Georgia Water Science Center
This report is available online in pdf format (15 MB): USGS SIR 2006-5077 (
)
Dorothy F. Payne, Alden M. Provost, Jaime A. Painter, Malek Abu Rumman, and Gregory S. Cherry
U.S. Geological Survey Scientific Investigations Report 2006-5077, 124 pages (Published 2006)
Regional ground-water flow and solute-transport models for the coastal area of Georgia, and adjacent parts of Florida and South Carolina were used to evaluate the effects of current and hypothetical ground-water withdrawal on ground-water flow and saltwater transport. The models were designed to simulate the flow system at different scales while being as consistent as possible in framework, hydraulic properties, pumpage distribution, and boundary conditions. Simulation results for future pumpage scenarios were compared with those during 2000 (the Base Case), or during 2100 for 2000 pumpage applied after 2000. The regional MODFLOW model assumes steady-state ground-water flow, and is calibrated to 1980 and 2000 pumping conditions. The SUTRA model of the Savannah, Georgia–Hilton Head Island, South Carolina, area is run as a transient simulation from a predevelopment (1885) steady-state flow field to 2004, and calibrated to water levels in September 1998 and estimated chloride values in 2000, 2002, 2003, and 2004.
Scenario A illustrates the effects of implementing an interim strategy for managing saltwater intrusion in the Upper Floridan aquifer of southeastern Georgia. Results show a combination of rises and declines in head from 1997 to 2000 in response to changes in the pumping patterns and only minor changes in the chloride distribution in the Hilton Head Island, South Carolina, area. Generally, water levels rose in the Savannah–Hilton Head Island area, and declined in the area north of the Gulf Trough.
Scenario B simulates the effect of a 36-million-gallon-per-day reduction of pumpage at a major pumping center in Camden County, Georgia. Results show that the largest recovery is limited in extent to the area surrounding St. Marys, Georgia, but a smaller water-level rise of 1–2 feet extends as far north as southern Chatham County, Georgia, and inland toward the Gulf Trough. Nearest the area where the wells were turned off, the model predicts a smaller recovery than indicated by observed water levels.
Scenarios C1 and C2 illustrate the relative effects of pumping in Chatham County, Georgia, and southern Beaufort County, South Carolina, on ground-water levels and saltwater distribution and movement in that area. Results indicate that pumping in southern Beaufort County has a smaller effect on saltwater-plume development than pumping in Chatham County for plumes west of Hilton Head Island, South Carolina. Results also indicate that the effect of pumping on the plume at the northern end of Hilton Head Island in either Chatham County or southern Beaufort County is small, although pumping in southern Beaufort County may have a slightly greater effect on plume growth than pumping in Chatham County. Furthermore, model results indicate that eliminating pumping in Chatham County would result in a greater water-level increase at the southern end of Hilton Head Island than elimination of pumping in southern Beaufort County, and that eliminating pumping in southern Beaufort County would result in a greater water-level increase at the northern end of Hilton Head Island than elimination of pumping in Chatham County.
Scenarios D1 and D2 simulate the effect of projected pumpage during 2000–2035, based on two estimates of future ground-water needs. Results from both scenarios show substantial water-level declines from 2000 to 2035 and an increase of inflow at the source-sink boundaries. For the solute-transport simulations, pumpage at 2035 was held constant until 2100. For both scenarios, chloride plumes expand during 2000–2100, but show limited expansion relative to plumes that develop for 2000 pumpage applied until 2100. Although the total pumpage difference between Scenarios D1 and D2 during 2035 is 477 million gallons per day, and pumpage in both scenarios is substantially larger than that during 2000, distance and hydraulic features, such as the Gulf Trough, and high hydraulic conductivity in the southwestern part of the model area, limit the effects of these differences on the extent of resultant plumes.
Results obtained using the ground-water flow and solute-transport models used in this study are subject to the limitations of the models and scenario conditions. For example, pumpage for Scenario B may not accurately represent conditions during 2002 when the industrial wells were turned off, and projected pumpage for 2010, 2020, and 2035 is substantially different in Scenarios D1 and D2. Generally, model results are less reliable for scenario conditions that are farther from calibration conditions. Conditions that differ substantially from calibration conditions may induce an unrealistic response from the model if the model assumptions are violated; for example, if influx from model boundaries is excessive. Excessive inflow from the model boundaries, particularly from the onshore area, may result in underestimated drawdown and inflow of saltwater. Model results are most reliable in areas where calibration data exist, and for the range of pumpage conditions for which the models are calibrated.
Abstract
Introduction
Purpose and Scope
Description of Study Area
Previous Coastal Sound Science Initiative Studies
Method of Study
Acknowledgments
Ground-Water Flow and Solute-Transport Models
Model Framework
Hydrologic-Unit Layering
Hydraulic Properties
Spatial Discretization
Boundary Conditions
Pumpage Distribution
Model Calibration
Base Case Simulations: Representation of the Flow System During 2000
Simulation of Ground-Water Management Scenarios
Scenario A: Representation of the Flow System During 1997
Fit of Simulated Heads to Observed Heads Using the MODFLOW Model
Ground-Water Flow
Solute Transport
Scenario B: Effects of an Industrial Well Field Shutdown
Scenarios C1 and C2: Relative Effects of Pumping in Chatham County, Georgia, and Southern Beaufort County, South Carolina
Scenario C1: Elimination of Pumping in Chatham County, Georgia
Ground-Water Flow
Solute Transport
Scenario C2: Elimination of Pumping in Southern Beaufort County, South Carolina
Ground-Water Flow
Solute Transport
Comparison of Head Gradients in the Savannah–Hilton Head Island Area, Scenarios C1 and C2
Scenarios D1 and D2: Projected Pumping to 2035
Scenario D1: Projection Based on Regional Economic Models, Inc
Ground-Water Flow
Solute Transport
Scenario D2: County Comprehensive Water–Supply Plan Projection
Ground-Water Flow
Solute Transport
Model Limitations
Summary
Selected References
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