Optimization of ground-water withdrawals in the Lower Patapsco aquifer, Waldorf, Maryland
2003, Andreasen, D.C.
Open File Report 2003-02-17
The north-central part of Charles County, Maryland is supplied by a network of wells screened in the Magothy and Lower Patapsco aquifers. The wells, operated by the Charles County Department of Utilities, supply water to the Waldorf area and to several communities located west of Waldorf. In 2001, averages of approximately 2.4 and 2.6 million gallons per day were pumped from the Magothy and Lower Patapsco aquifers, respectively. In response to pumping, water levels in both aquifers have declined. The greatest amount of drawdown has occurred in the Lower Patapsco aquifer with water levels as deep as 170 feet below sea level. Pumping water levels may be as deep as 550 feet below land surface, resulting in substantial pumping costs. The Lower Patapsco aquifer is a good source for municipal supply in the Waldorf area because it has relatively high transmissivity (up to 3,000 feet squared per day) and sufficient available drawdown (up to 550 feet in the eastern part of the Waldorf area in 2001). However, increased use, possibly as much as 5 million gallons per day if pumpage is shifted from the shallower Magothy aquifer, will cause additional drawdown and higher pumping costs. Additionally, increased withdrawals from the Lower Patapsco aquifer may cause water levels to fall below the 80-percent management level in the Indian Head-Bryans Road area, which is located west of the project area. To address these concerns, withdrawals from the Lower Patapsco aquifer were optimized to minimize drawdown. Optimization was performed using a three-dimensional ground-water-flow model (MODFLOW code) combined with linear programming (MODMAN and SuperLINDO codes). Optimum withdrawals were also determined given constraints that water levels not fall below management levels or pump intakes.