Simulation of groundwater flow and pumping scenarios for 1900–2050 near Mount Pleasant, South Carolina

Abstract

Groundwater withdrawals from the Upper Cretaceous-age Middendorf aquifer in South Carolina have created a large, regional cone of depression in the potentiometric surface of the Middendorf aquifer in Charleston and Berkeley Counties, South Carolina. Groundwater-level declines of as much as 249 feet have been observed in wells over the past 125 years and are a result of groundwater use for public water supply, irrigation, and private industry. To address the concerns of users of the Middendorf aquifer, the U.S. Geological Survey, in cooperation with Mount Pleasant Waterworks (MPW), recalibrated an existing groundwater-flow model to incorporate additional groundwater-use and water-level data since 2008. This recalibration process consisted of a technique of parameter estimation that uses regularized inversion and employs “pilot points” for spatial hydraulic property characterization. The groundwater-flow system of the Coastal Plain physiographic province of South Carolina and parts of Georgia and North Carolina was simulated using the U.S. Geological Survey finite-difference computer code MODFLOW-2000. After the model recalibration, the following six predictive water-management scenarios were created to simulate potential changes in groundwater flow and groundwater-level conditions in the Mount Pleasant, South Carolina, area: Scenario 1—maximize MPW reverse-osmosis plant capacity by increasing groundwater withdrawals from the Middendorf aquifer from 3.9 million gallons per day (Mgal/d), which was the amount withdrawn in 2015, to 8.58 Mgal/d; Scenario 2—same as Scenario 1, but with the addition of a 0.5 Mgal/d supply well in the Middendorf aquifer near Moncks Corner, South Carolina; Scenario 3—same as Scenario 1, but with the addition of a 1.5 Mgal/d supply well in the Middendorf aquifer near Moncks Corner, South Carolina; Scenario 4—maximize MPW well capacity by increasing withdrawals from the Middendorf aquifer from 3.9 Mgal/d (in 2015) to 10.16 Mgal/d; Scenario 5—minimize MPW surface-water purchase from the Charleston Water System by adding supply wells and increasing withdrawals from the Middendorf aquifer from 3.9 Mgal/d (in 2015) to 12.16 Mgal/d; and Scenario 6—same as Scenario 1, but with he addition of quarterly model stress periods to simulate seasonal variations in the groundwater withdrawals. Results from the simulations indicated further decline of groundwater levels creating cones of depressions near pumping wells in the Middendorf aquifer in the Mount Pleasant, South Carolina, area between 2015 and 2050 for all six scenarios. Simulation results from Scenario 1 showed an average decline of about 150 feet in the groundwater levels of the MPW production wells. Simulated hydrographs for two area observation wells illustrate the gradual decline in groundwater levels with overall changes in water-level altitudes of –92 and –33 feet, respectively. Simulated groundwater altitudes at a hypothetical observation well located in the MPW well field declined 121 feet between 2015 and 2050. Scenarios 2 and 3 have the same pumping rates as Scenario 1 for the MPW production wells; however, a single hypothetical pumping well was added in the Middendorf aquifer near the town of Moncks Corner, South Carolina. This hypothetical pumping well has a withdrawal rate of 0.5 Mgal/d for Scenario 2 and 1.5 Mgal/d for Scenario 3. A comparison to the 2050 Scenario 1 simulation indicates groundwater altitudes for Scenarios 2 and Scenario 3 are 3 feet and 8 feet lower, respectively, at the MPW production wells. Scenario 4 simulates the maximum pumping capacity of 10.16 Mgal/d for the MPW network of production wells. Simulated 2050 groundwater altitudes for this simulation declined to –359 feet. Simulated hydrographs for two observation wells show groundwater-level declines of 116 and 41 feet, respectively. Simulated differences in groundwater altitudes at a hypothetical observation well located in the MPW well field indicate a water-level decline of 164 feet between 2015 and 2050. Scenario 5 is a modification of Scenario 4 with the addition of two new MPW production wells. For this scenario, the MPW network of production wells were simulated the same as in Scenario 4, but withdrawals from the two new production wells were added in 2020. Simulated 2050 groundwater altitudes for this simulation declined to – 405 feet. Simulated hydrographs for two observation wells show groundwater-level declines of 143 and 51 feet, respectively. Simulated groundwater altitudes at a hypothetical observation well located in the MPW well field declined 199 feet between 2015 and 2050. Scenario 6 is a modification of Scenario 1, in which 140 additional quarterly stress periods were added to simulate MPW seasonal demands. Simulated groundwater altitudes for Scenario 6 declined to –353 feet during 2050. For Scenario 6, simulated hydrographs for two observation wells and the hypothetical observation well show similar groundwater-level declines as seen in Scenario 1, but with seasonal fluctuations of as much as 56 feet in the hypothetical observation well. Water budgets for the model area immediately surrounding Mount Pleasant, South Carolina, were calculated for 2015 and for 2050. The water budget for 2015 is equal for all of the scenarios because it represents the year prior to the hypothetical pumping beginning in 2016. The largest flow component in the 2015 water budget for the Mount Pleasant area is discharge to wells at a rate of 4.17 Mgal/d. Additionally, 0.23 Mgal/d flows laterally out of the Middendorf aquifer in this area of the model due to the regional horizontal hydraulic gradient. Flow into this zone consists predominantly of lateral flow within the Middendorf aquifer at 4.08 Mgal/d. Additionally, 0.02 Mgal/d is released into this zone from aquifer storage. Vertically, 0.06 Mgal/d flows down from the Middendorf confining unit located above the Middendorf aquifer, and 0.25 Mgal/d flows up from the Cape Fear confining unit below. The largest flow component in the 2050 water budget for all six scenarios is discharge to wells in the Mount Pleasant area at rates between 8.89 and 12.47 Mgal/d. Flow into this zone consists mostly of lateral flow between 8.47 and 11.77 Mgal/d within the Middendorf aquifer. Between 0.003 and 0.46 Mgal/d is released into this zone from aquifer storage. Between 0.004 and 0.15 Mgal/d flows laterally out of this zone into adjacent areas of the Middendorf aquifer due to the regional horizontal hydraulic gradient. Finally, between 0.15 and 0.22 Mgal/d flows vertically into this zone from confining units above and below the Middendorf aquifer.