Water resources of Washington County, Maryland
1991, Duigon, M.T. and Dine, J.R.
The western part of Washington County, underlain by strongly folded clastic sedimentary rocks, is steeper and more forested than most of the eastern part, where limestone and dolomite predominate. The eastern edge of the county is underlain by metamorphic rocks. Ground water occurs predominantly under unconfined, secondary-permeability conditions.
Unconsolidated sediments and overburden generally are not sufficiently permeable or are of insufficient saturated thickness to be used directly for water supplies. Ground-water levels range from near land surface to more than 150 feet below and they fluctuate from a few feet to tens of feet. Highest water levels occur during February through April, and lowest levels occur in October. Long-term trends are of lesser magnitude than seasonal variations. Reported well yields range from 0 to 385 gallons per minute, and specific capacities range from 0.00 to 86 gallons per minute per foot. Variability of well yields within geologic units is about as great as variability among units. Ground water is generally hard to very hard; total dissolved-solids concentrations range from about 20 to more than 2,000 milligrams per liter. Concentrations of trace elements, pesticides, and organic compounds generally were less than detection limits in the wells and springs sampled.
Streamflow characteristics are presented for 35 stations on 30 streams. Mean monthly flows at nine stations having long-term, continuous records range from about 2 to greater than 10,000 cubic feet per second. The highest streamflows generally occur in March or April, whereas the lowest occur in August through October. Mean annual flows range from approximately 13 to nearly 6,000 cubic feet per second. One-hundred-year peak flows at 33 stations range from 147 to 39,950 cubic feet per second. Seven-day, lO-year low flows at 28 stations (excluding the two stations on the Potomac River) range from 0.0 to 66 cubic feet per second, and, areally, from 0.000 to 0.425 cubic feet per second per square mile. Areally, annual high flows tend to be lower in the eastern drainage basins than in the western basins but low flows are generally greater, and flow-duration curves for the eastern basins are less steep than for the western basins . Most stream waters are calcium bicarbonate types. Nine trace elements were detected in stream-bottom materials from some of the 15 sites sampled. Seventeen pesticides and related compounds were detected and 11 were not detected in samples of stream-bottom materials obtained from 18 sites.
Hydrologic budgets were estimated for 28 drainage basins. An average annual budget is precipitation (39.6 inches) = subsurface runoff (9.6 inches) + surface runoff (5.5 inches) + evapotranspiration (24.5 inches) + change in storage (0.0 inches). Basins underlain by large proportions of carbonate rock are characterized by higher proportions of subsurface runoff compared to noncarbonate basins.