium.
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Corrective Action Plan Part 2
Riverbend Steam Station Ash Basin
2.2.1 Proposed Provisional Background Concentrations for Soil and
Groundwater
HDR used background soil and groundwater concentrations to determine site -specific
background concentrations for each COI. Because COls can be both naturally occurring and
related to the source areas, the selection of borings/monitoring wells used to establish
background concentrations is important in determining whether releases have occurred from the
source areas and to define the concentration of source -related compounds exceeding the
background concentrations for corrective action. Findings of the RBSS CSA indicate that some
of the previously installed wells thought to represent background data were determined to be in
locations downgradient or side gradient of the source areas and may not represent background
conditions for designation of naturally occurring COls.
During CAP Part 1, additional background wells were recommended to develop a more
complete understanding of the naturally occurring concentrations of COls in groundwater. For
the purpose of the CAP and for consistency with the CSA, only the background wells identified
in the CSA were used to develop RBSS background soil and groundwater concentrations.
Further refinement of the PPBCs is anticipated following the completion of additional
background well sampling events in 2016.
2.2.2 COI Occurrence and Distribution
The following soil COls for the RBSS site were identified in the CSA: arsenic, boron, cobalt,
iron, manganese, nickel, selenium, and vanadium as referenced in the CSA Table 8-4. Outside
the waste boundary, the primary soil COls are cobalt, iron, vanadium, and manganese.
Selenium occurs in localized areas outside the waste boundary. Arsenic is located only within
the waste boundary. Due to their widespread distribution, cobalt, iron, manganese, and
vanadium have naturally occurring concentrations that may exceed the soil (NC PSRGs for
POG) regulatory criteria.
The following groundwater COls for the RBSS site were identified in the CSA: antimony,
arsenic, boron, chromium, cobalt, iron, manganese, sulfate, thallium, TDS, and vanadium as
referenced in the CSA Table 10-6 and Table 10-8. Hexavalent chromium was added as a
groundwater COI in CAP Part 1. Arsenic, boron, and thallium were identified within the waste
boundary and planned ash basin excavation area that will be dewatered. Arsenic and boron
exceeded the 2L Standard and thallium exceeded the IMAC in only one groundwater sampling
location each during Round 1 sampling. Each of the locations is within the planned excavation
area. The absence of these COls in remaining areas at the RBSS site suggests that either their
presence in ash is limited, or that these COls are relatively immobile due to geochemical
properties and processes.
• Hexavalent chromium was identified within the waste boundary and primarily west and
northwest of the waste boundary. The highest concentration was located in the ash
storage area. CSA background well speciation sampling for hexavalent chromium was
not performed during Round 1 sampling.
• Sulfate and TDS concentrations support the conclusions made in the CSA and CAP
Part 1 that further evaluation in the vicinity of the cinder storage area is recommended.
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• Antimony, chromium, cobalt, iron, manganese, vanadium, and TDS exceeded their
respective 2L Standards or IMACs in at least one existing or CSA-identified background
monitoring well (MW-7D, MW-7SR, MW-7BR, BG-1 S, and BG-1 D). With the exception of
TDS, it is likely that these COls are in part related to natural background conditions and
will be evaluated further after the installation of additional background wells.
Round 2 Sampling
Round 2 groundwater, surface water, and AOW sampling activities were completed between
September 4 and 19, 2015. Groundwater analytical parameters and methods for Round 2 were
consistent with those employed for Round 1 in accordance with low flow sampling procedures
described in the CSA Report. The following subsections provide a comparison of Round 1 and
Round 2 groundwater flow and analytical results.
2.3.1 Groundwater
A total of 83 monitoring wells were sampled during the Round 2 sampling event including 49
groundwater assessment wells, 24 source area wells, and 10 CSA identified background wells.
Monitoring well locations are depicted on Figure 2-1.
2.3.1.1 Groundwater Water Levels
On September 15, 2015, all monitoring wells were manually gauged from the top of the PVC
casing using an electronic water level indicator accurate to 0.01 foot. Groundwater elevations
and contours based on Round 2 data for the shallow, deep, and bedrock flow layers are
depicted on Figures 2-2 through 2-4, respectively. Groundwater elevations measured during
the Round 2 water level gauging event were generally lower than those measured during the
Round 1 event; this is likely attributable to seasonal variations of the water table. Groundwater
flow directions based on Round 2 data are consistent with flow directions identified during
Round 1 water level gauging event documented in the CSA Report.
2.3.1.2 Horizontal and Vertical Gradients
Horizontal hydraulic gradients were derived using the Round 2 groundwater elevations for the
shallow, deep, and bedrock flow layers by calculating the difference in hydraulic heads over the
length of the flow path between two wells with similar well construction (e.g., both wells having
15-foot screens within the same water -bearing unit). Monitoring wells, groundwater elevations,
and length of flow paths used for horizontal hydraulic conductivity calculations are detailed in
Table 2-1. The average horizontal hydraulic gradients for Round 2 compared to Round 1 are
provided below.
• Shallow: Round 2 — 0.043 feet/foot; Round 1 — 0.032 feet/foot
• Deep: Round 2 — 0.031 feet/foot Round 1 — 0.028 feet/foot
• Bedrock: Round 2 — 0.021 feet/foot Round 1 — 0.032 feet/foot
Minor fluctuations were observed in the shallow and bedrock flow layers and may be attributable
to seasonal variations, but in general, horizontal hydraulic gradients were consistent with those
documented in the CSA Report.
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Vertical hydraulic gradients were calculated in Round 2 for 11 shallow and deep well pairs and
two deep and bedrock well pairs by taking the difference in groundwater elevation in each well
pair over the difference in mid -well screen of each well pair. Details regarding the vertical
gradients across the site were presented in the CAP Part 1 Report. In general, the gradients
calculated for Round 2 were consistent with those observed during Round 1 and are
summarized below.
A downward gradient between the shallow and deep zones generally exists across the Site. An
upward gradient was identified along the northern and eastern perimeter of the waste boundary.
This gradient is produced because the elevation of the water in the basin is 52 feet higher than
the ground surface at the base of the dike. This creates a pressure head resulting in an upward
vertical gradient at the wells downgradient of the dikes. Following excavation, this pressure
head will be reduced with removal of the water in the basin and it is anticipated that the vertical
gradient will be reduced. The vertical gradients between the deep and bedrock zones exhibited
a downward trend within the ash basin from the AB-3 monitoring well pair and a slight upward
trend was exhibited east of the ash basin from the GWA-7 well pair.
2.3.1.3 Groundwater Sampling
Groundwater samples were collected using low flow sample collection techniques, as outlined in
the Low Flow Sampling Plan developed for the Ash Basin Groundwater Assessment Program
and approved by NCDENR (CSA Report Appendix G). Field water quality parameters were
measured at sampling. The samples were submitted to a laboratory for analysis of total and
dissolved inorganic parameters (USEPA Methods 200.7/200.8, 245.7, and 218.7).
A 0.45-micron filter was used for collection of groundwater samples for dissolved concentration
analysis. During Round 2, additional sample volume was collected at select locations using a
0.1-micron filter and analyzed for dissolved constituent concentration along major flow paths
and at locations with constituent concentrations that may be affected by turbidity. The following
monitoring wells were sampled using the 0.1-micron filter.
• AB-2D
• AB-7D
• C-2S
• AB-2S
• AS-71
• C-21D
• AB-3BR
• AS-2D
• GWA-6S
• AB-3D
• AS-2S
• GWA-6D
• AB-5D
• AS-3D
• GWA-9S
• AB-6BRU
• AS-3SA
• GWA-9D
• GWA-21 S
• MW-9D
• GWA-23BR
• MW-9BR
• GWA-23D
• GWA-21 D
• GWA-23S
• GWA-21 BR
• MW-1 D
• AB-6S
• MW-1 S
Based on review of Round 2 analytical results, minimal to no differences in concentrations were
observed in the samples collected with the 0.45-micron and 0.1-micron filters. The analytical
results comparing the 0.45-micron and 0.1-micron filters are presented in Table 2-2.
Round 1 and Round 2 Source Area and Groundwater Data Comparison
Round 1 and Round 2 sample results are presented in Tables 2-3 through 2-7. Variation from
Round 1 to Round 2 cannot be further interpreted at this time as the data set consists of only
two comprehensive sampling events and therefore does not fully consider seasonal fluctuations,
or other temporal changes.
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2.3.2.1 Source Area Results
Ash Porewater
The ash basin is a permitted wastewater treatment facility and water in the basin is wastewater
not groundwater. The comparison to 2L Standards, IMACs, and NCDHHS HSLs is for
comparison purposes only.
Ash porewater samples were collected in Round 1 and Round 2 from locations within the source
area. Fluctuations in the total number of COls reported at individual wells were noted when
comparing Round 1 and Round 2. No strong correlation can be made between turbidity and the
number of COls exceeding the 2L Standard, IMACs, and NCDHHS HSLs. In some cases
turbidity increased and the number of COls decreased. Ash porewater sample results from
Round 1 and 2 are presented in Table 2-3.
Ash Basin Water
The ash basin is a permitted wastewater treatment facility and water in the basin is classified in
the NPDES permit as wastewater, not groundwater. Ash basin water is compared to both 213
and 2L Standards, IMACs and NCDHHS HSLs for comparison purposes only as ash basin
water is a source of groundwater and surface water impacts.
Two water samples (SW-1 and SW-2) were collected from within the ash basin Secondary Cell
in Round 1. These two locations were not sampled during Round 2, but were sampled in
November 2015 during Round 3. Aluminum, antimony, arsenic, chromium, cobalt, iron, lead,
manganese, nickel, thallium, and zinc concentrations exceeded their respective 213 Standards,
2L Standards, or IMACs in at least one of the two water samples collected from the ash basin
Secondary Cell. Ash basin water sample results from Round 1 and 3 are presented in
Table 2-4.
Groundwater within the Waste Boundary
A total of 24 monitoring wells were sampled within the waste boundary during the Round 2
sampling event. The following six wells were sampled during the Round 1 event but were not
sampled during Round 2 due to active excavation activities: A13-3S, AB-7S, AB-4S, A13-5S, AB-
5SL, and C-1 S. In general, the Round 2 groundwater concentrations within the waste boundary
are stable when compared to the Round 1 data with some COls exhibiting increasing and
decreasing concentrations. The TDS concentrations in Round 2 decreased from Round 1
concentrations in most of the monitoring wells installed in the ash storage area and the ash
basin area. The most significant decrease in TDS was exhibited in monitoring well AS-3SA,
which decreased from 519,000 pg/L in Round 1 to 92,000 pg/L in Round 2. However, the Round
2 TDS concentrations in ash basin well A13-2S increased from 30,000 pg/L to 197,000 fag/L but
did not exceed the 2L standard.
Other COI concentrations that increased in Round 2 include manganese in ash basin area wells
AB-2S and AB-6S; and chromium and iron in cinder storage area well C-2S. The Round 2
concentrations of these COI exceeded their respective 2L standards (Table 2-5).
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2.3.2.2 Groundwater Results
Background Wells
Groundwater samples were collected in Round 1 and Round 2 from background monitoring well
locations within the shallow, deep and bedrock flow layers (Table 2-6). Differences in the total
number of exceedances and COls reported at individual wells were noted when comparing
Round 1 to Round 2. Additional background wells are being installed and will be sampled
starting in spring 2016.
In general, background monitoring wells exhibited similar concentrations and constituents when
comparing data from the Round 1 and 2 sampling events. Background monitoring wells were
also sampled during Round 3 and Round 4. The results of these sampling events are
discussed in Section 2.4. Background monitoring wells will continue to be sampled and PPBCs
will be refined as the data set increases with additional sampling rounds.
Areas Outside of the Waste Boundary
This section provides a brief summary of groundwater analytical results outside the waste
boundary. The waste boundary is designated by the blue line that surrounds the ash
management area as presented on Figure 2-1.
HDR collected groundwater samples from 49 monitoring wells located outside the waste
boundary during Round 2 sampling (Table 2-7). The wells are installed either within the shallow,
deep or bedrock flow layers. The following COls exceeded the 2L standards or IMACs in
groundwater samples collected from wells outside the waste boundary in Rounds 1 and 2:
antimony, chromium, cobalt, iron, manganese, vanadium, thallium and TDS. This list of COls is
consistent with the COls that were reported in CAP Part 1.
Most of the shallow wells exhibited detections of cobalt, manganese, and vanadium in Round 1
and 2 sampling along with detections of iron in Round 2. Iron and manganese were not detected
in shallow wells MW-5S and MW-6, which are located to the northeast of the waste
management area. Vanadium concentrations exceeded the IMAC in all shallow, deep and
bedrock wells except shallow wells to the north, east, and south of the waste management area
(MW-1 S, GWA-8S, GWA-10S, GWA-21 S, GWA-22S, and GWA-23S).
2.3.2.3 Description of Groundwater Quality Standard Exceedances
Per CAMA, the CAP should include "A description of all exceedances of the groundwater quality
standards, including any exceedances that the owner asserts are the result of natural
background conditions." To address this requirement, COls identified during the Round 1 and
Round 2 sampling events were evaluated to determine if they are naturally occurring or
attributable to ash handling at the site.
Results of the COI evaluation are provided in Table 2-8. Only analytical results which exceed
their respective groundwater criteria are presented in this table.
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• Where the exceedance is less than the applicable PPBC, the cell is highlighted green.
The analytical results associated with the green highlighting are likely exceedances
considered attributable to natural background conditions.
Where the COI concentrations are greater than the applicable groundwater standard
and/or PPBC and the cell is highlighted orange. The analytical results associated with
the orange highlighted cells are considered to be exceedances associated with ash
handling at the site.
It is important to note that this evaluation only includes two sampling events and additional
sampling is needed to re-evaluate PPBCs and more appropriately assess COls compared to
PPBCs at the site. Areas of exceedances of COls attributable to ash handling at the RBSS site
are depicted on Figure 3-1.
2.3.3 Surface Water and Areas of Wetness
As part of the CSA activities AOW and surface water samples were collected at RBSS. Three
AOW locations (S-2, S-9, and S-11) were sampled during both Round 1 and 2. As reported in
CAP Part 1, during the CSA locations S-4, S-6, S-7, and S-8 were originally identified as AOW
samples. As discussed in CAP Part 1 Report, Section 2.4, due to the proximity of these
locations relative to Mountain Island Lake, these four AOW locations were compared to the
North Carolina surface water standards as specified in the 213 Standards and as amended
effective January 2015. These four locations are discussed as surface water samples in this
report. The results from the remaining AOW sample locations were compared to 2L Standards.
AOWs reported in CAP Part 1, cobalt, hexavalent chromium, iron, manganese, and vanadium
were identified as COls in AOWs during Round 1 sampling. These COls, except hexavalent
chromium, also exceeded their respective 2L Standards or IMACs in most sample locations
during Round 2. In addition, TDS exceeded the 2L Standard in AOW sample S-2. The TDS
concentration in sample S-2 increased from 195,000 pg/L in Round 1 to 51,600,000 pg/L in
Round 2.
Water flows discharging from AOWs were measured by field personnel at several AOW
locations during April 2014 and November 2015. In general, the AOW flows in November 2015
were lower than the flows measured in April 2014, although the flow at S-2 increased slightly.
AOW S-7 exhibited the greatest decrease in flow from 23 gallons per minute in April 2014 to
7.6 gallons per minute in November 2015. In general, the COls exhibited higher concentrations
during the lower water flow rates, and lower concentrations during greater water flow rates.
AOW locations S-3, S-5 and S-12 exhibited no flow during the November 2015 monitoring
event. The fluctuating flow rates are likely attributed to seasonal trends in precipitation and
water surface runoff infiltrating into the subsurface.
Four AOW samples (S-4, S-6, S-7, and S-8) and surface water sample SW-3 were collected
during the Round 1 and Round 2 sampling events. Surface water locations RBSW001 and
RBSW002 were only sampled during the Round 1 event. The surface water samples in Round 1
exhibited exceedances of 213 Standards for aluminum, chromium, cobalt, cadmium, copper,
iron, lead, manganese, selenium, thallium, vanadium and zinc. Concentrations of COls in
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Round 2 samples were higher than Round 1. Additional Cols exceeded their respective 213
standards during Round 2, including arsenic, barium, and nickel. The majority of these COls
were from sample locations S-6 and S-7 located near Mountain Island Lake northeast of the
primary ash basin.
Round 1 and Round 2 analytical results for the surface water and AOW samples are presented
in Table 2-9.
Following the removal of the source areas from RBSS and final grading as part of site
restoration, the current AOWs may be eliminated, modified, or remain as surface expressions of
groundwater in their current locations. Interim and Effectiveness Monitoring outlined in Section
9.0 recommends further inventory, identification and monitoring, if necessary, of AOWs.
2.4 Round 3 and Round 4 Background Well Sampling
In response to a Duke Energy request for clarification of guidance, NCDEQ provided a table
titled "Clarification of Attachment 1 Groundwater Assessment Plan Conditional Letters of
Approval Items Related to Speciation — May 22, 2015" by electronic mail.
In the responses provided in this table, NCDEQ requested that Duke Energy "plan to sample the
existing and newly installed background wells two (2) additional times during 2015 as part of an
anticipated corrective action measure to support USEPA tiered site analysis and statistical
analysis". The two additional sampling events referenced in this response correspond to
background sampling Round 3 and Round 4, performed in November and December 2015,
respectively.
The groundwater analytical parameters and methods are detailed in CSA Report Table 7-3.
Groundwater samples were collected in accordance with sampling procedures described in the
CSA. The results of the Round 3 and Round 4 background well sampling event are presented in
Table 2-5. Further evaluation of background sample results and PPBCs will be provided in
subsequent reports.
2.5 Well Abandonment
Due to source removal several monitoring wells within the source area were abandoned.
Monitoring wells AS-1 S, AS-1 D, AS-2D, AS-3D and AS-3SA have been abandoned. Well
abandonment forms can be found in Appendix A, Attachment 4.
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3 Site Conceptual Model
The SCM was initially presented in the CSA, and refined based on results from additional
sampling events and refined groundwater modeling. The SCM for RBSS was developed in
general accordance with ASTM International standard guidance document E1689-95
(Reapproved 2014), Standard Guide for Developing Conceptual Site Models for Contaminated
Sites (ASTM 2014), to describe and integrate processes that determine contaminant releases,
contaminant migration, and environmental receptor exposure to contaminants. The SCM is used
to integrate site information and determine whether additional information may be needed to
further understand site hydrogeologic and potential contaminant migration processes. The
model is also used to support selection of remedial alternatives and effectiveness of remedial
actions in reducing the exposure of environmental receptors to contaminants. The SCM was
developed using the six basic activities outlined in ASTM E1689-95:
• Identification of potential contaminants;
• Identification and characterization of the source(s) of contaminants;
• Delineation of potential migration pathways through environmental media;
• Establishment of background areas;
• Environmental receptor information; and
• Determination of system boundaries.
An expanded discussion of site geochemical controls on contaminant mobility and migration is
also provided in this section, as requested by the NCDEQ. A graphical representation of the
SCM is included as Figure 3-1.
3.1 Identification of Potential Contaminants
Potential contaminants (COls) were identified in the CSA Report and are summarized in
Section 2.1 of this report.
3.2 Identification and Characterization of Source
Contaminants
The source areas at RBSS are defined as the ash basin (Primary and Secondary cells), ash
storage area, and the cinder storage area (see Figure 2-1). Source characterization was
performed through the completion of soil and rock borings, installation of monitoring wells, and
collection and analysis of associated solid- and aqueous -matrix samples to identify physical and
chemical properties of ash, ash basin water, ash porewater, and AOWs outside the ash basins
(Figure 3-1). A geologic cross-section through the source areas is included as Figure 3-2.
Round 1 and Round 2 analytical results for ash porewater and ash basin water are provided in
Table 2-3 and 2-4.
Ash distribution and chemical and physical properties were evaluated through advancement and
sampling of 17 borings within the ash basin, 6 borings in the ash storage area, and 4 borings
within the cinder storage area. Ash within the ash basin was encountered to depths ranging
from the surface to approximately 76 feet below ground surface (bgs). Ash within the ash
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storage area was encountered from the surface to a maximum depth of 78 feet bgs. Within the
cinder storage area ash was encountered from the surface to a maximum depth of 14.5 feet
bgs.
Ash porewater was evaluated through the sampling of 7 monitoring wells installed within the ash
basin. Ash basin water was evaluated through sampling and analysis of two ash basin water
samples.
Based on the CSA results, groundwater impacts attributable to the source areas were identified
beneath the ash basin, ash storage area, and cinder storage area. The need for further
refinement of exceedences was identified and is being conducted in the area downgradient of
the cinder storage area. COI transport is generally in a northern, northwestern, and northeastern
direction toward Mountain Island Lake.
Analytical results of samples collected from the source areas were reviewed to identify COls, as
follows:
• Seven COls were identified in ash based on comparison to the NC PSRGs for POG:
antimony, arsenic, cobalt, iron, manganese, selenium, and vanadium.
• Eleven COls were identified in ash porewater samples based on comparison to 2L
Standards and IMACs: antimony, arsenic, boron, cobalt, iron, manganese, pH, sulfate,
thallium, TDS, and vanadium.
• Sixteen COls were identified in ash basin water samples based on comparison to both
2L Standards or IMACs for groundwater and 2B Standards/ USEPA National Water
Quality Criteria: aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium,
cobalt, copper, iron, lead, manganese, nickel, thallium, vanadium, and zinc.
3.3 Delineation of Potential Migration Pathways through
Environmental Media
3.3.1 Soil
The approximate horizontal extent of soil impacts was delineated during the CSA and is
generally limited to the area beneath the ash basin and one location along the waste boundary
south of the ash storage area. Where soil impacts were identified, the approximate vertical
extent of contamination beneath the ash/soil interface is generally limited to the uppermost soil
sample collected beneath ash. Cols identified in soil include arsenic, boron, cobalt, iron,
manganese, selenium, and vanadium. At the RBSS site, the soil located beneath the ash basin
could be considered as a potential secondary source. As discussed with NCDEQ, after
excavation, soils left on -site will be sampled and analyzed, and the analytical results will be
incorporated into the groundwater contaminant fate and transport model. If this evaluation
indicates that modification to the proposed CAP is required, Duke Energy will prepare and
submit a revised CAP.
Further assessment west of the cinder storage area is underway as recommended in the CSA
Report. Results of this assessment will be reported under separate cover.
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Groundwater
Site hydrogeologic conditions were evaluated through the sampling of 78 monitoring wells
during Round 1 sampling and 81 monitoring wells during Round 2 sampling. The wells were
screened within the shallow, deep, and bedrock flow layers beneath the site. Based on the site
investigation, the groundwater system in the natural materials (alluvium, soil, soil/saprolite, and
bedrock) at RBSS is consistent with the LeGrand slope -aquifer system and is an unconfined,
connected aquifer system.
In general, groundwater within the shallow, deep, and bedrock flow layers flows from the
southern extent of the RBSS site property boundary to the north, northeast, and northwest and
discharges into Mountain Island Lake. Flow contours developed from groundwater elevations
measured in the shallow and deep wells in the southeastern portion of the site depict
groundwater flow generally to the northeast discharging to Mountain Island Lake. Groundwater
flow direction in the shallow, deep flow layers based on water levels gauged during the Round 2
sampling event (August 2015) are shown on Figures 2-2 and 2-3, respectively.
The approximate horizontal extent of groundwater impacts is limited to beneath the waste
boundary and northeast of the ash basin. The extent of groundwater impacts in the following
areas requires refinement as noted below:
horizontal and vertical extent west of the ash and cinder storage areas near well GWA-
3SA/D; and
• horizontal and vertical extent to the south of the ash storage area.
The approximate vertical extent of groundwater impacts is generally limited to the shallow and
deep zones (although a transition zone as defined in the CSA is absent at RBSS), and vertical
migration of COls is impeded by the underlying bedrock.
The bedrock flow layer is defined by data obtained from the bedrock groundwater monitoring
wells (BR or BRU wells). Groundwater contours developed from the groundwater elevations in
the bedrock wells show groundwater flowing generally in a north/northwest direction from the
south side of the RBSS site discharging to Mountain Island Lake. Groundwater flow direction in
the bedrock flow layer is illustrated on Figure 2-4.
Following excavation of the ash basin, stabilization of groundwater elevations, and evaluation of
analytical results re-evaluation of the SCM will be required. Groundwater elevation
measurements, AOW inventory, and groundwater contours mapping will likely be warranted to
determine if changes that have occurred to groundwater or exposure routes described in the
SCM.
3.3.3 Surface Water and Sediment
Two surface water samples were collected from the intake channel: RBSW001 and RBSW002.
COls exceeding their respective 2B Standards include aluminum, cadmium, copper, lead, and
zinc. Surface water generally flows from the basins to Mountain Island Lake as shown on
Figure 3-1.
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Sediment samples were collected from 11 locations, including dry AOWs (S-1, S-3, S-10, and
S-12), during the CSA. COls exceeding the NC PSRGs for POG in sediment samples include
arsenic, barium, boron, cobalt, iron, manganese, and vanadium. Cobalt, iron, manganese, and
vanadium concentrations exceeded the NC PSRGs for POG, but are also naturally occurring
constituents in background soil. A summary of COls related to surface water and sediment from
AOW samples is provided in Section 2.3.2.
Ash basin water from within the waste boundary and groundwater will be removed as needed
during ash excavation activities. During excavation, new AOWs may arise and existing AOW
may no longer be present. Surface water for existing AOWs and sediment at dry AOWs
identified in the CSA may continue to contribute to groundwater through infiltration, reaching
groundwater and ultimately moving toward Mountain Island Lake. It is anticipated that the
decrease in ash basin water levels will cause a decrease in flows at these AOWs. Sediment that
exceeds the NC PSRGs for POG can also contribute to groundwater concentrations. The effect
of sediment COls on receptors is evaluated in the risk assessment (see Section 5). During
excavation, the processes that govern COI migration will fluctuate, and therefore, the influence
of surface water and sediment COI migration to Mountain Island Lake cannot be fully
ascertained until source removal is complete.
3.4 Establishment of Background Areas
Background areas at the RBSS site are located south and beyond the immediate boundary of
the ash storage area and south of Horseshoe Bend Beach Road (Figure 2-1). In addition to the
existing NPDES ash basin background compliance well, monitoring wells installed as
background wells during the CSA are not in locations that represent true upgradient,
background groundwater conditions (See Figures 2-2, 2-3, and 2-4). A detailed background
monitoring well assessment is presented in Appendix B in the CAP Part 1 Report. As a result of
this assessment, Duke Energy commenced installation of additional monitoring wells in January
2016 to aid in the evaluation of naturally occurring COls upgradient of the facility. Once the well
installation is complete and groundwater sampling results are available, refinement of
groundwater flow direction and distribution and influence of naturally occurring COls will be re-
evaluated.
3.5 Environmental Receptor Identification and Discussion
Duke Energy conducted a receptor survey of the area within 0.5 mile of the Compliance
Boundary in September 2014, and subsequently supplemented the receptor survey in
November 2014. Receptor locations identified during the surveys are shown on Figure 3-3.
Properties located within a 0.5-mile radius of the RBSS site generally consists of residential
properties, undeveloped land, and Mountain Island Lake (Figure 3-4). Properties north of
Mountain Island Lake are located in the Town of Huntersville, Mecklenburg County, North
Carolina. The Town of Huntersville identifies most of these properties as a park, nature
preserve, or wildlife refuge. A residential property is located to the northeast of the ash basin on
the northern side of Mountain Island Lake.
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Properties south of Mountain Island Lake are located in Mount Holly, Gaston County, North
Carolina. The majority of properties in this area are owned by Duke Energy and are associated
with RBSS. Residential properties are located south and southeast of the RBSS site (south of
Horseshoe Bend Beach Road).
One well was identified north of Mountain Island Lake. Based on the topography on the north
shore, groundwater flow is toward Mountain Island Lake. This observation, combined with the
distance away from the RBSS site, suggests that this well is not hydraulically connected with
groundwater at RBSS. No water supply wells (including irrigation wells and unused or
abandoned wells) were identified between the source area and Mountain Island Lake.
Mountain Island Lake supplies water to the Charlotte municipal area, as well as the towns of
Gastonia and Mount Holly, North Carolina. The Charlotte intake is located 3.4 miles
downstream of the RBSS site and the Gastonia and Mount Holly intakes are located
approximately 6.9 miles downstream of the RBSS site. Water supply intake locations are shown
on Figure 3-5.
3.6 Determination of System Boundaries
The site, waste, and Compliance Boundaries for the RBSS site are shown on Figure 2-1.
Spatially, the SCM for RBSS is bounded by Mountain Island Lake to the north and west and
topographic divides to the east and south of the site. The SCM extends into bedrock, which
inhibits vertical migration of COls at the site.
3.7 Site Geochemistry and Influence on COls
As excavation activities continue at RBSS, geochemistry in the ash management areas and the
excavation areas will change. Geochemistry described within this CAP Part 2 Report represents
a snapshot of what is occurring at RBSS.
Groundwater composition can be affected by an array of naturally occurring and anthropogenic
factors. Many of these factors can be causative agents for specific oxidation- reduction (redox)
processes or indicators of the implied redox state of groundwater as expressed by pH,
oxidation-reduction potential (ORP; expressed as Eh), and dissolved oxygen (DO).
Groundwater pH is affected by the composition of the bedrock and soil through which the water
moves as well as other factors, including lime -containing materials in well casings, exposure to
atmospheric carbon dioxide gas, and precipitation. In addition, metals and other elemental or
ionic constituents in groundwater, or the surrounding soil matrix, can act as electron donors or
acceptors as measured by ORP. The reactivity of different constituents can lead to oxidizing
(positive ORP) or reducing (negative ORP) environments in groundwater systems. DO in
groundwater can act as an oxidizing agent and is an indicator of a redox state.
Based on field measurements at RBSS, the predominant redox category is anoxic/mixed and
the predominant redox processes are ferrous iron/ferrous sulfate, so the reduced species
As(III), Se(IV), and Mn(IV) would be expected. The redox conditions appear to be controlled at
least partly by the SO4/S2 and Fe(III)/Fe(II) redox couples, and these redox couples should be
monitored to assess changing redox conditions.
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Chemical speciation measurements were determined for 37 groundwater and/or ash porewater
monitoring wells samples, and redox calculations were performed for each of those samples. Of
the 36 speciation samples, 14 were oxic and 19 were anoxic or mixed samples. Those
measurements indicate that pH ranges from 3.56 to 12.50 standard units (SU). It is anticipated
that high pH ranges are related to grout from the well installation and are not representative of
groundwater. In contrast, background well results indicate that pH ranges from 5.75 to 8.20 SU,
whereas pH within the ash basin materials ranges from 5.10 to 11.91 SU. A wide range of ORP
values was measured; in most cases, the ORP value ranges implied highly reduced (large
negative values) to highly oxidized (large positive values) environments. Standard (equilibrium)
electrode potentials for such reactions may be expected to be approximately -1,000 millivolts. In
contrast, measured ORP values at the RBSS site were never less than -270 millivolts.
Totals and speciation were measured in the lab or calculated for:
• Total arsenic, As(III), and As(V)
• Total chromium, Cr (III) and Cr (VI)
• Total iron, Fe (II), and Fe (III)
• Total manganese, Mn (II), and Mn (IV)
• Total selenium, Se (IV), and Se (VI)
Speciation analytical results are summarized below:
Reduced arsenic [As (III)] was detected less frequently than oxidized arsenic [AS (V)].
As (III) concentrations were about half of the As (V) concentrations. Also, As (III)
concentrations decreased in reducing conditions, which is counterintuitive, and may be
due in part to the limited number of As (III) samples available for comparison (two oxic
and four anoxic).
• Total chromium was detected in all samples except A13-613RU. Hexavalent chromium [Cr
(VI)] was detected in 32 of 37 speciation samples, including samples from anoxic
groundwater conditions. The average concentration of Cr (VI) was higher in anoxic
versus oxic groundwater conditions. There was also a corresponding increase in total
chromium concentrations in anoxic versus oxic groundwater conditions, which may in
part explain why Cr (VI) concentrations increased under these same conditions. Cr (VI)
concentrations were partly proportional to total chromium concentrations. Other factors,
such as redox influence on solid media phases that adsorb Cr (VI), also likely influenced
Cr (VI) concentrations at the RBSS site.
Iron speciation results showed similar trends as manganese. The reduced form of iron
[Fe(II)] was mainly present downgradient and southeast of the ash basin.
• Manganese speciation results did not exhibit the high reduced form of manganese
[Ms (II)] concentrations that would be expected in an anoxic (reducing) environment.
• Ash porewater speciation results indicate a favorability of the reduced form of selenium
[Se (IV)] over the oxidized form of selenium [Se (VI)].
In summary, the observed groundwater conditions at the RBSS site span oxidizing to
moderately reducing conditions. A review of equilibrium chemistry shows some oxidized species
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Corrective Action Plan Part 2
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(e.g., Cr (VI)) present in reduced conditions and some reduced species (e.g., Se (IV)) in oxic
conditions. In general, the observed groundwater conditions, showing a mixture of redox
conditions and variability in species, indicate a dynamic redox environment at the RBSS site
and that conditions are not in equilibrium for some COls.
Given this diverse range of conditions at the RBSS site, further evaluation and modeling of
geochemistry will be completed using the PHREEQC (Parkhurst and Appelo 2013) modeling
tool and groundwater transport and chemical transport modeling. The results of geochemical
and groundwater modeling and their relevance to remedial alternatives are presented in
Section 4.
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4 Modeling
4.1 Groundwater Model Refinement
The groundwater flow and fate and transport model was refined to incorporate post-CSA data.
Model refinements are summarized in the following sections. The refined groundwater flow and
transport model report was completed by HDR in conjunction with the University of North
Carolina at Charlotte (UNCC). An independent review of the refined RBSS model was
conducted by the Electric Power Research Institute (EPRI) and found that the model was
sufficient to meet the objective of predicting effects of corrective action alternatives on
groundwater quality. The refined groundwater flow and transport model report and the EPRI
review of the calibrated RBSS model are provided in Appendix B.
4.1.1 Flow Model Refinements
Transient transport simulations for all COls were calibrated and flow parameters were refined as
follows:
• Hydraulic conductivity measurements, obtained from slug test data collected during the
CSA, were utilized in the calibration of the flow model to better represent site -specific
conditions. This refinement led to reduction in the square root of the average square
error (also referred to as the root mean squared error, or RMS error) of the modeled
versus observed water levels for wells gauged in June 2015 to 5.5% compared to the
initial calibrated model of 9.95% in the CAP Part 1 model. The model calibration goal is
an RMS error less than 10% of the difference in head between the modeled and the
observed head across the model domain. The results are provided in Table 3 in
Appendix B.
• Recharge rates for the model were also refined within the ash basin footprint and also
for the area beyond the ash basin waste boundary. Recharge within the ash basin
footprint was calculated using Darcy's Law considering the approximate area of the
Primary and Secondary Cells, the approximate depth of water or saturated ash, and the
range of measured hydraulic conductivity values within the ash and fill. The mean annual
recharge in the Piedmont ranges from 4.0 to 9.7 inches per year (Daniel 2001). The
recharge rate applied in the groundwater model was 21.5 inches/year within the ash
basin footprint and 6.5 inches/year across the rest of the model domain. These model
refinements affect fate and transport of COls at the site and are representative of current
site conditions.
Historic basin water levels were considered during calibration of the flow model.
However, the current flow model is calibrated to steady-state conditions. Continued
refinement of the model to consider transient flow may enhance integration of historic
water level data within the ash basin.
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4.1.2 Fate and Transport Model Refinements
The groundwater fate and transport model was calibrated using refined parameters from the
groundwater flow model as discussed in Section 4.1.1 and presented below.
The initial model used conservative (low) sorption coefficient (Kd) values to achieve
calibration of the transport models for each COI. Subsequent to submittal of the CAP
Part 1 Report, UNCC and Geochemical, LLC each provided recalculated Kd values using
linear and Freundlich isotherms (Appendix C). Both sets of recalculated Kd values were
considered during refinement of the transport models for each COI. Use of the newly
derived COI Kd values in the fate and transport models resulted in improved calibration
of source concentrations to measured concentrations in downgradient wells. Note that
final Kd values used to calibrate the fate and transport models may have fallen outside
the recalculated upper and lower limits; however, adjustment of Kd values within the
model to achieve calibration is considered acceptable practice.
• The initial model used adjusted source area concentrations to achieve calibration at
downgradient monitoring wells. The flow model refinements discussed in Section 4.1.1
enabled refinement of the fate and transport model to better represent measured source
area porewater concentrations.
The initial model was not calibrated to background groundwater concentrations as
PPBCs were not developed in time for use in the model. The model has since been
refined to incorporate PPBCs for each COI. This refinement allows the model to account
for naturally occurring background concentrations and is particularly important for COls
whose PPBC is greater than the 2L Standard, IMAC, or NCDHHS HSL. However, the
model is limited in that it applies the PPBC across the entire site, as shown on individual
COI concentration figures in Appendix B.
Additional COls were added to the modeling scenario for COls identified within the
waste boundary including: Arsenic, boron, hexavalent chromium, sulfate, thallium, and
vanadium.
• The background concentrations for the COls were applied as initial concentrations.
Refinements to the groundwater model provide a more accurate representation of existing site
conditions and produce model results that more accurately depict closure scenarios at the site.
4.1.3 Summary of Modeled Scenarios
Two closure scenarios were modeled for RBSS: an Existing Conditions scenario with ash
sources left in place and an Excavation scenario with the accessible ash removed from the site.
These simulations predict flow and transport results using the model parameters calibrated for
existing conditions. Once the scenario for corrective action is selected, the model should be
revised and recalibrated to improve its accuracy and reduce its uncertainty. No modifications
were made to the previously modeled Existing Conditions scenario hydrogeologic parameters or
structure.
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4.1.3.1 Existing Conditions Scenario
The Existing Conditions scenario consists of using the calibrated model for steady-state
groundwater flow conditions and transient transport of COls under existing conditions across the
site to predict when steady-state concentrations are reached at the Compliance Boundary. COI
concentrations remain the same or increase initially for this scenario with source concentrations
being held at a constant value over time. Thereafter, the concentrations and discharge rates
remain constant. This scenario represents the most conservative case in terms of groundwater
concentrations on- and off -site, with COls discharging to surface water at a steady-state.
The time to achieve a steady-state concentration plume depends on the source zone location
relative to the Compliance Boundary and its loading history. Areas close to the Compliance
Boundary will reach a steady-state concentration sooner. The time to steady-state concentration
is also dependent on the sorptive characteristics of each COI. Sorptive COls will be transient for
a longer time period as their peak breakthrough concentration travels at a rate that is less than
groundwater pore velocity. Use of lower effective porosity values will result in shorter times to
achieve steady-state concentrations for both sorptive and non-sorptive Cols.
4.1.3.2 Excavation Scenario
In the Excavation scenario, the water in the ash basin is removed and the ash from the ash
basin, cinder storage area, and ash storage area is removed and transported off -site. In the
model, the constant concentration sources of ash above and below the water table are
removed. The flow parameters for this model scenario are identical to the Existing Conditions
scenario, except for removal of ash layers. This scenario assumes in -basin recharge rates
become equal to rates surrounding the ash basin (6.5 inches per year). Starting from the time
that excavation is complete, COls already present in the groundwater and not removed by
geochemical processes continue to migrate downgradient as clean water infiltrates from ground
surface and recharges the aquifer at the water table. The COls are removed from the saturated
zone beneath the source areas. COls with high Kd values will have migration retarded relative to
the ash porewater velocity as sorptive COls are attenuated by site materials. The model uses
the predicted concentration from the 2015 calibration as the initial COI concentrations.
4.1.4 Model Assumptions and Limitations
The model assumptions include the following:
• The steady-state flow model was calibrated to hydraulic heads measured at observation
wells in June 2015 and considered the ash basin water level. The model is not calibrated
to transient water levels over time, recharge, river flow, or river stage (i.e., Mountain
Island Lake) changes. A steady-state calibration does not consider groundwater storage
and does not calibrate the groundwater flux into adjacent surface water bodies.
• A single domain MODFLOW modeling approach was used for simulating flow in the
primary porous groundwater flow layers.
• During model calibration, the constant source concentrations at the ash basins and ash
storage area and cinder storage area reasonably match 2015 COI groundwater
concentrations.
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• For the purposes of numerical modeling and comparing closure scenarios, it is assumed
that the selected closure scenario is implemented in 2015.
• Predictive simulations were performed and steady-state flow conditions were assumed
from the time the ash basins and ash storage areas were placed in service through the
current time until the end of the predictive simulations (Year 2265).
• COI source zone concentrations within the waste boundary were applied uniformly within
each source area and assumed to be constant with respect to time for transport model
calibration.
• Since Mountain Island Lake is modeled as a constant head boundary in the numerical
model, it will not be possible to assess the effects of pumping wells or other groundwater
sinks that are near the river.
• Travel times are advective and do not account for sorption of COls to host rock, which
may cause the travel times to be reduced.
• The model does not predict co -precipitation of COls with iron and manganese.
Therefore, COI concentrations generated by the model may be over -estimated (Section
4.3); however, geochemical modeling was conducted to supplement the groundwater
modeling and is discussed further in Section 4.3.
• The model does not account for varying geochemical conditions such as pH and redox
potential that could affect COI mobility; however, geochemical modeling was completed
and is further discussed in Section 4.3.
4.1.5 Modeled Scenario Results
Constituent concentrations were analyzed at three downgradient monitoring wells (MW-3S,
MW-5S, and MW-6S) for all COls except hexavalent chromium (Appendix B, Figure 6).
Hexavalent chromium analysis was limited to downgradient monitoring well MW-91D (Appendix
B, Figure 7).
Closure scenario results are presented as predicted concentration versus time curves in
downgradient monitoring wells and as groundwater concentration maps for each of the seven
modeled COls on Figures 15 through 121 in Appendix B, as discussed in the following
subsections. Concentration contours and concentration breakthrough curves are referenced to
1957, the year that the ash basin became effective. Concentration contours and concentration
breakthrough curves are referenced to a time zero that represents the time the closure action
was implemented, which for the purposes of modeling is assumed to be 2015.
A summary of the modeled COI results at the Compliance Boundary is provided in Table 4-1. A
"+"indicates that the concentration of a given COI has exceeded its applicable 2L Standard,
IMAC, or NCDHHS HSL. A "-" indicates that the concentration of a given COI has not exceeded
its applicable 2L Standard, IMAC, or NCDHHS HSL.
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Corrective Action Plan Part 2
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Table 4-1 Summary of Modeled COI Results at the Compliance Boundary
Constituent
Appendix
B Figures
Flow
Layer
Existing Conditions
Scenario
Excavation Scenario
Year
0
(2015)
Year
100
(2115)
Year
0
(2015)
Year
100
(2115)
Antimony
IMAC
(1 pg/L)
15 — 26
Shallow
+
+
+
+
Deep
+
+
+
+
Bedrock
+
+
+
+
Arsenic
2L
(10 pg/L)
27 — 38
Shallow
Deep
Bedrock
Boron
2L
(700 pg/L)
39 — 50
Shallow
Deep
Bedrock
Chromium
2L
(10 pg/L)
51 — 62
Shallow
Deep
Bedrock
Cobalt
IMAC
(1 pg/L)
63 — 74
Shallow
+
+
+
+
Deep
+
+
+
+
Bedrock
+
+
+
+
Hexavalent
Chromium
NCDHHS HSL
(0.07 pg/L)
75 — 84
Shallow
+
+
+
+
Deep
+
+
+
+
Bedrock
+
+
+
+
Sulfate
2L
(250,000 pg/L)
85 — 96
Shallow
Deep
Bedrock
Thallium
IMAC
(0.2 pg/L)
97 — 108
Shallow
+
+
+
+
Deep
+
+
+
+
Bedrock
+
+
+
+
Vanadium
IMAC
(0.3 pg/L)
109 — 120
Shallow
+
+
+
+
Deep
+
+
+
+
Bedrock
+
+
+
+
The model predictions are summarized as follows:
In accordance with 15A NCAC 02L .0106 (k), a CAP may be approved by NCDEQ
without requiring groundwater remediation to the 2L Standards if seven conditions are
met. Condition (4) specifies that 2L Standards must be met at a location no closer than
one year time of travel upgradient of an existing or foreseeable receptor. For RBSS, the
receptor is considered to be Mountain Island Lake. To evaluate this condition, HDR and
UNCC conducted particle tracking using the excavation steady-state flow field to identify
the one-year travel time boundary. Particles were placed at select wells located near
Mountain Island Lake and also at the side -gradient ends of the ash basin. The advective
travel time one year from each well was performed using MODPATH and is shown on
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Corrective Action Plan Part 2
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Figure 14 in Appendix B. Based on the results of particle tracking, boron exceeds its 2L
Standard beyond the one-year travel time boundary, and thus, remediation would be
required to meet the 2L Standard. Thus, potential groundwater extraction wells were
added to the model to evaluate the degree of pumping required to provide hydraulic
control of the boron plume such that the 2L Standard is met at one year's advective
travel time upgradient of Mountain Island Lake. The monitoring locations are shown on
Figure 121 in Appendix B and are labeled as extraction wells for the purpose of the
simulation.
• The simulation was performed using six wells pumping at a rate of 3 gallons per minute.
Results of the simulation show that the modeled well configuration and pumping rate
would not adequately capture groundwater in the shallow zone that has been impacted
by the ash basin and other source areas. If Duke Energy were to pursue remediation
under 15A NCAC 02L .0106 (k), a more detailed modeling analysis would be needed to
predict recovery rates and design an efficient pumping recovery system.
• The model predicts that under the Existing Conditions and Excavation scenarios,
antimony, cobalt, thallium, and vanadium exceed their respective IMACs at Mountain
Island Lake. Also, hexavalent chromium is predicted to exceed the NCDHHS HSL at
Mountain Island Lake. For these Cols, the background concentrations used for
modeling exceed the applicable groundwater standards, so the actual impact of the site
sources on groundwater quality is in part related to background conditions. Further
sampling of background wells, statistical evaluation, and geochemical modeling will
provide further insight on contributions from the source area.
• Model predictions do not show that COI concentrations will be effectively reduced by ash
removal under the Excavation scenario. The COls that are predicted to exceed their
respective 2L Standard, IMAC, or NCDHHS HSL will not achieve compliance within the
time period modeled (2015-2265).
• The model predicts that under the Existing Conditions and Excavation scenarios,
arsenic, boron, chromium, and sulfate will not exceed their respective 2L Standards at
Mountain Island Lake.
Among the COls, sulfate and boron are similar in that both are considered conservative;
that is, neither of these COls has a strong affinity to attenuate or adsorb to soil/rock
surfaces. As a result, the model predicts similar behavior for sulfate, boron, and other
COls with low Kd values: rapid and nearly complete reduction to below the respective
standard or IMAC under the Excavation scenario.
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4.2 Surface Water Model Refinement
4.2.1 Methodology
The methodology to complete the surface water model in CAP Part 2 (Appendix D) is
consistent with CAP Part 1 and incorporates new groundwater modeling results addressed in
Section 4.1, including:
• Revisions to the Kd values;
• Updated groundwater flux data for input into the surface water model; and
• Additional COls based on review of Round 2 sampling data or as requested by NCDEQ.
New data were used to evaluate potential surface water impacts of COls in groundwater as they
discharge to surface water bodies adjacent to the RBSS site.
Groundwater to surface water interactions were completed using groundwater model output and
a surface water mixing model approach to evaluate potential surface water impacts of COls in
groundwater as they discharge to surface water bodies adjacent to the RBSS site and within the
Compliance Boundary.
Assessment of surface water quality was performed for concentrations and mass flux of COls
to Mountain Island Lake and separately for local groundwater loads to a small, semi -enclosed
basin (hereafter, "East Basin") located on the downstream (east) side of the RBSS site
(Figure 2-1). The East Basin receives limited upstream inflow through a narrow, shallow
channel and connects back to Mountain Island Lake downstream. Mountain Island Lake,
upstream of the East Basin inflow channel, is influenced by Cols from local groundwater inflow,
and COls in the upstream section will flow into the East Basin. The East Basin inflow channel
and the basin itself are influenced further by COls from local groundwater inflow and from the
discharge of a small, unnamed tributary (Figure 2-1).
Groundwater loading of COls to Mountain Island Lake and the East Basin were calculated as
the product of volumetric groundwater fluxes and corresponding COI concentrations calculated
with the groundwater model.
4.2.2 Results
The mixing model results indicate that impacts from groundwater exceedances do not cause
violation of 2B surface water quality standards at the edge of the mixing zones. The calculated
surface water COI concentrations in Mountain Island Lake downstream of the RBSS site and
separately for the East Basin are presented in Table 4-2 and Table 4-3. The river flows,
upstream surface water concentrations, groundwater flows, and groundwater COI
concentrations presented in Appendix B and Appendix D were used to complete these
calculations.
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Corrective Action Plan Part 2
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Table 4-2 Mountain Island Lake Surface Water Concentrations
Calculated Mixing Zone Conc. (Ng/L)
Water Quality Standard (Ng/L)
COI
Acute
Chronic
HH/WS
Acute
Chronic
HH / WS
Antimony
0.273
0.253
0.253 (nc)
NS
NS
640 / 5.6
Arsenic
0.272
0.252
0.250 (c)
340
150
10 / 10
Boron
27.5
25.3
25.0*
NS
NS
ns / ns
Total Chromium
0.633
0.514
0.501 *
NS
NS
ns / ns
Hexavalent
Chromium
0.573
0.508
0.500*
16
11
ns / ns
Cobalt
0.423
0.269
0.269 (nc)
NS
NS
4/3
Sulfate
1,233
580
580 (nc)
NS
NS
ns / 250,000
Thallium
0.055
0.051
0.051 (nc)
NS
NS
0.47 / 0.24
Vanadium
0.518
0.502
0.500*
NS
NS
ns / ns
Notes:
1. All COls are shown as dissolved except for total chromium
2. WS -water supply (15A NCAC 02B .0216, amended effective January 1, 2015)
3. HH - human health (15A NCAC 02B .0211, amended effective January 1, 2015)
4. c - carcinogen
5. nc - non -carcinogen
6. ns - no water quality standard
7. * - concentration calculated with annual mean river flow
Table 4-3 East Basin Surface Water Concentrations
Calculated Mixing Zone Conc. (pg/L)
Water Quality Standard (pg/L)
COI
Acute
Chronic
HH / WS
Acute
Chronic
HH/WS
Antimony
0.369
0.264
0.264 (nc)
NS
NS
640 / 5.6
Arsenic
0.360
0.263
0.251 (c)
340
150
10 / 10
Boron
40.1
26.8
25.1*
NS
NS
ns / ns
Total Chromium
1.157
0.579
0.504*
NS
NS
ns / ns
Hexavalent
Chromium
0.512
0.502
0.500*
16
11
ns / ns
Cobalt
1.206
0.365
0.365 (nc)
NS
NS
4/3
Sulfate
4,132
937
937 (nc)
NS
NS
ns / 250,000
Thallium
0.072
0.053
0.053 (nc)
NS
NS
0.47 / 0.24
Vanadium
0.601
0.512
0.501 *
NS
NS
ns / ns
Notes:
1. Mixing zone concentrations assume that 10% of upstream Catawba River flow enters the East Basin through the inflow
channel.
2. All COls are shown as dissolved except for total chromium
3. WS -water supply (15A NCAC 02B .0216, amended effective January 1, 2015)
4. HH - human health (15A NCAC 02B .0211, amended effective January 1, 2015)
5. c - carcinogen
6. nc - non -carcinogen
7. ns - no water quality standard
8. * - concentration calculated with annual mean river flow
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4.3 Geochemical Modeling
4.3.1 Objective
The objective of geochemical modeling is to describe the expected partitioning of COls between
aqueous and solid phases (i.e., between groundwater and soil and between ash porewater and
ash) and anticipated changes in phase distributions given variations in dissolved oxygen (DO),
pH, and TDS. Changes in DO affect the oxidation state of groundwater as measured by ORP,
which is generally expressed as Eh or electron activity (pE). Changes in pH affect the acidity of
groundwater and concurrently affects Eh. Changes in TDS affect ionic strength and ion
competition at sorption sites. COls evaluated for RBSS were: arsenic, antimony, boron,
chromium, cobalt, iron, manganese, pH, sulfate, TDS, thallium, and vanadium.
4.3.2 Methodology
Site -specific evaluations were performed for each of the monitoring wells using the United
States Geological Survey (USGS) PHREEQC (0.3.3) geochemical speciation code (Parkhurst
and Appelo 2013) and PhreePlot (Kinniburgh and Cooper 2011), a companion plotting package
that utilizes looping PHREEQC with a hunt and track approach to determine stability
boundaries. In using a single well approach, wells can be evaluated or grouped later based on
geochemical characteristics. The single well approach also allows fine resolution of
geochemical constituents and subtle differences between wells that have a significant bearing
on the overall geochemical characterization. Calculations were driven by measured
concentrations of COls and other analytes such as ORP alkalinity, and sodium, and other ions
in groundwater for each of the 103 wells monitored at the RBSS.
PHREEQC calculations were performed to construct Pourbaix (Eh -pH) diagrams to display the
dominant geochemical forms (i.e., species) that would be expected in groundwater in the
absence of adsorption under equilibrium conditions and allowing for most probable mineral
precipitation where appropriate. Measured ORP and pH values for each well were plotted on the
Pourbaix diagram for each COI to evaluate the likely distribution of species at the RBSS.
Additional PHREEQC calculations were performed to simulate anticipated geochemical
speciation that would occur for each COI in the presence of adsorption to soils. Further
simulations were performed to evaluate model and COI response to changes in DO, pH, and
TDS in the presence of sediment adsorption. Adsorption to soils was represented using a
surface complexation theory approach with hydrous ferric oxides (HFO) and hydrous aluminum
oxides (HAO) representing weak and strong binding sites, respectively. Values for HFO and
HAO were determined from extractions from actual site sediment that were also the basis for
measured Kd values for RBSS soils determined from adsorption experiments conducted by
UNCC.
To geochemically simulate changes to aquifers or test potential remediation strategies,
simulations in which DO, pH, redox, and TDS varied were utilized. These geochemical
simulations are termed titrations for this report. Each set of titrations provides an estimate of the
percentage of each COI that would be adsorbed as a function of changing DO, pH, redox, or
TDS along with relevant changes to the dominant species across the gradient. For these
titrations, TDS was evaluated along with select cations and anions known to be common in soils
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Corrective Action Plan Part 2
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and sediment at the site including sodium, calcium, chloride, potassium, and sulfate. Changes
to DO, pH, and TDS were utilized for titrations due to the affinity for numerous COls such as
metals to exist primarily as anionic or cationic species and their adsorption coefficient variations
to mineral surfaces, soils, sediment, rock, and ash. The titration method in geochemical
modeling can also account for mobility changes due to redox threshold changes and potential
mineral precipitation, indicated by saturation indices in outputs. Adsorption of anionic species is
typically greater at lower pH where anions are more strongly attracted to positively charged
surfaces (and vice versa regarding cationic species). Similarly, the solubility of mineral phases
is pH dependent and lower pH values tend to favor formation of more soluble cationic species
for most alkali elements, alkali earth elements, and transition metals. Methodologies are
discussed in further detail in Appendix E.
4.3.3 Assumptions
The following assumptions were incorporated in the PHREEQC modeling effort:
• Groundwater data were evaluated individually on a well -by -well basis.
• COI sorption in PHREEQC was represented by surface complexation. Surface
complexation models provide a molecular description of adsorption using an equilibrium
approach that defines surface species, chemical reactions, equilibrium constants, mass
and electric charge balances. A benefit of the surface complexation approach is that the
charge on both the adsorbing ion and the solid surface where sorption occurs.
• The surface complexation model was parameterized based on soil column tests and
extraction measurements reported by UNCC. The range of sorption properties was
parameterized as the minimum, mean, and maximum estimates for binding sites as
defined from soil extraction measurements. This range of sorption capacities was used
to develop pH, Eh, DO, TDS, and COI titrations.
• The dominant attenuation process is adsorption to hydrous metal oxides, particularly
HFO and HAO. HFO and HAO are representative of clay minerals and similar facies that
are abundant in soils, the transition zone, and bedrock.
• COI concentrations used in PHREEQC model were as reported in the database.
Analytical results qualified as non -detects or estimated values (U- and J-flagged values)
were used as reported without modification.
• Nitrogen values are assumed to be primarily nitrate and alkalinity results are primarily
bicarbonate, not carbonate.
• TDS is evaluated as a summary of sodium, potassium, magnesium, calcium, sulfate and
chloride ions. These constituents account for approximately 60% of the TDS value.
Chloride does not have sorption constants, so this is addressed as a component of TDS.
• Pourbaix diagrams and/or predominance plots were completed in PhreePlot or
Geochemist's Workbench for each COI to aid in demonstration of changes in Kd, pH,
and DO.
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4.3.4 Results
The modeling effort described above provides both qualitative and quantitative estimations of
the chemical speciation and adsorption behavior of several key COls. Relevant observations
from this modeling effort are as follows:
• The redox conditions vary widely at the site indicating that it has not reached equilibrium
or data is not representative of the conditions sampled. Additional groundwater results
will assist in refining the model further and confirm these findings should sampled data
not be representative of actual groundwater conditions
• Sorption of all of the aqueous groundwater species identified by the CSA would
consume only a fraction of the HFO and HAO sorption sites available in site soils. This
will be evaluated further under the Tier II I MNA evaluation to be completed after this
CAP.
• The observed site condition of limited solubility of arsenic, chromium, cobalt, and
selenium in site groundwater is confirmed by the modeling.
• Each of the pH, Eh, and TDS figures can be further evaluated to support monitored
natural attenuation (MNA) or remediation. In addition, pH adjustment could be
performed to make COls less soluble, thus limiting COI migration during excavation and
restricting the release of TDS and other metals.
• Soil sorptive capacities for COls such as boron are typically lower than COls such as
arsenic which have higher sorptive capacities.
4.4 Refined Site Conceptual Model
Groundwater and surface water models were revised to address comments from NCDEQ. An
updated SCM is provided on Figure 3-1. Based on updated groundwater and geochemical
model, the SCM has not changed significantly with the exception of the following:
COls represented in the groundwater model may be over -estimated since they did not allow for
co -precipitation of COls in the geochemical model. In terms of the SCM, groundwater fluxes
derived from the groundwater models may be conservatively high. Even with potentially over-
estimated modeling results, the surface water mixing models do not show exceedances of the
2B Standards. It is likely that, upon source removal, recreational receptors at Mountain Island
Lake will not be impacted.
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5 Risk Assessment
The purpose of the human health and ecological risk assessment is to characterize potential
risks to humans and ecological receptors associated with exposure to the coal ash -derived
constituents that may be present in groundwater, surface water, sediments, soil, and air due to
release(s) from the coal ash basin at RBSS.
Results of the risk assessment and the information provided on background conditions and
groundwater flow (including fate and transport model results) provided in the CAP will aid in
focusing remedial actions which, when implemented, will provide future conditions that are
protective of human health and the environment, as required by CAMA.
The risk assessment was completed using a methodology designed to be consistent with state
and federal guidance. This methodology represents a step -wise process whereby RBSS is
evaluated using the following methods:
• Step 1: Develop a conceptual site model (CSM), including receiving media, exposure
pathways, and human and ecological receptors.
• Step 2: Screen analytical data for the applicable site media by comparing screening
values identified in the risk assessment work plan to identify constituents of potential
concern (COPCs).
• Step 3: Develop site -specific human health risk -based concentrations (RBCs) for the
COPCs, derive exposure point concentrations (EPCs), and compare EPCs to RBCs to
draw conclusions about the significance of potential human health risks.
• Step 4: Develop a site -specific baseline ecological risk assessment (BERA) for the
COPCs and, where appropriate, derive RBCs based on the risk assessment results.
5.1 Step 1: Conceptual Site Model
The CSM includes a site description, information on current and anticipated future land uses,
sources and potential migration pathways through which coal ash -derived COPCs may have
been transported to other environmental media (receiving media), and the human and
environmental receptors that may come into contact with the receiving media. The CSM is
meant to be a living model that can be updated and modified as additional data become
available. Initial CSMs were presented on Figure 12-1 (human health) and Figure 12-2
(ecological) of the CSA Report. Updated CSMs are provided in Appendix F. The CSMs are
intended to identify potential exposure pathways and receptors that may be applicable at the
site.
For RBSS, the following receptors and exposure scenarios are identified in the human health
CSM (Figure 2-3 in Appendix F):
• Current/future on -site trespasser with potential exposure to dust in outdoor air, soil
remaining post -excavation, AOW water and AOW soil, on -site surface water, and on -site
sediment
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• Current/future commercial/industrial worker with potential exposure to dust in outdoor
air, soil remaining post -excavation, AOW water, AOW soil, on -site surface water, and
on -site sediment
• Current/future construction worker with potential exposure to dust in outdoor air, soil
remaining post -excavation, AOW soil, and groundwater
• Current/future off -site resident with potential exposure to on -site groundwater and off -site
surface water as potential sources of potable water
• Current/future off -site recreational swimmers, waders and boaters with potential
exposure to off -site surface water and off -site sediment
• Current/future recreational and subsistence fishers with potential exposure to off -site
surface water and off -site sediment, and fish ingestion for recreational purposes and
ingestion for subsistence fishers
The following ecological receptors and exposure scenarios are identified in the ecological CSM:
• Fish with potential exposure to surface water and benthic invertebrates with potential
exposure to surface water and sediments
• Aquatic birds with potential exposure to surface water, sediments, fish, and AOW water
• Aquatic mammals with potential exposure to surface water, sediment, fish, and AOW
water
5.2 Step 2: Risk -Based Screening
Groundwater, surface water, sediment, and soil data were evaluated during the CSA using risk -
based screening level concentrations for identified COPCs. Risk -based screening level
concentrations of COPCs were revised in the CAP Part 2 based upon additional groundwater,
surface water, sediment, and soil data collected in Round 1 and Round 2 sampling. Screening
levels are concentrations of constituents in environmental media (e.g., soil) considered to be
protective under most circumstances; their use requires a detailed understanding of the
underlying assumptions in the CSM, including land use and the presence of sensitive
populations. The presence of a constituent in environmental media at concentrations below the
media and constituent -specific screening level is generally assumed not to pose a significant
threat to human health or the environment. If a constituent exceeds the screening level, it does
not necessarily indicate adverse effects on human health or the environment; rather, it only
indicates that additional evaluation may be warranted. Screening levels are used in this
assessment to help identify COPCs, to be carried forward into the evaluation of human health
and ecological risk at the site.
5.3 Step 3: Human Health Risk Assessment
COPCs were evaluated through a comparison of EPCs to calculated RBCs. The comparison
was made through calculation of risk ratios for cancer and non -cancer effects. The total risk
ratios among all compounds were then summed.
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Risk ratios were calculated by first identifying whether COPC RBCs were based on cancer risk
or non -cancer hazard. For RBCs based on cancer risk, the risk ratio for each COPC was
calculated by dividing the EPC by the cancer -based RBC concentration. For RBCs based on
non -cancer risk, the risk ratio for each COPC was calculated by dividing the EPC by the non -
cancer -based RBC concentration.
A risk ratio less than 1 indicated that the EPC does not exceed the RBC, whereas a ratio
greater than 1 indicated that the EPC exceeds the RBC. Risk ratios were also used to evaluate
the cumulative receptor risk associated with each exposure point. Cumulative receptor risk was
calculated by summing the risk ratios among all COPCs on which the RBC was based.
In accordance with USEPA risk assessment guidance (USEPA 1991), the cumulative cancer
risks and non -cancer hazard indices were evaluated against the USEPA target cancer risk
range of 1.0E-06 to 1.0E-04 for potential carcinogens and target non -cancer hazard index of 1
for noncarcinogens (that act on the same target organ by the same mechanism of action);
cumulative cancer risks and hazard indices that are above these limits indicate further
evaluation may be deemed necessary.
For RBSS, the results of the human health risk assessment indicate that exposure to on -site
surface water, AOW water, AOW soil, sediment, and groundwater pose no unacceptable risk or
hazard for a trespasser and construction worker under the exposure scenarios developed in
Step 1. Exposure to on -site surface water, AOW water, AOW soil, and sediment for a
commercial/industrial worker results in a hazard index of 1.2E+00, but a target endpoint
analysis indicates target organ -specific hazard quotients (HQs) below 1; thus, on -site media
pose no unacceptable human health hazard under this scenario. No carcinogenic risk is above
1.0E-04 for the commercial/industrial worker. Similarly, off -site surface water and sediment pose
no unacceptable cancer risk or hazard for a recreational swimmer, wader, or boater under the
scenarios developed in Step 1. Consumption of fish (using on -site surface water concentrations
as surrogate data) by a recreational fisher and subsistence fisher results in hazard indices of
4.0E+00 and 1.2E+02, respectively; target endpoint analysis performed indicates HQs above 1
for selenium, thallium, and zinc for a recreational fisher and HQs above 1 for cadmium,
selenium, thallium, and zinc for a subsistence fisher. No carcinogenic risk is above 1.0E-04 for
either fisher.
Thus, the human health risk assessment indicates that potential risks are above risk targets for
the recreational fisher and subsistence fisher from ingestion of fish caught near the site. These
scenarios were evaluated using data for on -site surface water as a conservative surrogate in the
absence of off -site surface water data. Additional data regarding site -specific conditions as well
as evaluation of the very conservative nature of the exposure parameters and fish ingestion
models used in the risk assessment are needed to address these results.
5.4 Step 4: Ecological Risk Assessment
The purpose of a BERA is to: (1) determine whether unacceptable risks are posed to ecological
receptors from chemical stressors, (2) derive constituent concentrations that would not pose
unacceptable risks, and (3) provide the information necessary to make a risk management
decision concerning the practical need and extent of remedial action. The BERA was performed
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according to the traditional ecological risk assessment paradigm: Problem Formulation, Analysis
(Exposure and Effects Characterization), and Risk Characterization. Because of the many
combinations of conservative assumptions, an Uncertainty Evaluation section was also
included. The BERA generally adhered to the USEPA's "Ecological Risk Assessment Guidance
for Superfund: Process for Designing and Conducting Ecological Risk Assessments" (USEPA
1997) and the "Supplemental Guidance to ERAGS: Region 4, Ecological Risk Assessment"
(USEPA 2015a), as well as NCDENR's "Guidelines for Performing Screening Level Ecological
Risk Assessments" (NCDENR 2003).
Constituents of concern were selected based on factors including the type of source(s),
concentration, background levels, frequency of detection, persistence, bioaccumulation
potential, toxicity/potency, fate and transport (e.g., mobility to groundwater), and potential
biological effects. Screening consisted of comparing the maximum concentration of each
constituent in the applicable media to conservative environmental screening levels. Per Step 3
of the USEPA Region 4 ecological risk assessment guidance, COPCs that were retained using
the risk -based screening were evaluated using a multiple lines -of -evidence approach in the
BERA (USEPA 2015a, 2015b).
Risk characterization involved a quantitative estimation of risk followed by a description and/or
interpretation of the meaning of this risk. The purpose of the risk characterization was to
estimate potential hazards associated with exposures to COPCs and their significance. During
risk estimation, the exposure assessment and effects assessment were integrated to evaluate
the likelihood of adverse impacts to the wildlife receptors of interest (e.g., birds and mammals).
The risk estimate was calculated by dividing the dose estimate from the exposure assessment
by the applicable toxicity reference value (derived from the available literature) to obtain an HQ.
Receptors chosen for ecological risk assessment are often surrogates for the broad range of
potential ecological receptors in a given habitat. For RBSS, typical receptors were chosen for
their expected common presence in the habitats represented at the site, or because they are
common in the southeast and toxicity data are available. Aquatic or water -dependent receptors
include: fish, benthic invertebrates, aquatic birds (represented by mallard duck and great blue
heron), and aquatic mammals (represented by muskrat and river otter).
At RBSS, three ecological exposure areas were defined (Figure 2-5 in Appendix F). These
include:
• Ecological Exposure Area 1, located near an inlet in the area west of the steam station
• Ecological Exposure Area 2, located west of the ash basin Primary Cell and along
Mountain Island Lake
Ecological Exposure Area 3, located west, north and east of the ash basin Secondary
Cell and along the Catawba River/Mountain Island Lake
Potentially affected areas on -site are classified as aquatic and evaluated for exposure to site
COPCs. Ecological habitats are presented on Figure 2-6 in Appendix F.
Evaluation of surface water in Exposure Area 1 indicates a calculated HQ of 3 for a great blue
heron's exposure to selenium; using the No Observed Adverse Effects Level (NOAEL) as the
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toxicity reference value. Using the Lowest Observed Adverse Effect Level (LOAEL) toxicity
reference value, the great blue heron's selenium -based HQ decreases to 2. All other aquatic
wildlife receptors have chemical HQs below 1.
The evaluation of ecological exposures to surface water, AOW water and AOW sediment in
Exposure Area 2 indicates that no aquatic receptors have HQs above 1.
Evaluation of AOW water and AOW sediment in Exposure Area 3 indicates that the great blue
heron and muskrat have chemical HQs above 1. These include, for the great blue heron, a HQ
of 1 from barium, 2 from cobalt and 6 from vanadium using the NOAEL; the HQs decrease to
0.6 for barium, 3 for vanadium, but remains the same at 2 for cobalt under the LOAEL scenario.
For the muskrat, HQs include a value of 2 for aluminum, 3 for barium and 11 for manganese
exposure; the HQs decrease to 0.2, 2 and 8, respectively, using the LOAEL. Other aquatic
receptors have chemical HQs below 1.
Thus, the ecological risk assessment indicates that potential risks are above risk targets for
barium, cobalt, manganese, selenium and vanadium for some water -dependent mammals and
birds. Additional data and further refined assessment are needed to address uncertainties
associated with the evaluation of these scenarios including the occurrence of these ecological
receptors in the areas adjacent to the ash basins, and the conservative nature of the exposure
and toxicity assumptions used in the ecological risk characterization. However, since the
remediation of the ash basin area is planned, any future exposure would be eliminated and,
therefore, any subsequent ecological risks would be mitigated.
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6 Alternative Methods for Achieving
Restoration
6.1 Corrective Action Decision Process
This section discusses how remedial alternatives are evaluated and identifies the remedial
alternative selected to achieve restoration of groundwater quality at the RBSS site.
As described in Section 1, after removal of ash, soils left on -site will be sampled and analyzed,
and the analytical results will be incorporated into the groundwater contaminant fate and
transport model. If this evaluation indicates that modification to the proposed CAP is required,
Duke Energy will prepare and submit a revised CAP. Therefore, remediation of soils is not
discussed in this document.
As noted in Section 2, exceedances of 2B or 2L Standards or IMACs were measured at AOWs
adjacent to Mountain Island Lake. HDR and Duke Energy consider that the water in the Primary
and Secondary cells is the likely source of the water supplying these AOWs, and that
dewatering the ash basin will result in reduced or no flow at these AOWs. Duke Energy
proposes that remedial measures at these AOWs be deferred until after basin dewatering and
excavation of the ash. If at that time, the exceedances are still present, Duke Energy will
evaluate those conditions and develop corrective measures to address the exceedance(s). For
RBSS, the Plan for Identification of New Discharges was submitted to NCDEQ on May 8, 2015.
This plan was developed to address the requirements of North Carolina General Statute
(GS)130A-309.210 (d) Identification and assessment of discharges; correction of unpermitted
discharges, as modified by North Carolina Senate Bill 729. Identification of new discharges
(AOWs) and any associated sampling of the new discharge will be done in compliance with the
document referenced above."
6.1.1 Evaluation Criteria
The goal of groundwater corrective action in accordance with T15A NCAC 2L .0106 is:
"...where groundwater quality has been degraded, the goal of any required corrective
action shall be restoration to the level of the standards, or as closely thereto as is
economically and technologically feasible"...using best available technology 0), or to
an alternate standard (k) or using natural attenuation mechanisms (1).
The evaluation of best available methods for groundwater remediation is based on the objective
of meeting groundwater standards at the Compliance Boundary with consideration of
implementation, time, and cost. The methods may include one or a combination of best
available technologies and natural attenuation processes. For RBSS, the primary corrective
action is removal of source material by excavation and off -site disposal. The groundwater
corrective action alternatives discussed herein are evaluated to supplement source control.
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6.1.2 COls Requiring Corrective Action
Data from the CSA were evaluated in CAP Part 1 to identify the following groundwater Cols that
are considered for current or potential future corrective action: antimony, arsenic, boron,
chromium, cobalt, hexavalent chromium, iron, manganese, sulfate, thallium, TDS, and
vanadium. These Cols are considered for corrective action because they have been found to
exceed their applicable 2L Standards, IMACs, or NCDHHS HSLs, or may exceed their
applicable 2L Standards, IMACs, or NCDHHS HSLs in the future due to fluctuations of COI
concentrations as a result of closure activities. There are some locations that have exceedances
in both Round 1 and Round 2 results, while other locations do not present consistent
exceedances when comparing Round 1 and Round 2 results. For this reason, it is
recommended that additional groundwater sampling be conducted as recommended in Section
9 to confirm the effectiveness of proposed corrective action.
6.1.3 Potential Exposure Routes and Receptors
The Baseline Human Health and Ecological Risk Assessment (Appendix F) provides
information on the current knowledge of the RBSS site and conservative conditions assessment
of the potential risk associated with COls attributed to the currently defined sources at RBSS.
The primary source -to -receptor exposure route is leaching of ash porewater to groundwater.
Groundwater then migrates and reaches Mountain Island Lake prior to the Compliance
Boundary. There are no receptors to groundwater through public and private wells within 0.5
mile of the RBSS Compliance Boundary. The groundwater to surface water route is the primary
route of exposure of groundwater to receptors. A secondary source -to -receptor exposure route
may be infiltration of COls in water and/or sediment at AOWs to groundwater.
Localized groundwater mounding associated with the current hydraulic head in the ash basin
will be eliminated with the source control measures (i.e., excavation). The residual groundwater
concentrations are the focus of this CAP.
6.2 Alternative Evaluation Criteria
Alternative evaluation criteria are selected in general conformance with USEPA Office of Solid
Waste and Emergency Response Directive 9355.-27FS, "A Guide to Selecting Superfund
Remedial Actions" dated April 1990. This document provides threshold, balancing, and
modifying criteria for selection of a remedy.
Potential groundwater corrective action alternatives will be evaluated against the following
criteria:
• Effectiveness (Section 6.2.1)
• Implementability/Feasibility (Section 6.2.2)
• Environmental Sustainability (Section 6.2.3)
• Cost (Section 6.2.4)
• Stakeholder Input (Section 6.2.5)
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Effectiveness
Effectiveness is a comparison of the likely performance of applicable technologies taking into
consideration the following:
1. The estimated area and volumes of media to be treated.
2. Demonstrated reliability to achieve constituent remedial goals under site conditions.
3. Demonstrated reliability to reduce potential risk to human health and the environment in
a timely manner.
Specific effectiveness criteria include:
• Has the potential remedial alternative been demonstrated to be effective at similar sites?
• Does the remedial technology involve treatment that will permanently destroy target
constituents?
• Does the remedial technology involve treatment that will permanently detoxify target
constituents?
• Does the remedial technology involve treatment that will permanently reduce the mobility
of target constituents?
• Will the remedial alternative permanently remove contaminants from the site?
• Can the effectiveness of a potential remedial technology be monitored, measured, and
validated?
• Will a remedial technology reduce potential risk to human health when fully
implemented?
• Will a remedial technology reduce potential risk to the environment when fully
implemented?
• Will a remedial technology be protective of human health?
Technologies that are deemed to be less effective under site -specific conditions than otherwise
comparable technologies will be eliminated on the basis of effectiveness.
6.2.2 ImplementabiIity/Feasibility
The screening criteria of implementability evaluates whether implementation of a technology is
technically and administratively feasible. Specific implementability criteria include:
• Are the material resources and manpower readily available to fully implement the
remedial technology in a timely manner?
• Does the remedial technology require highly specialized resources and/or equipment?
• Is there sufficient on -site and off -site area to fully implement the remedy?
• Does the remedial technology require any permits, and can the permits be acquired in a
timely manner (e.g., wetlands permitting)?
• Can the remedial alternative be implemented safely?
• Can existing and future infrastructure support the remedial alternative?
• Will a remedial technology increase potential risk to the environment during
implementation?
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• Can a remedial technology meet all applicable or relevant and appropriate requirements
(USEPA 1997)?
Technologies that are deemed impractical under site -specific conditions will be eliminated on
the basis of implementability/feasibility.
6.2.3 Environmental Sustainability
A remedy is environmentally sustainable when it maximizes short-term and long-term protection
of human health and the environment through the judicious use of limited resources. Metrics
used to measure environmental sustainability include:
• Will constituents be treated to reduce toxicity or mobility, or will treatment transfer the
constituent from one media to another (e.g., discharge constituents in extracted
groundwater to surface water)?
• Is the carbon footprint (energy consumption) of otherwise comparable remedial
alternatives significantly different?
• Will source materials used in the remediation process be recycled or reclaimed?
• Will waste materials generated during the remediation process be recycled or
reclaimed?
• Will renewable sources of energy be used during the remediation process?
• Will natural habitat restoration, enhancement, or replacement be integral to the remedy?
Duke Energy considers environmental sustainability in their alternative evaluation criteria and
where appropriate will incorporate "green" remedial strategies in their evaluation. Green
remedial strategies consider all environmental effects of remedy implementation and
incorporating options to maximize new environmental benefit of cleanup operations (USEPA
2008). Green remediation reduces the demand placed on the environment during remedial
operations to avoid collateral damage to the environment. Green remediation strategies
minimize adverse impacts to other environmental media, such as:
• Air pollution caused by emission of carbon dioxide, nitrous oxide, methane, and other
greenhouse gasses emitted during remediation
• Imbalance to the local and regional hydrologic regimes
• Soil erosion and nutrient depletion causing changes to soil geochemistry
• Ecological diversity and population reductions
6.2.4 Cost
The criterion of cost has been evaluated by looking at the estimated capital cost and labor
required to implement technologies that will enhance future closure activities. The cost
evaluation considers design, construction, and operation and maintenance over a 30-year
period. Cost will not be the sole or primary basis for selecting a technology or remedial
alternative; however, cost will be considered when evaluating the alternatives.
6.2.5 Stakeholder Input
Appropriate stakeholders will be notified pursuant to 15A NCAC 02L .0114.
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6.3 Remedial Alternatives to Achieve Regulatory Standards
Source control is the primary corrective action for groundwater restoration at the site. As
required by CAMA, the ash basin at RBSS is being excavated due to the high priority
designation established by CAMA. Source removal at RBSS includes the export of ash for
landfill or beneficial use. The remedial alternatives described in this section were considered to
enhance source control measures at the RBSS site and improve the effectiveness of the
remedy
6.3.1 Groundwater Remediation Alternatives
Remedial alternatives for restoration of groundwater in accordance with T15A NCAC 2L.0106
include the following:
• Source Control. which can include:
o Ash removal to prevent COls from leaching into groundwater
o Placement of engineered cap to minimize infiltration and prevent Cols from
leaching into groundwater
o Slurry walls or grout curtains to prevent groundwater interaction with source
material
o In -situ solidification/stabilization to reduce or eliminate leaching of COls into
groundwater by mixing soil beneath source areas with pozzolanic materials (i.e.,
Portland cement or bentonite)
• Land Use Controls — State approval to restrict land use to prevent the use of surface
water and groundwater in the area.
Monitored Natural Attenuation — MNA involves regular monitoring of select groundwater
monitoring wells for specific parameters to ensure COI concentrations in groundwater
are decreasing. Dilution from recharge to shallow groundwater, mineral precipitation,
and COI adsorption will occur over time, thus reducing COI concentrations through
attenuation.
• Enhanced Attenuation. which can include:
o Addition of materials with high adsorptive capacity to the saturated zone to
increase the reduction of COI levels in groundwater
o Air sparging and adjusting pH to enhance precipitation of iron and manganese
oxide/hydroxide minerals to increase the reduction of COI levels in groundwater
o Bioremediation for removal of COls
Permeable Reactive Barriers — Involves trenching and placement of selected material in
the trench that would chemically bond and remove COls and reduce their levels in
groundwater.
• Water Treatment — Active in -situ groundwater remediation by injection of chemical
and/or air sparging, groundwater extraction and treatment, or passive groundwater
remediation through wetland construction.
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A detailed description of available remedial alternatives is documented in Appendix G.
6.3.2 Monitored Natural Attenuation Applicability to Site
A MNA Tier I and Tier II evaluation was conducted for the RBSS site by Geochemical, LLC and
is included in Appendix H. The following is a summary of the Tier I and Tier II evaluation.
MNA is a strategy and set of procedures used to demonstrate that physiochemical and/or
biological processes in an aquifer will reduce concentrations of COls to levels below regulatory
standards or criteria. The mechanisms that regulate their release from solids and movement
through aquifers are, for the most part, the same processes that provide chemical controls on
movement of CCR leachate in an aquifer. These processes attenuate the concentration of
inorganics in groundwater by depositing inorganics on aquifer solids removing and the
constituent from the groundwater. MNA is considered a viable remedial alternative for COls in
groundwater at the RBSS site.
The following groundwater COls were identified at RBSS: antimony, arsenic, boron, chromium,
cobalt, hexavalent chromium, iron, manganese, sulfate, thallium, TDS, and vanadium. Cobalt,
iron, manganese, and vanadium occur naturally in regional groundwater; however, these
constituents are still considered Cols because at certain locations, concentrations exceed their
respective 2L Standards or IMACs and were higher than observed background concentrations.
Sulfate and TDS are generally not attenuated by reactions with solids, but are reduced in
concentration by diffusion, mechanical mixing, or dilution.
Tier I analysis used two lines of evidence for attenuation:1) Solid -water pair comparison of COI
concentrations were performed, with a mutually rising relationship indicating attenuation
(USEPA 2007a); and 2) laboratory determination of the solid -water partitioning coefficient or Kd
value (USEPA 1999) was used as a measure of the susceptibility of COls to adsorb to solids
and be attenuated. Tier I analysis indicates that arsenic, boron, chromium, selenium, and
thallium should be carried through to Tier II analysis.
Following completion of a Tier I analysis, a conceptual model for COI attenuation involving
reversible and irreversible interaction with clay minerals, metal oxides, and organic matter is
proposed. A Tier II demonstration based on that conceptual model was partially executed. The
findings are as follows:
1. The samples evaluated for Kd determination were found to be representative of site -
specific conditions under which Cols would migrate.
2. Clay minerals and Fe-Mn-AI oxides were found in all samples. Organic matter is not a
significant sink for COls at RBSS.
3. Chemical extractions identified that COls were concentrated in soil samples exposed to
groundwater containing higher concentrations of COls, validating attenuation.
4. Chemical extractions were used to determine a probable range of Kd values that suggest
attenuation is taking place for arsenic, chromium, selenium and thallium.
5. Geochemical modeling performed titration tests for the COls. Arsenic is completely
adsorbed. Antimony and boron are the least adsorbed. If groundwater is maintained at a
pH range between 6-8 chromium, cobalt, manganese, selenium, and vanadium will have
the most effective adsorptive capacity.
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As documented in Appendix E, titration results for RBSS monitoring wells can be used to
support evaluation of MNA or remediation impacts. For example, titration results can be used to
help determine the expected impact that DO changes would have in response to addition of a
cap (leading to reduced infiltration and lower recharge DO), or the introduction of oxygen
creating a more oxic environment, addition of acid or base to adjust the pH to conditions that
prevent COls from being solubilizing, or impact due to excavation and the release of TDS and
other metals. Changes in redox can also occur in response to DO increases or decreases as
well as the introduction of inorganic oxidants from anthropogenic contamination or changes in
groundwater flow vectors. Results of the geochemical modeling support applicability of MNA as
an effective remedial alternative for the site.
Additional data collection is necessary to complete the Tier II/III assessment with respect to
specific attenuation mechanisms for each COI, and quantification of the magnitude of that
attenuation by specific media to support numerical modeling. The Tier III objective is to
"eliminate sites where site data and analysis show that there is insufficient capacity in the
aquifer to attenuate the contaminant mass to groundwater concentrations that meet regulatory
objectives or that the stability of the immobilized contaminant is insufficient to prevent
remobilization due to future changes in ground -water chemistry." The Tier III assessment will be
performed in general accordance with USEPA's "Monitored Natural Attenuation of Inorganic
Contaminants in Ground Water - Volume 2 Assessment for Non -Radionuclides Including
Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium"
(USEPA 2007b).
6.3.3 Site -Specific Alternatives Analysis
Source control (i.e., removal of ash) is currently underway at the RBSS site following NCDEQ
approval of the excavation plan. Approximately 4.6 million tons of ash will be transported to
permitted lined landfills and/or structural fills, or designated for beneficial reuse. The initial
phase of excavation work began in May 2015 and includes removal of materials in the northeast
corner of the ash storage area.
The majority of ash at the RBSS site is anticipated to be transported by rail to a lined clay mine
reclamation project in central North Carolina. This activity began in January 2016. Removal of
ash at the RBSS site is scheduled to be complete no later than August 2019. The earthen dams
will be removed and the non -impacted material will be used in site re -grading. Fill material (from
on -site and/or imported sources) will be used to fill the void left after ash removal and the area
will be re -graded and vegetated to establish a long-term, stable, erosion -resistant site condition.
As detailed in Appendix G, the following three remedial alternatives have been recommended
for consideration at the RBSS site to enhance or supplement the existing source removal
activities.
No Further Action — This alternative is provided to establish a baseline for comparison to
other alternatives. Under this alternative, there would be no corrective actions conducted
at the site to remove the source of COls other than the removal of the ash, and no
further corrective action would be taken for groundwater. This measure does not include
long-term monitoring or institutional controls.
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2. MNA — Groundwater monitoring would be continued until remedial objectives are met
(that is, groundwater concentrations are at or below applicable standards at the
Compliance Boundary). At RBSS, ash will be removed from the source areas. COls may
remain in groundwater and concentrations continually decrease over time. Attenuation
will occur over time due to natural processes, and its extent can be monitored between
the source and the Compliance Boundary.
3. MNA and Permeable Reactive Barrier at Key Locations — It is anticipated that removal of
the ash, the presumed source of most of the COls at the site, will resolve most of the
groundwater contamination at the RBSS site, although it will take time for groundwater
concentrations in downgradient areas to see decreasing concentrations. While COls
from the RBSS site do not currently result in 2B Standard exceedances in Mountain
Island Lake, if ash removal activities result in 2B exceedances of COls in the lake, a
permeable reactive barrier(s) could be constructed between the source area and the
lake. The barrier(s) would be designed to reduce contaminant loading for certain sorptive
COls. Pilot studies would be required, both for optimal placement and selection of
appropriate reactive or adsorptive media to incorporate into the permeable reactive
barriers. Laboratory tests would be required to confirm the effectiveness of the media on
the site -specific COls. Depth to bedrock at the RBSS site would need to be considered
as the relative deep bedrock layer may make installation of a permeable reactive barrier
technically infeasible at this site.
Selection of site -specific corrective actions will be based on results observed from the refined
groundwater model and modeling of remedial alternative actions for the site, as well as
evaluation of effectiveness, implementability, feasibility, environmental sustainability, and cost.
6.3.4 Site -Specific Recommended Approach
At the RBSS site, source removal is the primary corrective action and is anticipated to decrease
COI concentrations and number of COls at the site. Alternative 2 (MNA) is recommended as a
supplemental corrective action for the RBSS site. Based on results of the Tier I and Tier II
evaluation, MNA is an effective corrective action because COls will attenuate over time to
restore groundwater quality at the site and is protective of both human health and the
environment. MNA is a feasible corrective action and can be implemented at the site.
Implementation of MNA for a 30-year period is estimated to cost $6.7 million. Costs are
discussed further in Section 10. Select monitoring wells, surface water locations, and AOWs
will be used for MNA and will be monitored in accordance with the Interim Monitoring Plan and
the Effectiveness Monitoring Plan discussed further in Section 9. Groundwater quality and
effectiveness of MNA will need to be re-evaluated after excavation of source material is
complete. If results of MNA re-evaluation deem MNA is not a sufficient to reduce COI
concentrations within an acceptable time period, remedial alternatives should be reconsidered
and implemented to augment MNA.
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7 Selected Corrective Action(s)
Remedial alternatives were evaluated for achieving restoration of groundwater at the RBSS site
and are detailed in Section 6 of this report. Source control via excavation of ash from the ash
basin, ash storage area, and cinder storage area is the primary corrective action for the RBSS
site. MNA was determined to be the most appropriate corrective action to supplement source
control; however, groundwater quality will need to be monitored during excavation, and the
effectiveness of MNA should be re-evaluated after basin closure is complete. Post -excavation
MNA requires re-evaluation of the SCM, geochemical, hydrogeological, and surface water
models. If MNA is determined to not be an effective corrective action, then re-evaluation of
alternatives will be performed to determine the appropriate technology. Selection of MNA for
corrective action and conceptual design is discussed further in this section.
7.1 Selected Remedial Alternative for Corrective Action
COI transport in groundwater is primarily controlled by hydrogeologic and geochemical
conditions at the site (Section 3). COls enter the groundwater system through the shallow flow
layer within the source areas. Evaluation of the geochemical modeling indicated COls are
attenuated by a combination of sorption and/or precipitation and at a pH range between 6 and 8
SU. TDS and sulfate generally are not attenuated, but concentrations are reduced by diffusion,
mechanical mixing, and/or dilution. Arsenic, cobalt, and selenium were observed to have limited
solubility, meaning these constituents attenuate more readily. Groundwater fate and transport
model predictions presented in Appendix B are supported by findings of the geochemical
modeling presented in Appendix E. Based on review of the modeling, COls with sorption
coefficients similar to or greater than arsenic are immobilized by sorption and/or precipitation.
COls with sorption coefficients similar to or less than boron do not readily attenuate and easily
transport in groundwater.
7.2 Conceptual Design
7.2.1 Source Removal — Excavation
Excavation of ash at the RBSS site will remove the source areas which are identified as the ash
basin, ash storage area, and cinder storage area. Details regarding excavation can be found in
the Riverbend Steam Station Coal Ash Excavation Plan submitted to NCDENR by Duke Energy
in November 2014 (http://portal.ncdenr.org/web/wq/ca-excavation-plans). The excavation plan
covers the first 12 to 18 months of ash basin excavation activities, including the initiation of
basin dewatering, ash storage removal, and other permitted ash activities within the waste
boundary. The excavation plan is a living document and can be modified based on changing
site conditions. Currently, ash has been transported to three locations: Waste Management's
R&B Landfill in Homer, GA; Duke Energy's Marshall FGD Landfill in Mooresville, NC; and
Charah's Brickhaven Mine. The plan will be updated and submitted to NCDEQ annually or as
required during the excavation process. This CAP supplements the existing excavation plan by
proposing corrective action and a post -closure monitoring plan.
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7.2.2 Monitored Natural Attenuation
7.2.2.1 Demonstration of MNA
The use of MNA as a remedy involves the monitoring of select parameters to ensure Cols are
attenuating. Once the ash within the source areas is removed, groundwater quality will improve
over time due to dilution from the recharge to shallow groundwater, precipitation, and adsorption
of COIs.
Tier I and II analyses were conducted for RBSS (discussed in Section 6.3.2 and Appendix H).
A geochemical site conceptual model for COI attenuation involving reversible and irreversible
interaction with site samples containing clay minerals, metal oxides, and organic matter was
completed. The sampling was determined to be representative of material into and through
which the COls will migrate. The most significant finding was that precipitating iron and
manganese was removing other COls through co -precipitation and adsorption, thus confirming
that attenuation is occurring. In support of this reaction, clay minerals and Fe-Mn-AI oxides were
observed in samples.
Groundwater modeling did not take into consideration the removal of COls via co -precipitation
with iron oxides, which likely resulted in an over -prediction of COI transport, causing some of
the COls to exceed the 2L Standards or IMACs at the compliance boundary in the model output.
Surface water models have determined that even with over -prediction of COls to Mountain
Island Lake, exceedance of the 2B Standards will not occur.
Based on these predictions, site conditions are favorable for MNA to be implemented at the
RBSS site.
7.2.2.2 Verification of MNA
The MNA monitoring program and the data collection and evaluation to advance the Tier III
assessment should be implemented during excavation (removal) activities. The monitoring will
continue through the removal effort and be maintained until water quality meets remedial
objectives whether that be 2L/2B Standards or a site -specific standards based on background
conditions, as applicable. The site monitoring requirements are discussed in Section 9.
The current groundwater monitoring network along with monitoring wells scheduled to be
installed to address the need for additional assessment at the site are suitable for
characterization of COls and monitoring effectiveness of MNA at RBSS. Monitoring wells within
the waste boundary will be abandoned as part of closure activities. Well abandonment reports
are located in Appendix A. With the exception of background wells no new wells are proposed
at this time.
If COls or COI concentrations are observed to increase during the MNA monitoring,
effectiveness of MNA should be re-evaluated. If warranted, additional remedial alternatives
should be evaluated and implemented as necessary and as described above.
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8 Recommended Interim Activities
Several interim activities will occur at the RBSS site to address additional assessment needs
identified in the CSA and CAP Part 1 Reports. Interim activities to be completed include the
installation of additional monitoring wells and monitoring of groundwater during excavation
activities.
8.1 Well Installation
Eight additional groundwater assessment wells, one replacement well, and six background wells
are currently being installed to address data needs identified in the CSA. These wells are being
installed to better assess background conditions upgradient of the ash storage areas at the
RBSS site and to further delineate COls. Additional well locations are presented on Figure 8-1.
Assessment wells are being placed in shallow and deep locations in the vicinity of the cinder
storage area. The purpose of these wells is to better delineate COls in the vicinity of the cinder
storage area. The wells will also better define groundwater flow direction in the vicinity of the
cinder storage, and Mountain Island Lake.
Concurrent with the additional assessment wells, six new background wells are being installed
on Duke Energy property hydrogeologically upgradient of the source area. These wells will
increase the understanding of naturally occurring COI concentrations at RBSS.
The new background wells are designated BG-4S, BG-4D, and BG-4BR; and BG-5S, BG-5D,
and BG-SBR. The new assessment wells are designated GWA-11 S, GWA-11 D, GWA-12S,
GWA-12D, GWA-13S, GWA-13D, GWA 14S, GWA 14D, and replacement well MW-2S.
8.2 Additional Groundwater Sampling and Analyses
The additional assessment wells and background wells will be incorporated into the
groundwater monitoring network in 2016 and sampled concurrently and in accordance with the
Interim and Effectiveness Monitoring Plans. Review of the data will be used to refine the
understanding of natural background concentrations of COls, refine the existing PPBCs, and to
delineate the vertical and horizontal extent of COI impacts.
The Interim and Effectiveness Monitoring Plans associated with RBSS are described in
Section 9.
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9 Interim and Effectiveness Monitoring Plans
Interim and Effectiveness Monitoring Plans (Monitoring Plans) provide detailed information on
field activities to be performed during collection of groundwater and AOW samples at the RBSS
site. The Monitoring Plans are intended to evaluate the effectiveness of the proposed corrective
actions; monitor the movement of contaminants in groundwater during and after excavation of
the RBSS site's ash basin Primary and Secondary Cells, the ash storage area, and the cinder
storage area; and address the need to evaluate baseline conditions and seasonal variation in
groundwater and AOWs at RBSS. These Monitoring Plans replace the monitoring plan provided
in the CSA Report (Section 16 - Interim Monitoring Plan).
Protocols for groundwater and AOW sample collection, analysis, and reporting are consistent
between the Monitoring Plans. This sampling and analysis will be completed in accordance with
the Monitoring Plans presented below, the CSA Work Plan, and the Low Flow Sampling Plan
(CSA Report Appendix G).
9.1 Interim Monitoring Plan
The Interim Monitoring Plan has been developed to provide baseline seasonal analytical data
associated with RBSS. The Interim Monitoring Plan will be implemented at RBSS through the
first half of 2016. The Interim Monitoring Plan establishes data quality objectives (DQOs) and
sampling requirements associated with sampling frequency, sampling locations, and analytical
requirements. Upon completion of the Second Quarter 2016 sampling event, monitoring
activities will be conducted in accordance with the Effectiveness Monitoring Plan described in
Section 9.2.
9.1.1 Data Quality Objectives
The following DQOs are associated with the Interim Monitoring Plan:
• Monitor the extent of groundwater contamination in and around the ash basin, ash
storage area, and cinder storage area and evaluate seasonal trends associated with
COIs.
Monitor the movement of COls within groundwater and the interaction of groundwater
with AOWs.
• Determine seasonal groundwater flow direction and elevations throughout RBSS and
monitor potential changes to groundwater flow direction and elevation as the result of
closure activities.
The DQOs will be met through the following activities:
Perform groundwater and AOW sampling at the locations depicted on Figure 2-1 and
identified in Table 9-1 through the first half of 2016. These monitoring events, planned
for the First and Second Quarters 2016, will be combined with analytical data from
Rounds 1 through 4 (collected in 2015) to evaluate seasonal water quality conditions at
the RBSS site. Additional assessment wells are being installed at the RBSS site in First
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Quarter 2016 and may be added to the interim monitoring network following installation.
If monitoring indicates that excavation activities are mobilizing COls towards off -site
receptors, more frequent sampling of select wells will be considered.
• Perform groundwater static water level measurements at monitoring wells on an annual
basis.
• Perform total depth measurements at monitoring wells on an annual basis.
Prepare reports documenting sampling results and analysis for submittal to NCDEQ, as
specified in Section 9.1.3 below.
9.1.2 Sampling Requirements
9.1.2.1 Sample Frequency
To meet the DQOs and evaluate seasonal fluctuations in COI concentrations, sampling of CSA,
compliance, voluntary, and background groundwater monitoring wells and AOW locations will
be conducted at the RBSS site in the First and Second Quarters of 2016. Sampling frequency
will be revised as described in the Effectiveness Monitoring Plan (Section 9.2) upon completion
of the Second Quarter 2016 sampling event.
9.1.2.2 Sample Locations
Groundwater monitoring well and AOW locations to be sampled during the interim monitoring
are identified in Table 9-1 and depicted on Figure 2-1. Wells may be added to the sampling
program through the installation of additional monitoring wells or removed as excavation of the
ash basin, ash storage area, and cinder storage area leads to well abandonment. NCDEQ will
be notified and NCDEQ's approval will be obtained prior to the abandonment of these
monitoring wells.
9.1.2.3 Analytical Requirements
Analytical parameters will be consistent with those specified in the approved Work Plan.
Analytes for monitoring wells and AOWs include total and dissolved metals, alkalinity, calcium,
chloride, hexavalent chromium, potassium, magnesium, nitrate, sodium, sulfate, total combined
radium, total combined uranium, total dissolved solids, total organic carbon, and total
suspended solids. In addition, samples will be analyzed for ammonia for use in evaluation of
MNA as a corrective action. Analytical services will be provided by a North Carolina certified
laboratory. Chemical analytes, analytical methods, bottle requirements, and preservatives are
provided in Table 9-2.
9.1.3 Reporting
Validated analytical results from the First Quarter 2016 sampling event are proposed to be
transmitted to NCDEQ within 90 days of completion of the sampling event. Within 120 days of
completion of the Second Quarter 2016 sampling event, Duke Energy proposes to submit a
groundwater monitoring report to NCDEQ that summarizes the results from the First and
Second Quarter 2016 sampling events.
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9.2 Effectiveness Monitoring Plan
The Effectiveness Monitoring Plan has been developed to monitor select wells during
excavation to develop baseline analytical data for use with future MNA analysis. The
Effectiveness Monitoring Plan will be implemented at select locations following completion of the
Second Quarter 2016 sampling event and will continue through 2020 (five years of CAP -related
sampling). The Effectiveness Monitoring Plan may be modified within the first five-year period if
additional corrective actions are implemented. Note that ash removal from within the waste
boundary will likely preclude accurate analysis of MNA processes through August 2019.
9.2.1 Data Quality Objectives
The following DQOs are associated with the Effectiveness Monitoring Plan:
• Monitor the effectiveness of the approved remedy (excavation and MNA)
• Monitor changes in groundwater and AOW COI concentrations as the result of
excavation within the waste boundary.
• Monitor the potential migration of COls within groundwater and the expression of
groundwater as AOWs.
• Collect additional analytical data from new background wells to establish new PPBCs.
• Monitor seasonal groundwater flow direction and elevations, and monitor potential
changes to groundwater flow direction and elevation resulting from closure activities.
The DQOs will be met through the following activities:
• Perform groundwater and AOW sampling, including MNA parameters at select
monitoring well and AOW locations.
• Perform groundwater static water level measurements at CSA, compliance, and
voluntary monitoring wells concurrent with groundwater sampling described above.
• Perform total depth measurements at CSA, compliance, and voluntary monitoring wells
on an annual basis.
• Prepare reports documenting sampling results and analysis for submittal to NCDEQ.
9.2.2 Sampling Requirements
Following the Second Quarter 2016 sampling event, four seasonal sampling events, including
CSA Rounds 1 and 2, will have occurred at the RBSS site. Results from the four seasonal
sampling events will be evaluated to establish an MNA sampling network of select monitoring
well and AOW locations. Results will also be evaluated to determine the need for increased
frequency of sampling in beyond the waste boundary to monitor potential migration of COls as
the result of excavation activities. Additional monitoring of the MNA network will be proposed to
confirm the effectiveness of MNA as a proposed corrective action and to determine if additional
sampling is required at locations outside the MNA network to monitor site conditions.
9.2.2.1 Sample Frequency
Following the establishment of the MNA network, one additional sampling event of these
locations will be conducted in 2016 in conjunction with the Third Quarter 2016 NPDES
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compliance sampling event. Beginning in 2017, samples from the MNA network will be collected
three times per year, in conjunction with the NPDES compliance monitoring to correlate the
results from the MNA network sampling with the NPDES results. Sampling frequency
associated with the Effectiveness Monitoring Plan will be re-evaluated every five years. Upon
completion of the first five-year sampling cycle, the potential for semi-annual sampling
frequency will be evaluated. Additional sampling beyond the MNA network, including
excavation -specific monitoring associated with the ash management area, will be evaluated
after completion of the Second Quarter 2016 sampling event and will be proposed prior to the
subsequent event sampling event.
In order to establish PPBCs using the additional background wells described in Section 8, an
additional monitoring event will be collected in 2016 from all RBSS background monitoring wells
in order to obtain four rounds of analytical data in 2016. Based on the sampling schedule
proposed in the Interim Monitoring Plan and the scheduled effectiveness monitoring to be
conducted concurrent with the compliance monitoring (scheduled for October 2016), it is
anticipated this additional background sampling event would be conducted in the Third Quarter
2016.
9.2.2.2 Sample Locations
Sampling locations associated with the Effectiveness Monitoring Plan will be established after
collection of the Second Quarter 2016 sampling event, following evaluation of four seasonal
sampling events. Sample locations will be proposed to NCDEQ prior to implementation of the
Effectiveness Monitoring Plan.
9.2.2.3 Analytical Requirements
Samples collected from the MNA network will be analyzed for the parameters described in the
Interim Monitoring Plan in Section 9.1. Changes to the analytical requirements may be
proposed upon evaluation of the seasonal monitoring results obtained during the CSA and
interim monitoring.
9.2.3 Reporting
Monitoring reports analyzing the results from each sampling event will be submitted to NCDEQ
within 120 days of completion of each sampling event. Results from the additional background
monitoring event will be included in the next scheduled monitoring report.
9.3 Sampling and Analysis
9.3.1 Monitoring Well Measurements and Inspection
Groundwater sampling will be conducted at monitoring well locations associated with the CSA
and additional assessment wells, as described in the Monitoring Plans above.
During each sampling event, these wells will be measured for static water levels. These
measurements will be taken within one 24-hour period and prior to sampling to minimize
temporal variations. The depth to water measurements, along with date and time will be
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recorded on a dedicated field form, a field notebook, and/or electronically via the iForms
program or comparable system.
Monitoring wells will be measured for total depth during the Second Quarter 2016 sampling
event and during the first sampling event in subsequent years. Measurements will be recorded
on a dedicated field form, a field notebook, and/or electronically via the iForms software
program or comparable system.
The thickness of sediment accumulated in each monitoring well will be calculated once a year,
during the second sampling event of the year. Sediment thickness will be calculated by
comparing current total depth with historical data. If more than 1 foot of sediment exists, wells
will be redeveloped with a bailer or pump prior to the third sampling event. In addition, wells may
be redeveloped if turbidity readings below 10 Nephelometric Turbidity Units (NTU) cannot be
achieved during sample purging. Wells where turbidity less than 10 NTU cannot be obtained
may still be sampled in accordance with the Low Flow Sampling Plan.
Each monitoring well will be inspected while performing water level measurements for damage
to the casing, protective monuments, and bollards. Well caps and locks will be inspected to
determine whether they are in good working order and functioning properly. Flush -mounted
wells will be inspected for any damage by vehicular traffic and to ensure that the rubber seal is
functioning properly.
9.3.2 Sample Collection
9.3.2.1 Monitoring Well Purging
All monitoring wells will utilize low flow (minimal drawdown) groundwater purging and sampling
methods, in accordance with the Low Flow Sampling Plan. The low flow technique will be used
to determine when a well has been adequately purged and is ready to sample by monitoring the
pH, specific conductance, temperature, ORP, and turbidity. The volume of water that is removed
will also be observed and recorded. Wells with slow recharge rates, excessive draw down, or
that require a higher pumping rate may be purged using the volume -averaging method.
An adequate purge is achieved when the pH, specific conductance, ORP, and temperature of
groundwater have stabilized and the turbidity is below 10 NTU.
9.3.2.2 Groundwater Sample Collection
After purging and stabilization are accomplished, laboratory -supplied sample containers will be
carefully filled using the same method utilized for purging. Appropriate sample containers,
quantities, and preservatives for the various analyses are listed in Table 9-2.
9.3.2.3 AOW Sample Collection
Grab samples will be collected from each AOW location. Water quality parameters (pH, specific
conductance, ORP, temperature, and turbidity) will be measured from each location. After water
quality parameters have been collected and recorded, AOW samples will be collected by slightly
submersing the lip of the sample container under the water surface. Samples collected to be
analyzed for dissolved target analyte list metals will be field filtered through a 0.45-micron filter
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on the day of collection using a peristaltic pump. Filled laboratory -supplied sample containers
will be labeled and placed in a cooler on ice (4°C) and maintained at that temperature until
delivery to the laboratory.
9.3.2.4 Sample Naming Convention
Samples are identified by the following convention: site code, sample identification name,
sample matrix, and date code. The codes are further explained below.
• The two -digit Site Code for RBSS is RV.
• Sample Identification Name will be the sample location name (i.e., BG-1 D).
• Sample Matrix
o NS — Normal Sample
o FD —
Field Duplicate
o AMB
— Ambient Blank
o FB —
Filter Blank
o EB —
Equipment Blank
• Date Code is a four digit code indicating the quarter and year a sample was collected
For example, a groundwater sample collected from monitoring well BG-1 D in February 2016
would be designated RV-BG-1 D-NS-1 Q1 6. If a field duplicate was also collected from that
location it would be designated RV-BG-1 D-FD-1 Q16.
9.3.2.5 Waste Handling
Purge water and decontamination water will be discharged to the ground surface at the RBSS
site. Other investigation derived waste, including disposable tubing and gloves, will be bagged
and disposed of as part of the RBSS site's municipal solid waste.
9.3.2.6 Chain of Custody and Sample Delivery
All samples will be tracked using chain -of -custody procedures. A separate chain -of -custody
form will be filled out by each sample team and accompany each cooler shipped. Samples will
be hand delivered or shipped to the contract laboratory.
9.3.3 Quality Assurance/Quality Control
In addition to laboratory and other quality assurance/quality control procedures, field quality
control measures are implemented to ensure that data meet project requirements. The following
field quality control procedures are utilized for this project:
• Field Duplicates — Field duplicate samples will be collected by filling two identical sets
of sample containers with water from the same sample location for each of the planned
analyses. Field duplicates will be given unique sample numbers.
Equipment Blanks — Equipment blanks measure the cleanliness of field sampling
equipment. Equipment blanks will be created by pouring reagent -grade water over a
decontaminated pump and collecting the water in appropriate sample containers.
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Equipment blanks receive all the tests which are to be performed on the associated
samples.
Ambient Blanks — Field blanks are also filled with reagent grade water. Field blanks are
created in the field and are intended to measure background contamination in the field.
Field blanks will be created by filling the appropriate sample containers while at the site
collecting other water samples.
• Filter Blanks — Filter blanks evaluate the possible addition of chemicals from the filter to
the sample. Filter blanks are created in the field and are intended to measure potential
contamination in the filters. Filter blanks will be created by filling the appropriate sample
containers with reagent grade water run through a new filter.
Trip Blanks — Trip blanks are created by the laboratory and accompany low level
mercury samples the entire time they are being shipped and are out in the field.
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10 Implementation Cost and Schedule
CAMA Section §1 30A-309.21 1 (b)(1) requires implementation of corrective action within 30 days
of CAP approval.
10.1 Implementation Cost
The recommended corrective action at RBSS following source removal is MNA. A summary of
costs for the selected remedy is provided in Table 10-1. Note that the actual cost will be
dependent on the actual conditions that exist following excavation and completion of closure
activities. Therefore, these values represent an estimate for reference purposes.
Table 10-1 Estimated Capital and Annual Costs for Corrective Action - MNA
Proposed Activity
Total
Capital Costs - Monitoring Well Installation
Monitoring Well Installation —15 new wells
$126,000
Site Prep and Erosion Sediment Control
$30,000
Field Management (15%)
$23,000
Well Install Reporting (GW-1s/construction records)
$5,000
Project Management (10%)
$18,000
Contingency (20%)
$41,000
Total Capital Costs
$243,000
Annual Costs - Monitoring/Reporting
Lab Analysis
$19,000
Data Validation
$15,000
Reporting
$60,000
Equipment and Expendables
$9,000
Sampling Labor
$37,000
Project Management (10%)
$14,000
Escalation to Mid -Point (4%)
$6,000
Annual Monitoring/Reporting Costs
$160,000
Total Capital/Annual Costs for Project Duration*
$6,700,000
*Note: this total project cost includes the annual cost over the project duration of 30 years with a 4.25% discount factor per year.
10.2 Implementation Schedule
The Interim Monitoring Plan will be implemented at the RBSS site during sampling activities
conducted in the first half of 2016. Details of interim monitoring are discussed in Section 9.
MNA will be re-evaluated after closure activities are completed using results of the interim
monitoring. Beginning in September 2016, effectiveness monitoring events will coincide with
compliance monitoring events required by the NPDES permit, which are conducted in January,
May, and September.
MNA effectiveness and groundwater quality monitoring results at the RBSS site will be
evaluated and used to assess the effectiveness of MNA as a remedial alternative. Based on the
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resulting monitoring data, recommendations will be made regarding modifications in the
monitoring program to ensure representative data are being collected, changes in the
implementation of the selected remedy, or if other alternatives need to be considered.
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11 References
ASTM. 2014. E1689-95 (Reapproved 2014), Standard Guide for Developing Conceptual Site
Models for Contaminated Sites. ASTM International.
Daniel, C. C. III. 2001. Estimating ground -water recharge in the North Carolina Piedmont for
land use planning [abs.], in 2001 Abstracts with Programs, 50th Annual Meeting,
Southeastern Section, April 5-6, 2001: Raleigh, N.C., The Geological Society of
America, v. 33, no. 2, p. A-80.
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