Phase III Assessment. Potential Impacts of Uranium Mining in Virginia on Drinking Water Sources

Transcription

1 Phase III Assessment Potential Impacts of Uranium Mining in Virginia on Drinking Water Sources FINAL December 2013

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3 Uranium Mining in Virginia Table of Contents 1. INTRODUCTION DATA COLLECTED AND GENERATED FOR 2-D SIMULATIONS Improved Bottom Topography for Lake Gaston Lake Gaston and Pea Hill Creek Bridge Crossings Pea Hill Creek Discharge Hydrographs for Wet and Dry Periods Pea Hill Creek Pump Station Water Intake Wind Data for Dry and Wet Years Lake Water Surface Data SETUP OF 2-D SIMULATIONS AND SIMULATION SCENARIOS Lake Water Surface Data SIMULATION RESULTS CONCLUSIONS REFERENCES APPENDIX A. SIMULATION RESULTS Table of Figures Figure 1-1. Location of Coles Hill in Virginia and Downstream Drinking Water Sources... 1 Figure 2-1. Detailed View of the Computational Mesh Near the Junction with Pea Hill Creek Tributary (Mesh Colored Based on the Initial Bed Elevation)... 6 Figure 2-2. Lake Gaston Storage Volume and Surface Area as a Function of Elevation (NVGD 29)(Source: Dominion Power)... 7 Figure 2-3. Comparison of Storage Volume (left) and Lake Surface Area (right) versus Water- Surface Elevation Curves Obtained from the Computational Mesh with Those in Figure Figure 2-4. Bridges Crossing Lake Gaston and the Pea Hill Creek Branch... 8 Figure 2-5. U.S. Highway 1 Bridge... 8 December 2013 Page i

4 Phase III Assessment Figure 2-6. Interstate 85 Bridge... 9 Figure 2-7. Railroad Bridge... 9 Figure 2-8. Eaton Ferry Bridge Figure 2-9. Bridges Crossing over Pea Hill Creek Branch of Lake Gaston Figure Bridge on State Route Figure Opening Under the Bridge Viewed from the North Side (NCDOT, 2011) Figure Cross Section of the Streambed under Bridge (NCDOT 2011) Figure Comparison of Bed Elevation Contours from the Computational Mesh and the Fishing Map for the Passage under Bridge Figure Route 626 Bridge Figure Route 667 Bridge Figure Longitudinal Cross Sections of the Bridges for Route 667 and Route Figure Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 626 Bridge Figure Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 667 Bridge Figure Map of Pea Hill Creek Watershed and the Location of the Pump Station Water Intake Figure Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on Allen Creek Figure Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on the Meherrin River Figure Estimated Pea Hill Creek Discharges for the 2-Year Dry Year from January 1, 2001, to May 31, Figure Estimated Pea Hill Creek Discharges for the Wet Year from September 1, 1996, to August 31, Figure Location of the Water Intake in Google Earth Image and on the Mesh Figure Pump Station Intake Discharge Used for Dry-Year Simulations Figure Pump Station Intake Discharge Used for Wet-Year Simulations Figure Location of the Observation Station Halifax County Airport (KRZZ) (Courtesy of Google Maps) Figure Wind Roses for Dry (left) and Wet (right) Two-Year Periods Figure Hourly Water-Surface Elevation Measurements at Lake Gaston for the Dry Years Figure Hourly Water-Surface Elevation Measurements at Lake Gaston for the Wet Years Page ii December 2013

5 Uranium Mining in Virginia Figure Wet Year Hourly Outflow Discharge at Gaston Dam, Estimated Using Equation Figure Dry Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24- hour Moving Average Figure Wet Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24- hour Moving Average Figure Gaston Dam Dry-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data Figure Gaston Dam Wet-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data Figure Gaston Dam and its Appurtenances Figure 3-1. Dry Year Flow Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-2. Wet Year Flow Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-3. Dry-Year Sediment Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-4. Wet-Year Sediment Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-5. Radium Radioactivity Concentration Entering Lake Gaston Figure 3-6. Thorium Radioactivity Concentration Entering Lake Gaston Figure 4-1. Water Column Contaminant Concentrations at the City of Virginia Beach Pump Station Water Intake on Pea Hill Creek Figure A-1 Locations Selected for Evaluation in Lake Gaston and its Branches (Tributaries) Figure A-2 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-3 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-4 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-5 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-6 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-7 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main December 2013 Page iii

6 Phase III Assessment Figure A-8 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-9 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main Figure A-10 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Outflow Figure A-11 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-A Figure A-12 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-A Figure A-13 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-B Figure A-14 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-C Figure A-15 Bed Sediment Contaminant Mass in Branch A of Lake Gaston Figure A-16 Bed Sediment Contaminant Mass in Branch B of Lake Gaston Figure A-17 Bed Sediment Contaminant Mass in Branch C of Lake Gaston Figure A-18 Bed Sediment Contaminant Mass in The Entire Reservoir of Lake Gaston Figure A-19 Bed Sediment Contaminant Mass in the Main Channel of Lake Gaston Table of Tables Table 3-1. Properties of Tailings Used in Phase II Assessment and the Partition Coefficients Assumed for the Two Scenarios Table 3-2. List of 2-D Simulations Performed During the Phase III Assessment Page iv December 2013

7 Uranium Mining in Virginia 1. INTRODUCTION In 2010, the City of Virginia Beach initiated an assessment to understand the potential impacts of mining uranium in Virginia on drinking water sources. Uranium reserves, including the Coles Hill area reserves that are currently proposed for mining, are located in Pittsylvania County, upstream of the John H. Kerr Reservoir (Kerr Reservoir) and Lake Gaston in southern Virginia, as shown in Figure 1-1. As the figure shows, the Banister River, Kerr Reservoir, and Lake Gaston are sources of drinking water for several communities. Coles Hill Figure 1-1. Location of Coles Hill in Virginia and Downstream Drinking Water Sources Uranium milling and extraction produces vast quantities of waste material, known as tailings, which are typically stored in above-ground impoundments (also known as tailings containment cells). Tailings retain about 85 percent of the original radioactivity for hundreds of thousands of years because certain radioactive materials, such as radium (radium-226) and thorium (thorium-230), are not extracted during the uranium milling process. The first phase of the assessment (Phase I Assessment) evaluated the potential for contaminated sediment (with radium and thorium radioactivity) to reach Kerr Reservoir if a catastrophic failure of a tailings containment cell in the vicinity of the potential uranium mining at Coles Hill were to occur. Although no site-specific data for the potential Coles Hill site was available, the Phase I Assessment used a range of published data associated with uranium mining in the United States, flood hydrographs December 2013 Page 1

8 Phase III Assessment with various occurrence probabilities derived from recorded streamflow data, and a number of failure scenarios to develop an understanding of the range of potential impacts of a tailings containment cell failure. The results of the Phase I Assessment were published in February 2011 (Baker, 2011). The Phase I Assessment used unsteady, one-dimensional (1-D) numerical modeling/simulation of hydrodynamics, sediment transport, and the transport and fate of contaminants for the Banister and Roanoke Rivers and Kerr Reservoir using the CCHE1D model developed by the National Center for Computational Hydroscience and Engineering (NCCHE) at the University of Mississippi (for detailed explanation see Baker, 2011). The 1-D simulations assumed a uniform transport of water and sediments through Kerr Reservoir. Potential lateral or non-uniform transport of water and sediments, especially in and around tributaries to the reservoir, was not modeled. Additionally, the Phase I Assessment did not extend beyond Kerr Reservoir. In the second phase (Phase II Assessment), likely failure scenarios based on more refined parameters and a better characterization of potential mixing in Kerr Reservoir and Lake Gaston were incorporated into the assessment. To achieve this objective, two-dimensional (2-D) models were developed for Kerr Reservoir and Lake Gaston using the CCHE2D software package, which was also developed by the NCCHE at the University of Mississippi (for detailed explanation see Baker, 2012). These 2-D models allowed the simulation of the possible interaction of the flow in the main channel with the tributaries, as well as the residence time of the contaminants (radium, thorium and uranium) in the reservoirs, by taking into account the lateral mixing processes. The 1-D models developed in the Phase I Assessment were used for the Banister and Roanoke Rivers to estimate inputs for the 2-D model of Kerr Reservoir. The Phase II Assessment was based on the following assumptions: 1) a constant water-surface elevation for Lake Gaston, 2) no water being withdrawn from the City of Virginia Beach s water intake on Pea Hill Creek after containment cell failure, and 3) no tributary inflow to the reservoirs. The results of the Phase II Assessment were published in February 2012 (Baker, 2012). For the Phase II Assessment, simulations were performed for a combination of two sets of hydrologic data (2-year wet and dry periods) and two sets of partition coefficients for three contaminants (radium, thorium, and uranium). To better model the downstream impacts of uranium tailings released into the Banister River as a result of a tailings containment cell failure, the estimations of the initial radioactivity of tailings and the solid/liquid partition coefficient (K d ) for the three contaminants (radium, thorium, and uranium) were revised using information reported in a preliminary economic assessment report (the Lyntek Report) prepared specifically for the Coles Hill uranium property in 2010 by Lyntek, Inc., for Virginia Uranium, Inc. and Virginia Energy Resources, Inc. (Baker, 2012). This Phase III Assessment refines aspects of the 2-D modeling performed for Lake Gaston during the Phase II Assessment to determine the movement of radionuclides by using updated topographic data for the lake, wind shear, an assumption of the continuing operation of the City of Virginia Beach s Pea Hill Creek water intake after a tailings dam failure, Lake Gaston water-surface fluctuation, and tributary inflow into Pea Hill Creek. The Phase III Assessment uses the results of the 1-D simulations of the Banister and Roanoke Rivers and the 2-D simulations of Kerr Reservoir from the Phase I and II Assessments. Therefore, the water discharge, suspended sediment load discharge, and contaminant discharge (radium and thorium) from Kerr Reservoir (as computed during the Phase II Assessment) are used as inflow boundary conditions for Lake Gaston. Only the 2-D modeling of Lake Gaston was modified. Uranium was not modeled in the Phase III Assessment because Phase II simulation results showed that uranium concentrations do not exceed the MCL anywhere in Lake Gaston and its branches in any of the scenarios. Page 2 December 2013

9 Uranium Mining in Virginia The scope of the Phase III Assessment can be summarized as follows: 1. Improved bottom topography for Lake Gaston: The lake bottom (under water) topographies of Kerr Reservoir and Lake Gaston were not available in digital format. They were digitized from hardcopy maps with 10-foot contours. The topography above the water-surface elevation was obtained from U.S. Geological Survey (USGS) 10-meter Digital Elevation Model (DEM) data. Because of the resolution of these sources, some details, especially in shallow areas with depths less than 10 feet and at the bridge crossings in Lake Gaston, were not represented with the desired accuracy in the Phase II Assessment model. In the Phase III Assessment, the existing Lake Gaston models for these areas were refined to better represent the topography of the lake bottom. Attention was specifically focused on shallow areas, especially near the shores, where alternating wetting and drying takes place as the watersurface levels in the lake fluctuate; around the locations where side branches exchange water with the main lake channel; and under the bridge crossings in Lake Gaston. 2. Influence of wind on the diffusion and dispersion of contaminants in Lake Gaston: In the Phase II Assessment, the potential impact of the wind on the dispersion of contaminants (radium, thorium and uranium) in Lake Gaston was not taken into account. If the wind speed is sufficiently high and the direction is favorable, the wind shear stress on the water surface may generate currents faster than the flow speed due to the inflow discharge alone, especially during the times when inlet and outlet discharges are low. In the Phase III Assessment, the wind field (wind speed at 10 meters above ground and wind direction) taken from a meteorological station near Lake Gaston Dam was used over the surface of the lake. The 2-D model used for the simulations calculated the wind shear stress on the water surface from the site-specific wind field data. 3. Fluctuations of the water surface in Lake Gaston: In the Phase II Assessment, the water-surface elevation of Lake Gaston was assumed to be constant, due to the lack of reliable outflow data. The outflow discharge at the dam was computed based on the incoming discharge and a constant lake level. Neglecting the fluctuations of the lake surface elevation in the simulations may have attenuated exchanges between the main channel and the tributaries to a certain degree. In the Phase III Assessment, the water-surface elevation of Lake Gaston was allowed to fluctuate by imposing an hourly lake elevation time series at the dam, which was obtained by filtering the hourly time series data recorded by and obtained from the dam owner, Dominion Virginia Power, using a 24-hour moving average. The outflow discharge was computed to match the lake elevations at the dam. 4. Operation of the City of Virginia Beach s Pea Hill Creek Pump Station Intake: In the Phase II Assessment, it was assumed that the City of Virginia Beach s Pea Hill Creek intake and pump station would be shut down immediately after a failure of the tailings dam; therefore, water intake withdrawal was not included in the 2-D model of Lake Gaston. December 2013 Page 3

10 Phase III Assessment In the Phase III Assessment, the operation of the pump station water intake was represented in the model by incorporating the time series of recorded pump station water intake data for 2-year periods to represent the dry-year and wet-year simulations. 5. The Inflow Hydrograph for Pea Hill Creek: The Phase II simulations did not take into account any tributary inflow entering Lake Gaston. To improve the hydrodynamics of the flow in the Pea Hill Creek tributary, where the City of Virginia Beach pump station water intake is located, and to improve the computed sediment and contaminant concentrations, the inflow discharge contributed by Pea Hill Creek was considered. In the Phase III Assessment, the discharge time series of Pea Hill Creek was considered as an inflow hydrograph. Pea Hill Creek is not gaged, which means that the inflow to the creek is not recorded. The time series of the inflow to Pea Hill Creek was estimated using other streams in the vicinity of the Pea Hill Creek that have a flow gage and similar watershed characteristics. In summary, this Phase III Assessment provides additional refinement to the following: The bottom topography of Lake Gaston and within Pea Hill Creek. The impact of Lake Gaston s surface level fluctuations on the dispersion of radionuclides along Lake Gaston and within Pea Hill Creek. The impact of wind on the dispersion of radionuclides in Pea Hill Creek. The impact of the tributary inflow on the movement of radionuclides in Pea Hill Creek. The impact of the City of Virginia Beach s pump station water intake on the movement of radionuclides in Pea Hill Creek. Page 4 December 2013

11 Uranium Mining in Virginia 2. DATA COLLECTED AND GENERATED FOR 2-D SIMULATIONS 2.1 Improved Bottom Topography for Lake Gaston As discussed in Section 1.2, the Phase III Assessment utilized a higher resolution of the Lake Gaston bottom topography, particularly in the lake s shallow areas with depths less than 10 feet (3 meters) and produced a better representation of the bridge crossings along Lake Gaston and Pea Hill Creek (in the Phase II Assessment, bridge crossings were not taken into account). To obtain more resolution, the number of mesh elements for the modeling was increased from (114,540) to (169,500), which represents an approximately 48-percent increase. The smallest size was 1.4 meters, and the largest size was 411 meters. Figure 2-1 shows the computational mesh and provides a detailed view of the mesh near the junction with the Pea Hill Creek branch. As this figure shows, the mesh has been considerably refined to correctly model the bed topography near the shallow areas and the openings under the bridges. Further explanation of the improved computational mesh for better modeling of the bridge openings is provided in the next section. In refining the mesh, considerable efforts were taken to match the volume and area curves for the lake (which were obtained from Dominion Virginia Power and are provided in Figure 2-2 for water-surface elevations varying from 130 to 200 feet). An accurate representation of the lake volume and surface in the numerical mesh is important for correctly capturing the hydrodynamics of the lake and computing the concentration levels of sediment and the contaminants (radium and thorium radioactivity). The curves obtained from the new, refined numerical mesh, showing storage volume versus water-surface elevation and lake surface area versus water-surface elevation, are plotted in Figure 2-3, together with the curves from Figure 2-2. As can be seen in Figure 2-3, the curves have good correlation. The numerical mesh represents the storage volume and lake surface area of Lake Gaston with sufficient accuracy. The storage volume computed from the computational mesh closely follows the one given in Figure 2-2, although the computed volume is slightly lower. The sudden jumps in the plot of elevation versus surface area are due to the fact that for a small increase in water level, many cells suddenly may become wet and contribute to the lake surface, even though in reality some of these areas are only partially covered with water. This occurs due to the contour interval of the source maps used for generating the mesh. A higher mesh density (i.e., smaller mesh elements) and a smaller interval between the contour lines would reduce these sudden jumps. 2.2 Lake Gaston and Pea Hill Creek Bridge Crossings Six bridges that cross Lake Gaston and Pea Hill Creek tributary (Figure 2-4) were considered when preparing the numerical mesh for the 2-D model for conditions that would potentially restrict flow. These bridge crossings were not included in the Phase II Assessment. The U.S. Highway 1 bridge (Figure 2-5) provides an almost unobstructed opening over the entire width of Lake Gaston, except for five bridge piers located at about 128-foot intervals. In the 2-D model, the piers were neglected, and the entire width of the lake was assumed to be unobstructed. December 2013 Page 5

12 Phase III Assessment N Lake Gaston Main Channel Figure 2-1. Detailed View of the Computational Mesh Near the Junction with Pea Hill Creek Tributary (Mesh Colored Based on the Initial Bed Elevation) Page 6 December 2013

13 Uranium Mining in Virginia Figure 2-2. Lake Gaston Storage Volume and Surface Area as a Function of Elevation (NVGD 29)(Source: Dominion Power) Figure 2-3. Comparison of Storage Volume (left) and Lake Surface Area (right) versus Water-Surface Elevation Curves Obtained from the Computational Mesh with Those in Figure 2-2 December 2013 Page 7

14 Phase III Assessment Figure 2-4. Bridges Crossing Lake Gaston and the Pea Hill Creek Branch Figure 2-5. U.S. Highway 1 Bridge Page 8 December 2013

15 Uranium Mining in Virginia The Interstate 85 bridge (Figure 2-6) has an opening of approximately 864 feet and 12 piers, each separated by 72 feet. For this crossing, the influence of the piers was neglected, and the entire bridge opening was assumed to be unobstructed. Figure 2-6. Interstate 85 Bridge A railroad bridge (Figure 2-7) crosses Lake Gaston about 1.2 miles downstream of the Interstate 85 bridge. It has a span of about 2,854 feet and is supported by a number of bridge piers at 63-foot intervals. The influence of the piers was neglected, and the cross section was assumed to be unobstructed over the entire span of the bridge, except where it passes over a high ground dividing the river into two branches. Figure 2-7. Railroad Bridge December 2013 Page 9

16 Phase III Assessment Eaton Ferry Bridge (Figure 2-8) crosses the main channel of Lake Gaston over a width of approximately 4,596 feet. Only a 1,167-foot-long reach of the bridge, supported by piers placed at foot intervals, provides passage for the water. The remaining portion of the bridge crossing is built over an embankment that fully obstructs the flow. In the 2-D model, the influence of the piers was neglected, and the 1,167-foot-long reach was modeled as unobstructed. Figure 2-8. Eaton Ferry Bridge Three bridges cross the Pea Hill Creek tributary of Lake Gaston, as shown in Figure 2-9 (Route 667, Route 626, and River Road). The City of Virginia Beach pump station (water intake) is located on this tributary. Therefore, a particular emphasis was placed on detailed modeling of the bed topography under these three bridges. The constructional details of the three bridges were obtained from the Virginia Department of Transportation (VDOT) and the North Carolina Department of Transportation (NCDOT). The following list summarizes the references used for the details of each bridge: 1. Bridge on SR 1214 (River Road) Bridge Inspection Report by the NCDOT, Division of Highways, Bridge Management Unit 2. Route 626 Structure # 6181 Plan # Route 667 Structure # 6183 Plan # Page 10 December 2013

17 Uranium Mining in Virginia Bridge on SR 1214 (Figure 2-10) has a narrow open span of 150 feet, supported by three groups of piers (Figure 2-11), that controls the exchanges between the main channel and Pea Hill Creek. The cross section of the streambed under the bridge is shown in Figure Figure 2-13 depicts the contour lines from the computational mesh developed for the model with those provided in the source map. As this figure shows, the topography of the passage under the bridge is correctly represented. Figure 2-9. Bridges Crossing over Pea Hill Creek Branch of Lake Gaston December 2013 Page 11

18 Phase III Assessment The other two bridges (Route 626 and Route 667) are upstream of the City of Virginia Beach pump station water intake (Figure 2-14 and Figure 2-15). They both have very narrow open spans that create contractions. The Route 626 bridge span is 118 feet wide, and the Route 667 bridge span is 85 feet wide. The plans obtained from the VDOT show the bridge length, width, piers, and finished ground (see Figure 2-16). The Approximate Existing Ground elevation shown in these plans was used to model the topography in the numerical mesh by using sufficiently small elements. Figure 2-17 and Figure 2-18 compare the contour lines obtained from the computational mesh for the passages under the bridges for Route 626 and Route 667, respectively, with those given in the source map. As the figures show, the topography of the passage under the bridge is correctly represented. Figure Bridge on State Route 1214 Figure Opening Under the Bridge Viewed from the North Side (NCDOT, 2011) Page 12 December 2013

19 Uranium Mining in Virginia Figure Cross Section of the Streambed under Bridge (NCDOT 2011) Figure Comparison of Bed Elevation Contours from the Computational Mesh and the Fishing Map for the Passage under Bridge December 2013 Page 13

20 Phase III Assessment Figure Route 626 Bridge Figure Route 667 Bridge Page 14 December 2013

21 Uranium Mining in Virginia Figure Longitudinal Cross Sections of the Bridges for Route 667 and Route 626 Figure Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 626 Bridge December 2013 Page 15

22 Phase III Assessment Figure Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 667 Bridge 2.3 Pea Hill Creek Discharge Hydrographs for Wet and Dry Periods Pea Hill Creek, whose watershed is shown in Figure 2-19, flows into Lake Gaston south of Gasburg, Virginia. The total drainage area of Pea Hill Creek at the mouth and confluence with Lake Gaston is 27.1 square miles. The drainage area of Pea Hill Creek at the water supply intakes is 22.2 square miles. Pea Hill Creek is an ungaged stream so no discharge hydrographs are available for the dry (June 1, 2001, to May 31, 2003) and wet (September 1, 1996, to August 31, 1998) years. Therefore, the discharge hydrographs for the water entering Lake Gaston during the dry and wet years needed to be estimated by transferring discharges from two nearby gaged streams using the drainage area ratio method. Allen Creek, near Boydton, Virginia (Station ), is the closest gaged stream with watershed characteristics most similar to those of Pea Hill Creek. Allen Creek is a 53.4-square-mile watershed in Mecklenberg County, Virginia about 35 miles northwest of the Pea Hill Creek watershed. Discharge data are available at this station for 30-minute increments from February 1, 2001, to May 31, These data were used to estimate discharges at Pea Hill Creek for the dry year. No data for Allen Creek are available for the wet year from September 1, 1996, to August 31, Meherrin River, near Lawrenceville, Virginia (Station ), is a gaged stream in Brunswick County, Virginia. It was used to estimate discharges for the wet year from September 1, 1996, to August 31, Meherrin River is a 552-square-mile watershed about 20 miles north of the Pea Hill Creek watershed. Although the Meherrin River is much larger than Pea Hill Creek, it provided the best alternative gaged watershed for estimating the discharges needed for Pea Hill Creek due to proximity and similar meteorological conditions Page 16 December 2013

23 Uranium Mining in Virginia Figure Map of Pea Hill Creek Watershed and the Location of the Pump Station Water Intake The unit discharges for Pea Hill Creek were estimated using a drainage area ratio between Pea Hill Creek and the two nearby gaging stations. For example: Discharges for Pea Hill Creek for the dry year were estimated as 22.2/53.4 = * unit discharge for Allen Creek near Boydton. Discharges for Pea Hill Creek for the wet year were estimated as 22.2/552 = * unit discharges for Meherrin River. The USGS collected water quality data for the Pea Hill arm of Lake Gaston from 1988 to 1990 (Woodside, 1994). As part of the water quality data collection, USGS made several discharge measurements from August 1987 to October The measurements were taken on Pea Hill Creek at Route 665 near Gasburg, where the drainage area is 7.2 square miles. These discharge measurements were used to evaluate the drainage area ratio method of estimating unit discharges on Pea Hill Creek. The discharge measurements were increased by a drainage area ratio to be indicative of the discharges on Pea Hill Creek at the water intakes, where the drainage area is 22.2 square miles. The adjusted discharge measurements were plotted against the mean daily discharges for the nearby gaging stations (Allen Creek and Meherrin River) for the day of the discharge measurement at Pea Hill Creek. This adjustment allowed a comparison to the drainage area ratio method that was being used to estimate December 2013 Page 17

24 Phase III Assessment discharges at the water intakes, where the drainage area is 22.2 square miles. The relation of the discharge measurements on Pea Hill Creek to the mean daily discharges for Allen Creek is shown in Figure The relation of discharge measurements on Pea Hill Creek to mean daily discharges for Meherrin River is shown in Figure The blue trend line (y = x ) in Figure 2-20 represents the relation between the measured discharges on Pea Hill Creek made from 1987 to 1991 to the daily discharges at Allen Creek for the day of the measured discharge on Pea Hill Creek. The R 2 value is , indicating that the daily discharges at Allen Creek are highly correlated with the measured discharges on Pea Hill Creek. The red trend line (y = x) in Figure 2-20 represents the drainage area relation being used to estimate unit discharges on Pea Hill Creek. That is, the discharges at Allen Creek were multiplied by 22.2/53.4 = to obtain the corresponding discharges for Pea Hill Creek. The close agreement between the two trend lines in Figure 2-20 indicates that multiplying the drainage area ratio by the discharge amounts at Allen Creek is a reasonable way to estimate the discharges for Pea Hill Creek. The drainage area ratio method was considered a better approach than using the measured discharges at Pea Hill Creek, particularly for estimating the larger discharges, because the range of discharge measurements on Pea Hill Creek was limited. 100 Pea Hill Creek measured discharge, in cfs 10 1 y = x R² = y = x Pea Hill Creek vs. Allen Creek Power (Pea Hill Creek vs. Allen Creek) Allen Creek daily discharge, in cfs Relation based on DA Power (Relation based on DA) Figure Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on Allen Creek In Figure 2-21, the blue trend line (y = x ) represents the relation between the measured discharges on Pea Hill Creek made from 1987 to 1991 to the daily discharges at the Meherrin River for Page 18 December 2013

25 Uranium Mining in Virginia the day of the measured discharge on Pea Hill Creek. The R 2 value is , indicating that the daily discharges at the Meherrin River are highly correlated with the measured discharges on Pea Hill Creek. The red trend line in Figure 2-21 (y = x) represents the drainage area relation that is being used to estimate unit discharges on Pea Hill Creek. That is, the discharges at Meherrin River were multiplied by 22.2/552 = to obtain the corresponding discharges for Pea Hill Creek. The close agreement between the two trendlines in Figure 2-21 indicates that multiplying the drainage area ratio by the discharge amounts at the Meherrin River is a reasonable way to estimate the discharges for Pea Hill Creek. The drainage area ratio method was considered a better approach than using the discharge measurements on Pea Hill Creek, particularly for estimating the larger discharges, because the range of discharge measurements on Pea Hill Creek was limited. 100 Pea Hill Creek measured discharge, in cfs 10 1 y = x y = x R² = Meherrin Creek daily discharge, in cfs Pea Hill Creek vs. Meherrin Creek Relation based on DA Power (Pea Hill Creek vs. Meherrin Creek) Power (Relation based on DA) Figure Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on the Meherrin River The estimated Peak Hill Creek discharge hydrographs for the water entering Lake Gaston during the 2-year dry and wet periods are plotted in Figure 2-22 and Figure As these figures show, the dry year has very few high-discharge events until about October During the remaining portion of the 2-year period from October 2002 to May 2003, several high-discharge flood events occur, with the last one having a peak discharge of 3,068 cubic feet per second (cfs). The wet year has a large number of flood events, almost regularly scattered throughout the 2-year period. The hydrographs shown in Figure 2-22 and Figure 2-23 were used as the upstream boundary condition in the CCHE2D model of Lake Gaston and were imposed at the upstream end of Pea Hill Creek (after the discharge unit was converted to cubic meters per second). December 2013 Page 19

26 Phase III Assessment Figure Estimated Pea Hill Creek Discharges for the 2-Year Dry Year from January 1, 2001, to May 31, 2003 Page 20 December 2013

27 Uranium Mining in Virginia Figure Estimated Pea Hill Creek Discharges for the Wet Year from September 1, 1996, to August 31, 1998 December 2013 Page 21

28 Phase III Assessment 2.4 Pea Hill Creek Pump Station Water Intake The City of Virginia Beach (City) operates a pump station that withdraws water from the Pea Hill Creek tributary of Lake Gaston. The pump station has a capacity of 60 million gallons of water per day (MGD). There are two intake structures at the Lake Gaston pump station. Figure 2-24 shows the location of these two intakes in the Google Earth image and in the computational mesh. In the computational mesh, the intake is represented by a number of cells that are defined as sink-type cells, through which a discharge can be extracted based on a discharge-versus-time data series. Figure Location of the Water Intake in Google Earth Image and on the Mesh When the pump station began operating in 1998, the City began recording data on the average monthly amount of water pumped. In 2005, the City began recording daily data regarding the amount of water pumped by the pump station. Because daily pump data were needed for the Phase III Assessment and the wet and dry year periods used for this assessment occurred prior to 2005, daily pump station data recorded for the following 2-year periods were selected to be representative of the hydrographs used for the wet and dry year periods. Data from June 1, 2010, to May 31, 2012, for the dry year (June 1, 2001, to May 31, 2003). Data from January 2010 to December 31, 2011, for the wet year (September 1, 1996, to August 31, 1998). Page 22 December 2013

29 Uranium Mining in Virginia The pump station intake data used for the dry and wet year simulations are plotted in Figure 2-25 and Figure 2-26, respectively. It is important to note that, based on consultation with the City, the pump station data still reflects the current pumping expectations. The maximum discharge is m 3 /s (60 MGD). The data series does not contain discharge values for August 28 and 29, 2011, when the station was shut down due to a storm, or for February 13-16, 2012, when the station was closed for maintenance. Figure Pump Station Intake Discharge Used for Dry-Year Simulations Figure Pump Station Intake Discharge Used for Wet-Year Simulations December 2013 Page 23

30 Phase III Assessment 2.5 Wind Data for Dry and Wet Years There are several weather stations and airports around the Lake Gaston area. These sources were searched to find wind data for the dry and wet years. The wind data for the following two stations offered the longest periods of data (downloaded from the National Oceanic and Atmospheric Administration (NOAA) website): Mecklenburg-Brunswick Regional Airport (MBRA) from January 1997 to December Halifax County Airport (HCA) from January 1999 to December The MBRA data has a 4-month gap during the wet year, and the HCA data are only available for the dry year. Further investigation revealed that Weatherbank, Inc. had wind data for both dry and wet years from the observation station for Halifax County Airport (KRZZ), which is located at approximately N/ W, at an elevation of 256 feet above sea level (Figure 2-27). These data, which include various detailed weather parameters in addition to wind speed and wind direction, were obtained for the wet and dry years from September 1996 to August 1998 and from June 2001 to May The wind speed is given in miles per hour. The wind direction angle is measured clockwise from north, which corresponds to zero degrees. The dataset also contains records with wind direction angles equal to 370 degrees, which indicates a constantly changing wind direction. The wind roses for the dry and wet years are plotted in Figure 2-28 (showing the frequency of windspeed and direction). During the dryyear, the dominant wind directions are North (N) and West-Southwest (WSW). During the wet year, the dominant wind directions are North (N) and South (S), with some wind energy also at all angles from the western half of the wind rose. To use the wind data obtained from Weatherbank, Inc. in the CCHE2D model, the data was converted into a time series of wind speed components in the horizontal plane in units of meters per second. Data records with 370 degrees were assumed to take on the same direction as the previous record with a valid direction angle. These two time series of the horizontal components constituted the input data for wind for the CCHE2D model. Page 24 December 2013

31 Uranium Mining in Virginia Halifax County Airport - KRZZ Figure Location of the Observation Station Halifax County Airport (KRZZ) (Courtesy of Google Maps) ws: wind speed Figure Wind Roses for Dry (left) and Wet (right) Two-Year Periods December 2013 Page 25

32 Phase III Assessment 2.6 Lake Water Surface Data Lake Gaston receives water released from Kerr Reservoir located just upstream of Lake Gaston. Gaston Dam was built in 1963 by the Virginia Power and Electric Company (now Dominion Virginia Power) as a single-use reservoir to generate hydropower. The surface area of Lake Gaston is approximately equal to 40 percent of Kerr Reservoir; therefore, Lake Gaston does not provide storage capability. Dominion Virginia Power synchronizes the operation of its turbines at Gaston Dam based on water releases from Kerr Dam. When Kerr generates hydropower, the water released into Lake Gaston is allowed to flow through and is released at Gaston Dam, which also generates power 3 (see also Wishnant et al. 2009). Thus, since Gaston Dam is operated as run-of-kerr, the timing and the discharges from Gaston Dam generally mirror those from Kerr Dam upstream. There is very little re-regulation (storage and subsequent release) of Kerr Dam releases at Gaston Dam. Dominion is required by the Federal Energy Regulatory Commission (FERC) to maintain the lake elevation fluctuation at Lake Gaston to within approximately 1 foot at all times, except during flood events and spawning season, when the limits are 4 feet and 2 feet, respectively. Hourly Lake Gaston water-surface elevation data for the dry (June 1, 2001, to May 31, 2003) and wet (September 1, 1996, to August 31, 1998) years were obtained from Dominion Resources. These data are plotted in Figure 2-29 and Figure 2-30, respectively. Before using the data for the 2-D flow simulations, the water balance for Lake Gaston was checked to see if it was consistent with the net inflow and outflow, which are the daily inflow discharges from Kerr Reservoir, the estimated inflow discharge from Pea Hill Creek, and the pumping outflow from the water intake. If the prism storage during wave propagation is neglected, the simple hydrologic water balance in Lake Gaston is given by the following ordinary differential equation. ds dt = ΣI ΣO 1 where Sis the volume of water stored in the lake, t is time, and ΣI and ΣO are the total incoming and total outgoing discharges. Incoming discharges are the releases from Kerr Reservoir, Q KR, and the contribution by Pea Hill Creek, Q PHC. ΣI = Q KR + Q PHC Page 26 December 2013

33 Uranium Mining in Virginia Figure Hourly Water-Surface Elevation Measurements at Lake Gaston for the Dry Years Figure Hourly Water-Surface Elevation Measurements at Lake Gaston for the Wet Years December 2013 Page 27

34 Phase III Assessment The outgoing discharges are the discharge released through the turbines for hydroelectric energy production, Q out, and the releases from spillway or bottom outlets (Q SB ) during floods or for other reasons: ΣO = Q out + Q SB 3 Equation 1 can then be rewritten as: ds dt = Q KR + Q PHC Q out Q SB 4 During the dry and wet years of interest, no releases were made from the spillway or the bottom outlet (Q SB = 0). Therefore, the hourly outflow from the dam can be computed as: Q out = Q KR + Q PHC ds dt 5 In discretized form, the above equation becomes: (Q out ) n = (Q KR ) n + (Q PHC ) n Sn S n 1 6 t In this equation, the superscript n refers to the current time step and (n 1) to the previous time step. Since simulations use hourly data, t is in hours. The storage values S n and S n 1 can be obtained from the Lake Gaston storage-versus-elevation curve given in Figure 2-2, using the water-surface elevations Z n and Z n 1, which are taken from hourly water-surface elevation data. Figure 2-31 shows the wet year hourly outflow discharge at Gaston Dam, estimated using Equation 6. As shown in Figure 2-31, outflow discharges varying from m 3 /s to 3000 m 3 /s were obtained when recorded inflow discharges and lake elevation data were used. Negative discharges are not possible. Reasons for apparent negative discharges could be the noise in the hourly water-surface elevation data, the neglect of other incoming discharges from other tributaries, and/or the neglect of the prism storage. It was, therefore, decided to smooth the observed water-surface elevation at Gaston Dam using a 24-hour moving average method. Filtered water-surface elevation data for the wet and dry years are plotted in Figure 2-32 and Figure 2-33, respectively. Page 28 December 2013

35 Uranium Mining in Virginia Estimated hourly outflow discharge 3000 hourly outflow discharge 2000 Flow Discharge (m 3 /s) Day Figure Wet Year Hourly Outflow Discharge at Gaston Dam, Estimated Using Equation 6 Figure Dry Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24-hour Moving Average December 2013 Page 29

36 Phase III Assessment Figure Wet Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24-hour Moving Average The filtered (24-hour moving average) hourly water-surface elevation time series were used to estimate the outflow discharges using Equation 6. The results for the dry year, with and without pumping, are plotted in Figure The results for the wet year, with and without pumping, are plotted in Figure As these figures indicate, the negative discharges still exist but have been significantly reduced. It is also interesting to note that the City of Virginia Beach pumping discharge is too small to significantly affect the outflow discharge values. Given that no major streams contribute to Lake Gaston, it is likely that the negative discharges are the result of noise and/or errors in the water-surface elevation data. Despite the observations regarding some level of negative discharge, it was decided to use the filtered water-surface elevations in Figure 2-32 and Figure 2-33 as the boundary conditions for the dry- and wet-year simulations, respectively. The intakes for the Gaston Dam turbines are located on the right side of the dam, facing downstream (Figure 2-36). To represent the flow hydrodynamics near the dam as realistically as possible, the outflow discharge boundary condition was placed at the location of the intakes for the turbines. Page 30 December 2013

37 Uranium Mining in Virginia Figure Gaston Dam Dry-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data Figure Gaston Dam Wet-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data December 2013 Page 31

38 Phase III Assessment Figure Gaston Dam and its Appurtenances Page 32 December 2013

39 Uranium Mining in Virginia 3. SETUP OF 2-D SIMULATIONS AND SIMULATION SCENARIOS As discussed in Section 1, some aspects of the 2-D modeling developed for Lake Gaston in the Phase II Assessment were further refined in the Phase III Assessment. This work was performed to determine the movement of radionuclides as a result of updated topography of the lake, wind shear, continuing operation of the water intake after a tailings dam failure, Lake Gaston s water-surface fluctuation, and tributary inflow. Relevant data used and generated in Phase II Assessment were also used in the Phase III Assessment. These data are summarized as follows (taken from Baker 2012): Uranium tailings properties (Table 3-1). Tailings dam failure conditions. 1-D modeling results of hydrodynamics, sediment transport and contaminant transport and fate in Banister River and Roanoke River. Particle size distribution of sediment released from Kerr Reservoir into Lake Gaston. (Simulations consider only the sediment load from Kerr Reservoir release; sediment loads from tributaries are not considered in the Phase III Assessment.) Boundary conditions at the upstream boundary of Lake Gaston (time series of flow discharge, sediment discharge and contaminant concentrations released from Kerr Reservoir), which are provided in Figures 3-1 through 3-6. Table 3-1. Properties of Tailings Used in Phase II Assessment and the Partition Coefficients Assumed for the Two Scenarios December 2013 Page 33

40 Phase III Assessment Figure 3-1. Dry Year Flow Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-2. Wet Year Flow Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Page 34 December 2013

41 Uranium Mining in Virginia Figure 3-3. Dry-Year Sediment Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model Figure 3-4. Wet-Year Sediment Discharges Entering Lake Gaston at the Upstream Boundary, as Computed in the 2-D Simulation of Kerr Reservoir Using the CCHE2D Model December 2013 Page 35

42 Phase III Assessment SCENARIO 1 (S1) K-Outflow_Ra_WY_S1 SCENARIO 2 (S2) K-Outflow_Ra_WY_S2 1.00E E+02 WET PERIOD (WY) Radioactivity Concentration pci/l 1.00E E E E-02 Radioactivity Concentration pci/l 1.00E E E E E E Day Day K-Outflow_Ra_DY_S1 K-Outflow_Ra_DY_S2 1.00E E+02 DRY PERIOD (DY) Radioactivity Concentration pci/l 1.00E E E E-02 Radioactivity Concentration pci/l 1.00E E E E E E Day Day Figure 3-5. Radium Radioactivity Concentration Entering Lake Gaston Page 36 December 2013

43 Uranium Mining in Virginia SCENARIO 1 (S1) K-Outflow_Th_WY_S1 SCENARIO 2 (S2) K-Outflow_Th_WY_S2 1.00E E+02 WET PERIOD (WY) Radioactivity Concentration pci/l 1.00E E E E-02 Radioactivity Concentration pci/l 1.00E E E E E E Day Day K-Outflow_Th_DY_S1 K-Outflow_Th_DY_S2 1.00E E+02 DRY PERIOD (DY) Radioactivity Concentration pci/l 1.00E E E E-02 Radioactivity Concentration pci/l 1.00E E E E E E Day Day Figure 3-6. Thorium Radioactivity Concentration Entering Lake Gaston December 2013 Page 37