Maryland Department of Natural Resources


Water-supply potential of the Coastal Plain aquifers in Calvert, Charles, and St. Mary's Counties, Maryland, with emphasis on the Upper Patapsco and Lower Patapsco aquifers

2007, Drummond, D.D.

Report of Investigations 76


A study was conducted of the water-supply potential of the aquifer system in Calvert, Charles, and St. Mary’s Counties. The water needs of this area are supplied by the Piney Point, Aquia, Magothy, Upper Patapsco, and Lower Patapsco aquifers. Declining water levels and elevated arsenic concentrations in the Aquia aquifer have prompted water-supply managers to consider shifting some ground-water withdrawals from the Aquia aquifer to the deeper Upper Patapsco and Lower Patapsco aquifers. This report presents an assessment of the potential for these aquifers to meet future water demands of the area, given several scenarios for ground-water development. Emphasis was placed on the Upper Patapsco and Lower Patapsco aquifers, for which critical information has been lacking, to assess their water-bearing properties throughout the region.

Data were collected from existing sources and from six test wells drilled during this project, on lithology, hydraulic parameters, geophysical logs, pumpage, water levels, and water quality. Hydrogeologic maps were constructed that show the structure, transmissivity, and head distribution of the Upper Patapsco and Lower Patapsco aquifers throughout Southern Maryland, and data for pumpage and water quality were tabulated. Connectivity of individual sand bodies within the Upper Patapsco and Lower Patapsco aquifers was demonstrated using sand-percentage analysis and hydraulic continuity relations. Hydrogeologic cross sections were also constructed to show the vertical distribution of aquifers and confining units in the region.

A ground-water flow model was developed that simulates water levels in five major aquifers in Southern Maryland. The flow model was calibrated using historical pumpage and water levels, and was then used to estimate future water levels through 2030 based on future pumpage scenarios compiled in conjunction with county planning departments. Flow-model scenarios were evaluated primarily based on the Maryland Department of the Environment regulated 80-percent management level, which restricts regional heads from declining to the tops of aquifers. Consideration was also given to the possibility of land subsidence; and of ground-water withdrawals in the deeper confined parts of aquifers reducing heads in the shallow unconfined parts of those aquifers, which may create the potential for stream-flow depletion, wetland degradation, and river-water intrusion.

Flow-model simulations indicate that projected water demand in Calvert and St. Mary’s Counties through 2030 could be met by increased pumpage from the Aquia aquifer without reducing water levels below the 80-percent management level. Shifting a portion of public-supply withdrawals from the Aquia aquifer to the Patapsco aquifers would result in an increase in available drawdown in the Aquia aquifer in many areas of Calvert and St. Mary’s Counties, with minimal impact on future water levels in the Patapsco aquifers in Charles County.

In Charles County, withdrawals from the Magothy aquifer in the Waldorf area cannot be increased significantly above 2002 amounts without lowering water levels below the 80-percent management level by 2030. The relatively shallow depth of the Patapsco aquifers and the proximity of major pumping centers to outcrop/recharge areas limit productive capacity. Future pumpage scenarios result in drawdowns exceeding the 80-percent management level at several locations, such as Indian Head and La Plata.

Simulated future drawdowns indicate the potential for river-water intrusion into the Upper Patapsco and Lower Patapsco aquifers from the Potomac River in the Indian Head area. Simulated drawdowns also indicate the potential in shallow portions of the Patapsco aquifers for lowering the water table, which could reduce base flow to streams and reduce the amount of water available in wetlands where ground-water inflow provides moisture for plants. These issues could not be specifically addressed in the context of a large regional study, but require additional examination. Alternative water-supply options should be evaluated in Charles County, such as utilizing the Patuxent aquifer, or replacing current production well fields with new wells in the Patapsco aquifers located farther southeast.