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Home My WebLink AboutNC0004961_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 FIGURE 1 rs �IgNtnfiSs ,�-• �t -- �,-� 'fimlrr r k��50 i+ Y. _j � #sty=�i� • , � �A �� FA Nil 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