HomeMy WebLinkAboutNC0004961_Proposed GW Assessment Work Plan_20121101
HDR Engineering, Inc. of the Carolinas
Transmitted via email:
Andrew.Pitner@ncdenr.gov
November 1, 2012
Mr. Andrew Pitner, P.G.
North Carolina Department of Environment and Natural Resources
Division of Water Quality
Aquifer Protection Section
610 East Center Avenue, Suite 301
Mooresville, NC 28115
Subject: Duke Energy Carolinas, LLC
Riverbend Steam Station Ash Basin
NPDES Permit NC0004961
Proposed Groundwater Assessment Work Plan
Dear Mr. Pitner:
In a letter dated March 16, 2012, the Division of Water Quality (DWQ) Aquifer Protection
Section (APS) requested that Duke Energy, LLC (Duke Energy) begin additional assessment
activities at the Riverbend Steam Station ash basin. North Carolina Administrative Code
(NCAC) Title 15A Chapter 02L (g) groundwater quality standards were exceeded for pH, iron,
and manganese at the compliance boundary at certain wells at the Riverbend Steam Station ash
basin.
On behalf of Duke Energy, HDR Engineering, Inc. of the Carolinas (HDR) submits the attached
proposed groundwater assessment work plan.
The proposed groundwater assessment work plan describes the tasks for assessment of the
groundwater exceedances. The proposed groundwater assessment work plan also describes the
refinements that will be made to the groundwater model used to predict groundwater
concentrations at the compliance boundary for monitoring wells MW-9, MW-10, and MW-13.1
The proposed groundwater assessment will include data collected from the February 2013
sampling event. The completed assessment will be submitted approximately 12 weeks after the
February 2013 sampling event.
Annual Report Submittal
Monitoring wells MW-9, MW-10, and MW-13 are located inside the compliance boundary of
the ash basin. The concentrations for these wells at the compliance boundary are developed by
1 Supplemental Groundwater Monitoring Report, Riverbend Steam Station Ash Basin, NPDES Permit NC0004961,
Duke Energy Carolinas, LLC, January 16, 2012, Altamont Environmental Inc. Project Number 2370.04.
Mr. Andrew Pitner
November 1, 2012
Page 2
groundwater modeling. A report providing the modeled concentrations at the compliance
boundary is submitted annually. The annual report containing the modeled concentrations, based
on the groundwater sampling performed in 2012, would typically be submitted in January 2013.
As discussed above, the groundwater model used to predict the concentrations at the compliance
boundary will be refined in work performed for the assessment.
Based on our telephone call of October 26, 2012, on behalf of Duke Energy, HDR requests that
the submittal of the annual report containing the results of the predictive groundwater modeling
be deferred until after submittal of the proposed assessment. The proposed submittal date for the
annual report, based on the refined groundwater model, is 3 weeks after submittal of the
assessment.
If you have questions or require additional information, please contact me at (828) 891-6296.
Sincerely,
HDR Engineering, Inc. of the Carolinas
William Miller, P.E.
Sr. Environmental Engineer
Enclosures: Duke Energy Carolinas, LLC
Riverbend Steam Station Ash Basin
NPDES Permit NC0004961
Proposed Groundwater Assessment Work Plan
cc: Ed Sullivan, Duke Energy Carolinas, LLC (via email)
Dayna Herrick, Duke Energy Carolinas, LLC (via email)
Allen Stowe, Duke Energy Carolinas, LLC (via email)
Tim Hunsucker, Duke Energy Carolinas, LLV (via email)
Robert Caccia, Duke Energy Carolinas, LLC (via email)
HaR Engineering, Inc. of the Carolinas
DUKE ENERGY CAROLINAS, LLC
RIVERBEND STEAM STATION ASH BASIN
NPDES PERMIT NC0004961
PROPOSED GROUNDWATER ASSESSMENT
WORK PLAN
Prepared for:
DUKE ENERGY CAROLINAS, LLC
Charlotte, North Carolina
Prepared by:
HDR ENGINEERING, INC. OF THE CAROLINAS
Charlotte, North Carolina
NOVEMBER 1, 2012
RIVERBEND STEAM STATION ASH BASIN
NPDES PERMIT NC0004961
PROPOSED GROUNDWATER ASSESSMENT WORK PLAN
TABLE OF CONTENTS
Section Title Page No.
1. INTRODUCTION ................................................................................................... 1
2. SITE DESCRIPTION .............................................................................................. 3
2.1Plant Description ..........................................................................................................3
2.2Ash Basin Description ..................................................................................................3
3. REGULATORY REQUIREMENTS ............................................................................ 5
4. SITE HYDROGEOLOGY ........................................................................................ 7
5. DESCRIPTION OF GROUNDWATER MONITORING SYSTEM .................................... 9
6. GROUNDWATER MONITORING RESULTS ........................................................... 11
7. GROUNDWATER ASSESSMENT WORK PLAN ...................................................... 12
8. GROUNDWATER MODEL REFINEMENT PLAN .................................................... 15
9. ASSESSMENT REPORT AND PROPOSED SCHEDULE ............................................ 17
10. REFERENCES ..................................................................................................... 18
Figures
1 SITE LOCATION MAP
2 SITE LAYOUT MAP
3 GENERIC MODEL CROSS SECTION WITH REFINED MESH
iii
RIVERBEND STEAM STATION ASH BASIN
NPDES PERMIT NC0004961
PROPOSED GROUNDWATER ASSESSMENT WORK PLAN
LIST OF TABLES
Table Title Page No.
TABLE 1 – GROUNDWATER MONITORING REQUIREMENTS ...............................................5
TABLE 2 – EXCEEDANCES OF 2L STANDARDS FOR IRON, MANGANESE, AND pH ......11
TABLE 3 – MONITORING WELLS WITH 2L EXCEEDANCES ................................................12
APPENDIX
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
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). The
steam station generates electricity by burning coal. The coal ash residue from the coal
combustion process is disposed of in the station’s ash basin. The discharge from the ash basin is
permitted by the North Carolina Department of Environment and Natural Resources (NCDENR)
Division of Water Quality (DWQ) under the National Pollution Discharge Elimination System
(NPDES) Permit NC0004961.
The NPDES permit, issued January 18, 2011, Condition A(11) requires groundwater monitoring
to be performed around the ash basin. Attachment XX to the NPDES permit lists the
groundwater monitoring wells to be sampled, constituents to be analyzed and contains
requirements for the sampling frequency and reporting of results. The NPDES permit requires
these monitoring wells to be sampled three times per year with the analytical results submitted to
the DWQ. In 2010 and 2011, groundwater monitoring wells (monitoring wells) were installed
by Duke Energy at locations around the ash basin in response to these requirements.
Groundwater monitoring has been performed in accordance with the permit conditions beginning
in December 2010. Exceedances of the North Carolina Administrative Code (NCAC) Title 15A
Chapter 02L (g) groundwater quality standards (2L Standards) for pH, iron, and manganese have
been measured in groundwater samples collected at 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.
The compliance boundary for groundwater quality for the Riverbend ash basin is defined in
accordance with NCAC Title 15A Chapter 02L .0107(a) (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. Monitoring wells MW-7SR, MW-7D, MW-8S, MW-8I, MW-8D, MW-11SR, MW-
11DR, MW-14, and MW-15 are located at or near the compliance boundary. Analytical results
from sampling these wells are submitted to NCDENR within 60 days after each sampling event.
As a result of site access conditions, monitoring wells MW-9, MW-10, and MW-13 were
Section 1 Introduction
2
installed inside of the compliance boundary. These monitoring wells are also sampled three
times per year, but compliance with 2L Standards for these locations is determined by
groundwater modeling. A report is submitted annually with the results of the groundwater
modeling along with the analytical results from the sampling.
In a letter dated March 16, 2012, DWQ Aquifer Protection Section (APS) requested that Duke
Energy begin additional assessment activities at stations where measured and modeled
concentrations of groundwater constituents exceed 2L groundwater quality standards at the
compliance boundary. This letter is included as Appendix A.
HDR Engineering, Inc. of the Carolinas (HDR) has prepared this proposed groundwater
assessment work plan on behalf of Duke Energy. This document presents a proposed work plan
for performing additional assessment of these exceedances
3
Section 2
Site Description
2.1 Plant Description
Riverbend is a coal-fired electricity generating facility with a capacity of 454 megawatts located
in Gaston County, North Carolina near the town of Mt. Holly. The station is located on the south
bank of 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.
The four-unit station, which began commercial operation in 1929, is named after a bend in the
Catawba River. Riverbend is considered a cycling station and is brought online to supplement
supply when electricity demand is highest.
2.2 Ash Basin Description
The ash basin system consists of a Primary Cell and a Secondary Cell, separated by an
intermediate dike. The initial ash basin at Riverbend originally consisted of a single-cell basin
commissioned in 1957 and was expanded in 1979. The single basin was divided by constructing
a divider dike to form two separate cells in 1986.
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. The Primary Cell is impounded by an earthen
embankment dike, referred to as Dam #1, located on the west side of the Primary Cell. The
Secondary Cell is impounded by an earthen embankment dike, referred to as Dam #2, located
Section 2 Site Description
4
along the northeast side of the Secondary Cell. The toe areas for both dikes are in close
proximity to Mountain Island Lake.
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.
The ash basin system is an integral part of the station’s wastewater treatment system. The ash
basin receives inflows from the following sources:
Ash removal system
Station yard drain sump
Stormwater flows
Due to the cycling nature of station operations, inflows to the ash basin are 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 Cell 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 discharges through the discharge tower into Mountain Island Lake. The ash
basin pond elevation is controlled by the use of concrete stop logs.
1 The datum for all elevation information presented in this report is NAVD88.
5
Section 3
Regulatory Requirements
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
conditions set forth in the permit. The NPDES permitting program 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 1. Attachment XX also provides requirements for well location and well
construction.
TABLE 1 – GROUNDWATER MONITORING REQUIREMENTS
WELL
NOMENCLATURE
PARAMETER DESCRIPTION FREQUENCY
Monitoring Wells:
MW-7SR, MW-7D, MW-
8S, MW-8I, MW-8D,
*MW-9, *MW-10, MW-
11SR, MW-11DR, *MW-
13, MW-14, MW-15
Antimony Chromium Nickel Thallium
February, June,
October
Arsenic Copper Nitrate Water Level
Barium Iron pH Zinc
Boron Lead Selenium
Cadmium Manganese Sulfate
Chloride Mercury TDS
Note: Monitoring wells marked with * are located inside of the compliance boundary.
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. All monitoring wells are
Section 3 Regulatory Requirements
6
sampled three times per year. 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
is determined by using predictive calculations or a groundwater model to demonstrate
compliance. 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 at MW-9, MW-10, and MW-
13 were submitted to DWQ in January 2012.2
2 Supplemental Groundwater Monitoring Report Riverbend Steam Station Ash Basin, NPDES Permit NC0004961,
Duke Energy Carolinas, LLC, January 16, 2012.
7
Section 4
Site 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, where 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
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).
Section 4 Site Hydrogeology
8
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 generally
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.
9
Section 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. These wells are
sampled in February, June, and October.
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.
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
auger refusal.3
Monitoring wells MW-7SR and MW-7D 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
3 Amended Ash Basin Monitoring Well Installation Report, Riverbend Steam Station, MACTEC Project No. 6228-
10-5284, March 30, 2011.
Section 5 Description of Groundwater Monitoring System
10
dividing the Primary Cell and the Secondary Cell. Monitoring wells MW-13, MW-14, and MW-
15 are located downgradient of the Secondary Cell.
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. These wells are not shown on Figure 2. No samples are
currently collected from these wells.
11
Section 6
Groundwater Monitoring Results
Through September 2012, the monitoring wells at Riverbend have been sampled a total of six
times.4 These monitoring wells were sampled in:
December 2010
February 2011
June 2011
October 2011
February 2012
June 2012
With the exception of iron, manganese, and pH, the results for all monitored parameters and
constituents were less than the 2L Standards. Table 2 lists the range of exceedances for iron,
manganese, and pH from the sampling events listed above.
Table 2 – Exceedances of 2L Standards for Iron, Manganese, and pH
Iron Manganese pH
2L Std 300 µg/L 50 µg/L 6.5 - 8.5 SU
Well ID
Range of
Exceedances
µg/L
Range of
Exceedances
µg/L
Range of
Exceedances
SU
MW-7SR 285 - 790 113 - 413 5.1 - 5.4
MW-7D No Exceedances No Exceedances 5.6 – 5.8
MW-8S No Exceedances 126 - 144 4.9 – 5.2
MW-8I 643 - 976 32 - 290 6.3 – 6.4
MW-8D 777 – 1,480 174 - 671 No Exceedances
MW-9* 381 – 1,870 25 - 147 6.0 – 6.4
MW-10* 72 – 1,420 48 - 355 5.3 – 5.4
MW-11SR 39 - 486 17 - 384 5.6 – 6.1
MW-11DR No Exceedances 87 - 168 5.6 – 5.8
MW-13* 18,300 – 20,600 10,000 – 11,200 6.0 – 6.4
MW-14 58 - 935 43 - 353 No Exceedances
MW-15 45 - 399 52 - 86 5.1 – 5.3
Note: Monitoring wells marked with * are located inside of the compliance boundary.
4 The analytical results for the October 2012 sampling event had not been finalized at the time this plan was
prepared.
12
Section 7
Groundwater Assessment Work Plan
Exceedances of the 2L Standards for iron, manganese, and pH have been measured at the
following monitoring wells:
TABLE 3 – MONITORING WELLS WITH 2L EXCEEDANCES
Monitoring Well Locations Monitoring Well
At or Near the Compliance Boundary
MW-7SR, MW-7D, MW-8S, MW-8I
MW-8D, MW-11SR, MW-11DR, MW-14,
MW-15
Inside of the Compliance Boundary MW-9, MW-10, MW-13
As described earlier in this document, compliance with groundwater standards for monitoring
wells located at or near the compliance boundary is determined by comparing analytical results
to the 2L Standards. Compliance with groundwater standards for monitoring wells located inside
of the compliance boundary are determined by predictive calculations or groundwater modeling.
The proposed work plan for performing the assessment of 2L exceedances at the monitoring
wells will be the same. As described in Section 8, the groundwater model used to predict the
concentrations of the monitoring wells inside of the compliance boundary will be refined.
The proposed assessment work plan will evaluate the exceedances of iron, manganese, and pH to
determine if the exceedances are naturally occurring, or if they are caused by particulate matter
which is preserved in the sample as a result of well construction and/or sampling procedures.
The assessment work plan report will include the following tasks:
Task 1 Develop A Site Hydrogeologic Conceptual Model - 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.
Task 2 Discuss Site History and Land Uses – A discussion of the site history and site land
uses will be developed.
Section 7 Groundwater Assessment Work Plan
13
Task 3 Review Available Data on Ash Basin Water Quality- 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.
Task 4 Review Mountain Island Lake Water Level Data – 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 data in the
monitoring wells adjacent to Mountain Island Lake to better understand the influence of
the lake on the monitoring wells.
Task 5 Review Location and Number of Background Monitoring Wells - The site
hydrogeologic conceptual model and other data will be reviewed to determine if the
location and number of background wells is sufficient.
Task 6 Evaluate well construction information – Well installation records will be reviewed
to determine if well construction methods are contributing to the exceedances.
Task 7 Evaluate exceedances against background well results – 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.
Task 8 Evaluate exceedances against turbidity values – 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.
Section 7 Groundwater Assessment Work Plan
14
Task 9 Evaluate sampling flow rates – 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.
Task 10 Collect and analyze filtered and non-filtered samples – 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.
Task 11 Collect Reduction/Oxidation Field Parameters – 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.
Task 12 Perform Statistical Data Analyses of the Sampling Results - 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.
15
Section 8
Groundwater Model Refinement Plan
Compliance with groundwater standards for monitoring wells located inside of the compliance
boundary were determined by predictive calculations or groundwater modeling. 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 at MW-9, MW-
10, and MW-13 were submitted to the DWQ in January 2012.5
The groundwater model prepared for the January 2012 submittal utilized MODFLOW to perform
the hydraulic flow modeling and MT3D6 to perform the fate and transport modeling. The MT3D
model uses the flow field developed by MODFLOW to simulate the transport of contaminants.
The model(s) consisted of a single layer of cells with a domain that extended along a cross
section from the subject monitoring well to the compliance boundary.
As requested in the DWQ APS letter dated March 16, 2012, the groundwater model will be
refined. The model will utilize MODFLOW and MT3D to predict the concentrations of iron and
manganese at the compliance boundary and will contain the following refinements:
The model development will incorporate information from the site hydrogeologic
conceptual model described in Section 7.0.
Where appropriate, the model boundary of the cross sections will be extended to include
the ash basin dike, located hydraulically upgradient of the subject monitoring well to the
compliance boundary. Piezometric and water level data from the dikes will be utilized.
Additional layers of cells will be added to the cross section.
A review of the existing slug test data will be performed to determine if additional testing
is necessary.
Recharge from infiltration of precipitation will be incorporated into the model.
5 Supplemental Groundwater Monitoring Report Riverbend Steam Station Ash Basin, NPDES Permit NC0004961,
Duke Energy Carolinas, LLC, January 16, 2012.
6 Model developed by S.S. Papadopulos & Associates, Inc. for the United States Environmental Protection Agency.
Section 8 Groundwater Model Refinement Plan
16
Figure 3 shows an example of a refined mesh for a generic cross-section. The actual mesh size
and modeling will be adjusted as appropriate to the available data and associated analyses.
As with the January 2012 submittal, the groundwater model report will include sensitivity
analyses to account for uncertainties associated with the heterogeneities of lithology, structure,
aquifer parameters, groundwater elevations, water quality, interactions with surface water bodies,
and other parameters.
The groundwater model report will be prepared by Dr. John Daniels, P.E., Interim Chair,
Department of Civil and Environmental Engineering, University of North Carolina at Charlotte.
Dr. Daniels will perform the work under contract with Duke Energy and the groundwater model
report will be included as an attachment to the groundwater assessment prepared and submitted
by HDR.
17
Section 9
Proposed Schedule for Groundwater Assessment
Report
The groundwater assessment report will present the results of the work proposed in Section 7 and
in Section 8. The proposed schedule is to submit the groundwater assessment report 12 weeks
after the next scheduled groundwater sampling event at Riverbend. The next sampling event at
Riverbend is scheduled to take place in February 2013.
18
Section 10
References
Harry E. LeGrand, 2004, A Master Conceptual Model for Hydrogeological Site Characterization
in the Piedmont and Mountain Region of North Carolina, A Guidance Manual, North
Carolina Department of Environment and Natural Resources Division of Water Quality,
Groundwater Section.
FIGURES
fm
HDR Engineering, Inc.
of the Carolinas
License Number: F-0116
440 South Church Street Charlotte, NC 28202
SITE LOCATION MAP
RIVERBEND STEAM STATION
DUKE ENERGY CAROLINAS, LLC
GASTON COUNTY, NORTH CAROLINA
DATE
November 1, 2012
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NOTES:
1. PARCEL DATA FOR THE SITE WAS OBTAINED FROM DUKE ENERGY REAL ESTATE AND IS APPROXIMATE.
2. ASH STORAGE BOUNDARY AND CINDER STORAGE BOUNDARY ARE APPROXIMATE.
3. AS -BUILT MONITORING WELL LOCATIONS PROVIDED BY DUKE ENERGY.
4. SHALLOW MONITORING WELLS (S) - WELL SCREEN INSTALLED ACROSS THE SURFICIAL WATER TABLE.
5. DEEP MONITORING WELLS (D) - WELL SCREEN INSTALLED IN THE TRANSITION ZONE BETWEEN COMPETENT BEDROCK AND THE REGOLITH.
6. TOPOGRAPHY DATA FOR THE SITE WAS OBTAINED FROM NC DOT GEOGRAPHIC INFORMATION SYSTEM (GIS) WEB SITE.
7. ORTHOPHOTOGRAPHY WAS OBTAINED FROM NC ONEMAP GIS WEB SITE (DATED 2009).
8. THE COMPLIANCE BOUNDARY IS ESTABLISHED ACCORDING TO THE DEFINITION FOUND IN 15A NCAC 02L .0107 (a).
,fth Engineering, Inc.
b
l a ME 005%
,�
LEGEND:
— — — — — ® ®
PROPERTY BOUNDARY
COMPLIANCE BOUNDARY
COMPLIANCE BOUNDARY COINCIDENT
WITH DUKE ENERGY PROPERTY BOUNDARY
WASTE BOUNDARY
ASH OR CINDER STORAGE AREA
TOPOGRAPHIC CONTOUR
YYY
COMPLIANCE GROUNDWATER MONITORING WELL
SCALE (FEET)
300'
0 300600'
/' = 600'
SITE LAYOUT MAP
RIVERBEND STEAM STATION
DUKE ENERGY CAROLINAS, LLC
GASTON COUNTY, NORTH CAROLINA
DATE
November 1, 2012
FIGURE 2
Figure 3 – Generic Model Cross Section with Refined Mesh 5 foot x 5 foot mesh (40 layers x 300 columns x 1 row) Constant Head Boundary = 643.5 ft (Elevation of Mt. Island Lake) Constant Head Boundary = 714 ft (Elevation of secondary ash basin) Approximate Compliance Boundary
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
- A-74
HCDENR
North Carolina Department of Environment and Natural
Beverly Eaves Perdue
Governor
Attn: Ed Sullivan and Allen Stowe
Water Management
Duke Energy corporation
EC 13 K I Post Office Box 1006
Charlotte, NC 28201-1006
Dear Mr. Sullivan and Mr. Stowe:
Division of water Quality
Charles Wakild, P.E.
Director
FILE
Resources
Dee Freeman
Secretary
March 16, 2012
RE: Additional Assessment of Groundwater
Alen, Buck, Marshall & Riverbend Stations
We appreciate your meeting with staff of the Mooresville Regional Office of the Division of
Water Quality's Aquifer Protection Section (APS) on March 2nd to discuss groundwater monitoring
efforts at the subject facilities in the Mooresville Region.
To summarize the meeting, APS has requested that Duke begin additional assessment
activities at each of the stations where measured and modeled concentrations of groundwaiter
constituents exceed 2L groundwater quality standards at the compliance boundary.
Additional assessment activities may be specific to issues at certain wells and include, but
are not limited to: statistical evaluation of constituents that may be naturally occurring, refining
groundwater models for Riverbend and Allen to better describe the flow systems, continued well
development and evaluation of well location and construction, and the assessment of horizontal and
vertical extent of constituents in excess of standards.
We appreciate your cooperation and willingness to continue assessment and will loop forward
to a plan outlining a timeline for these activities. Feel free to contact me at (704) 235-2180 or via
email atndrew.'itner,ncdenr.�ov should you have any questions.
Sin ely, r
Andrew H. Pitner, P. �
Regional Aquifer Protection Supervisor
Cc: APS Raleigh (Debra Watts by email)
SWPS Raleigh (Sergei Dhernikov by email)
William Miller, Altamont Environmental (by email)
Division of Water Quality l Aquifer Protection Section 1 Mooresville Regional Office One
610 East Center Avenue, Suite 301, Mooresville, forth Carolina 23115 NbfthCarolina
Phone: 704-063-1699 t Fax: 704- 3-6040
internet: www.ncwaterquality.org )Vatffrallff
An Equal Opporturky 1 Affirmative Action Employer