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

FINAL December 2013

December 2013 Page i

Uranium Mining in Virginia

Table of Contents 1. INTRODUCTION ................................................................................................................... 1

2. DATA COLLECTED AND GENERATED FOR 2-D SIMULATIONS ............................... 5

2.1 Improved Bottom Topography for Lake Gaston .......................................................... 5

2.2 Lake Gaston and Pea Hill Creek Bridge Crossings ...................................................... 5

2.3 Pea Hill Creek Discharge Hydrographs for Wet and Dry Periods ............................. 16

2.4 Pea Hill Creek Pump Station Water Intake ................................................................ 22

2.5 Wind Data for Dry and Wet Years ............................................................................. 24

2.6 Lake Water Surface Data ............................................................................................ 26

3. SETUP OF 2-D SIMULATIONS AND SIMULATION SCENARIOS ............................... 33

2.1 Lake Water Surface Data ............................................................................................ 38

4. SIMULATION RESULTS .................................................................................................... 40

5. CONCLUSIONS ................................................................................................................... 43

6. REFERENCES ...................................................................................................................... 44

APPENDIX A. SIMULATION RESULTS .................................................................................. 45

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 2-2 ....... 7

Figure 2-4. Bridges Crossing Lake Gaston and the Pea Hill Creek Branch ................................................. 8

Figure 2-5. U.S. Highway 1 Bridge .............................................................................................................. 8

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Phase III Assessment

Figure 2-6. Interstate 85 Bridge .................................................................................................................... 9

Figure 2-7. Railroad Bridge .......................................................................................................................... 9

Figure 2-8. Eaton Ferry Bridge ................................................................................................................... 10

Figure 2-9. Bridges Crossing over Pea Hill Creek Branch of Lake Gaston................................................ 11

Figure 2-10. Bridge 650061 on State Route 1214 ...................................................................................... 12

Figure 2-11. Opening Under the Bridge 650061 Viewed from the North Side (NCDOT, 2011) ............... 12

Figure 2-12. Cross Section of the Streambed under Bridge 650061 (NCDOT 2011) ................................ 13

Figure 2-13. Comparison of Bed Elevation Contours from the Computational Mesh and the Fishing Map for the Passage under Bridge 650061 .................................................................................... 13

Figure 2-14. Route 626 Bridge ................................................................................................................... 14

Figure 2-15. Route 667 Bridge ................................................................................................................... 14

Figure 2-16. Longitudinal Cross Sections of the Bridges for Route 667 and Route 626 ............................ 15

Figure 2-17. Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 626 Bridge .................................................................................. 15

Figure 2-18. Comparison of Elevation Contours from the Computational Mesh and the Source Map for the Passage under the Route 667 Bridge .................................................................................. 16

Figure 2-19. Map of Pea Hill Creek Watershed and the Location of the Pump Station Water Intake ....... 17

Figure 2-20. Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on Allen Creek .............................................................................................................................................. 18

Figure 2-21. Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on the Meherrin River ............................................................................................................................... 19

Figure 2-22. Estimated Pea Hill Creek Discharges for the 2-Year Dry Year from January 1, 2001, to May 31, 2003 ................................................................................................................................. 20

Figure 2-23. Estimated Pea Hill Creek Discharges for the Wet Year from September 1, 1996, to August 31, 1998 ............................................................................................................................. 21

Figure 2-24. Location of the Water Intake in Google Earth Image and on the Mesh ................................. 22

Figure 2-25. Pump Station Intake Discharge Used for Dry-Year Simulations ........................................... 23

Figure 2-26. Pump Station Intake Discharge Used for Wet-Year Simulations .......................................... 23

Figure 2-27. Location of the Observation Station Halifax County Airport (KRZZ) (Courtesy of Google Maps) ................................................................................................................................ 25

Figure 2-28. Wind Roses for Dry (left) and Wet (right) Two-Year Periods ............................................... 25

Figure 2-29. Hourly Water-Surface Elevation Measurements at Lake Gaston for the Dry Years .............. 27

Figure 2-30. Hourly Water-Surface Elevation Measurements at Lake Gaston for the Wet Years ............. 27

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Figure 2-31. Wet Year Hourly Outflow Discharge at Gaston Dam, Estimated Using Equation 6 ............. 29

Figure 2-32. Dry Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24-hour Moving Average .................................................................................................................... 29

Figure 2-33. Wet Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24-hour Moving Average .................................................................................................................... 30

Figure 2-34. Gaston Dam Dry-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data ............................................................................... 31

Figure 2-35. Gaston Dam Wet-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data ............................................................................... 31

Figure 2-36. Gaston Dam and its Appurtenances ....................................................................................... 32

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 .......................... 34

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 ........................ 34

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 .......................... 35

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 .......................... 35

Figure 3-5. Radium Radioactivity Concentration Entering Lake Gaston ................................................... 36

Figure 3-6. Thorium Radioactivity Concentration Entering Lake Gaston .................................................. 37

Figure 4-1. Water Column Contaminant Concentrations at the City of Virginia Beach Pump Station Water Intake on Pea Hill Creek ..................................................................................................... 41

Figure A-1 Locations Selected for Evaluation in Lake Gaston and its Branches (Tributaries) .................. 45

Figure A-2 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main01 ........................................................................................................................ 48






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Figure A-10 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Outflow ....................................................................................................................... 56

Figure A-11 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-A1 .................................................................................................................. 57

Figure A-12 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-A3 .................................................................................................................. 58

Figure A-13 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-B1................................................................................................................... 59

Figure A-14 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-C1................................................................................................................... 60

Figure A-15 Bed Sediment Contaminant Mass in Branch A of Lake Gaston ............................................ 61

Figure A-16 Bed Sediment Contaminant Mass in Branch B of Lake Gaston ............................................. 62

Figure A-17 Bed Sediment Contaminant Mass in Branch C of Lake Gaston ............................................. 63

Figure A-18 Bed Sediment Contaminant Mass in The Entire Reservoir of Lake Gaston .......................... 64

Figure A-19 Bed Sediment Contaminant Mass in the Main Channel of Lake Gaston ............................... 65

Table of Tables Table 3-1. Properties of Tailings Used in Phase II Assessment and the Partition Coefficients

Assumed for the Two Scenarios .................................................................................................... 33

Table 3-2. List of 2-D Simulations Performed During the Phase III Assessment ...................................... 39

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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.

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

Coles Hill

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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 (Kd) 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.



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 water- surface 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.



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.



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 166×690 (114,540) to 234×725 (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.



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)

Lake Gaston Main Channel

N



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



Figure 2-4. Bridges Crossing Lake Gaston and the Pea Hill Creek Branch

Figure 2-5. U.S. Highway 1 Bridge



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



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 183.5-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 650061 on SR 1214 (River Road) – Bridge Inspection Report by the NCDOT, Division of Highways, Bridge Management Unit

2. Route 626 – Structure # 6181 – Plan #179-06

3. Route 667 – Structure # 6183 – Plan #179-07



Bridge 650061 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 2-12. 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



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 2-10. Bridge 650061 on State Route 1214

Figure 2-11. Opening Under the Bridge 650061 Viewed from the North Side (NCDOT, 2011)



Figure 2-12. Cross Section of the Streambed under Bridge 650061 (NCDOT 2011)

Figure 2-13. Comparison of Bed Elevation Contours from the Computational Mesh and the Fishing Map for the

Passage under Bridge 650061



Figure 2-14. Route 626 Bridge

Figure 2-15. Route 667 Bridge



Figure 2-16. Longitudinal Cross Sections of the Bridges for Route 667 and Route 626

Figure 2-17. Comparison of Elevation Contours from the Computational Mesh and the Source Map for the

Passage under the Route 626 Bridge



Figure 2-18. 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 02079640), 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, 20031. 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, 1998.

Meherrin River, near Lawrenceville, Virginia (Station 02051500), 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, 19982. 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.

1 http://ida.water.usgs.gov/ida/available_records.cfm?sn=02079640 2 http://ida.water.usgs.gov/ida/available_records.cfm?sn=02051500



Figure 2-19. 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 = 0.4157 * unit discharge for Allen Creek near Boydton.

• Discharges for Pea Hill Creek for the wet year were estimated as 22.2/552 = 0.0406 * 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 1991. 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



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 2-20. The relation of discharge measurements on Pea Hill Creek to mean daily discharges for Meherrin River is shown in Figure 2-21.

The blue trend line (𝑦 = 0.7937𝑥0.7494) 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 R2 value is 0.8623, indicating that the daily discharges at Allen Creek are highly correlated with the measured discharges on Pea Hill Creek. The red trend line (𝑦 = 0.4157𝑥) 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 = 0.4157 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.

Figure 2-20. Comparison of Measured Discharges on Pea Hill Creek to Daily Discharges on Allen Creek

In Figure 2-21, the blue trend line (𝑦 = 0.0239𝑥1.0175) 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

y = 0.7937x0.7494 R² = 0.8623

y = 0.4157x

0.1

1

10

100

0.1 1 10 100 1000

Pea

Hill

Cree

k m

easu

red

disc

harg

e , i

n cf

s

Allen Creek daily discharge , in cfs Pea Hill Creek vs. Allen Creek Relation based on DAPower (Pea Hill Creek vs. Allen Creek) Power (Relation based on DA)



the day of the measured discharge on Pea Hill Creek. The R2 value is 0.8475, 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 = 0.0406𝑥) 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 = 0.0406 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.

Figure 2-21. 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 2-23. As these figures show, the dry year has very few high-discharge events until about October 2002. 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).

y = 0.0239x1.0175 R² = 0.8475

y = 0.0406x

0.1

1

10

100

10 100 1000 10000

Pea

Hill

Cree

k m

easu

red

disc

harg

e , i

n cf

s

Meherrin Creek daily discharge , in cfs Pea Hill Creek vs. Meherrin Creek Relation based on DAPower (Pea Hill Creek vs. Meherrin Creek) Power (Relation based on DA)



Figure 2-22. Estimated Pea Hill Creek Discharges for the 2-Year Dry Year from January 1, 2001, to May 31, 2003



Figure 2-23. Estimated Pea Hill Creek Discharges for the Wet Year from September 1, 1996, to August 31, 1998



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 2-24. 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).



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 2.243 m3/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 2-25. Pump Station Intake Discharge Used for Dry-Year Simulations

Figure 2-26. Pump Station Intake Discharge Used for Wet-Year Simulations



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 2005.

• Halifax County Airport (HCA) from January 1999 to December 2005.

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 36.4395N/77.7093W, 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 2003.

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.



Figure 2-27. Location of the Observation Station Halifax County Airport (KRZZ) (Courtesy of Google Maps)

Figure 2-28. Wind Roses for Dry (left) and Wet (right) Two-Year Periods

Halifax County Airport - KRZZ

ws: wind speed



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 power3 (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.

𝑑𝑆𝑑𝑡

= 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, 𝑄𝐾𝑅 , and the contribution by Pea Hill Creek, 𝑄𝑃𝐻𝐶 .

Σ𝐼 = 𝑄𝐾𝑅 + 𝑄𝑃𝐻𝐶 2

3 http://www.virginiaplaces.org/watersheds/lakegaston.html



Figure 2-29. Hourly Water-Surface Elevation Measurements at Lake Gaston for the Dry Years

Figure 2-30. Hourly Water-Surface Elevation Measurements at Lake Gaston for the Wet Years



The outgoing discharges are the discharge released through the turbines for hydroelectric energy production, 𝑄𝑜𝑢𝑡 , and the releases from spillway or bottom outlets (𝑄𝑆𝐵) during floods or for other reasons:

Σ𝑂 = 𝑄𝑜𝑢𝑡 + 𝑄𝑆𝐵 3

Equation 1 can then be rewritten as:

𝑑𝑆𝑑𝑡

= 𝑄𝐾𝑅 + 𝑄𝑃𝐻𝐶 − 𝑄𝑜𝑢𝑡 − 𝑄𝑆𝐵 4

During the dry and wet years of interest, no releases were made from the spillway or the bottom outlet (𝑄𝑆𝐵 = 0). Therefore, the hourly outflow from the dam can be computed as:

𝑄𝑜𝑢𝑡 = 𝑄𝐾𝑅 + 𝑄𝑃𝐻𝐶 −𝑑𝑆𝑑𝑡

5

In discretized form, the above equation becomes:

(𝑄𝑜𝑢𝑡)𝑛 = (𝑄𝐾𝑅)𝑛 + (𝑄𝑃𝐻𝐶)𝑛 −𝑆𝑛 − 𝑆𝑛−1

∆𝑡

6

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 𝑆𝑛 and 𝑆𝑛−1 can be obtained from the Lake Gaston storage-versus-elevation curve given in Figure 2-2, using the water-surface elevations 𝑍𝑛 and 𝑍𝑛−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 -3000 m3/s to 3000 m3/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.



Figure 2-31. Wet Year Hourly Outflow Discharge at Gaston Dam, Estimated Using Equation 6

Figure 2-32. Dry Year Hourly Water-Surface Elevation Data in Lake Gaston, Filtered Using a 24-hour Moving

Average

-3000

-2000

-1000

0

1000

2000

3000

0 100 200 300 400 500 600 700

Flow

Disc

harg

e (m

3 /s)

Day

Estimated hourly outflow discharge

hourly outflow discharge



Figure 2-33. 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 2-34. The results for the wet year, with and without pumping, are plotted in Figure 2-35. 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.



Figure 2-34. Gaston Dam Dry-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered Water-Surface Elevation Data

Figure 2-35. Gaston Dam Wet-Year Outflow Discharge (With and Without Pumping) Computed Using Filtered

Water-Surface Elevation Data



Figure 2-36. Gaston Dam and its Appurtenances



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



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

WET

PER

IOD

(WY)

SCENARIO 1 (S1)DR

Y PE

RIO

D (D

Y)SCENARIO 2 (S2)

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Ra_WY_S1

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Ra_WY_S2

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Ra_DY_S1

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Ra_DY_S2



Figure 3-6. Thorium Radioactivity Concentration Entering Lake Gaston

WET

PER

IOD

(WY)

SCENARIO 1 (S1)DR

Y PE

RIO

D (D

Y)SCENARIO 2 (S2)

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Th_WY_S1

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Th_WY_S2

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Th_DY_S1

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

0 100 200 300 400 500 600 700 800

Radi

oact

ivity

Con

cent

ratio

n pC

i/L

Day

K-Outflow_Th_DY_S2



These data from the Phase II Assessment and the new data collected and generated as discussed in Section 2 were used to set up the 2-D model and run simulation scenarios.

2.1 Lake Water Surface Data

Scenarios for the Phase III Assessment included combinations of several parameters, as listed below:

Hydrology:

• Wet Year - September 1, 1996, to August 31, 1998

• Dry Year - June 1, 2001, to May 31, 2003

Partition Coefficient:

• Low: Radium-226=10 mL/g; Thorium-230=2,500 mL/g; Uranium=50ml/g

• High: Radium-226=3,000 mL/g; Thorium-230=10,000 mL/g; Uranium=1,000 ml/g

Pump Station:

• Operational

• Shut down

As described in Section 1, the effects of wind, water-surface fluctuations in Lake Gaston, bridge crossings, improved topography, and inflow from Pea Hill Creek were also incorporated into the simulations. The scenarios simulated in the Phase III Assessment are listed in Table 3-2.

Unsteady flow hydrodynamics and sediment transport simulations were performed for the wet and dry years. For each period, two simulations were performed. One is without the City of Virginia Beach pump station operating and the other is with the City of Virginia Beach pump station operating. The four unsteady flow hydrodynamic simulations are labeled as 1a, 1b, 5a, and 5b.

A corresponding unsteady, non-equilibrium and non-uniform sediment transport simulation was performed for each of the four hydrodynamics simulations. The four sediment transport simulations are labeled as 2a, 2b, 6a, and 6b in Table 3-2.

Two contaminant transport and fate simulations, corresponding to two sets of 𝐾𝑑 values, were carried out for each of the four hydrodynamic and sediment transport simulation pairs. Therefore, there are eight contaminant transport and fate simulations for radium and thorium, labeled 3a, 3b, 4a, and 4b for the wet year and 7a, 7b, 8a, and 8b for the dry year as shown in Table 3-2.



Table 3-2. List of 2-D Simulations Performed During the Phase III Assessment

No Name Simulation U/S Bnd D/S Bnd Period Kd Flow Sediment Ra-226 Th-230

1a G-WY-Pumping FlowUnsteady flow with water intake pumping

Outflow from Kerr Dam

Water surface elevation at Gaston Dam

Wet Year N/A Yes No N/A N/A

1bG-WY-NoPumping Flow

Unsteady flow without water intake pumping



Wet Year N/A Yes No N/A N/A

2aG-WY-Pumping Sediment

Suspended sediment transport with water intake pumping


Ouflow from Gaston Dam

Wet Year N/A No Yes N/A N/A

2bG-WY-NoPumping Sediment

Suspended sediment transport without water intake pumping



Wet Year N/A No Yes N/A N/A

3a G-WY-Pumping--S1Chemica l with water intake pumping



Wet Year Scenario 1 No No Yes Yes

3b G-WY-Pumping-S2Chemica l with water intake pumping




4a G-WY-NoPumping-S1Chemica l without water intake pumping




4b G-WY-NoPumping-S2Chemica l without water intake pumping




5a G-DY-Pumping FlowUnsteady flow with water intake pumping



Dry Year N/A Yes No N/A N/A

5bG-DY-NoPumping Flow

Unsteady flow without water intake pumping



Dry Year N/A Yes No N/A N/A

6aG-DY-Pumping Sediment

Suspended sediment transport with water intake pumping



Dry Year N/A No Yes N/A N/A

6bG-DY-NoPumping Sediment

Suspended sediment transport without water intake pumping



Dry Year N/A No Yes N/A N/A

7a G-DY-Pumping--S1Chemica l with water intake pumping



Dry Year Scenario 1 No No Yes Yes

7b G-DY-Pumping-S2Chemica l with water intake pumping




8a G-DY-NoPumping-S1Chemica l without water intake pumping




8b G-DY-NoPumping-S2Chemica l without water intake pumping




Scenario Elements Simulation and Results



4. SIMULATION RESULTS

As described in Section 3, eight simulations were performed for each contaminant (radium and thorium) by combining two hydrographs (wet year and dry year), two sets of partition coefficients (Scenario 1 and Scenario 2), and two City of Virginia Beach Pea Hill Creek pumping options (operational and shut down). Therefore, 16 simulation runs were carried out for the Phase III Assessment.

A number of locations in Lake Gaston were selected, and the results at these locations were plotted as time series graphs for each contaminant (radium radioactivity and total radioactivity from radium and thorium) to show the estimated range of water column and sediment radioactivity concentrations at each location for all scenarios and hydrologic conditions. These time series graphs are provided in Appendix A.

Figure 4-1 shows the results obtained for the sum concentrations of dissolved and suspended radium and total radioactivity (due to radium and thorium) in the water column at the location of the pump station for the City of Virginia Beach’s water intake. The results shown in these figures indicate each hydrological condition (dry and wet year) and the operational condition of the pump station (operational or shut down) by colored bands. The width of the band represents the potential range of contaminant levels due to the two sets of partition coefficients used in this study. The top of the band represents Scenario 1 (low partition coefficient/high solubility), and the lower limit of the band represents Scenario 2 (high partition coefficient/low solubility).

The dashed red lines represent the maximum contaminant levels (MCLs) for the associated parameter, as established by the Safe Drinking Water Act, which consist of the following:

• Radium 226 and Radium 228 (combined), 5 pCi/L

• Gross Alpha Emitters, 15 pCi/L (used for total radium and thorium radioactivity)

As Figure 4-1. Water Column Contaminant Concentrations at the City of Virginia Beach Pump Station Water Intake on Pea Hill Creekshows, the radium concentrations near Pea Hill Creek in Lake Gaston for the dry year exceed the MCL value for approximately 400 days, when the pump station is not operational. The duration of the radium concentrations above the MCL increases when the pump station is in operation (approximately 500 days). During wet-year conditions, however, the radium concentrations do not exceed the MCL.

Total radioactivity concentrations under dry-year conditions, when the pump station is not in operation, remain at or slightly above Gross Alpha Activity MCL for approximately 300 days following the containment cell failure and then decline below 15 pCi/L for the rest of the simulation. However, the total radioactivity concentrations exceed the Gross Alpha Activity MCL for about 400 days when the pump station is operational. The total radioactivity concentrations under wet-year conditions do not exceed the MCL.



Figure 4-1. Water Column Contaminant Concentrations at the City of Virginia Beach Pump Station Water

Intake on Pea Hill Creek

Dry Year No Pumping Dry Year With Pumping Wet Year No Pumping Wet Year With Pumping Radium MCL

Dry Year No Pumping Dry Year With Pumping Wet Year No Pumping Wet Year With Pumping Gross Alpha Activity MCL



The results of the simulations for Lake Gaston led to the following general observations:

• The maximum radium radioactivity concentration in the main body of Lake Gaston is not influenced by the City’s pump station water intake in Pea Hill Creek.

• The variation of the water-surface elevation in Lake Gaston generated a strong hydrodynamic interaction between the main body of Lake Gaston and its tributaries. This is especially true for Pea Hill Creek, which is connected to the main channel by a narrow opening under Bridge 650061. This narrow opening leads to a phase difference between the water-surface elevations in the main channel and those in Pea Hill Creek, which favors the interaction and leads to relatively large velocities in the narrow opening.

• The strong hydrodynamic interaction between the main body of Lake Gaston and Pea Hill Creek also increases the rate of sediment and contaminant flow into Pea Hill Creek. When the City’s pump station is operational during dry-year conditions, total radioactivity concentrations exceed the MCL (15 pCi/L) for approximately 500 days at the pump station for the low partition coefficient values (more soluble radium and thorium) compared to when the pump station is not operating.

• The radium radioactivity concentration at the Pea Hill Creek pump station water intake rises above the MCL (5 pCi/L) for all dry-year simulations, regardless of whether the pump station is in operation or not. Total radioactivity concentration at the same location rises above the MCL (15 pCi/L) for dry-year simulations with the lower partition coefficient scenario, regardless of the pumping at the intake. In case of simulations with the higher partition coefficient scenario, the total radioactivity concentration approaches but does not exceed the MCL level. The maximum radium and total radioactivity concentrations are higher with pumping than without pumping.

• The radium and total radioactivity concentration levels at the pump station water intake remain below the MCL for wet-year simulations, regardless of the combination of the partition coefficient scenario and the pumping option.

• The period of exceedance during which the contaminant remains above the MCL value in the water column is longer for dry-year simulations than for wet-year simulations. For a given hydrologic period, the period of exceedance of contaminant concentration in the water column is longer for the lower partition coefficient scenario than for the higher partition coefficient scenario.



5. CONCLUSIONS

The following conclusions were drawn, based on the results of the simulations completed as part of the Phase III Assessment:

• The partial release of the contents of only one containment cell at the proposed mining site at Coles Hill is likely to result in contaminant concentrations above allowable Safe Drinking Water Act levels (Maximum Concentration Levels, or MCLs) in Lake Gaston and at the pump station intake for the City of Virginia Beach.

• The variation in water-surface elevations in Gaston Lake has pronounced impact on the level of radioactivity concentrations in the water column in Pea Hill Creek than when assumed constant as reported in the Phase II Assessment. In dry-year conditions, the maximum radium radioactivity concentrations in the water column at the City of Virginia Beach pump station intake remain above the MCL value for 1.2 to 1.5 years, depending on whether the pump station is shut down or operational.

• Although the operation of the City of Virginia Beach pump station does not influence the maximum radioactivity concentrations in the main body of Lake Gaston, continued water intake at the pump station could increase radioactivity concentrations above the MCL in Pea Hill Creek, particularly at the location of the pump station intake.

• The majority of the contaminated sediments settle in the main body of Lake Gaston for all the scenarios. Therefore, the deposition of contaminated sediment in tributaries is quite small.

Based on the Phase III Assessment projections, the City of Virginia Beach may have to cease pumping water from Lake Gaston for up to 1.5 years. The inability to withdraw water from Lake Gaston could result in severe water shortages for the cities of Virginia Beach, Chesapeake, and Norfolk.



6. REFERENCES

Baker (Michael Baker Jr., Inc.), 2011, Phase I Assessment: Potential Impacts of Uranium Mining in Virginia on Drinking Water Sources, Final Report, February, 2011.

Baker (Michael Baker Jr., Inc.), 2012, Phase II Assessment: Potential Impacts of Uranium Mining in Virginia on Drinking Water Sources, Final Report, February, 2012.

NCDOT, 2011, Bridge Inspection Report, NC Department of Transportation, Division of Highways, Bridge Management Unit, Bridge Number 650061, Northampton County, 2/9/2011.

Richard B. Whisnant, Gregory W. Characklis, Martin W. Doyle, Victor B. Flatt, Jordan D. Kern, "Operating Policies and Administrative Discretion at the John H. Kerr Project - A Component of a Study of Operations at the John H. Kerr Project pursuant to Section 216 of Public Law 91-611," October 31, 2009, p.5, http://sogweb.sog.unc.edu/Water/images/a/ae/FinalReportKerr216DiscretionaryAnalysis.pdf

Rico, M., Benito, G., Díez-Herrero, A. 2008. Floods from Tailings Dam Failures. Journal of Hazardous Materials, 154, 79–87.

Woodside, M.D., 1994, Land Use in, and Water Quality of, the Pea Hill Arm of Lake Gaston, Virginia and North Carolina, 1988-90: U.S. Geological Survey Water-Resources Investigations Report 94-4140, 54 p.

http://sogweb.sog.unc.edu/Water/images/a/ae/FinalReportKerr216DiscretionaryAnalysis.pdf



APPENDIX A. SIMULATION RESULTS

To present the results, a number of locations were selected in Lake Gaston. Results at these locations were plotted as time series graphs for each contaminant (radium radioactivity and total radioactivity from radium and thorium) to show the range of radioactivity concentrations simulated in the water column and bed sediments at each location for all scenarios and hydrologic conditions. It is important to note that the values extracted for each selected location constitute the spatial average of a number of computational cells around the selected location, rather than the results for a single point. Lake Gaston was partitioned into a main channel (G-Main) and three principal tributaries (G-Branch-A, G-Branch-B, and G-Branch-C), as shown in Figure A-1. This figure also shows the 13 locations selected along the main channel (G-Main) and the principal tributaries. For the Phase III Assessment, 16 simulations were performed for two hydrographs (wet year and dry year), two sets of partition coefficients (Scenario 1 and Scenario 2), and two City of Virginia Beach Pea Hill Creek pumping options (operational and shut down) for radium and thorium. For each simulation, results are presented at each selected location for radium and total radioactivity (radium radioactivity and total radioactivity from radium and thorium) where the dashed lines represent the maximum contaminant levels (MCL) under the Safe Drinking Water Act.

Figure A-1 Locations Selected for Evaluation in Lake Gaston and its Branches (Tributaries)



For each simulation, the time series of contaminant concentrations (radium and total radioactivity from radium and thorium) with a sampling interval of 6 hours (4 values per day) are presented at these selected monitoring locations. Area-integrated instantaneous (every 6 hours) and cumulative concentrations of radium, and thorium are plotted for the main channel and selected branches of the reservoir. In addition, the time series of the instantaneous (every 6 hours) concentrations of the two contaminants are also plotted for selected locations.



A.1 Results of Two-Dimensional Simulations for the Transport and Fate of Contaminants in Lake Gaston



Figure A-2 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Main01








































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-A1





Figure A-12 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-A3





Figure A-13 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-B1





Figure A-14 Water Column Contaminant Concentrations in the Main Channel of Lake Gaston at Location G-Branch-C1





Figure A-15 Bed Sediment Contaminant Mass in Branch A of Lake Gaston

Dry Year No Pumping Dry Year With Pumping Wet Year No Pumping Wet Year With Pumping




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



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