Reports
Ground-water occurrence in the Maryland Piedmont
1969, Nutter, L.J. and Otton, E.G.
Report of Investigations 10
Abstract
Factors governing the occurrence of ground water in the crystalline rocks of the Maryland Piedmont are complex and variable. The crystalline rocks consist of schist, gneiss, granite, quartzite, marble, metagabbro, and other rock types of lesser importance. The rocks of the area range in geologic age from Precambrian to early Paleozoic.
Analysis of the hydrologic cycle, an important factor governing the occurrence of ground water, indicates that effective ground-water recharge is about 11 inches per year, or about 500,000 gpd (gallons per day). Much of the ground water in the area is stored in the weathered zone (saprolite), having an average thickness of about 45 feet. Many wells yield water occurring in joints, fractures, and other crevices in the rocks; the occurrence and distribution of these features are difficult to predict, but the drainage pattern and surface topography of the Piedmont reflects the existence of major joint systems in the rocks. Statistical analysis of well yields and specific capacities shows that wells situated in valleys or draws yield three to four times as much water as wells on hilltops or divides. Based on an analysis of more than 1,300 well records, little significant difference was observed for the yield of wells in different rock types, although, based on mean yields, wells in marble are the most productive. The mean yield of all wells is about 9 gpm (gallons per minute) and the mean specific capacity is 0.6 gpm per foot of drawdown. Most of these wells were drilled for domestic use.
The ability of the crystalline rocks to transmit and store water is shown by aquifer tests to be extremely variable. Coefficients of transmissibility range from less than 100 to 35,000 gpd per foot. Coefficients of storage, based on aquifer tests, range from 0.02 to 0.05. However, the long-term gravity yield of the regolith (the zone contributing to the base flow of streams) is about 8 percent.
Electric, gamma-ray, temperature, and caliper logs of several wells in the crystalline rocks provide a useful means of identifying water-bearing zones and identifying changes in the character of the rocks.
Statistical analysis of wells grouped by 100-foot depth intervals indicates a decreasing probability of obtaining a specified yield with increasing depth below 100 feet. A map in the report shows the extreme areal variability of the yields of wells in the crystalline rocks, and indicates specific areas where the wells are of lower than average productivity. Wells in other areas are shown to have higher than average productivity.
