NC0004961_Text - GW Assessment_20130531

GROUNDWATER ASSESSMENT
DUKE ENERGY CAROLINAS, LLC
RIVERBEND STEAM STATION
ASH BASIN
NPDES PERMIT NC0004961

Prepared for:
DUKE ENERGY CAROLINAS, LLC
Charlotte, North Carolina

Prepared by:
HDR ENGINEERING, INC. OF THE CAROLINAS
Charlotte, North Carolina

May 31, 2013
REPORT VERIFICATION
PROTECT: GROUNDWATER ASSESSMENT
DUKE ENERGY CAROLINAS, LLC
RIVERBEND STEAM STATION
ASH BASIN
NPDES PERMIT NCOOO4961
This document has been reviewed for accuracy and quality commensurate with the intended
application.
Prepared by:
Checked by:
Approved by:
Project Manager: Ty Ziegler, PE
Date: My 3 020 / 3
Date: Ar,, 31, 2.013
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HDR Engineering, e Carolinas
440 South. Church St., Suite 1000
Charlotte, NC 28202
North Carolina Engineering Firm Number F-0I 16

ii
GROUNDWATER ASSESSMENT
RIVERBEND STEAM STATION
ASH BASIN
NPDES PERMIT NC0004961

TABLE OF CONTENTS

Section Page No.
SECTION 1 INTRODUCTION ................................................................................... 1
SECTION 2 BACKGROUND .................................................................................... 2
2.1 Plant and Ash Basin Description ..................................................................................2
2.2 Ash Basin Description ..................................................................................................2
2.3 Site Geology and Hydrogeology ..................................................................................3
2.4 Regulatory Requirements for Groundwater Monitoring ..............................................4
2.5 Description of Groundwater Monitoring System .........................................................6
SECTION 3 GROUNDWATER EXCEEDANCES............................................................ 8
3.1 Groundwater Quality Exceedances ...............................................................................8
SECTION 4 ASSESSMENT OF GROUNDWATER EXCEEDANCES ................................. 10
4.1 Site Hydrogeologic Conceptual Model -Task 1 .........................................................10
4.1.1 Geologic/Soil Framework ...........................................................................................11
4.1.2 Hydrologic Framework ...............................................................................................12
4.1.3 Hydrologeologic Information for Groundwater Model ..............................................14
4.1.4 Strat igraphic Unit Properties ......................................................................................17
4.1.5 Groundwater Model Refinement Plan ........................................................................17
4.2 Discuss Site History and Land Uses -Task 2 ..............................................................18
4.3 Review Available Data on Ash Basin Water Quality - Task 3 ...................................19
4.4 Review Mountain Island Lake Water Level Data -Task 4 .........................................24
4.5 Review Location-Number of Background Monitoring Wells - Task 5 ......................26
4.6 Evaluate Well Construction Information - Task 6 ......................................................28
4.7 Evaluate Exceedances Against Background Well Results -Task 7 ............................30
TABLE OF CONTENTS
(Continued)

iii
4.8 Evaluate Exceedances A gainst Turbidity Values -Task 8 ..........................................34
4.9 Evaluate Sampling Flow Rates - Task 9 .....................................................................36
4.10 Collect and Analyze Filtered and Unfiltered samples - Task 10 ................................37
4.11 Collect Reduction/Oxid ation Field Parameters - Task 11 ..........................................38
4.11A Collect Soil Samples – Analyze for Iron and Manganese Content-Task 11A ..........40
4.12 Perform Statistical Data Analyses of the Sampling Results - Task 12 .......................42
SECTION 5 CONCLUSIONS AND RECOMMENDATIONS ............................................ 45
Conclusions ..........................................................................................................................45
Recommendations ................................................................................................................50
REFERENCES ..................................................................................................... 51

TABLE OF CONTENTS
(Continued)

iv
FIGURES

Figure 1-1 Site Location Map
Figure 2.5-1 Compliance Groundwater Monitoring Wells
Figure 2.5-2 Compliance and Voluntary Groundwater Monitoring Well Locations
Figure 4.1 -1 Geologic Map
Figure 4.1 -2 Water Level Elevations – Groundwater and Surface Water Locations
Figure 4.1 -3 Site Plan with Cross Sections
Figure 4.1 -4 Cross Section A’-A’
Figure 4.1 -5 Cross Section B’-B’ and C’-C’
Figure 4.2 -1 Site Property
Figure 4.2 -2 1948 Paw Creek Topo
Figure 4.3 -1 Piper Diagrams – Ash Basin Sample TOWER 0.3M and Upgradient
Monitoring Wells
Figure 4.3 -2 Piper Diagram – Ash Basin Sample TOWER 0.3M and Downgradient
Monitoring Wells
Figure 4.3 -3 Stiff Diagrams – Background Monitoring Wells MW-7SR, MW-7D and
TOWER Sample
Figure 4.3 -4 Stiff Diagrams – Upgradient Monitoring Wells MW-8S, MW-8I, MW-8D
and TOWER Sample
Figure 4.3 -5 Stiff Diagrams – Primary Cell Downgradient Monitoring Wells MW -9,
MW-10 and TOWER Sample
Figure 4.3 -6 Stiff Diagrams – Secondary Cell Downgradient Monitoring Wells
MW-11SR, MW-11DR and TOWER Sample
Figure 4.3 -7 Stiff Diagrams – Downgradient Monitoring Wells MW -13, MW-14,
MW-15 and TOWER Sample
Figure 4.3 -8 Stiff Diagrams – Voluntary Monitoring Wells MW -4S, MW-4D and
TOWER S ample

TABLE OF CONTENTS
(Continued)

v
FIGURES (Continued)

Figure 4.3 -9 Stiff Diagrams – Voluntary Monitoring Wells MW -5S, MW-5D and
TOWER Sample
Figure 4.3 -10 Cation/Anion Concentrations – Upgradient MW’s and TOWER
Figure 4.3 -11 Ratio of Sulfate/Chloride in MW’s and TOWER
Figure 4.4 -1 Location Map
Figure 4.4 -2 MW-13 and Mountain Island Lake Water Level
Figure 4.4 -3 MW-13 and Mountain Island Lake Water Level March 7-8, 2013
Figure 4.4 -4 MW-13, Mountain Island Lake Water Level, and Rainfall
Figure 4.5 -1 MW-7SR/D and MW-8S/I/D Groundwater Elevations
Figure 4.5 -2 MW-9, MW-10, MW-11DR/SR, MW-13, MW-14 and MW-15
Groundwater Elevations
Figure 4.7 -1 pH – Shallow Monitoring Wells
Figure 4.7 -2 pH – Deep Monitoring Wells
Figure 4.7 -3 Background Wells MW-7SR & MW -7D – Iron
Figure 4.7 -4 MW-7D vs Deep Wells – Iron
Figure 4.7 -5 MW-7SR vs Shallow Wells – Iron
Figure 4.7 -6 MW-7SR vs MW-8I – Iron
Figure 4.7 -7 MW-7SR vs MW-9 – Iron
Figure 4.7 -8 MW-7SR vs MW-10 – Iron
Figure 4.7 -9 MW-7SR vs MW-11SR – Iron
Figure 4.7 -10 MW-7SR vs MW-13 – Iron
Figure 4.7 -11 MW-7SR vs MW-14 – Iron
Figure 4.7 -12 MW-7SR vs MW-15 – Iron
Figure 4.7 -13 Background Wells – Manganese
Figure 4.7 -14 MW-7D vs Deep Wells – Manganese
Figure 4.7 -15 MW-7SR vs Shallow Wells – Manganese

TABLE OF CONTENTS
(Continued)

vi
FIGURES (Continued)

Figure 4.7 -16 MW-7SR vs MW-8S – Manganese
Figure 4.7 -17 MW-7SR vs MW-8I – Manganese
Figure 4.7 -18 MW-7D vs MW-8D – Manganese
Figure 4.7 -19 MW-7SR vs MW-9 – Manganese
Figure 4.7 -20 MW-7SR vs MW-10 – Manganese
Figure 4.7 -21 MW-7SR vs MW-11SR – Manganese
Figure 4.7 -22 MW-7D vs MW-11DR – Manganese
Figure 4.7 -23 MW-7SR vs MW-13 – Manganese
Figure 4.7 -24 MW-7SR vs. MW-14 – Manganese
Figure 4.7 -25 MW-7SR vs MW-15 – Manganese
Figure 4.8 -1 MW-7SR – Iron and Turbidity
Figure 4.8 -2 MW-7SR – Manganese and Turbidity
Figure 4.8 -3 MW-8S – Manga nese and Turbidity
Figure 4.8 -4 MW-8I – Iron and Turbidity
Figure 4.8 -5 MW-8I – Manganese and Turbidity
Figure 4.8 -6 MW-8D – Iron and Turbidity
Figure 4.8 -7 MW-8D – Manganese and Turbidity
Figure 4.8 -8 MW-9 – Iron and Turbidity
Figure 4.8 -9 MW-9 – Manganes e and Turbidity
Figure 4.8 -10 MW-10 – Iron and Turbidity
Figure 4.8 -11 MW-10 – Manganese and Turbidity
Figure 4.8 -12 MW-11SR – Iron and Turbidity
Figure 4.8 -13 MW-11SR – Manganese and Turbidity
Figure 4.8 -14 MW-11DR – Manganese and Turbidity
Figure 4.8 -15 MW-13 – Iron and Turbidity
Figure 4.8 -16 MW-13 – Manganese and Turbidity

TABLE OF CONTENTS
(Continued)

vii
FIGURES (Continued)

Figure 4.8 -17 MW-14 – Iron and Turbidity
Figure 4.8 -18 MW-14 – Manganese and Turbidity
Figure 4.8 -19 MW-15 – Iron and Turbidity
Figure 4.8 -20 MW-15 – Manganese and Turbidity

TABLE OF CONTENTS
(Continued)

viii
TABLES

Table 2.4-1 Groundwater Monitoring Requirements
Table 3.1-1 Field Data Parameters
Table 3.1-2 Groundwater Field and Analytical Results – Compliance Wells
Table 3.1-3 Groundwater Field and Analytical Results – Voluntary Wells
Table 3.1-4 Exceedances of 2L Standards
Table 4.1-1 Hydraulic Conductivity ‘k’
Table 4.1-2 Parameters for Soil
Table 4.1-3 Parameters for Rock
Table 4.3-1 Field and Analytical Results – Ash Basin Water Quality Sample
Table 4.3-2 Analytical Results – Ash Basin Cations and Anions
Table 4.3-3 Groundwater Analytical Results – Cations and Anions
Table 4.10-1 Filtered and Unfiltered Analytical Results
Table 4.11A-1 Summary of Soil Analytical Results

APPENDICES

Appendix A Letter from Andrew H. Pitner, P.G, Regional Aquifer Protection Supervisor,
NCDENR, Division of Water Quality, to Ed Sullivan and Allen Stowe, Water
Management, Duke Energy Corporation, Dated March 16, 2012
Appendix B Letter from Andrew H. Pitner, P.G., Regional Aquifer Protection Supervisor,
NCDENR Division of Water Quality, to Ed Sullivan and Allen Stowe, Water
Management, Duke Energy Corporation, Dated January 10, 2013
Appendix C Statistical Analysis for Groundwater Metals Riverbend Steam Station Ash Basin
February 2013 Sampling Event
Appendix D Soil Sampling Laboratory Analytical Report and Chain of Custody Form

1
Section 1
Introduction
Duke Energy Carolinas, LLC (Duke Energy), owns and operates the Riverbend Steam Station
(Riverbend), located near Mt. Holly, in Gaston County, North Carolina (see Figure 1-1). The
steam station has previously generated electricity by burning coal; however , as of April 1, 2013,
the facility has been retired from service. Exceedances of the North Carolina Administrative
Code (NCAC) Title 15A Chapter 02L (g) groundwater quality standards (2L Standards) have
been measured in groundwater samples collected at the compliance boundary from groundwater
monitoring wells at the Riverbend ash basin. In a letter dated March 16, 2012, the North
Carolina Department of Environment and Natural Resources (NCDENR) Division of Water
Quality (DWQ) Aquifer Protection Section (APS) requested that Duke Energy begin additional
assessment activities at stations where measured and modeled concentrations of groundwater
constituents exceed the 2L Standards at the compliance boundary. A copy of this letter is
included as Appendix A.
HDR Engineering, Inc. of the Carolinas (HDR) prepared the proposed groundwater assessment
work plan, dated November 1, 2012, and submitted the document to DWQ on behalf of Duke
Energy. In a letter dated January 10, 2013, DWQ notified Duke Energy that DWQ generally
agreed with the assessment approaches outlined in the work plan. A copy of this letter is
included in Appendix B.

2
Section 2
Background

2.1 Plant and Ash Basin Description
The recently retired Riverbend Steam Station was a coal-fired electricity generating facility
located in Gaston County, North Carolina near the town of Mt. Holly. The four -unit station,
which began commercial operation in 1929, is named after a bend in the Catawba River. The
station is located along the Catawba River on Mountain Island Lake. Mountain Island Lake is a
reservoir used for hydroelectric generation and is owned by Duke Energy and operated as part of
the Catawba-Wateree Project Federal Energy Regulatory Commission (FERC) Project No. 2232.
Duke Energy also owns and operates the reservoirs upstream and downstream of Mountain
Island Lake as part of the Catawba-Wateree Project. Lake Norman is located on the Catawba
River upstream of Mountain Island Lake and has a surface area of approximately 32,475 acres.
Lake Wylie is located downstream of Mountain Island Lake and has a surface area of
approximately 13,443 acres. Mountain Island Lake has a surface area of approximately 3,281
acres.
2.2 Ash Ba sin Description
The ash basin is located approximately 2,400 feet to the northeast of the power plant, adjacent to
Mountain Island Lake, as shown on Figure 1-1. The Primary Cell is impounded by an earthen
dike, referred to as Dam #1, located on the west side of the Primary Cell. The Secondary Cell is
impounded by an earthen dike, referred to as Dam #2, located along the northeast side of the
Secondary Cell.
The surface area of the Primary Cell is approximately 41 acres with an approximate maximum
pond elevation of 724 feet.1 The surface area of the Secondary Cell is approximately 28 acres
with an approximate maximum pond elevation of 714 feet. The full pond elevation of Mountain
Island Lake is approximately 646.8 feet.

1 The datum for all elevation information presented in this report is NAVD88.
Section 2 Background

3
The ash basin system was an integral part of the station’s wastewater treatment system. The ash
basin received the following inflows:
• Ash removal system
• Station yard drain sump
• Stormwater flows
During station operations, due to the cycling nature of station operations, inflows to the ash basin
were highly variable.
The inflows from the ash removal system and the station yard drain sump are discharged through
sluice lines into the Primary Cell. The discharge from the Primary Ce ll to the Secondary Cell is
through a concrete discharge tower located near the divider dike.
Effluent from the ash basin is discharged from the Secondary Cell, through a concrete discharge
tower, to Mountain Island Lake. The concrete discharge tower drains through a 30 -inch
corrugated metal pipe (CMP) into a concrete lined channel that discharges to Mountain Island
Lake. The ash basin pond elevation is controlled by the use of concrete stop logs.
2.3 Site Geology and Hydrogeology
Riverbend and its associated ash basin system are located in the Charlotte Belt of the Piedmont
physiographic province (Piedmont). The following generalizations on the site hydrogeology are
taken from A Master Conceptual Model for Hydrogeological Site Characterization in the
Piedmont and Mountain Region of North Carolina (LeGrand 2004).

Piedmont bedrock primarily consists of igneous and metamorphic bedrock. The fractured
bedrock is overlain by a mantle of unconsolidated material known as regolith. The regolith
includes, whe re present, the soil zone, a zone of weathered, decomposed bedrock known as
saprolite, and where present, alluvium. Saprolite, the product of chemical and mechanical
weathering of the underlying bedrock, is typically composed of clay and coarser granular
material up to boulder size, and may reflect the texture of the rock from which it was formed.
The weathering product of granitic rocks may be quartz-rich and sandy-textured, whereas rocks
Section 2 Background

4
poor in quartz and rich in feldspar and other soluble minerals form a more clayey saprolite. The
regolith serves as the principal storage reservoir for the underlying bedrock (LeGrand 2004).

A transition zone may occur at the base of the regolith between the soil-saprolite and the
unweathered bedrock. This transition zone of partially weathered rock is a zone of relatively
high permeability, compared to the overlying soil-saprolite and the underlying bedrock (LeGrand
2004).

Groundwater flow paths in the Piedmont are almost invariably restricted to the zone underlying
the topographic slope extending from a topographic divide to an adjacent stream. LeGrand
describes this as the local slope aquifer system. Under natural conditions the general direction of
groundwater flow can be approximated from the surface topography (LeGrand 2004).

Groundwater recharge in the Piedmont is derived entirely from infiltration of local precipitation.
Groundwater recharge occurs in areas of higher topography (i.e., hilltops) and groundwater
discharge occurs in lowland areas bordering surface water bodies, marshes, and floodplains
(LeGrand 2004).

The site is located on the north side of Horseshoe Bend Beach Road. This road runs generally
west to east and is located along a local topographic divide. The topography at the site genera lly
slopes downward from that divide to Mountain Island Lake. Groundwater at the site appears to
generally flow from areas of higher topography toward Mountain Island Lake.

The site geology and hydrogeology is discussed in Section 4.1 Site Hydrogeologic Conceptual
Model.

2.4 Regulatory Requirements for Groundwater Monitoring
The NPDES program regulates wastewater discharges to surface waters, to ensure that surface
water quality standards are maintained. Riverbend operates under NPDES Permit NC0004961 ,
which authorizes discharge of cooling water (Outfall 001) and ash basin discharge (Outfall 002)
to the Catawba River in accordance with effluent limitations, monitoring requirements, and other
Section 2 Background

5
conditions set forth in the permit. The NPDES permitting pro gram requires that permits be
renewed every five years. The most recent NPDES permit renewal at the Riverbend station
became effective on January 18, 2011.
In addition to surface water monitoring, the NPDES permit requires groundwater monitoring.
Permit Condition A (11) Attachment XX, Version 1.1, dated June 15, 2011, lists the groundwater
monitoring wells to be sampled, the parameters and constituents to be measured and analyzed,
and the requirements for sampling frequency and results reporting. These requirements are
provided in Table 2.4-1. Attachment XX also provides requirements for well location and well
construction.
The compliance boundary for groundwater quality at the Riverbend ash basin site is defined in
accordance with T15 A NCAC 02L .0107(a) as being established at either 500 feet from the
waste or at the property boundary, whichever is closest to the waste. Sampling at the
groundwater wells commenced December 2010. Since that time, all monitoring wells are
required to be sampled three times per year (February, June, and October). Analytical results are
submitted to DWQ approximately 60 days after each sampling event for all monitoring wells
except MW-9, MW-10, and MW-13.
Monitoring wells MW -9, MW-10, and MW-13 are located inside of the compliance boundary.
These monitoring wells are also sampled three times per year, and compliance with 2L Standards
(at the compliance boundary) is determin ed by using predictive calculations or a groundwater
model. For these three monitoring wells, Duke Energy used a groundwater model to predict the
concentrations at the compliance boundary. The predicted results from the groundwater model
and the analytical results for samples collected during the 2011 sampling events at MW-9, MW-
10, and MW-13 were submitted to DWQ in January 2012.2 The annual report for samples
collected during the 2012 sample events with the predicted model results will be submitted
subsequent to the submittal of this assessment.

2 Supplemental Groundwater Monitoring Report Riverbend Steam Station Ash Basin, NPDES Permit NC0004961,
Duke Energy Carolinas, LLC, January 16, 2012.
Section 2 Background

6
2.5 Description of Groundwater Monitoring System
As discussed in Section 3, groundwater monitoring is performed in accordance with the
requirements of the NPDES permit. The groundwater monitoring system for the ash basin system
consists of the following monitoring wells: MW -7SR, MW-7D, MW-8S, MW-8I, MW-8D,
MW-9, MW-10, MW-11SR, MW-11DR, MW-13, MW-14, and MW-15. Monitoring well
MW-7D was installed in 2006. All other compliance monitoring wells were installed in 2010
and 2011.
The locations for the monitoring wells were selected in consultation with the DWQ APS. The
locations of the monitoring wells, the waste boundary, and the compliance boundary are shown
on Figure 2 .5-1.
Monitoring wells MW -7SR and MW-7D are located to southeast (i.e., upgradient) of the Primary
Cell and are considered by Duke Energy to represent background water quality. Monitoring
wells MW -8S, MW-8I, and MW-8D are located to the south of an ash storage area and to the
north of Horseshoe Bend Beach Road. Monitoring well MW-9 is located to the north of a cinder
storage area. MW-10 is located downgradient of the Primary Cell. Monitoring wells MW -11SR
and MW -11DR are located northwest of the dike dividing the Primary Cell and the Secondary
Cell. Monitoring wells MW -13, MW-14, and MW-15 are located downgradient of the
Secondary Cell.
Monitoring wells MW -7SR, MW-8S, MW-9, MW-10, MW-11SR, MW-13, MW-14, and MW-
15 were installed by rotary drilling methods using hollow stem augers, with the well screen
installed above auger refusal to monitor the shallow aquifer within the saprolite layer. The
screen lengths for these wells range from 15 feet to 20 feet. Monitoring well MW -8I was also
installed by rotary drilling methods using hollow stem augers, with the well screen installed at an
intermediate depth in the surficial aquifer at 98 feet to 118 feet below ground surface (bgs). The
screen for monitoring well MW -8D was installed immediately above auger refusal and screened
from 156 feet to 166 feet bgs to monitor the transition zone. The 5-foot long screen for
monitoring well MW -11DR was installed in the fractured bedrock zone immediately below
Section 2 Background

7
auger refusal.3 Monitoring well MW -7D was installed using hollow stem augers and mud rotary
drilling techniques to complete the well with a 5 -foot long well screen from 95 to 100 feet bgs
within the fractured bedrock zone.4
With the exception of monitoring wells MW -9, MW-10, and MW-13, the ash basin monitoring
wells were installed at or near the compliance boundary. Monitoring wells MW-9, MW-10, and
MW-13 are located where it was not possible to access the compliance boundary. Therefore,
these monitoring wells are installed inside of the 500-foot compliance boundary.
The monitoring wells at Riverbend are equipped with dedicated bladder-type pumps and are
sampled in accordance with the approved groundwater sampling and analysis plan.
Groundwater monitoring wells MW -1S, MW-1D, MW-2S, MW-2D, MW-3S, MW-3D, MW-4S,
MW-4D, MW-5S, MW-5D, MW-6S, and MW-6D were installed by Duke Energy in 2006 as
part of a voluntary monitoring system. Thes e voluntary wells are shown on Figure 2 .5-2. No
groundwater samples are currently collected from these wells; however, groundwater samples
were collected from selected wells during the February 2013 sampling event.

3 Amended Ash Basin Monitoring Well Installation Report, Riverbend Steam Station, MACTEC Project No. 6228-
10-5284, March 30, 2011.
4 Ash Basin Drilling Services for: Riverbend Steam Station, Mount Holly, North Carolina, ARCADIS G&M of
North Carolina, Inc. January 7, 2007.

8
Section 3
Groundwater Exceedances

3.1 Groundwater Quality Exceedances
As noted in the NCDENR letter dated March 16, 2012, exceedances of the 2L Standards were
reported for groundwater monitoring wells during the 2010 - 2012 monitoring events. These
monitoring wells were sampled in:
• December 2010
• February 2011
• June 2011
• October 2011
• February 2012
• June 2012
• October 2012
• February 2013
Groundwater monitoring results for the most recent sampling event (February 4 -5, 2013) were
submitted by Duke Energy as required. These results are also provided in Table s 3.1-1 and 3.1-
2. Voluntary groundwater monitoring wells MW -4S, MW-4D, MW-5S, and MW-5D were also
sampled during this event. The analytical results for these wells are included in Table 3.1-3.
With the exception of iron, manganese, and pH, the results for all monitored parameters and
constituents were less than the 2L Standards. Table 3.1-4 lists the range of exceedances for iron,
manganese, and pH.
Iron and manganese are listed in the 2L Standards; however the se substances are listed by the
EPA as having non-mandatory water quality standards. The EPA has established National
Secondary Drinking Water Regulations in Title 40 of the Code of Federal Regulations (CFR)
Part 143 National Secondary Drinking Water Standards (40 CFR 143). In 40 CFR 143, the EPA
established non-mandatory water quality standards for 15 contaminants. Iron and manganese are
included in this list of contaminants having secondary maximum contaminant levels or
standards. EPA does not enforce these secondary maximum contaminant levels. The secondary
Section 4 Assessment of Groundwater Exceedances

9
standards are established only as guidelines to assist public water systems in managing their
drinking water for aesthetics and other considerations, such as taste, color, and odor.

10
Section 4
Assessment of Groundwater Exceedances

The assessment of the groundwater exceedances at the Riverbend Ash Basin was performed as
stated in the proposed assessment work plan. Comments provided by the DWQ on the proposed
work plan in the January 10, 2013 letter were incorporated into the assessment work plan. These
comments included the following:
• DWQ commented that there may be value in sampling monitoring wells that are part of
the voluntary well network to suppor t the modeling effort and other site activities.
• DWQ commented that only non-filtered samples are acceptable for compliance purposes.
• DWQ requested the collection of redox parameters, dissolved oxygen (DO) and oxidation
reduction potential (ORP), at all mo nitoring wells.
The approach used in this assessment was developed to comply with the process described in the
flow chart contained in a DWQ Memorandum dated June 17, 2011, Policy for Compliance
Evaluation of Long-Term Permitted facilities with No Prior Groundwater Monitoring
Requirements. In general the evaluation process for exc eedances depicted on the flowchart
contained in the memorandum is to determine if the exceedance is in excess of naturally
occurring background concentrations. As part of the process, the evaluation include s an array of
hydrogeologic features, site specific features, well location and construction, groundwater flow
direction, compliance boundaries, and contaminant sources.
The italicized text in the following sections contains the wording from the proposed groundwater
assessment work plan for the particular task.
4.1 Site Hydrogeologic Conceptual Model -Task 1
Available reports and data on site geotechnical, geologic, and hydrologic conditions will be
reviewed and used to develop a site hydrogeologic conceptual model. The NCDENR document,
Hydrogeologic Investigation and Reporting Policy Memorandum , dated May 31, 2007, will be
used as general guidance.
Section 4 Assessment of Groundwater Exceedances

11
4.1.1 Geologic/Soil Framework
Riverbend and its associated ash basin system are located in the Charlotte Terrane of the
Carolina Zone (Pippin and others, 2008), or as described in the older belt terminology, the
Charlotte Belt of the Piedmont physiographic province (Piedmont) (North Carolina Geological
Survey, 1985). The Charlotte terrane is characterized by mostly felsic to mafic plutonic rocks
which intrude a suite of mainly metaigneous rocks and minor metasedimentary rocks (Pip pin and
others, 2008).

The site location, overlaid on the Geologic Map of the Charlotte 1º x 2º Quadrangle, North and
South Carolina (Goldsmith and others 1988), is presented as Figure 4.1-1. Based on the location
of the site, the underlying bedrock at the site is composed of metamorphosed quartz diorite and
tonalite (mqd). The mqd unit is described as gray, usually medium- to coarse-grained, generally
foliated rock composed dominantly of plagioclase, quartz biotite, hornblende, and epidote.
Biotite, hornblende, and epidote are commonly associated in clots replacing original mafic
phenocrysts; clots may be smeared out, thus defining foliation (Goldsmith and others 1988).

The soils that overlie the bedrock in the area have generally formed from the in-place weathering
of the parent bedrock. The fractured bedrock is overlain by a mantle of unconsolidated material
known as regolith. The regolith, where present, includes the soil zone, a zone of weathered,
decomposed bedrock known as saprolite, and alluvium. Saprolite, the product of chemical and
mechanical weathering of the underlying bedrock, is typically composed of silt and coarser
granular material up to boulder size, and may reflect the texture of the rock from which it was
formed . The weathering products of felsic rocks may be sandy-textured and rich in quartz
content, while mafic rocks form a more clayey saprolite (LeGrande, 2004).

Based on a review of the monitoring well insta llation logs provided by Duke Energy, the soils
comprising the sapr olite layer onsite were characterized as ranging from silty clay to partially
weathered rock consisting of fine to medium grained sand with coarse quartz and feldspar parent
rock material. Bedrock encountered onsite consists of granite, quartzite, and gne iss with quartz,
mica, and mafic materials (MACTEC and ARCADIS).

Section 4 Assessment of Groundwater Exceedances

12
4.1.2 Hydrologic Framework

The groundwater system in the Piedmont Province, in most cases, is within a system comprised
of two interconnected layers, or mediums: 1) residuum/saprolite and weathered rock (regolith)
overlying and 2) fractured crystalline bedrock (Heath 1980; Harned and Daniel 1992). Within
the regolith layer a thoroughly weathered and structureless material termed residuum occurs near
the ground surface with the degree of weathering decreasing with depth. The residuum grades
into a coarser-grained material that retains the structure of the parent bedrock and is termed
saprolite. Beneath the saprolite, partially weathered bedrock occurs with depth until sound
bedrock is encountered. This mantle of residual soil, saprolite, and weathered rock is a
hydrogeologic unit that covers and crosses various types of rock (LeGrand 1988). It provides an
intergranular medium through which the recharge and discharge of water from the underlyin g
fractured rock occurs. The bedrock layer consists of fractured, nonporous crystalline bedrock.
The fractures control both the hydraulic conductivity and storage capacity of the rock mass.

A transition zone at the base of the regolith has been interpreted to be present in many areas of
the Piedmont. The zone consists of partially weathered/fractured bedrock and lesser amounts of
saprolite that grades into bedrock and has been described as “being the most permeable part of
the system, even slightly mo re permeable than the soil zone” (Harned and Daniel 1992). The
zone thins and thickens within short distances and its boundaries may be difficult to distinguish.
It has been suggested that the zone may serve as a conduit of rapid flow and transmission of
contaminated water (Harned and Daniel 1992).

Piedmont topography is characterized by gently rounded sloped hills and valleys. Recharge
typically occurs on upland areas and slopes, while groundwater discharge is concentrated in
surface water bodies and lowland areas. LeGrand’s (1988; 2004) conceptual model of the
groundwater setting in the Piedmont incorporates the above two medium system into an entity
that is useful for the description of groundwater conditions. That entity is the surface drainage
basin that contains a perennial stream or river (LeGrand 1988). Each basin is similar to adjacent
basins and the conditions are generally repetitive from basin to basin. Within a basin, movement
of groundwater is generally restric ted to the area extending from the drainage divides to a
Section 4 Assessment of Groundwater Exceedances

13
perennial stream or river (Slope-Aquifer System; LeGrand 1988; 2004). Rarely does
groundwater move beneath a perennial stream or river to another more distant stream (LeGrand
2004).

Therefore, in most cases in the Piedmont, the groundwater system is a two medium system
(LeGrand 1988) restricted to the local drainage basin. The groundwater occurs in a system
composed of two interconnected layers: residuum/saprolite and weathered rock overlying
fr actured crystalline rock separated by the transition zone. Typically, the residuum/saprolite is
partly saturated and the water table fluctuates within it. Water movement is generally through
the fractured bedrock. The near-surface fractured crystalline rocks can form extensive aquifers.
The character of such aquifers results from the combined effects of the rock type, fracture
system, topography, and weathering. Topography exerts an influence on both weathering and
the opening of fractures, while the weathering of the crystalline rock modifies both transmissive
and storage characteristics. The aquifer system in the piedmont typically exists in an unconfined
or semi-confined condition in the bedrock zone. Under natural conditions the general direction
of groundwater flow can be approximated from the surface topography. Groundwater moves
both vertically down through the regolith and parallel to the bedrock surface to areas where
groundwater discharges as seepage into streams, lakes, or other surface water bodies.

The Riverbend Steam Station is located on a peninsula in the Catawba River, on the north side of
Horseshoe Bend Beach Road (Figure 2.2-1). This road runs generally west to east and is located
along a local topographic divide. Based on the slope-aquifer system, groundwater at the site is
expected to flow downward from this topographic divide, to the ash basins, and discharge into
Mountain Island Lake (Catawba River).

The water elevation measurements in the groundwater monitoring wells, primary and secondary
cells of the ash basin, and Mountain Island Lake are presented on Figure 4.1 -2. Based on a
review if this data it appears that the slope-aquifer system applies to the Riverbend site. Based
on the water level elevations provided by Duke Energy, and ground surface topography,
groundwater flow direction across the site appears to be predominantly from the ash basins to the
north and west towards Mountain Island Lake.
Section 4 Assessment of Groundwater Exceedances

14

The ash basin ponds establish a perched water condition affecting the local groundwater table
surrounding the basins. Monitoring wells MW -1S, MW-1D, MW-3D, MW-5D, MW-6S, and
MW-6D, located downgradient of the primary and secondary ash ponds, have had groundwater
elevations under artesian conditions (water level above ground surface) due to the presence of
the basins.

Based on the groundwater levels measured in the shallow and deep well pairs, it appears that an
upward vertical gradient exists in the location of monitoring wells MW -11SR/DR, MW-5S/D,
and MW -7SR/D. Monitoring wells MW -8S/I/D and MW-4S/D appear to be located in areas
with downward vertical gradients.

The Riverbend site receives an average of approximately 44 inches of rain per year. Rainfall is
the source of groundwater recharge and monthly averages can vary significantly as droughts and
floods are common (LaGrande 2004). The average recharge rate of the surficial aquifer at the
site is expected to be 300,000 gallons per day per square mile (Heath 1994).

Monitoring wells MW -8S and MW-7S located topographically upgradient of the ash basins have
had groundwater elevation variations of as much as approximately four feet, while the
monitoring wells located adjacent to Mountain Island Lake (MW-9, MW-10, MW-13) have had
variations in groundwater elevations measur ed at approximately one foot or less. The wells
located along Mountain Island Lake also may be influenced by the frequent water level changes
in the lake due to the operation of upstream and downstream hydroelectric stations (i.e., Cowans
Ford Hydroelectric Station on Lake Norman and Mountain Island Hydroelectric Station on
Mountain Island Lake, respectively).

4.1.3 Hydrologeologic Information for Groundwater Model
As described in Section 8 Groundwater Model Refinement Plan of the proposed groundwater
assessment work plan, dated November 1, 2013, the groundwater model used to model
compliance at groundwater monitoring wells MW -9, MW-10, and MW-13 will be refined based
Section 4 Assessment of Groundwater Exceedances

15
on information developed from the site hydrogeologic conceptual model. The refined
groundwater is being developed by Dr. John Daniels, P.E. under contract with Duke Energy.
The model results will be submitted three weeks after submittal of this assessment.
The site -specific data reviewed for inclusion in the refined groundwater model is limited to:

• the areas west of the ash basin primary cell in the vicinity of monitoring wells MW -9 and
MW-10 (i.e., between the primary cell and Mountain Island Reservoir) and,
• the area northeast of the ash basin secondary cell in the vicinity of monitoring well MW -
13 (i.e., between the secondary cell and Mountain Island Reservoir).

The area of study was defined to support the hydrogeologic cross-sections at these locations.

Monitoring well and piezome ter installation records and soil boring lo gs provided by Duke
Energy were reviewed to determine the characteristics of the subsurface conditions onsite . The
location of the monitoring wells, piezometers, soil borings , and plan view location of the cross-
sections are depicted on Figure 4.1 -3.

The cross-sections depicted on Figure 4.1-4 and 4.1-5 are based on:
• 1992 topographic contours
• hydrostratigraphic layer thicknesses determined by HDR based on soil boring,
monitoring well, and piezometer installation logs,
• location of sluiced ash based on 2011 bathymetry survey of the ash basin,
• fill associated with the ash basin dikes is based on boring data and construction drawings,
• the elevation of the lakebed based on the 2005 bathymetric survey of Mountain Island
Lake , and
• water surfaces based on surface and groundwater measurements

The water elevation data within the primary and secondary cells of the ash basin and the
piezometer data collected within the dikes were measured on January 19, 2013. Groundwater
elevations in monitoring wells MW -9 and MW-10 were measured on February 4, 2013. The
Section 4 Assessment of Groundwater Exceedances

16
groundwater elevations in monitoring wells MW -4S, MW-4D, MW-5S, MW-5D, and MW-13
were measured on February 5, 2013.

Based on Standard Penetration Testing (SPT) of soil and rock core recovery (REC) and rock
quality designation (RQD) in the boreholes, the groundwater system was divided into four
hydrostratigraphic layers:

1. A – Sluiced Ash
2. F – Fill (embankments)
3. S-M1/M2 – Alluvium/Soil/Saprolite; REC < 50%
4. D – Bedrock; REC > 85% and RQD > 50%

A transition zone of highly fractured bedrock was observed in only two of the borings which
were installed at depths which encounter this zone. The transition zone is present at the site;
however, not enough data to define the elevations and thicknesses of the zone are available to
include it as a stand-alone hydrostratigraphic layer. Therefore, the transition zone has been
included in the bedrock (D) layer.

Figure 4.1 -4 depicts cross-section A’-A’ which initiates within the ash basin primary cell and
extends west through monitoring well MW -9 toward Mountain Island Lake. The area within the
ash basin primary cell is filled with sluiced ash (A) in the location of cross -section A’-A’ to an
approximate elevation of 722 feet which equates to an ash thickness of approximately 40 feet to
55 feet. The dike itself is comprised of compacted fill (F) material.

The alluvium/soil/saprolite (S) layer in this cross -section ranges from a maximum thickness of
approximately 65 feet to 80 feet beneath the primary basin to a minimum thickness of
approximately 30 feet to 40 feet in the vicinity of monitoring well MW-9.

Figure 4.1 -5 depicts cross-section B’-B’ which initiates within the ash basin primary cell,
extends northwest through monitoring well M W-10, and terminates in Mountain Island Lake.
The sluiced ash (A) within the ash basin primary cell in the location of cross -section B’-B’
Section 4 Assessment of Groundwater Exceedances

17
ranges in thickness from approximately 15 feet to 30 feet in the area behind the dike. The dike
itself is comprised of compacted fill (F) material.

The alluvium/soil/saprolite (S) layer in this cross -section ranges from a maximum thickness of
approximately 100 feet beneath the primary basin to a minimum thickness of approximately 20
feet beneath Mountain Island Lak e.

Figure 4.1 -5 depicts cross-section C’-C’ which initiates within the ash basin secondary cell,
extends northeast through monitoring well MW -13, and terminates in Mountain Island Lake.
The sluiced ash (A) within the ash basin secondary cell in the location of cross-section C’-C’ is
approximately 15 feet deep. The dike itself is comprised of compacted fill (F) material.

The alluvium/soil/saprolite (S) layer in this cross -section ranges from a maximum thickness of
approximately 50 feet at the approximate toe of the dike to a minimum thickness of
approximately 15 feet beneath the base of the dike.

4.1.4 Stratigraphic Unit Properties
Tables 4.1-1, 4.1-2, and 4.1-3 present the parameter values and assumptions for the stratigraphic
units used in the groundwater model. The values for stratigraphic layer A are being provided by
UNCC.
4.1.5 Groundwater Model Refinement Plan
The refined groundwater model is briefly described below:
A site specific, two-dimensional model domain will be prepared for a vert ical cross section at
each of the three wells (MW-9, MW-10, and MW-13) where compliance monitoring is required.
This refined groundwater model utilizes MODFLOW and MT3D to predict the concentrations of
iron and manganese at the compliance boundary as follows:
• The model incorporates information from the site hydrogeologic conceptual model
described as part of the groundwater assessment work plan.
Section 4 Assessment of Groundwater Exceedances

18
• Where appropriate (and as identified in the model report), the model boundary of the
cross sections were extended to include the ash basin dike, located hydraulically
upgradient of the subject monitoring well(s) to the compliance boundary. Piezometric and
water level data from the dikes were utilized.
• Additional layers of cells were added to the cross sections.
• Recharge from infiltration of precipitation was incorporated into the model.
4.2 Discuss Site History and Land Uses -Task 2
A discussion of the site history and site land uses will be developed.
The Riverbend site is located at 175 Steam Plant Road, Mount Holly, North Carolina, in Gaston
County, and encompasses approximately 340.7 acres. The site is generally wooded along the
Catawba River and to the east of the Secondary Cell. The buildings and other structures
associated with the power production facilities are located on the north side of Horseshoe Bend
Beach Road (SR1912). Duke Energy also owns property on the south side of Horseshoe Bend
Beach Road where a lake access boat ramp is located. Figure 4.2-1 shows the site with the
property boundary.
Duke Energy began commercial operations at the site in 1929 with the operation of coal-fired
Units 1 -4. Coal-fired Units 5 – 7 began operations in 1952 through 1954. Units 1 -3 were retired
from service in the 1970’s. Units 4 – 7 were retired from service on April 1, 2013. All of the
coal-fired units are located in a single powerhouse . The air pollution control system for the coal-
fired units at Riverbend did no t include a flue gas desulfurization (scrubber) system. Coal was
delivered to the station by a railroad line. Figure 4.2-2 shows the site location on the 1948 USGS
Paw Creek Quadrangle.
Duke Energy also operated 4 combustion turbines at the site from 1969 until October 2012.
The se units could be fired by either natural gas or oil and are located to the west of the coal-fired
units.
Other areas of the site are occupied by facilities supporting the production or transmission of
power. The site cont ains two switchyard s and associated transmission lines. The Lark
Section 4 Assessment of Groundwater Exceedances

19
Maintenance Center is located at the Riverbend site to the west of the coal-fired units . The Lark
facility is an advanced machining and welding shop that supports various Duke Energy power
plants.
During station operations the ash from the coal combustion was sluiced to the ash basin through
a pipe, discharging in the south-west corner of the Primary Cell. Located to the west of the ash
basin is a cinder storage area (±10.7 acres). An earthen dike is located on the north side of the
cinder storage area. An ash storage area is located to the southwest of the Primary Cell. The
material in the ash storage area was removed from the basin to maintain the NPDES permit
required free-water volume. The ash storage area is ± 22.3 acres in area.
As described in Section 2.2, the ash basin system consists of a Primary Cell and a Secondary
Cell, separated by an intermediate dike. The original ash basin at Riverbend consisted of a
single -cell basin commissioned in 1957 . The ash basin was created by constructing two earthen
dikes across the lower ends of two natural valleys. Duke Energy refers to these dikes as Dike #1,
which now impound s the Primary Cell, and Dike #2, which now impound s the Secondary Cell.
As shown on the Figure 4.2-2, a stream type feature was located in the valley where Dike #2 was
constructed.
In 1979, the Intermediate Dike was constructed, separating the original ash basin into a Primary
Cell and a Secondary Cell. The waste boundary shown on Figure 2.3-1 encompasses an area of
±134.7 acres and includes the cinder storage area, the ash storage area, and the two ash basin
cells. As described in Section 2.2, the surface area of the Primary Cell is approximately 41 acres
with an approximate maximum pond elevation of 724 feet, and t he surface area of the Secondary
Cell is approximately 28 acres with an approximate maximum pond elevation of 714 feet.
4.3 Review Available Data on Ash Basin Water Quality - Task 3
Available data on ash basin water quality will be reviewed to determine if a suitable
“fingerprint” of ash basin water quality can be determined. If a suitable “fingerprint” of ash
basin water quality can be determined, the parameters and constituents associated with the
“fingerprint” will be used with the analytical results from the monitoring wells to determine if
the exceedances in the monitoring wells can be attributed to impacts from the ash basin.
Section 4 Assessment of Groundwater Exceedances

20
Background Information on Ash
Coal ash is produced from the combustion of coal. The coal is dried, pulverized, and conveyed
to the burner area of a boiler. Unburned material that forms large particles and falls to the
bottom of the boiler is called bottom ash. Smaller particles of ash, fly ash, are carried upward in
the flue gas and are captured by an air pollution control device, such as an electrostatic
precipitator. Approximately 70% to 80% of the ash produced during coal combustion is fly ash.
(EPRI 1993) Typically 65% to 90% of fly ash has particle sizes that are less than 0.010
millimeter (mm). Bottom ash particle diameters can vary from approximately 38 mm to 0.05
mm.
The specific mineralogy of coal ash varies based on many factors including the chemical
composition of the coal (which is dire ctly related to the geographic region where the coal was
mined), the type of boiler where the combustion occurs (the thermodynamics of the boiler), and
air pollution control technologies employed. The major elemental composition of fly ash
(approximately 95% by weight) is composed of mineral oxides of silicon, aluminum, iron,
calcium. Oxides of magnesium, potassium, titanium and sulfur comprise approximately 4% by
weight. (EPRI 1995) Trace elemental composition typically is approximately 1% by weight and
may include arsenic, antimony, barium, boron, cadmium, chromium, copper, manganese,
mercury, nickel, lead, selenium, silver, thallium, zinc, and other elements.
The geochemical factors controlling the reactions associated with leaching of ash is complicated.
The oxidation-reductions and precipitation-dissolution reactions that occur in a complex
environment such as an ash basin are poorly understood. In addition to the variability that might
be seen in the mineralogical composition of the ash, based on different coal types, different age
of ash in the basin, etc., it would be anticipated that the chemical environment of the ash basin
would vary o ver time and over distance and depth, increasing the difficulty of making specific
predictions related to concentrations of specific constituents.
Riverbend Ash Collection and Conveyance
At Riverbend , electrostatic precipitators are used to remove fly ash from the flue gas. The fly ash
is collected in hoppers. Bottom ash is collected in the bottom of the boilers. After collection,
both fly ash and bottom ash are sluiced to the ash basin using conveyance water withdrawn from
Section 4 Assessment of Groundwater Exceedances

21
the Catawba River. During operations, t he sluice lines discharged the water/ash slurry and other
flows to the southwest corner of the Primary Cell.
Ash Basin Water Quality Data – February 2013 Sample Event
During the February 2013 sample event, Duke Energy collected a water quality sample adjacent
to the concrete discharge tower in the Secondary Cell. The sample was collected at a depth of
0.3 meters. This sample location was designated as TOWER-0.3M (TOWER). In addition to
the analyses for the parameters monitored in the groundwater monitoring program, the sample
was analyzed for the parameters listed in Table s 4.3-1 and 4.3-2. The purpose of analyzing these
additional constituents was to observe the ionic composition of the ash basin water relative to the
ionic composition of groundwater in the background, upgradient, and downgradient monitoring
wells.
In addition to the analyses for the parameters monitored in the groundwater monitoring program,
the compliance groundwater monitoring wells and MW -4S, MW-4D, MW-5S, and MW-5D were
analyzed for alkalinity (as calcium carbonate), calcium, magnesium, potassium, and sodium. The
results for this analyses, along with the results for chloride, nitrate (as nitrogen), and sulfate from
the compliance monitoring wells are included in Table 4.3 -3. Iron was measured at 0.059 mg/L
and manganese was measured at 0.017 mg/L in the TOWER sample. The pH in the TOWER
sample was measured to be 7.0.
In addition to presenting the results in tabular form, a graphical presentation of the ionic
composition data, in the form of a Piper diagram is found on Figure s 4.3-1 and 4.3-2. Piper
diagrams can be useful when comparing dissolved major cations such as calcium, magnesium,
sodium, and potassium) and anions (bicarbonate/carbonate, chloride, and sulfate ) in groundwater
samples . The TOWER sample data is also displayed on the Piper diagrams. The concentrations
presented on these figures are expressed in terms of milliequivalents per liter (meq/L).
Figure 4.3-1 shows the data from the background monitoring wells MW -7SR and MW-7D, and
the upgradient monitoring wells MW -8S, MW-8I, and MW-8D, plotted with the data from the
TOWER sample on a Piper diagram. As shown on Figure 4.3-1, the ionic composition of the
sample results from background and upgradient wells is different from the ionic composition
from the TOWER sample. When compared to these monitoring wells, the TOWER sample is
Section 4 Assessment of Groundwater Exceedances

22
higher in calcium, chloride, magnesium, and sulfate. Figure 4.3-2 shows the downgradient
monitoring wells plotted with the results for TOWER on a Piper diagram. The ionic composition
of the TOWER sample appears to have more similarities to the ionic composition of the
downgradient wells than to the ionic composition of the background wells. The downgradient
wells have increased proportions of sulfate, chloride, calcium, and magnesium compared to the
background wells.
The ionic composition of groundwater samples for the February 2013 sampling event are also
presented in the form of Stiff diagrams on Figures 4.3-3 through 4.3 -9. O bservations on these
figures concerning the comparison of the ionic composition of the TOWER sample and the
monitoring wells are provided below. As with the Piper diagrams, the concentrations presented
on these figures are expressed in terms of meq/L.
• Up gradient Wells - Figures 4.3 -3 and 4.3-4: The ionic composition of the upgradient
monitoring wells (MW -7SR, MW-7D, MW-8S, MW-8I, MW-8D) is different from the
ionic composition of the TOWER sample, particularly with respect to concentrations of
sulfate and chloride , with the TOWER sample having higher concentrations of sulfate
and chloride.
• Primary Cell Downgradient Wells - Figure 4.3 -5: The ionic composition of the
downgradient monitoring wells (MW -9, MW-10) is similar in appearance to the
composition of the TOWER sample, particularly with respect to concentrations of
magnesium, sodium + potassium, bicarbonates (in MW-10 only), chloride, and sulfate.
The concentration of bicarbonate + carbonate in MW -9 is greater than measured in the
TOWER sample and in MW-10.
• Secondary Cell Downgradient Wells MW-11SR and MW-11DR - Figure 4.3 -6: The
ionic composition of the downgradient monitoring wells MW -11SR and MW-11DR is
similar to the composition of the TOWER sample with respect to concentrations of
calcium, sod ium + potassium, bicarbonate + carbonate, and chloride. The concentrations
of sulfate and magnesium in MW -11SR and MW-11DR are greater than the
concentration measured in the TOWER sample.
Section 4 Assessment of Groundwater Exceedances

23
• Secondary Cell Downgradient Wells MW-13, MW-14, MW-15 - Figure 4 .3-7: The ionic
composition of the downgradient monitoring well MW-13 is similar to the TOWER
sample with respect to calcium and chloride, generally similar in sulfate and in
magnesium, and lower than TOWER in concentrations of sodium + potassium. MW-13
has a greater concentration of bicarbonate + carbonate than measured in TOWER. The
ionic composition of the downgradient monitoring well MW -14 is similar to the
TOWER sample with respect to concentrations of calcium and chloride. MW-14 has
higher concentra tions of magnesium, sulfate and bicarbonate than measured in TOWER.
The ionic composition of MW-15 is similar to that of TOWER with respect to
magnesium, sodium + potassium, sulfate, and chloride. MW-15 has generally lower
concentrations of calcium and bicarbonate + carbonate than measured in TOWER.
• Secondary Cell Downgradient Voluntary Wells MW-4S, MW-4D - Figure 4.3 -8: The
ionic composition of the downgradient monitoring well MW -4S is similar to the
TOWER sample with respect to concentrations of magnesium, bicarbonate + carbonate,
and generally similar in concentrations of calcium. MW-4S had concentrations of
sodium + potassium and sulfate that were greater than those measured in the TOWER
sample. Concentrations of magnesium, calcium, sulfate, bicarbonate + carbonate, and
chloride were greater in MW-4D than those measured in the TOWER sample.
• Secondary Cell Downgradient Voluntary Wells MW-5S, MW-5D - Figure 4.3 -9: The
ionic composition of the downgradient monitoring well MW -5S is similar to the
TOWER sample with respect to concentrations of magnesium, chloride, and sodium +
potassium. Concentrations of sulfate and bicarbonate + carbonate were lower in MW-5S
than in TOWER. Concentrations of magnesium, calcium, sulfate, bicarbonate +
carbonate, and chloride were greater in MW-5D than those measured in the TOWER
sample.
Figure 4.3-10 presents the concentrations of sulfate, calcium, chloride, sodium, potassium, and
magnesium in TOWER and in the upgradie nt monitoring wells . These constituents would be
expected to be present in the form of ionic compounds in the groundwater and in the ash basin
water. In general, ionic compounds formed with sodium, potassium, chloride, and sulfate would
be expected to be soluble in water, with limited exceptions. As shown on Figure 4.3-10, sulfate
Section 4 Assessment of Groundwater Exceedances

24
is present in the highest concentration in the TOWER sample, followed in decreasing order of
concentrations by calcium, chloride, sodium, potassium, and magnesium.
Sulfate and chloride were selected as constituents used as qualitative indicators, or as a signature
of ash basin water quality observed in groundwater wells. Sulfate and chloride were found in the
TOWER sample at concentrations greater than found in the site background wells . Sulfate and
chloride are typically non-reactive and mobile in the typical groundwater environment. Although
no site specific data is available on soil-water partition coefficient (Kd) terms for sulfate or
chloride, these constituents would be expected to have low values for Kd, and therefore would be
expected to be used as indicators of ash basin impacts to groundwater.
Although calcium concentrations were greater than chloride concentrations in the TOWER
sample, calcium was not selected as a constituent indicating a signature of ash basin water
quality because calcium would likely be involved in precipitation and/or cation exchange
processes.
Figure 4.3-10 shows that the concentrations of sulfate and chloride in the upgradient monitoring
wells are low relative to the concentrations found in the TOWER sample. The ratio of the
concentrations of sulfate to the concentrations of chloride (sulfate/chloride) in the groundwater
monitoring wells and in the TOWER sample ar e shown on Figure 4.3-11. The values for the
sulfate/chloride ratio s in the background well pair (MW -7SR, MW-7D) and in the upgradient
wells (MW -8S, MW-8I, MW-8D) range from 0.1 at MW-7SR and MW-8S to 1.0 at MW-8D.
The sulfate/chloride ratio in the TOWER sample is 4.0. The sulfate/chloride ratios in the
downgradient wells range from 2.74 at MW-13 to 13.33 at MW-9. Based on using the
sulfate/chloride ratio data in Figure 4.3-4, the downgradient wells appear to have similar
groundwater quality with regards to these constituents.
4.4 Review Mountain Island Lake Water Level Data -Task 4
Mountain Island Lake is located adjacent to the Riverbend ash basin. The hydroelectric
generating station on Mountain Island Lake is operated in conjunction with the upstream
reservoir, Lake Norman. The operation the hydroelectric stations on Lake Norman and
Mountain Island Lake cause changes in the water levels in Mountain Island Lake on a frequent
basis. The water level changes in Mountain Island Lake will be reviewed with the water level
Section 4 Assessment of Groundwater Exceedances

25
data in the monitoring wells adjacent to Mountain Island Lake to better understand the influence
of the lake on the monitoring wells.
Cowans Ford Hydroelectric Station (Cowans Ford) is part of the main dam that creates Lake
Norman and is located 8 river miles upstream of Riverbend ash basin (Figure 4.4 -1). Cowans
Ford discharges water directly into Mountain Island Lake. Mountain Island Hydroelectric
Station is part of the dam that creates Mountain Island Lake and is located 8.5 river miles
downstream of Riverbend ash basin. The operations of both hydroelectric stations depend on a
number of factors including generation needs and rainfall, both near the reservoirs and also in the
larger Catawba-Wateree drainage basin.
Monitoring well MW -13 is located downgradient from the Secondary Cell of the ash basin and is
approximately 100 feet from the bank of Mountain Island Lake (based on field measurements
made on April 15, 2013 by HDR). The top of casing at MW-13 is approximately 2.3 feet above
the ground surface with an elevation of 654.11 feet. The screened interval for the well is 2.85 to
17.85 feet below the ground surface (bgs), placing the screen interval from elevation 648.96 feet
to 633.96 feet. Duke Energy installed a pressure transducer in MW-13 to record data every 15
minutes for the period of February 14, 2013 to March 22, 2013. The pressure data was corrected
for barometric pressure before being converted into a water surface elevation. The groundwater
elevation in MW -13 over this period ranged from 643.93 feet to 645.70 feet. During this time ,
the water level data for the Secondary Cell remained near elevation 710 ft.
The water surface elevation of Mountain Island Lake is measured on the upstream side of
Mountain Island Dam and typically varies from 643.5 feet to 647.0 feet over the course of a year.
Operations and water elevation data was collected from February 14 through March 21, 2013
and provided by Duke Energy’s Hydro Central Operations Center. The water surface elevation
data for Mountain Island Reservoir was compared to the water surface elevation data collected
by the pressure transducer in MW -13.
Figure 4.4 -2 shows the water surface elevations for the Cowans Ford tailwater, Mountain Island
Lake at Mountain Island Dam, and monito ring well MW-13 at Riverbend. Over the period of
February 4 to March 22, 2013, Cowans Ford generation increased the tailwater elevation in
Section 4 Assessment of Groundwater Exceedances

26
varying amounts ranging between 1.5 and 5 feet. This increase in the water surface elevation of
Mountain Island Lak e results in a corresponding increase in the groundwater elevation measured
in monitoring well MW -13. The influence is more apparent when looking at relatively short
time period s. Figure 4.4-3 shows two days with morning generation at Cowans Ford (March 7
and 8, 2013). The water elevation in MW-13 increased from 644.45 feet at 5:45AM on March
7, 2013 to 644.89 feet at 12:00PM on the same day. Similarly, Figure 4.4 -4 shows the water
elevation increased in MW-13 from 644.35 feet at 10:45PM on February 25, 2013 to 645.67 at
2:00PM on February 26, 2013. This is an increase in water elevation in MW-13 of 1.32 feet over
approximately 15.25 hours.
The water elevation recorded in monitoring well MW-13 appears to be correlated with the
elevation of Mountain I sland Lake. An increase in Cowans Ford tailwater level increases water
surface elevation of Mountain Island Lake, thereby influenc ing the water elevation in MW-13.
The water elevation data in MW-13 from February 14 through 23 shows a minor influence while
March 4 to 16 shows sharper, more distinct increases (see Figure 4.4-2).
Rainfall data recorded at 15 -minute intervals was also collected at the two hydroelectric stations,
Cowans Ford and Mountain Island. Figure 4.4-4 shows rainfall data with the water surface
elevations for MW-13 and Mountain Island Lake. The quantity of rainfall did not exceed one
inch for any single day during the period depicted in Figure 4.4-4. Two of the larger rainfall
events, March 5 and 18, appear to have little influence on the water level in MW-13.
The water elevation in MW-13 appears to experience direct and rapid changes in elevation due to
changes in elevations of Mountain Island Lake.
4.5 Review Location-Number of Background Monitoring Wells - Task 5
The site hydrogeologic conceptual model and other data will be reviewed to determine if the
location and number of background wells is sufficient.
Monitoring wells MW -7SR and MW-7D have historically been considered by Duke Energy to
represent background water quality. The locations of the monitoring wells are depicted on
Figure 2.3 -1. Based on the ground surface topography in the vicinity of monitoring wells MW-
Section 4 Assessment of Groundwater Exceedances

27
7SR/D and the slope-aquifer system (see Section 4.1), groundwater flow direction in the location
of these wells is expected to be generally to the north and northeast towards Mountain Island
Lake and the water quality measured in these wells is expected to represent the groundwater
quality of the recharge area upgradient of the wells and Horseshoe Bend Beach Road.
Based on surface topography, the potential exists for a groundwater divide to be present between
the Primary Cell of the ash basin and monitoring wells MW -7SR/D. The ground surface at this
well pair is approximately 40 feet above the water surface in the Primary Cell and 50 feet above
the secondary cell water surface. Water surface elevations at the site are presented on Figure
4.1-2. Based on the groundwater levels measured in the shallow and deep well pairs it appears
that an upward vertical gradient exists in the location of monitoring wells MW -7SR/D.
Based on their location, monitoring wells MW -8S, MW-8I, and MW-8D appear to be upgradient
of the Primary and Secondary Cells of the ash basin. The ground surface at this well pair is
approximately 30 feet above the water surface in the Primary Cell and 40 feet above the
Secondary Cell water surface.
Based on the ground surface topography in the vicinity of monitoring wells MW -8S/I/D and the
slope-aquifer system, the groundwater flow direction at thes e wells is expected to be generally to
the west, away from the ash basin. Based on the groundwater levels measured in the shallow,
intermediate, and deep wells it appears that a downward vertical gradient exists near monitoring
wells MW -8S/I/D.
The hist orical groundwater elevation measurements at monitoring wells MW -7SR/D and MW-
8S/I/D were plotted (see Figure 4.5-1) to determine groundwater level fluctuations over time.
Groundwater level elevation changes in these wells have varied b y 2.26 feet in monitoring well
MW-7S and up to 3.98 feet in monitoring well MW -7D; decreasing over the period of record.
The variation in groundwater elevations measured in monitoring wells MW -8S/I/D have also
been over three feet of elevation change during the monitoring period. The relatively large
variation in groundwater elevations measured in these wells suggests that these wells are not
being stabilized by the static water level in the ash basin.
Section 4 Assessment of Groundwater Exceedances

28
The remaining compliance monitoring wells (MW -9, MW-10, MW-11SR, MW-11DR, MW -13,
MW-14, and MW-15) are located topographically downgradient of the Primary and Secondary
Cells of the ash basin. The historical groundwater elevation measurements for the
topographically downgradient compliance monitoring wells were plotted (see Figure 4.5 -2) to
observe groundwater level fluctuations over time. Groundwater level elevation changes in these
wells have varied between 0.55 feet in monitoring well MW-11DR to 1.1 feet in monitoring well
MW-13. The relatively small, long-term variatio n in groundwater elevations measured in these
wells suggests that the water levels in these wells are being influenced by the by the static water
level in the ash basin and/or by the water level in Mountain Island Lake.
As noted on Figure 4.4-1, the Rive rbend site is located entirely within a single geologic unit and
based on this information, additional background monitoring wells are not needed to account for
differing geologic conditions. The topographic data and water elevation data suggest that
monitoring wells MW -7SR and MW -7D adequately represent background water quality
conditions at the site.
4.6 Evaluate Well Construction Information - Task 6
Well installation records will be reviewed to determine if well construction methods are
contributing to the exceedances.
The groundwater monitoring well construction records were reviewed for monitoring wells MW-
7SR, MW-7D, MW-8S, MW-8I, MW-8D, MW-9, MW-10, MW-11SR, MW-11DR, MW-13,
MW-14, and MW-15. With the exception of monitoring well MW-7D, the monitoring wells
were installed by MACTEC and A.E. Drilling Services, LLC between October 2010 and
February 2011.5 Monitoring well MW -7D was installed by ARCADIS G&M of North Carolina,
Inc., and Geologic Exploration on December 4, 2006.6

5 Amended Ash Basin Monitoring Well Installation Report, Riverbend Steam Station, MACTEC Project No. 6228-
10-5284, March 30, 2011.
6 Ash Basin Drilling Services for: Riverbend Steam Station, Mount Holly, North Carolina, ARCADIS G&M of
North Carolina, Inc. January 7, 2007.

Section 4 Assessment of Groundwater Exceedances

29
Monitoring wells MW -7SR, MW-8S, MW-9, MW-10, MW-11SR, MW-13, MW-14, and MW-
15 were installed by rotary drilling methods using hollow stem augers, with the well screen
installed above auger refusal to monitor the shallow aquifer within the saprolite layer. The
screen lengths for these wells range from 15 feet to 20 feet. Monitoring well MW -8I was also
installed by rotary drilling methods using hollow stem augers, with the well screen installed at an
intermediate depth in the surficial aquifer at 98 feet to 118 feet below ground surface (bgs). The
screen for monitoring well MW -8D was installed immediately above auger refusal and screened
from 156 feet to 166 feet bgs to monitor the transition zone.
Monitoring wells MW -7D and MW-11DR were installed using hollow stem augers until auger
refusal was encountered. Monitoring well MW-7D was completed using mud rotary drilling
techniques to complete the well with a 5 foot long well screen from 95 to 100 feet bgs. The 5 -
foot long screen for monitoring well MW -11DR was installed from 35 to 40 feet bgs into the
fractured bedrock zone immediately below auger refusal using HQ -sized rock coring techniques.
A review of the well construction records found that the screen filter material for all monitoring
wells, except MW-7D, was #1 sand. For monitoring well MW-7D #2 sand was used as the
screen filter material.
Based on a review of the well construction records for monitoring wells MW-7SR, MW-7D,
MW-8S, MW-8I, MW-9, MW-11SR and MW -11DR, the wells appear to have been constructed
according to North Carolina Administrative Code (NCAC) Title 15A 02C .0108 with regards to
borehole depth, well construction materials, minimum packing material thicknesses above the
well screen, bentonite seal thicknesses, and annular space materials and thickness.
Monitoring wells MW -10, MW-13, MW-14, and MW-15 were installed with less than three feet
of concrete or cement grout between the ground surface and the top of the bentonite seal as
specified in NCAC Title 15A 02C .0108 (i). Due to the shallow depth to groundwater in these
locations, only one foot of concrete or cement grout could be placed above the bentonite seal
during the construction of these monitoring wells.
Section 4 Assessment of Groundwater Exceedances

30
Although the grout thickness above the bentonite seal is less than required in NCAC Title 15A
02C .0108 (i) for monitoring wells MW -10, MW-13, MW-14, and MW-15, it is not likely that
this construction variance is contributing to the 2L Standard exceedances measured in these
wells.
The borehole depths during the construction of monitoring wells MW -8D, MW-10, MW-13, and
MW-14 were extended to depths of approximately 34 feet, 7 feet, 7 feet, and 22 feet, respectively
below the bottom of the well screens. Monitoring well construction standard NCAC 15A 02C
.0108 (d) requires any portion of the borehole that extends to a depth greater than the depth to be
monitored to be grouted completely. During the construction of these monitoring wells the
borehole void below the bottom of the monitoring well screen was filled with well gravel pack
#1 sand.
Although well filter sand and not grout was used to fill the void beneath the well screen as
specified in NCAC Title 15A 02C .0108 (d) in monitoring wells MW-8D, MW-10, MW-13, and
MW-14, it is not likely that this construction variance is contributing to the 2L Standard
exceedances measured in these wells.
4.7 Evaluate Exceedances Against Background Well Results -Task 7
The analytical results from the wells with exceedances will be evaluated against results from the
site background wells to determine if the exceedances can be attributed to background water
quality conditions.
The analytical results from groundwater sampling at Riverbend show exceedances for iron,
manganese, and pH (Table 3.1-4). Monitoring wells MW -7SR, MW-8I, MW-8D, MW-9, MW-
10, MW-11SR, MW-13, and MW-15 exceeded the 2L Standard for all three parameters. The 2L
Standard for manganese and pH were exceeded at MW-8S and MW-11DR. Monitoring well
MW-14 exceeded the 2L Standard for iron and manganese, while MW-7D only exceeded for pH.
Background Monitoring Wells MW-7D and MW-7SR
Monitoring well MW-7SR is the background well for shallow and intermediate wells and
monitoring well MW-7D is the background well for deep wells. The measurements for pH have
been consistently below the 2L Standard range of 6.5 to 8.5 standard units (SU) for both wells
Section 4 Assessment of Groundwater Exceedances

31
and both wells show a decreasing trend as shown on Figure s 4.7-1 and 4.7-2. The analytical
results for iron and manganese at MW-7SR since 2008 have been measured at concentrations
greater than and less than the respective 2L Standards as shown in Figures 4.7 -3 and 4.7-13. The
general trend at MW-7SR is decreasing iron and manganese concentrations from 2011 to 2013.
The a nalytical results for iron and manganese at MW-7D have not been measured at
concentrations which attain or exceed the laboratory method repo rting limit over the monitoring
period as shown on Figures 4.7 -3 and 4.7-13, respectively.
Exceedances of 2L Standards for pH
Exceedances for pH have been measured at monitoring wells MW-7SR, MW-7D, MW-8S, MW-
8I, MW-8D, MW-9, MW-10, MW-11SR, MW-11-DR, MW -13, and MW-15. The ranges of
values for the exceedances are listed in Table 3.1-4. Figures 4.7 -1 and 4.7-2 show the pH values
measured at the site. Values have been within a range of 0.5 SU over the period of sampling for
most wells. MW -8I and MW-8D have shown a decreasing trend over the two most recent
monitoring events. For the February 2013 sampling event, t he only well with pH values less
than the background wells levels was MW-8S.

Exceedances of 2L Standards for Iron
The 2L Standard for iron is 300 micrograms per liter (µg/L). Historically, e xceedances of the 2L
Standard for iron have been measured at monitoring wells MW-7SR, MW-8I, MW-8D, MW-9,
MW-10, MW-11SR, MW-13, MW-14, and MW-15 over the monitoring period . However,
during the February 2013 sampling event, iron exceedances were only measured in wells MW-
8I, MW-8D, MW-9 and MW-13. The historic al range of exceedance values are provided in
Table 3.1-4. Figure 4.7-3 shows the historical iron concentrations measured in background wells
MW-7SR and MW-7D. Figure 4.7-4 shows the historical iron concentrations measured in the
deep monitoring wells. Figure 4.7-5 shows the historical concentrations of iron measured in the
shallow monitoring wells.
MW-7SR - Iron concentrations have been generally decreasing over the monitoring
period and have been less than the 2L Standard during the last two sampling events
(Figure 4.7 -3).
Section 4 Assessment of Groundwater Exceedances

32
MW-8I – Iron concentrations have exceeded the 2L Standard over the monitoring period
but have been decreasing over the period of February 2012 to February 2013 (Figure 4.7-
6).
MW-8D – Iron concentrations have been consistently in excess of the 2L sta ndard but
variable over the monitoring period (Figure 4.7 -4).
MW-9 - Iron concentrations have been variable over the monitoring period ranging from
an initial concentration and maximum of 2,440 µg/L to a minimum of 381 µg/L (Figure
4.7-7).
MW-10 - Iron concentrations have been variable over the monitoring period , decreasing
from an initial maximum of 1,710 µg/L to a minimum of 72 µg/L in the June 4, 2012
sampling event. Iron concentrations during t he last 3 sampling events have been less
than the 2L Standard (Figure 4.7-8).
MW-11SR – The only iron concentration exceedance of the 2L Standard occurred during
the first sampling event . Iron concentrations have been less than the 2L Standard during
the last 6 sampling events (Figure 4.7 -9).
MW-13 – Iron concentrations have generally been consistently in excess of the 2L
standard over the monitoring period , ranging from a maximum of 23,000 µg/L to a
minimum of 13,500 µg/L (Figure 4.7 -10).
MW-14 – Only t hree exceedances for iron have occurred (i.e., the first two sampling
event s and the February 2012 sampling event). The three most recent events measured
iron concentrations at less than the 2L Standard. (Figure 4.7-11).
MW-15 – The only iron exceedance was measured during the February 2012 sampling
event. During t he last three sampling events, iron has been measured at concentrations
less than 60 µg/L (Figure 4.7 -12).

Exceedances of 2L Standards for Manganese
The 2L Standard for manganese is 50 µg/L. Exceedances for manganese have been observed at
monitoring wells MW-7SR, MW-8S, MW-8I, MW-8D, MW-9, MW-10, MW-11SR, MW-
11DR, MW-13, MW-14, and MW-15 over the monitoring period . However, in the February
2013 sampling event, exceedances for manganese were only measured at MW-8S, MW-8I, MW-
Section 4 Assessment of Groundwater Exceedances

33
8D, MW-11DR, and MW-13. Figure 4.7-13 shows the historic al concentrations of manganese
at the background wells. Figure 4.7-14 shows the historical concentrations of manganese
measured in the deep monitoring wells. Figure 4.7 -15 shows the historical concentrations of
manganese measured in the shallow monitoring wells.
MW-7SR - Manganese concentrations have been generally decreasing over the
monitoring period and were less than the 2L Standard during the February 2013 sampling
event (Figure 4.7 -15).
MW-8S – Manganese concentrations have been generally decreasing over the monitoring
period from February 2012 to February 2013 but have been consistently in excess of the
2L Standard (Figure 4.7 -16).
MW-8I – Manganese concentrations have been less than the 2L Standard during three of
the last four sampling events , increasing to 168 µg/L in the most recent sampling event
(Figure 4.7 -17).
MW-8D – Manganese concentrations have generally been decreasing over the
monitoring period, however, the most recent sampling event showed an increase to 143
µg/L (Figure 4.7 -18).
MW-9 - Manganese concentrations have generally been decreasing from an initial
maximum of 282 µg/L to a low of 25 µg/L in both the June and October 2012 sampling
events. The manganese concentrations measured during the last three sampling events
have been less than the 2L Standard (Figure 4.7 -19).
MW-10 - Manganese concentrations have been variable over the monitoring period from
a maximum of 355 µg/L to a minimum of 33 µg/L during the February 4, 2013 sampling
event (Figure 4.7 -20).
MW-11SR - Manganese concentrations exceeded the 2L Standard during the first two
sampling events and have been less than the 2L Standard during the last five events
(Figure 4.7 -21).
MW-11DR - Manganese concentration have generally been consistent over the
monitoring period from an initial maximum of 168 µg/L to a minimum of 87 µg/L during
the October 3, 2012 sampling event (Figure 4.7-22).
Section 4 Assessment of Groundwater Exceedances

34
MW-13 – Manganese concentrations have generally been consistent over the monitoring
period, ranging from a maximum of 11,200 µg/L to a minimum of 10,000 µg/L (Figure
4.7-23).
MW-14 - Manganese concentrations have been variable over the monitoring period from
a maximum of 353 µg/L to a minimum of 18 µg/L in the February 4, 2013 sampling
event . Concentrations measured during the last two sampling events have been less than
the 2L Standard (Figure 4.7 -24).
MW-15 - Manganese concentrations have generally been consistent over the monitoring
period, ranging from a maximum of 86 µg/L to a minimum of 46 µg/L. Concentrations
measured in the last two sampling events have been less than the 2L Standard (Figure
4.7-25).
4.8 Evaluate Exceedances Against Turbidity Values -Task 8
Exceedances will be evaluated with turbidity values measured during sampling to determine if
the exceedances are a result of sediment or particulate matter which is preserved in the sample
as a result of well construction or sampling methods.
The Environmental Protection Agency (EPA) recommends that when possible, and especially
when sampling for contaminants that may be biased by the presence of turbidity, the turbidity
values in the stabilized well should be less than 10 Nephelometric Turbidity Units (NT U) (US
EPA 2002). Monitoring wells with analytical results exceeding the 2L Standards for iron and/or
manganese have been individually plotted along with turbidity measurements made at the time
the wells were sampled (Figures 4.8 -1 through 4.8 -20).
MW-7SR – Turbidity values have ranged from a maximum of 6.3 NTU to a minimum of
0.8 NTU during the June 5, 2012 sampling event (Figures 4.8 -1 and 4.8-2). Manganese
concentrations measured during the last sampling event and the iron concentrations
measured during three of the last four sampling events have been less than their
respective 2L Standards. There appears to be a general correlation between turbidity and
measured iron and manganese concentrations, with concentrations of iron and manganese
generally de creasing with decreasing turbidity values .
Section 4 Assessment of Groundwater Exceedances

35
MW-8S - Turbidity values have ranged from a maximum of 18.0 NTU in the February
2011 sampling event to a minimum of 2.8 NTU in the February 2013 sampling event
(Figure 4.8 -3). There appears to be a general correlation between turbidity and measured
manganese concentrations, with concentrations of manganese generally decreasing as
turbidity decreases.
MW-8I - Turbidity values have ranged from a maximum of 38.7 NTU to a minimum of
17.7 NTU. During three of the last four sampling events , manganese has been measured
at concentrations less than the2L Standard. There appears to be a general correlation
between turbidity and measured iron concentrations, with concentrations of iron
generally increasing and decreasing along with turbidity values (Figure 4.8 -4). However,
manganese concentrations do not appear to be correlated with turbidity (Figure 4.8 -5).
MW-8D - Turbidity values have ranged from a maximum of 217 NTU in the initial
sampling event to a low of 25 NTU in the June 7, 2011 sampling event and appear to be
correlated with iron concentrations (Figures 4.8 -6). Manganese concentrations do not
appear to be correlated with turbidity (Figure 4.8-7).
MW-9 - Turbidity values have ranged from a maximum of 139 NTU in the initial
sampling event to a low of 12.6 NTU in the October 4, 2011 sampling event and appear
to be correlated with iron and manganese concentrations (Figures 4.8 -8 and 4.8-9).
Manganese has been measured at concentrations less than the 2L Standard during the last
three sampling events.
MW-10 - Turbidity values have ranged from a maximum of 182 NTU to a minimum of
3.2 NTU during the June 4, 2012 sampling event and appear to be correlated with iron
and manganese concentrations (Figures 4.8 -10 and 4.8-11). Iron during the last three
sampling events and manganese during two of the last three sampling events have been
measured at concentrations less than their respective 2L Standards.
MW-11SR - Turbidity values have ranged from a maximum of 11 NTU to a minimum of
1.4 NTU during the October 3, 2012 sampling event and appear to be correlated with iron
and manganese concentrations (Figures 4.8 -12 and 4.8-13). Analytical results have
remained below the 2L Standard for iron since the second sampling event in June 2011
and for manganese since the third sampling event in October 2011 .
Section 4 Assessment of Groundwater Exceedances

36
MW-11DR - Turbidity values have ranged from a maximum of 3.4 NTU to a low of 0.2
NTU during the June 4, 2012 sampling event and do not appear to be correlated with
manganese concentrations (Figure 4.8 -14).
MW-13 - Turbidity values have ranged from a maximum of 7.7 NTU to a minimum of
2.4 NTU during the October 3, 2012 sampling event and do not appear to be correlated
with iron or manganese concentrations (Figure 4.8 -15 and 4.8-16).
MW-14 - Turbidity values have ranged from a maximum of 25.6 NTU during the initial
sampling event to a minimum of 1.9 NTU during the June 5, 2012 sampling event. As
shown on Figure 4.8 -17 and 4.8-18, with the exception of the February 2012 sampling
event, the iron and manganese concentrations have generally been decreasing with
decreasing turbidity values.
MW-15 - Turbidity values have ranged from a maximum of 13.2 NTU to a minimum of
2.7 NTU during the June 5, 2012 sampling event. The iron concentrations appear to
generally be correlated with turbidity (Figure 4.8-19) while the manganese concentrations
do not appear to be correlated with turbidity (Figure 4.8 -20).
4.9 Evaluate S ampling Flow Rates - Task 9
Sampling collection flow rates will be evaluated to determine if the flow rates are affecting
results. Low flow sampling techniques will be evaluated for selected wells.
The purge methods, pumping rates (where applicable) and evacuated well volumes for each of
the wells is presented on Table 3.1-1. The majority of the monitoring wells sampled during the
February 2013 monitoring event were purged prior to sampling using dedicated bladder pumps
and conventional purging techniques of evacuating three to five well volumes prior to collection
of the groundwater samples while measuring indicator parameters. Duke Energy describes the
conventional purging technique as adjusting t he flow rates to provide a “low energy” stream
from the discharge tubing with a general flow rate of less than 300 milliliters per minute
(ml/min). The purging/sampling flow rates have decreased over the last several years in an effort
to lower turbidity values and more accurately represent metals concentrations in the groundwater
Section 4 Assessment of Groundwater Exceedances

37
samples.7 Evacuated well volumes for the conventionally purged wells ranged from 1.49 gallons
in monitoring well MW -15 to 8.53 gallons in MW-7D during the February 2013 sampling event.
Low flow purge methods and sampling were performed during collection of groundwater
samples from monitoring wells MW -8I, MW-8D, MW-4S, MW-4D, MW-5S and MW-5D
during the February 2013 sampling event. Flow rates for these wells ranged from 58 ml/min at
MW-4S to 336 ml/min at MW-5D. During low -flow purging and sampling, the groundwater is
pumped into a flow-through chamber at flow rates that minimize or stabilize water level
drawdown within the well and continues until indicator parameters have stabilized.
Sampling flow rates may have a direct affect on turbidity levels in groundwater samples
collected from monitoring wells. Monitoring wells MW -8D, MW-9, and MW-4S were noted by
Duke Energy personnel as being turbid at the time of sampling. Monitorin g wells MW -8D and
MW-4S were collected utilizing low flow sampling techniques. Monitoring well MW-9 was
collected using conventional purge techniques which may have resulted in an increased turbidity
level of the groundwater sample collected from this well compared to low-flow purging and
sampling.
4.10 Collect and Analyze Filtered and Unfiltered samples - Task 10
Groundwater samples collected for compliance monitoring are not filtered in the field. In order
to provide additional information for the assessment of exceedances, both unfiltered and filtered
samples will be collected and analyzed for iron and manganese. The field filtration will be
performed with an in-line, sealed, 0.45 micron filter.
During the February 4 -5, 2013 sampling event, filte red and non-filtered groundwater samples
were collected at the compliance wells. The samples were analyzed for iron and manganese.
Table 4.10-1 presents a summary of the analytical results.
Iron Results
Monitoring wells MW -8I, MW-8D, MW-9, and MW-13 had exceedances of the 2L Standard for
iron measured during the February 2013 sampling event. The filtered concentrations of iron at

7 Electronic mail correspondence from Tim Hunsucker, Sr. Scientist, Duke Energy Water & Natural Resources /
Water Programs to William Miller, PE, Senior Engineer, HDR Engineering Inc. of the Carolinas dated
September 27, 2012.
Section 4 Assessment of Groundwater Exceedances

38
MW-8I, MW-8D, and MW-9 were less than the unfiltered results. As discussed in Section 4.8,
the iron concentrations at monitoring wells MW -8I, MW-8D, and MW-9 appear to be correlated
with turbidity results. The concentrations measured in the filtered samples at MW -8I, MW-8D,
and MW -9 were below the 2L Standard for iron. There was no appreciable reduction in the iron
concentration mea sured at MW-13 in the filtered sample results .
Manganese Results
Monitoring wells MW -8S, MW-8I, MW-8D, MW-11DR, and MW-13 had exceedances of the
2L Standard for manganese measured during the February 2013 sampling event. In wells MW-
8I and MW-8D the concentrations in the filtered results were less than the unfiltered results . As
discussed in Section 4.8, the manganese concentrations at monitoring wells MW-8I, MW-8D,
and MW -9 appear to be correlated with turbidity results. There was no appreciable reduction in
manganese concentrations for MW-13 and slight increases in manganese concentrations for the
filtered results at MW -8S and MW -11DR. The filtered results for manganese at MW-8I and
MW-8D were below the 2L Standard.
4.11 Collec t Reduction/Oxidation Field Parameters - Task 11
Reduction/oxidation (redox) processes can alternately mobilize or immobilize metals associated
with naturally occurring aquifer materials. Iron and manganese are commonly associated with
lakes and the associated sediments. The redox conditions associated with the aquifer/lake
system may be a factor in the concentrations of iron and manganese observed at selected
monitoring well locations. Additional field parameters (dissolved oxygen, reduction/oxidation
potential) will be collected at selected wells to characterize the reduction/oxidation conditions at
these locations. A discussion of the redox conditions at the wells will be provided.
The most commo n source of iron in gr ound water is from weathering of ir on bearing mineral
oxides and rocks. Iron occurs in the enviro nme nt as a cation and may take one of two common
forms: insolub le ferric ir on (Fe3+) or solub le ferrous ir on (Fe2+). The solubility of iron and othe r
metals is controlled by the oxidation/reduction (Eh)-pH conditions. In oxic settings, insolub le
ferric ir on Fe 3+ can be retained as an iron oxide coating on soils, weathered rocks, and bedrock
fractures. If the Eh-pH conditions move towards anoxic conditions (reducing cond itions), the
otherwise insolub le ferric ir on is reduced to the solub le ferrous ir on Fe2+ and readily dissolves
Section 4 Assessment of Groundwater Exceedances

39
into gr ound water. Because deeper wells tend to be le ss oxygenated than shallow wells, dissolve d
ir on concentrations may in ma ny cases (though not all) be higher in deeper rather than shallow
wells. Ferric ir on (Fe3+) may also be released in gr ound water as ferrous iron (Fe2+) in soggy,
wa terlogged wetland areas characterized by low dissolved oxygen (DO ). 8
Manganese (Mn) occurs in the enviro nme nt as a cation and may take one of three common
forms: Mn2+, Mn3+, or Mn4 +. Manga nese in the enviro nmen t behave s ve ry similarly to iron. For
example, insolub le ma nga nes e oxides become dissolved in gr ound water to the solub le form of
manga nese (Mn2+) und er low Eh-pH conditions (red ucing cond itions). Howeve r, the solub le
fo rm of ma nga ne se is often more stable than its counterpart fe rrous ir on in anoxic gr ound water.
High iron and ma nga nese can co-occur, but this is not alwa ys the case. N evertheless, in general,
the factors discussed above associa ted with ir on are similar to those of ma nga ne se.9
During the February 2013 sampling event , oxidation/reduction potential (ORP or Eh) and DO
were measured by Duke Energy personnel at each of the groundwater monitoring wells sampled
using a Hydrolab ® MS5 water quality multiprobe . The results of the measurements prior to
sampling (stabilized values) are presented on Table 3.1 -1.
The ORP values for the background wells (MW -7SR, MW-7D) and upgradient wells (MW -8S,
MW-8I, MW-8D) had relatively high values ranging from 171 millivolts (mV)10 in MW -8D, to
444 mV in MW-8S. The DO at these wells ranged from 3.98 milligrams per liter (mg/L) in
MW-8D to 9.88 mg/L in MW-8I. The low DO and ORP readings in MW -8D are likely
attributable to the greater depth at which this groundwater sample is being collected (i.e.,
reduced environment).
The wells downgradient of the ash basin generally had lower ORP and DO measurements
compared to the background and upgradient wells with the exception of MW -4S which had an

8 Evaluating Metals in Groundwater at DWQ Permitted Facilities: A Technical Assistance Document for DWQ
Staff. North Carolina Department of Environment and Natural Resources, Department of Water Quality, July
2012. 9 Evaluating Metals in Groundwater at DWQ Permitted Facilities: A Technical Assistance Document for DWQ
Staff. North Carolina Department of Environment and Natural Resources, Department of Water Quality, July
2012. 10 Oxidation/reduction (ORP) data reported is in millivolts (mV)-standard hydrogen electrode (SHE).

Section 4 Assessment of Groundwater Exceedances

40
ORP of 365 mV and a DO measurement of 7.86 mg/L. Monitoring well MW -4S also had high
turbidity (>1000 NTUs). The remaining downgradient wells had ORP ranging from 118 mV in
MW-11SR to 384 mV in MW-5S. DO measurements ranged from 0.11 mg/L in MW -4D to 3.82
mg/L in monitoring well MW -5D.
At voltages between +300mV and +200 mV and DO concentrations of 0.1 mg/L 11 iron would be
reduced to the soluble Fe2+ form. The ORP measurement at monitoring well MW -13 was 178
mV with a corresponding DO concentration of 0.19 mg/L, indicating conditions that would be
favorable for soluble Fe2+. Groundwater pH can also affect which metals may be present in
soluble or insoluble forms. Measurements of ORP can be influenced by contact with the
atmosphere and normal purge sampling techniques may interfere with accurate measurements of
Eh.
4.11A Collect Soil Samples – Analyze for Iron and Manganese Content-
Task 11A
Soil samples will be collected from soil borings performed adjacent to monitoring wells MW-9,
MW-10 and MW-13. One soil sample in each boring will be collected from above the observed
water level in the adjacent monitoring well. Up to two soil samples in each boring will be
collected from the depth of the screened interval in the adjacent well (within the shallow
aquifer). The samples will be collected from different soil types where applicable. The borings
will be logged for soil lithology by a Professional Geologist. The soil samples will be analyzed
for total concentrations of iron and manganese, cation exchange capacity, redox potential,
nitrate, sulfate, and pH. A discussion of the iron and manganese concentrations detected in the
soils associated with the three monitoring wells will be provided.
On April 15 and 16, 2013, one soil boring was performed adjacent to each of monitoring wells
MW-9, MW-10, and MW-13. The locations of the monitoring wells are depicted on Figure 2.3-
1. The groundwater elevations in each of the monitoring wells were measured by Duke Energy
on April 3, 2013, and the information provided to HDR to assist in estimating the groundwater
level in the monitoring wells adjacent to the borings. The borings were performed using track

11 Textbook of Limnology, Gerald A. Cole, second Edition, C.V. Mosby Company, 1979.
Section 4 Assessment of Groundwater Exceedances

41
and trailer mounted drill rigs with 6¼ inch inside diameter augers and logged for soil lithology
during advancement.
Soil samples were collected in each boring using a split-spoon sampler from one depth above the
measured water level in the adjacent monitoring well and from up to two locations within the
depth of the screened interval of the adjacent monitoring well. Collected soil samples were
submitted to Pace Analytical Services, Inc. for analysis for:
• total concentrations of iron and manganese by EPA Method 6010
• cation exchange capacity by EPA Method 9081
• pH by EPA Method 9045
• oxidation/reduction potential by Method SM 2580B
• nitrate and sulfate concentrations by Method SW9056A.

Soil boring SB -9A was performed approximately 10 feet west of monitoring well MW-9. The
measured depth to groundwater in monitoring well MW -9 was 0.76 feet below ground surface
(bgs). The soil sample identifications, sample depths, and soil types collected from SB-9A were:
• SB-9A-1.5’, collected 0.5 to 1.5 feet bgs, brown sandy silt
• SB-9A-16.5’, collected 15 to 16 feet bgs, light brown to gray silty clay
• SB-9A-26.5’, collected 25 to 26.5 feet bgs, light brown silty fine to medium sand
Soil boring SB -10A was performed approximately 8 feet northeast of monitoring well MW-10.
The measured depth to groundwater in monitoring well MW -10 was 8.08 feet bgs. The soil
sample identifications, sample depths, and soil types collected from SB-10A were:
• SB-10A-5’, collected 4 to 5 feet bgs, light brown sandy silt
• SB-10A-13.5’, collected 12 to 13.5 feet bgs, light brown clayey silt
• SB-10A-20’, collected 18.5 to 20 feet bgs, light brown micaceous sandy silt
Soil boring SB -13A was performed approximately 11 feet west of monitoring well MW -13.
The measured depth to groundwater in monitoring well MW -13 was 7.91 feet bgs. The soil
sample identifications, sample depths, and soil types collected from SB-13A were:
• SB-13A-6’, collected 5 to 6 feet bgs, reddish brown clayey silt
Section 4 Assessment of Groundwater Exceedances

42
• SB-13A-15’, collected 13.5 to 15 feet bgs, dark brown clay

The summary of the a nalytical results for the collected soil samples are presented in Table
4.11A-1. The NCDENR inactive hazardous sites branch (IHSB) protection of groundwater
preliminary soil remediation goals (PSRGs) have been included on this table for reference. A
copy of the laboratory analytical report and chain-of-custody forms are included in Appendix D.
The analytical results for the soil samples collected from soil boring SB -9A had iron
concentrations ranging from 25,500 milligrams per kilogram (mg/kg) to 33,900 mg/kg and
manganese concentrations ranging from 117 mg/kg to 649 mg/kg.
The analytical results for the soil samples collected from soil boring SB -10A had iron
concentrations ranging from 22,400 mg/kg to 27,900 mg/kg and manganese concentrations
ranging from 133 mg/kg to 197 mg/kg.
The analytical results for the soil samples collected from soil boring SB -13A had iron
concentrations ranging from 42,300 mg/kg to 42,500 mg/kg and manganese concentrations
ranging from 523 mg/kg to 967 mg/kg.
The iron and manganese concentrations measured in the collected soil samples were greater than
the IHSB PSRGs for iron and manganese. The concentrations of iron and manganese measured
in the soil samples collected from soil bo ring SB-13A were greater than the concentrations
measured in soil borings SB-9A and SB-10A.
4.12 Perform Statistical Data Analyses of the Sampling Results - Task 12
Statistical analyses of groundwater monitoring results will be performed to determine if the
exceedances can be attributed to contamination or if the exceedances can be attributed to
naturally occurring background concentrations. The NCDENR document, Evaluating Metals in
Groundwater at DWQ Permitted Facilities: A Technical Assistance Document for DWQ Staff,
dated July 2012, will be used as general guidance. After approval of the proposed assessment
work plan, Duke proposes to meet with DWQ regional staff to discuss the specific statistical
analyses that will be employed.
Section 4 Assessment of Groundwater Exceedances

43
A statistical analysis of the groundwater monitoring results was performed and a copy of the
detailed report describing the analysis is located in Appendix C.
The report outlines the methods used to determin e statistically significant increases (SSI) and
trends in the groundwater monitoring data from the February 4, 2013 sampling event. Statistical
analysis was conducted for parameters that ha ve been sampled at least eight times at each
monitoring well. The data were statistically analyzed using Starpoint Software, Inc.’s
ChemPoint  and ChemStat software packages. Groundwater data (analytical results) of the
downgradient wells (both shallow and intermediate wells) were compared to the pooled
background well (MW-7SR) groundwater data (inter-well test) to determine whether a SSI exists
between the background wells and the compliance wells. A similar process was performed
comparing the analysis results from the downgradient deep wells to the deep background well
(MW-7D). This statistical analysis method is in accordance with the EPA Statistical Analysis of
Groundwater Monitoring Data at RCRA Facilities – Unified Guidance, March 2009.
Confidence intervals were calculated when a parameter is determined to have a SSI over the
background well data. The prediction interval is calculated to include observations from the
same population with a specified confidence. Intra-well prediction interval is developed based
on a 99% confidence that future observations will fall within the range. If any future observation
exceeds the prediction interval, this is considered statistically significant evidence that the
observation is not representative of the background data group.
Parameters are considered as exceeding the compliance limits whenever the lower value of the
99% confidence interval (LCL) exceeds the compliance limits (2L Standard).
The summary of the results of the statistical inter -well and the intra -well analys es for iron and
manganese is repeated below:
Inter-well Analysis :

1. Iron concentrations in wells MW -8S, MW-8I, MW-8D, MW-9, MW-10, MW-11SR,
MW-13, and MW-15 showed SSI above background concentrations. Wells MW-11DR
and MW -14 did not show a SSI above the background concentrations. Iron data showed
Section 4 Assessment of Groundwater Exceedances

44
decreasing trends in concentrations , or no trends, at all wells with the exception of MW-
13, which showed an increasing trend in concentrations.
2. Manganese at three wells (MW-8D, MW-11DR, and MW-13) showed SSI above the
background data; but the data showed no trend over time.
Intra -well Analysis :
1. The 99 % LCL for the iron concentrations at MW-8I, MW-8D, MW-9, and MW-13 were
greater than 2L Standard for iron of 300 µg/L.
2. The 99% LCL for the manganese concentrations at MW-8D, MW-11DR, and MW-13
were greater than the 2L Standard for manganese of 5 0 µg/L.
This indicates that these wells consistently have results in excess of the 2L Standards.

45
Section 5
Conclusions and Recommendations

Conclusions
Exceedances of 2L Standards for pH, iron, and manganese have been measured at the monitoring
wells noted in Table 3 .1-4.
Iron and manganese are listed in the 2L Standards; however the se substances are listed by the
EPA as having non-mandatory water quality standards. The EPA has established National
Secondary Drinking Water Regulations in Title 40 of the Code of Federal Regulations (CFR)
Part 143 National Secondary Drinking Water Standards (40 CFR 143). In 40 CFR 143, the EPA
established non-mandatory water quality standards for 15 contaminants. Iron and manganese are
included in this list of contaminants having secondary maximum contaminant levels or
standards. EPA does not enforce these secondary maximum contaminant levels. The secondary
standards are established only as guidelines to assist public water systems in managing their
drinking water for aesthetics and other considerations, such as taste, color, and odor.
Exceedances of 2L Standards for pH
Based on a review of the pH values in the monitoring wells as compared to the pH values in
the background monitoring wells, the exceedances do not appear to be related to impacts
from the ash basin.

Exceedances of 2L Standards for Iron and Manganese - Upgradient Monitoring Wells
MW-7SR – Monitoring well MW -7SR is the shallow background well for the site.
Groundwater monitoring well MW-7SR has historical exceedances for iron that have
decreased to concentrations less than the 2L Standard for the two most recent sampling
event s. Prior to the February 2013 sampling event, four of the previous five sampling events
measured manganese concentrations less than the 2L Standard. Based on the location of the
well and the information evaluated in Section 4, the iron and manganese exceedances at this
well should be considered to be from naturally occurring sources and do not appear to be
related to impacts from the ash basin.
Section 5 Conclusions and Recommendations

46
MW-8S – Groundwater monitoring well MW-8S has historical exceedances for manganese.
Manganese concentrations have been generally decreasing from February 2012 to February
2013 but have consistently been in excess of the 2L Standard. There appears to be a general
correlation between turbidity and measured manganese concentrations, with manganese
concentrations generally decreasing with decreasing turbidity values . Based on the location
of the well and the information evaluated in Section 4, the manganese exceedances at this
well should be considered to be from naturally occurring sources and do not appear to be
related to impacts from the ash basin.
MW-8I – Groundwater monitoring well MW-8I has historical exceedances for iron and
ma nganese. Iron concentrations have been generally decreasing from February 2012 to
February 2013. Manganese has been measured at concentrations less than the 2L Standard
during three of the last four sampling events (Figure 4.7 -17), increasing in the most recent
sampling event. There appears to be a general correlation between turbidity values and
measured iron concentrations. Based on the location of the well and the information
evaluated in Section 4, the iron and manganese exceedances at this well should be considered
to be from naturally occurring sources and do not appear to be related to impacts from the
ash basin.
MW-8D – Groundwater monitoring well MW-8D has historical exceedances or iron and
manganese. Both iron and manganese concentrations have been consistent over the
monitoring period. There appears to be a general correlation between turbidity values and
measured iron concentrations, but the manganese concentrations do not appear to be
correlated with turbidity. Based on the location of the well and the information evaluated in
Section 4, the iron and manganese exceedances at this well should be considered to be from
naturally occurring sources and do not appear to be related to impacts from the ash basin.
Exceedances of 2L Standards for Iron and Manganese – Downgradient Monitoring
Wells
MW-9 – Groundwater monitoring well MW-9 has historical exceedances of iron and
manganese. Iron concentrations are variable and have been greater than the 2L Standard over
Section 5 Conclusions and Recommendations

47
the mon itoring period. Manganese concentrations have been below the 2L Standard during
the last three sampling events. There is no evidence of redox conditions at this well to
account for the iron and manganese exceedances. A review of the concentrations of
constituents measured in the TOWER sample does not indicate impacts from the ash basin;
however, this may not be conclusive due to the anticipated variability over area and depth in
the geochemistry in the ash basin. Turbidity values have ranged from a maximum of 139
NTU in the initial sampling event to a minimum of 12.6 NTU in the October 4, 2011
sampling event . Turbidity values appear to be correlated with iron and manganese
concentrations. Manganese results were below the 2L Standard in the February 2013
sampling event. Based primarily on a review of the variability in the turbidity results and the
apparent correlation with the variability in the iron and manganese concentrations, the iron
and manganese exceedances at this well should be considered to be from naturally occurring
sources and do not appear to be related to impacts from the ash basin.
MW-10 – Groundwater monitoring well MW-10 has historical exceedances for iron and
manganese . Iron concentrations have been variable over the monitoring period, but show a
decreasing trend over time. Iron concentrations have been lower than the 2L Standard during
the past three sampling events. Manganese concentrations have also varied over the
monitoring period, but like iron, show a generally decreasing trend. Iron and manganese
concentrations appear to be correlated with turbidity values. Iron and manganese results were
below their respective 2L Standards in the February 2013 sampling event.
There is no evidence of redox conditions at this well to account for the iron and manganese
exceedances. A review of the concentrations of constituents measured in the TOWER sample
do not indicate impacts from the ash basin, however, this may not be conclusive due to
anticipated variability in geochemistry of the ash basin over area and depth. Turbidity values
have ranged from a maximum of 182 NTU in the initial sampling event to a minimum of3.2
NTU in the June 4, 2012 sampling event and appear to be correlated with iron and
manganese concentrations. Based primarily on a review of the variability in the turbidity
results and the apparent correlation with the variability in the iron and manganese results, the
Section 5 Conclusions and Recommendations

48
iron and manganese exceedances at this well should be considered to be from naturally
occurring sources and do not appear to be related to impacts from the ash basin.
MW-11SR – Groundwater monitoring well MW-11SR has historical exceedances of iron
and manganese. The only iron concentration exceedance of the 2L Standard occurred during
the first sampling event . Iron concentrations have been less than the 2L Standard during the
last six sampling events . Manganese concentrations exceeded the 2L Standard during the
first two sampling events and have been less than the 2L Standard during the past five events .
Iron and manganese concentrations appear to be correlated with turbidity values. Based
primarily on a review of the variability in the turbidity result s and the apparent correlation
with the variability in the iron and manganese results, the iron and manganese exceedances at
this well should be considered to be from naturally occurring sources and do not appear to be
related to impacts from the ash basin.
MW-11DR – Groundwater monitoring well MW-11DR has historical exceedances of
manganese. Manganese concentrations have been generally consistent over the monitoring
period and are in excess of the 2L Standard. Turbidity values do not appear to be correlated
with manganese concentrations. DO concentrations of 0.95 mg/L and an ORP of 232 mV-
NHE were measured during the February 2013 sampling event. The low DO concentrations
are consistent with reduced conditions that could cause naturally occurring manganese to be
reduced to a dissolved state which would increase concentrations in the groundwater at this
location. See the Recommendations.
MW-13 – Groundwater monitoring well MW-13 has historical exceedances of iron and
manganese. Iron concentrations have generally been consistent over the monitoring period,
ranging from a maximum of 23,000 µg/L to a minimum of 13,500 µg/L. Manganese
concentrations have generally been consistent over the monitoring period, ranging from a
maximum of 11,200 µg/L to a minimum of 10,000 µg/L. This well is located approximately
100 feet from the shore of Mountain Island Lake and as discussed in Section 4.4, the water
elevation in the well is likely influenced by the frequent changes in elevation of Mountain
Island Lake as a result of hydroelectric operations. Soil samples collected and analyzed for
Section 5 Conclusions and Recommendations

49
iron and manganese found concentrations above those measured at MW-9 and MW-10. As
discussed in Section 4.11, measurement of DO and ORP during the February 2013 sampling
event found evidence that reduc ed conditions exist at this well. Reduc ed conditions can
cause naturally occurring iron and manganese to be reduced to a dissolved state, potentially
increasing the concentrations measured in the groundwater at this location. See the
Recommendations.
MW-14 – Groundwater monitoring well MW -14 has historical exceedances of iron and
manganese. The results for iron and manganese in the February 2013 sampling event were
below the 2L Standard. There have been only three exceedances of iron at this location (the
first two sampling event s and in February 2012). The last two events measured iron
concentrations below the 2L Standard. Manganese concentrations have been variable over
the monitoring period . Manganese concentrations measured during the last two sampling
events have been less than the 2L Standard. The iron concentrations appear to generally be
correlated with turbidity values while the manganese concentrations do not appear to be
correlated with turbidity values . DO and ORP values during the February 2013 sampling
event indicated the possibility that reducing conditions exist at this well. Although these
measurements are not indicative of strongly reduc ed conditions, measurement of these
parameters using normal sampling techniques can bias results. Reduc ed conditions can cause
naturally occurring iron and manganese to be reduced to a dissolved state, potentially
increasing the concentrations in the groundwater at this location. See the Recommendations.

MW-15 – Groundwater monitoring well MW -15 has historical exceedances of iron and
manganese. The results for iron and manganese during the February 2013 sampling event
were below the 2L Standards. The only iron exceedance measured was during the February
2012 sampling event. Iron concentrations measured during the last three sampling events
were below 60 µg/L. Manganese concentrations have generally been consistent and in excess
of the 2L Standard over the monitoring period . Concentrations measured in the last two
sampling events have been less than the 2L Standard. Measurements of DO and ORP during
the February 2013 sampling event indicate the possibility of reduced conditions at this well.
Although these measurements are not indicative of strongly reducing conditions,
Section 5 Conclusions and Recommendations

50
measurement of these values using normal sampling techniques can bias results. Reduc ed
conditions can cause naturally occurring iron and manganese to change to a dissolved state,
potentially increasing the concentrations in the groundwater at this location. See the
Recommendations.

Recommendations
HDR presents the following recommendations to assess the exceedances at the Riverbend ash
basin.
1. MW-11DR, MW-13, MW-14, and MW-15 – Collection of oxidation-reduction
parameters (Eh, dissolved oxygen) should be continued. Measurement of ORP can be
influenced by contact with the atmosphere, and normal purge sampling techniques may
interfere with accurate measurement of Eh. During t he next sampling event , the well
purging should be performed using low flow sampling techniques to improve accuracy of
ORP measurements. Groundwater samples should be analyzed for the same set of
parameters and at the same wells sampled in the February 2013 sampling event.
2. Voluntary monitoring wells MW -4S and MW-4D were sampled during the February
2013 sampling event. The samples collected at both of these wells experienced high
turbidity, with samples from MW -4S having turbidity in excess of 1000 NTU and
samples from MW -4D turbidity measured at 43.6 NTU. HDR recommends that these
wells be redeveloped to reduce the turbidity and resampled as in Recommendation 1.

The results of the sampling recommended above should be evaluated and submitted to
NCDENR DWQ as an amendment to the groundwater assessment.

51
References

A Master Conceptual Model for Hydrogeological Site Characterization in the Piedmont and
Mountain Region of North Carolina. LeGrand, Harry E. Sr., 2004

Policy for Compliance Evaluation of Long-Term Permitted facilities with No Prior Groundwater
Monitoring Requirements

Physical and Hydraulic Properties of Fly Ash and Other By-Products From Coal Combustion,
Electric Power Research Institute (EPRI) TR-1001999, February 1993.

Coal Ash Disposal Manual: Third Edition, EPRI TR-104137, January 1995.

Ground-Water Sampling Guidelines for Superfund and RCRA Project Managers, Ground Water
Forum Issue Paper, Douglas Yeskis and Bernard Zavala, EPA 542-S-02-001, Office of Solid
Waste and Emergency Response, May 2002.
ARCADIS G&M of North Carolina, Inc., Ash Basin Drilling Services for: Riverbend Steam
Station, Mount Holly, North Carolina, January 7, 2007.

Goldsmith, R., Milton, D. J. and Horton, J. W;, Jr. 1988. Geologic map of the Charlotte 1o x 2o
quadrangle, North Carolina and South Carolina: United States Geological Survey,
Miscellaneous Investigations Series, Map I-1251-E, scale 1:250,000.

Harned, D. A. and Daniel, C. C., III, 1992, The transition zone between bedrock and regolith:
Conduit for contamination?, p. 336-348, in Daniel, C. C., III, White, R. K., and Stone, P. A.,
eds., Groundwater in the Piedmont: Proceedings of a Conference on Ground Water in the
Piedmont of the Eastern United States, October 16-18, 1989, Clemson University, 693p.

Heath, R. C., 1980, Basic elements of ground-water hydrology with references to conditions in
North Carolina: U. S. Geological Survey Water-Resources Open-File Report 80-44, 86p.

Heath, Ralph. C., 1994, Ground-Water Recharge in North Carolina: Prepared for the
Groundwater Section, Division of Environmental Management, North Carolina Department of
Environment, Health, and Natural Resources 45, 52p.

LaGrand, H.E., 1988, Region 21, Piedmont and Blue Ridge, p.201-208, in Black, W., Rosenhein,
J. S., and Seaber, P. R., eds., Hydrogeology: Geological Society of America, The Geology of
North America, v. O -2, Boulder, Colorado, 524p.

LeGrand, Harry, Sr. 2004, A Master Conceptual Model for Hydrogeological Site
Characterization in the Piedmont and Mountain Region of N orth Carolina, North Carolina
Department of Environment and Natural Resources.

MACTEC, Amended Ash Basin Monitoring Well Installation Report, Riverbend Steam Station,
MACTEC Project No. 6228-10-5284, March 30, 2011.

Section 5 Conclusions and Recommendations

52
North Carolina Geologica l Survey, 1985, Geologic map of North Carolina: Raleigh, North
Carolina Geological Survey, scale 1:500,000.

Pippin, Charles, G., Chapman, Melinda, J., Huffman, Brad A., Heller, Matthew J., and Schelgel,
Melissa E., 2008, Hydrogeologic Setting, Ground -Water Flow, and Ground-Water Quality at
the Langtree Peninsula Research Station, Iredell County, North Carolina, 2000-2005, United
States Geological Survey, Prepared in cooperation with the North Carolina Department of
Environment and Natural Resources, Division of Water Quality.

FIGURES

TABLES

APPENDICES

APPENDIX A
Letter from Andrew H. Pitner, P.G, Regional Aquifer Protection Supervisor, NCDENR, Division
of Water Quality, to Ed Sullivan and Allen Stowe, Water Management, Duke Energy
Corporation, Dated March 16, 2012

64
APPENDIX B
Letter from Andrew H. Pitner, P.G., Regional Aquifer Protection Supervisor, NCDENR Division
of Water Quality, to Ed Sullivan and Allen Stowe, Water Management, Duke Energy
Corporation, Dated January 10, 2013

APPENDIX C
Statistical Analysis for Groundwater Metals Riverbend Steam Station Ash Basin February 2013
Sampling Event

APPENDIX D
Soil Sampling Laboratory Analytical Report and Chain of Custody Form