Simulated changes in water level in the Piney Point aquifer in Maryland
1979, Williams, J.F.
Report of Investigations 31
A two-dimensional finite-difference computer model of the freshwater part (less than 250 milligrams per liter chloride) of the Piney Point aquifer in Maryland was developed to simulate and predict drawdown in the aquifer.
The Piney Point aquifer is of Eocene age and is composed of fine to very coarse sand varying from a few feet to more than 120 feet in thickness. It contains cemented, interbedded shell layers, and is highly glauconitic. The aquifer has no known outcrop area. Hydrogeologic information pertaining to the Piney Point is presented as a series of maps which show the potentiometric surface in the prepumping stage, the potentiometric surface in 1952 and 1976, the water-level change between 1952 and 1976, the available drawdown as of 1976, the transmissivity, the thickness, and the subsurface structure.
The Piney Point aquifer is a major source of water for several cities, communities, industries, housing subdivisions, and hundreds of individual homeowners in southern and eastern Maryland. The aquifer is also used as a water source in parts of Virginia, Delaware, and New Jersey. The total pump age of the Piney Point in Maryland has increased from an estimated 0.5 million gallons per day in 1900, to an average of 4.33 million gallons per day between June 1975 and June 1976. A complex picture of water-level decline and rise has recently developed due to changes in the withdrawal rates from the major pumping centers.
The Piney Point was modeled as a confined aquifer recharged by leakage from an overlying aquifer which is separated from the Piney Point by semi-confining material. The calibration scheme consisted of simulating historical pumpage from an initially flat potentiometric surface. Pumpage was simulated over a period of 86 years (1890-1976) and was subdivided into seven separate pumping periods of various durations. Calibration was obtained by comparing computed versus measured water-level changes for the periods 1952-76, 1970-74, 1974-75, and 1975-76.
Values for aquifer characteristics used in the model to represent the Piney Point are:
Transmissivity- 100 to 6,200 square feet per day, and storage coefficient-3x10-4. The
values used to represent the semi-confining material were: Vertical hydraulic conductivity-
1x10-8 to 1x10-12 feet per second, specific storage- 6x10