Reports

Hydrogeology, model simulation, and water-supply potential of the Aquia and Piney Point-Nanjemoy aquifers in Calvert and St. Mary's Counties, Maryland

1997, Achmad, G. and Hansen, H.J.

Report of Investigations 64

Abstract

Calvert County and St. Mary's County are rapidly growing areas located in southern Maryland. The Aquia aquifer and the Piney Point-Nanjemoy aquifer are the chief sources of ground water in the two-county area. In 1994 withdrawals from the Piney Point-Nanjemoy aquifer totaled about l.9 million gallons per day (Mgal/d) in Calvert County and 3.5 Mgal/d in St. Mary's County. Aquia withdrawals in 1994 were about 3.6 Mgal/d in Calvert County and 4.5 Mgal/d in St. Mary's County. Aquia water levels in the Lexington Park-Solomons area have declined substanti ally, forming a large cone-of-depression that in 1994 was at least 131 ft below sea level at its deepest. Because substantial growth is anticipated over the next 25 years, a new ground-water flow model was constructed to simulate future water-level trends through 2020, using several alternative pumping scenarios.

The input and hydrogeologic framework of the flow model were constructed based on an earlier modeling investigation (1983) of the Aquia and Piney Point-Nanjemoy aquifer system. The new flow model was updated with hydrogeologic data collected since the completion of the previous model. The hydrogeologic framework consists of three layers; each layer includes an aquifer and its underlying confining unit. From top to bottom, the hydrogeologic framework consists of: 1) the HolocenePleistocene Surficial Aquifer modeled as a water-table aquifer (aquifer 1) and its underlying confining unit (Upper Confining Bed); 2) the Piney Point-Nanjemoy aquifer modeled as a confined aquifer (aquifer 2) and its underl ying confining unit (Middle Confining Bed); and 3) the Aquia aquifer modeled as a confined aquifer (aquifer 3) and its underl ying confining unit (Lower Confining Bed) modeled as a no-flow boundary. Hydrologic properties assigned to the model in Calvelt and St. Mary's Counties include transmissivity values for the Piney Point-Nanjemoy aquifer and Aquia aquifer ranging between 100 to 700 feet squared per day (ft²/d) and 500 to 2,000 ft²/d, respectively. The confining beds range in thickness between 60 and 270 ft and were assigned vertical hydraulic conductivity values of 4 and 5 x 10.4 feet per day (ft/d).

The model was calibrated by matching simulated water levels with water levels measured in 198 observation wells. Steady-state calibration was performed to obtain a stabilized head distribution that simulated pre-pumping conditions. The transient model was calibrated by matching simulated water levels against 1952, 1980, and 1982 data. The calibrated model was verified by matching simulated data against 1991, 1992, 1993, and 1994 water levels. The goodness of fit between simulated and measured water levels is considered acceptable with an average deviation about the regression line ranging between 3.1 and 7.7 ft.

Three pumping scenarios for major ground-water appropriators were simulated: (1) projected 1995 to 2020 pumpage rates based on county water plans and population growth estimates; (2) pumpage using the current (1995) annual average groundwater appropriation permit (GAP) allocation rates from 1995 to 2020; and (3) pumpage using the Current (1995) maximum GAP allocation rates applied as annual averages from 1995 to 2020. In the maximum GAP scenario total pumpage was increased from 8.7 Mgal/d (1995) to 11.1 Mgal/d (2020) in Calvert County and from 11 .9 Mgal/d (1995) to 13.0 Mgal/d (2020) in St. Mary's County. The maximum GAP simulation represents the maximum rates permitted for the month of highest demand used as an annual average. Future domestic pumpages used for the 1995 to 2020 simulation were based on estimated population increases by sanitary district (in St. Mary's County) and planning area (in Calvert County). All three pumping scenarios used the same rates of domestic pumpage.

The potential for increased ground-water withdrawals at six Aquia well fields, three in Calvert County and three in St. Mary's County, was evaluated by comparing drawdowns obtained from the Maximum GAP scenario with pelmitted management levels (80% of available drawdown). The well fields are located at Chesapeake Ranch Estates, Solomons, and Prince Frederick in Calvert County and at Lexington Park, Patuxent Naval Air Test Center, and Leonardtown in St. Mary's County. At each well field water levels simulated to 2020 were maintained above the management water level, although relatively deep. Simulated water levels representing model-cell averages ranged from 106 ft below sea level at Prince Frederick to 235 ft below sea level in Lexington Park. Additional drawdowns calculated for selected pumping wells ranged between 10 and 144 ft depending on the number of wells assigned to a model cell. In other areas of the Atlantic Coastal Plain comparable loss of head has reportedly caused aquifer compaction and moderate land subsidence (less than 0.75 ft).

In Calvert and St. Mary's Counties Piney Point-Nanjemoy water levels remain substantially above Aquia levels, particularly surrounding the major well fields. In the future large appropriators should be restricted from using the Piney Point-Nanjemoy aquifer, leaving it to accommodate self-supplied, domestic usage. In areas where Aquia domestic wells predominate, water levels could be stabilized by allocating major withdrawals to deeper, more productive aquifers such as the Magothy and Upper Patapsco. For example, in the northern half of Calvert County, the Magothy aquifer has reported transmissivity values three or more times greater than the overlying Aquia aquifer.

The continued collection of accurate pumpage, water-level, and land-subsidence data is essential to manage increasing water demand effectively in Calvert and St. Mary's Counties. Observation wells serve as an early-warning system to alert managers to unforeseen water-level trends, as well as providing a basis to verify the model projections discussed in this report.

Downloads and Data

Report of Investigations 64 (pdf, 93.9 MB)
Plate 1 (pdf, 2.7 MB)
Plate 2 (pdf, 2.8 MB)