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Scientific Investigations Report 2015–5072

Prepared in cooperation with the City of Rincon, Georgia

Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Georgia

By Gregory S. Cherry and John S. Clarke

Thumbnail of and link to report PDF (5.16 MB)Abstract

Steady-state simulations using a revised regional groundwater-flow model based on MODFLOW were run to assess the potential long-term effects on the Upper Floridan aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) at well (36S048) near the City of Rincon in coastal Georgia near Savannah. Simulated pumping of well 36S048 at a rate of 1,000 gallons per minute (gal/min; or 1.44 million gallons per day [Mgal/d]) indicated a maximum drawdown of about 6.8 feet (ft) in the UFA directly above the pumped well and at least 1 ft of drawdown within a nearly 400-square-mile area (scenario A). Induced vertical leakage from the UFA provided about 99 percent of the water to the pumped well. Simulated pumping of well 36S048 indicated increased downward leakage in all layers above the LFA, decreased upward leakage in all layers above the LFA, increased inflow to and decreased outflow from lateral specified-head boundaries in the UFA and LFA, and an increase in the volume of induced inflow from the general-head boundary representing outcrop units. Water budgets for scenario A indicated that changes in inflows and outflows through general-head boundaries would compose about 72 percent of the simulated pumpage from well 36S048, with the remaining 28 percent of the pumped water derived from flow across lateral specified-head boundaries.

Additional steady-state simulations were run to evaluate a pumping rate in the UFA of 292 gal/min (0.42 Mgal/d), which would produce the equivalent maximum drawdown in the UFA as pumping from well 36S048 in the LFA at a rate of 1,000 gal/min (called the drawdown offset; scenario B). Simulated pumping in the UFA for the drawdown offset produced about 6.7 ft of drawdown, comparable to 6.8 ft of drawdown in the UFA simulated in scenario A. Water budgets for scenario B also provided favorable comparisons with scenario A, indicating that 69 percent of the drawdown-offset pumpage (0.42 Mgal/d) in the UFA originates as increased inflow and decreased outflow across general-head boundaries from overlying units in the surficial and Brunswick aquifer systems and that the remaining simulated pumpage originates as flow across general- and specified-head boundaries within the UFA.

A steady-state simulation representing implementation of drawdown-offset-pumping reductions totaling 292 gal/min at Rincon UFA production wells 36S034 and 36S035 and pumping from the new LFA well 36S048 at 1,000 gal/min (scenario C) resulted in decreased magnitude and areal extent of drawdown in the UFA compared with scenario A. In the latter scenario, the LFA well was pumped without UFA drawdown-offset-pumping reductions. Water budgets for scenario C yielded percentage contributions from flow components that were consistent with those from scenario B. Specifically, 69 percent of the increased pumping in scenario C originated from general-head boundaries from overlying units of the surficial and Brunswick aquifer systems and the balance of flow was derived from general- and specified-head boundaries in the UFA. In all scenarios, the placement of model boundaries and type of boundary exerted the greatest control on overall groundwater flow and interaquifer leakage in the system.

First posted May 22, 2015

For additional information, contact:
Director, South Atlantic Water Science Center
North Carolina–South Carolina–Georgia
U.S. Geological Survey
720 Gracern Road, Suite 129
Columbia, SC 29210
http://ga.water.usgs.gov/

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Suggested citation:

Cherry, G.S., and Clarke, J.S., 2015, Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan aquifer at Rincon, Effingham County, Georgia: U.S. Geological Survey Scientific Investigations Report 2015–5072, 35 p., http://dx.doi.org/10.3133/sir20155072.

ISSN 2328-0328 (online)



Contents

Abstract

Introduction

Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer

Summary

Selected References

Appendix 1. Regional Groundwater Model


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