Reports
A digital simulation model of the Aquia Aquifer in Southern Maryland
1976, Kapple, G.W. and Hansen, H.J.
Information Circular 20
Abstract
A digital finite-difference method was used to model about 3,800 square miles (9,840 square kilometres) of the Aquia aquifer underlying southern Maryland. The Aquia Formation (Paleocene) is a confined aquifer throughout the project area, except in outcrop, and reaches depths of 500 feet (152 metres) below sea level. Aquifer test results indicate that transmissivity ranges from 670 to 4,000 feet squared per day (62 to 372 metres squared per day) and thickness ranges from 75 to 200 feet (23 to 61 metres). The majority of the pump age has occurred in the vicinity of the Patuxent Naval Air Training Station (PNATS) which is the focus for much of the work done on this project.
Over most of the project area the Aquia is overlain by the Marlboro Clay Member of the Nanjemoy Formation, a relatively tight, homogeneous confining bed 15 to 30 feet (5 to 9 metres) thick. Laboratory values of hydraulic conductivity range from 3.4 x 10-5 to 4.5 X 10-4 feet per day (1.0 x 10-5 to 1.4 X 10-4 metres per day) . The lower confining bed, the Brightseat Formation, is sandier and more permeable and ranges in thickness from 10 to 35 feet (3 to 11 metres).
The model was calibrated for the PNATS area by comparing observed and calculated water-level records for the period 1943-51. The cone of depression had stabilized during this period. Fair agreement between observed and calculated drawdown was achieved by adjusting the value of the hydraulic conductivity of the confining bed which was found to be the most important parameter controlling recharge. After calibration, the model was used to predict future water-level declines based on hypothetical rates of withdrawal from pumping centers in the vicinity of the PNATS. The final, lO-year prediction run, involving a total pumpage of 21 million gallons per day (79,490 cubic metres per day), resulted in more than 400 feet (122 m) of drawdown and partial dewatering of the aquifer over a 12 square mile (31 square kilometre) area of the PNATS.
