Reports
The availability of ground water in western Montgomery County, Maryland
1981, Otton, E.G.
Report of Investigations 34
Abstract
Western Montgomery County is underlain by pre-Triassic crystalline phyllite and schist, by Triassic shale, siltstone, and sandstone, by terrace deposits of Tertiary (?) age, and by surficial alluvial deposits of Quaternary age. Water occurs chiefly in fractures and crevices in the rocks under water-table or modified artesian conditions. The source of all the ground water is local precipitation, amounting annually to about 42 inches.
Yields of wells range from a few to 400 gallons per minute. Specific capacity, a better index of the productivity of wells, ranges widely. The median specific capacity of 21 crystalline-rock wells is 0.18 gallon per minute per foot, and the median specific capacity of 80 wells in the consolidated sediments is 0.15, suggesting little significant difference between the two major rock types. Median transmissivity of the crystalline rocks is 35 square feet per day and the median transmissivity of the Triassic sedimentary rocks is 17 square feet per day. Storage coefficients range widely, but the mean value in the Triassic rocks at the Poolesville well field is about 0.01.
In 1978, well pumpage at the N.I.H. Animal Center averaged 28,000 gallons per day; pumpage from four wells at Poolesville during 1978 averaged 246,000 gallons per day. Ground-water use in the remainder of the area in 1978 is estimated to have been 250,000 gallons per day.
During dry periods of 60 days or more, the yield of the four public-supply wells at Poolesville declines substantially. In 1978, this decline amounted to about 36 percent, based on the comparative yield of wells during two 60-day periods. Additional well supplies are needed by the town to prevent future water crises.
Chemical quality of the water is suitable for most uses. Water from the crystalline rocks is mildly acidic and soft. Water from the Triassic rocks is commonly mildly alkaline and has a greater hardness. Because of the relatively thin soil zone and the fractured nature of the underlying rocks, chances of polluting the aquifers are relatively great. Possible pollutants might be septic-tank effluent, road salt, chemical fertilizers, and insecticides and herbicides. Because of the relative ease of movement of water in fractured rocks, the effects of spills of hazardous or toxic wastes could be intensified.
