Georgia Water Science Center
This report is available online in pdf format (8 MB): USGS SIR 2006-5195 (
)
Gregory S. Cherry
U.S. Geological Survey Scientific Investigations Report 2006-5195, 156 pages (Published 2006)
Ground-water flow under 2002 hydrologic conditions was evaluated in an eight-county area in Georgia and South Carolina near the Savannah River Site (SRS), by updating boundary conditions and pumping rates in an existing U.S. Geological Survey (USGS) ground-water model. The original ground-water model, developed to simulate hydrologic conditions during 1987–92, used the quasi-three-dimensional approach by dividing the Floridan, Dublin, and Midville aquifer systems into seven aquifers. The hydrogeologic system was modeled using six active layers (A2–A7) that were separated by confining units with an overlying source-sink layer to simulate the unconfined Upper Three Runs aquifer (layer A1). Potentiometric-
surface maps depicting September 2002 for major aquifers were used to update, evaluate, and modify boundary conditions used by the earlier ground-water flow model.
The model was updated using the USGS finite-difference code MODFLOW–2000 for mean-annual conditions during 1987–92 and 2002. The specified heads in the source-sink layer A1 were lowered to reflect observed water-level declines during the 1998–2002 drought. These declines resulted in a decrease of 12.1 million gallons per day (Mgal/d) in simulated recharge or vertical inflow to the uppermost confined aquifer (Gordon, layer A2). Although ground-water pumpage in the study area has increased by 32 Mgal/d since 1995, most of this increase (17.5 Mgal/d) was from the unconfined Upper Three Runs aquifer (source-sink layer A1) with the remaining 14.5 Mgal/d assigned to the active layers within the model (A2–A7).
The simulated water budget for 2002 shows a decrease from the 1987–92 model from 1,040 Mgal/d to 1,035 Mgal/d. The decreased ground-water inflows and increased ground-water withdrawal rates reduced the simulated ground-water outflow to river cells in the active layers of the model by 43 Mgal/d. The calibration statistics for all layers of the 2002 simulation resulted in a decrease in the root mean square (RMS) of the residuals from 10.6 to 8.0 feet (ft). The residuals indicate 83.3 percent of the values for the 2002 simulation met the calibration error criteria established in the original model, whereas 88.8 percent was within the specified range for the 1987–92 simulation. Simulated ground-water outflow to the Savannah River and its tributaries during water year 2002 was 560 cubic feet per second (ft3/s), or 86 percent of the observed gain in mean-annual streamflow between streamflow gaging stations at the Millhaven, Ga., and Augusta, Ga. At Upper Three Runs Creek, simulated ground-water discharge during 2002 was 110 ft3/s, or 83 percent of the observed streamflow at two streamflow gaging stations near the SRS. These results indicate that the constructed model calibrated to 1987–92 conditions and modified for 2002 dry conditions is still representative of the hydrologic system.
The USGS particle-tracking code MODPATH was used to generate advective water-particle pathlines and their associated time-of-travel based on MODFLOW simulations for 1987–92, 2002, and each of four hypothetical ground-water management scenarios. The four hypothetical ground-water management scenarios represent hydrologic conditions for (1) reported pumping for 2002 and boundary conditions for an average year; (2) reported pumping for 2002 with SRS pumping discontinued and boundary conditions for an average year; (3) projected 2020 pumping and boundary conditions for an average year; and (4) projected 2020 pumping and boundary conditions for a dry year. The MODPATH code was used in forward-tracking mode to evaluate flowpaths from areas on the SRS and in backtracking mode to evaluate further areas of previously documented trans-river flow on the Georgia side of the Savannah River. Trans-river flow is a condition in which the local head gradients might allow migration of contaminants from the SRS into the underlying aquifers and beneath the Savannah River into Georgia.
The analysis of ground-water flowpaths using MODPATH was conducted by establishing five zones in which particles were seeded into model cells based on the following criteria: (1) occurrence of recharge from the source-sink layer A1 (Upper Three Runs aquifer) into layer A2 (Gordon aquifer), (2) downward flow from layer A2 (Gordon aquifer) into layers A3–A5 (Dublin aquifer system), (3) delineated areas of contamination or storage of hazardous materials on the SRS, and (4) defined surface-water drainage divides. Selected areas near streams on the SRS were not considered for the analysis because of localized flow regimes to nearby streams. In the case of trans-river flow areas, particles were placed in cells located on the western side of the Savannah River floodplain near Flowery Gap Landing in Burke County, Ga., and backtracked to recharge areas on the SRS.
The most influential factors controlling particle movement are vertical and lateral head gradients and pumping distribution within the active layers of the model. MODPATH results indicate that Upper Three Runs Creek and the alluvial valley of the Savannah River are the dominant sinks or areas of ground-water discharge with time-of-travel ranging from 20 to greater than 2,000 years (yr). Simulated ground-water flowpaths for each of the four ground-water management scenarios were generally limited to areas within the SRS boundary because this is the area of concern for contaminant transport.
Five particle seed zones were established in which individual particles were observed from their point of recharge to discharge areas located along local streams within the boundaries of the SRS or the Savannah River. The median time-of-travel listed below for each of the five zones represents a range for the five simulations (2002, Scenarios A, B, C, and D). In general, the elimination of pumping at the SRS (Scenario B) reduces the time-of-travel for particles to reach the discharge areas. In zone 1, median time-of-travel from recharge areas to discharge areas located along Upper Three Runs Creek and west of the SRS boundary ranges from 217 to 264 yr. In zone 2, median time-of-travel from recharge areas to discharge areas located along Upper Three Runs Creek, Pen Branch, Fourmile Branch, and the Savannah River ranges from 524 to 593 yr. In zone 3, median time-of-travel from recharge areas to discharge areas located along Upper Three Runs Creek ranges from 834 to 1,150 yr. In zone 4, median time-of-travel from recharge areas to discharge areas located along the South Carolina side of the Savannah River ranges from 395 to 404 yr. In zone 5, median time-of-travel from recharge areas to discharge areas located along Lower Three Runs Creek and the Savannah River ranges from 1,310 to 1,350 yr. The longer travel times are generally associated with particles that penetrate deeper into the underlying aquifers before moving laterally toward discharge areas.
For the backtracking analysis of particles, three model cells located near Flowery Gap Landing (covering about 1 square mile) on the Georgia side of the Savannah River were chosen based on results from forward-tracking analysis (zone 2), indicating these cells as common discharge areas. Of the 300 particles released in these three cells, as few as 88 particles (29 percent, 2002, Scenario C) to as many as 110 particles (37 percent, Scenario B) backtrack to recharge areas on the SRS (trans-river flow). Of the particles exhibiting trans-river flow, the median time-of-travel along pathlines range from 366 to 507 yr with north of the Pen Branch Fault. Backtrack time-of-travel for the shortest flowpaths ranged from 79 to 82 yr from trans-river flow to interstream areas located north of Fourmile Branch with 10 percent of these particles reaching endpoints at about 100 yr. The results indicate that simulations with active SRS pumping centers (1987–92, 2002, Scenarios A, C, and D) allowed fewer particles to migrate to the Georgia side of the Savannah River. If these SRS production wells are deactivated (Scenario B), the number of particles migrating to trans-river zones increases to 110 and the median time-of-travel decreases to about 370 yr with a shortest time-of-travel period of about 80 yr.
Generally, time-of-travel for particles migrating downward through the Gordon confining unit (C1) and then moving laterally through layer A2 (Gordon aquifer) to discharge areas ranges from 20 to 200 yr. For particles migrating deeper into layers A3 through A5 (Dublin aquifer system), time-of-travel generally ranges from 200 to 1,000 yr. Eliminating pumping on the SRS (Scenario B) reduces the depth of penetration of particles and shortens the pathways to discharge areas with median time-of-travel decreased from 15 to 70 yr in zones 1 and 2. The second most influential factor controlling particle movement is the adjustment of heads in the source-sink layer A1, which affects the amount of recharge entering the system. In areas of trans-river flow, from 29 to 37 percent of the particles placed in three grid cells located near Flowery Gap Landing backtrack to recharge areas on the SRS. For these particles, the shortest travel time from 80 to 415 yr was for particles moving laterally through layer A2 (Gordon aquifer) and upward into the base of the source-sink layer A1 (Upper Three Runs aquifer) in areas south of Fourmile Branch.
Abstract
Introduction
Purpose and Scope
Description of Study Area
Previous Investigations
Methods of Study
Acknowledgments
Hydrologic, Climatic, and Ground-Water Use Conditions, 1992–2002
Precipitation
Ground-Water Use
Ground-Water Levels
Streamflow
Simulation of Steady-State Ground-Water Flow, 2002
Updating the Model to 2002 Conditions
Simulated Water Budget
Simulated Water-Level Changes
Particle-Tracking and Time-of-Travel Analysis
Zone 1
Zone 2
Zone 3
Zone 4
Zone 5
Trans-River Flow
Simulation of Ground-Water Management Scenarios, 2002 and 2020
Adjustments to Boundary Conditions
Projected Pumping
Comparison of Scenarios
Limitations of Digital Simulation and Particle Tracking
Summary and Conclusions
Selected References
Appendix A. Scenario A—Simulation of 2002 Average Conditions
Appendix B. Scenario B—Simulation of 2002 Average Conditions without Savannah River Site pumping
Appendix C. Scenario C—Simulation of 2020 Average Conditions
Appendix D. Scenario D—Simulation of 2020 Dry Conditions
Appendix E. Observed and Simulated Ground-Water Levels, 1987–92 and 2002
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