Reports
Digital simulation and prediction of water levels in the Magothy Aquifer in Southern Maryland
1977, Mack, F.K. and Mandle, R.J.
Report of Investigations 28
Abstract
A digital model was developed of the Magothy aquifer, an important source of water in an 8OO-square mile area ofthe Coastal Plain of Maryland. The model was developed as part of a program to furnish planners with information on the availability of water from areas in which demands for water are increasing in response to rapid increases in population.
Data obtained by test drilling, development of an observation well network, and searches of unpublished and published pumpage and water-level data were used to develop a calibrated digital model of the aquifer. The digital model is based on a computer program which uses the finite-difference method of approximating the ground-water flow equation and, with appropriate initial and boundary conditions, solves the resulting simultaneous algebraic equations using the iterative alternating-direction implicit procedure (IADIP). The model was calibrated by adjusting various hydrologic parameters until computed water-level declines caused by historical pumpage compared favorably with water-level declines measured in several observation wells. The model provides a method for evaluating the impact pumping rates would have on water levels in the aquifer.
Stressing the model with pumping from hypothetical well fields designed to utilize the most favorable hydrologic factors - high transmissivity and greatest available drawdown-indicated that the projected average daily demands for water for the year 2000 could not be met by pumpage solely from the Magothy. However, because the transmissivity and the amount of available drawdown vary considerably within the project area, there is a great difference in the amount of water available from one part of the area to another. Southern Anne Arundel and northern Calvert Counties have adequate quantities of water available from the Magothy, while Prince Georges and Charles Counties would have to seek additional supplies of water, possibly from deeper aquifers, by the year 2000.
At the Chalk Point power-generation plant in southern Prince Georges County, where the expected rate of pumping from the Magothy aquifer by year 2000 is 0.83 million gallons per day, the model predicts a relatively steep sided cone of depression having drawdowns of about 50 feet at a distance of 1 mile away, and less than 5 feet of drawdown at a distance of 10 miles away.
