The water resources of Howard and Montgomery Counties
1954, Dingman, R.J., Meyer, G., and Martin, R.O.R.
Howard and Montgomery Counties are in central Maryland, just west of a line joining Washington, D. C., and Baltimore. They are within the Piedmont province except for a narrow zone in the Coastal Plain province along the eastern edge of Howard County. The Piedmont part is underlain by crystalline rocks of pre-Cambrian(?) and early Paleozoic age and, in the western part of Montgomery County, by consolidated sedimentary rocks of Late Triassic age. The Coastal Plain part of Howard County is underlain by unconsolidated sedimentary rocks of Early Cretaceous age. In places thin unconsolidated sedimentary deposits of Tertiary and Quaternary ages cap hills, form valley-side terrace deposits, and occur as valley alluvium.
Approximately 4,500,000 gallons of ground water are pumped daily in Howard and Montgomery Counties. As most of the area is underlain by crystalline rocks, they are utilized more extensively for ground-water supplies than are the sedimentary rocks. The ground water occurs essentially under water-table conditions, but artesian conditions occur locally. Ground water is stored and transmitted chiefly through fractures in the unweathered crystalline and indurated sedimentary rocks, and through intergranular interstices in the weathered mantle rock and the unconsolidated sedimentary rocks.
Depths of wells in the crystalline rocks range from 20 to 750 feet and yields range from a fraction of a gallon to about 180 gallons per minute. Specific capacities range from less than 0.1 to 7.5 gallons per minute per foot of drawdown. The magnitude of well yields is related to depth of the well, its topographic position and geologic setting, and the thickness of the weathered-rock mantle in its vicinity.
Stream-flow and precipitation data for the Rock Creek Basin show that over long periods of time the discharge from or effective recharge to, the ground-water reservoir of the basin is about 20 percent of precipitation and loss by evaporation and transpiration is about 71 percent of precipitation.
Measurements of water levels in observation wells show no appreciable net change in most wells. Locally, heavy pumping or cessation of heavy pumping have results in declines and rises, respectively, in water levels. Seasonal fluctuations correlate with precipitation and changes in rates of evaporation and transpiration.
The chemical character of the ground water is related to the chemical composition of the rocks. In general, the water is low in mineral content and is satisfactory for most uses; but locally the water is corrosive, contains large amounts of iron, or is hard.