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Increasing water use from the Upper Cretaceous Middendorf aquifer in South Carolina has 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 up to 249 feet (ft) 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, recalibrated an existing groundwater flow model to incorporate additional groundwater-use and water-level data that have been compiled 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, SC area: (1) maximize Mount Pleasant Waterworks reverse-osmosis plant capacity by increasing groundwater withdrawals from 3.9 million gallons per day (Mgal/d) in 2015 to 8.6 Mgal/d from the Middendorf aquifer; (2) same as Scenario 1, but with the addition of a 0.5 Mgal/d supply well in the Middendorf aquifer near Moncks Corner, SC; (3) same as Scenario 1, but with the addition of a 1.5 Mgal/d supply well in the Middendorf aquifer near Moncks Corner, SC; (4) maximize Mount Pleasant Waterworks well capacity by increasing withdrawals from the Middendorf aquifer from 3.9 Mgal/d in 2015 to 10.2 Mgal/d (5) minimizing Mount Pleasant Waterworks 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 12.2 Mgal/d; and (6) same as Scenario 1, but with the 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 ft 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 32 ft, respectively. Simulated groundwater altitudes at an imaginary well located in the MPW well field declined 120 ft 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, SC. This hypothetical 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 ft and 8 ft lower, respectively, at the Mount Pleasant Waterworks 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 as low as -357 ft. Simulated hydrographs for observation wells CHN-14 and BRK-431 show groundwater-level declines of 164 and 40 feet, respectively. Simulated differences in groundwater altitudes at an imaginary well located in the MPW well field indicate a water-level decline 164 ft between 2015 and 2050. Scenario 5 is a modification of Scenario 4 with the addition of two new MPW production wells, MPW 1 and MPW 2. For this scenario, the MPW network of production wells were simulated the same as Scenario 4, but withdrawals from the two new production wells (MPW 1 and MPW 2) were added in 2020. Simulated 2050 groundwater altitudes for this simulation declined to as low as -403 ft. Simulated hydrographs for observation wells CHN-14 and BRK-431 showgroundwater-level declines of 50 and 143 ft, respectively. Simulated groundwater altitudes at a hypothetical well located in the MPW well field declined 198 ft between 2015 and 2050. Scenario 6 is a modification of Scenario 1, where 140 quarterly additional stress periods were added to simulate MPW seasonal demands. Simulated groundwater altitudes for Scenario 6 declined to 351 ft during 2050. For Scenario 6, simulated hydrographs for observation wells CHN-14, BRK-431, and the hypothetical well show similar groundwater-level declines seen in Scenario 1, but with seasonal fluctuations of as much as 56 ft in the hypothetical well. Water budgets calculated for the model area immediately surrounding Mount Pleasant, SC, 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 4.39 million gallons per day (Mgal/d). Additionally, 0.22 Mgal/d flows laterally out of this zone within the Middendorf aquifer due to the regional horizontal hydraulic gradient. Flow into this zone consists predominantly of lateral flow within the Middendorf aquifer at 4.07 Mgal/d. Additionally, 0.02 Mgal/d is released into this zone from aquifer storage. Vertically, 0.08 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 within the Middendorf aquifer, between 8.48 and 11.77 Mgal/d. Between 0.003 and 0.46 Mgal/d is released into this zone from aquifer storage. Between 0.003 and 0.15 Mgal/day 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 gal/d flows vertically into this zone from confining units above and below the Middendorf aquifer.

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