Reports

Delineation of zones of transport for public-supply wells in St. Mary's County, Maryland

2003, Achmad, G. and Fewster, T.B.

Open File Report 2003-02-16

Key Results

Ground-water flow models designated as the Western area model and the Lexington Park area model were developed to simulate particle pathlines that delineate zones of transport for 102 water-supply wells, withdrawing from the confined Aquia and Piney Point-Nanjemoy aquifers in St. Mary's County, Maryland. Zones of transport, defined as the area surrounding a pumping well that is bounded by an isochrone contour through which a contaminant may travel and reach a pumping well, can be used to develop well-head protection areas surrounding public water-supply welis. These areas help to focus the search for improperly abandoned wells that could potentially provide a pathway for contamination from the land surface to the confined aquifer. Withdrawals by major users (typically greater than 10,000 gallons per day) were simulated using the annual average rates allocated by ground-water appropriation permit. These withdrawals totaled 4.776 million gallons per day from the Aquia aquifer and 0.517 million gallons per day from the Piney Point-Nanjemoy aquifer. Pumpage by self-supplied domestic and minor users was estimated at 1.18 million gallons per day for the Aquia aquifer and 3.34 million gallons per day for the Piney Point-Nanjemoy aquifer. Zones of transport were delineated for public water-supply wells operated by the St. Mary's County Metropolitan Commission, Leonardtown Board of Commissioners, Patuxent Naval Air Test Center, and other selected ground-water users. Key results of the study inciude:

Aquia aquifer

The Aquia aquifer is the principal source of ground water for public suppliers in St. Mary's County. Zones of transport were delineated for 88 water-supply wells, 13 of which pumped at simulated rates greater than 100,000 gallons per day. Withdrawals from those wells created large cones of depression in the Lexington Park, Leonardtown, and County Lakes areas, which influenced regional ground-water flow in St. Mary's County. The 20-year zones of transport associated with these sites extended outward radially in mostly circular or ellipsoidal patterns to distances ranging from 1,150 feet to 2,350 feet from the pumping wells. The remaining users, simulated at lower rates, generated zones of tansport that were relatively small and elongated in the up-gradient direction. The shape of these zones is strongly influenced by the regional ground-water gradient and extend approximately 300 feet to 800 feet from the pumping wells.

Piney Point-Nanjemoy aquifer

Zones of transport were delineated for nine St. Mary's County Metropolitan Commission wells and five Patuxent Naval Air Test Center wells pumping from the Piney Point-Nanjemoy aquifer. Three St. Mary's County Metropolitan Commission wells (Great Mills, Hilis Mobile Home Park, and Town Creek #6A) and one Patuxent Naval Air Test Center well (building #532) were simulated using pumpage greater than 100,000 gallons per day. These large withdrawals created local cones of depression and generated particle pathlines that propagated outward in a radial direction. The 20-year zones of transport extended 1,500 feet to 1,800 feet from the pumping wells. The remaining public water-supply wells were simulated at lower rates and created smaller cones of depression. The lower rates of withdrawal generated particle pathlines that were generally elongated in an up-gradient direcfion. Their pathlines track the regional flow pattern. Twenty (20)-year pathlines extended approximately 500 feet to 1,000 feet from the pumping wells.

Downloads and Data

Open-File Report 2003-02-16 (pdf, 35 MB)

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