Reports
Hydrogeology of the upper Chesapeake Bay area, Maryland, with emphasis on aquifers in the Potomac Group
1984, Otton, E.G. and Mandle, R.J.
Report of Investigations 39
Abstract
To gain a better understanding of the ground-water hydrology of aquifers in the Potomac Group on Maryland's Eastern Shore, six test holes were drilled through the Coastal Plain deposits in Cecil and Kent Counties. A similar test hole was drilled in Queen Annes County near Chestertown, Maryland , before the beginning of this study. Data from these holes are included here.
At each test site the Coastal Plain sediments were penetrated to crystalline basement rock, the altitude of which ranges from 1,121 feet below sea level at Stillpond Neck to 2,059 feet below sea level at Massey, Maryland. Thickness of the Potomac Group ranges from 1,038 feet at Stillpond Neck to 1,538 feet at Massey. Geophysical logs were run in each test hole to (1) help evaluate geologists' and drillers' logs, (2) help determine the depth and relative distribution of sand in the Potomac Group, (3) help determine the total dissolved solids of the ground water and (4) to locate appropriate zones in which to screen the observation wells.
Sand-percentage and total sand-thickness maps show that the sand percentage of the Potomac Group decreases with distance from the Fall Line , whereas total sand thickness increases. This is because of the greater increase in total thickness of the Potomac Group. Transmissivity and sand-unit thickness and sand percentage seem to be directly related. Most areas of reported high transmissivity are nearer to the Fall Line , where sand percentages are highest and individual sand units are generally thickest. Also, the sands close to the Fall Line tend to have higher porosities than the more deeply buried sands occurring farther away.
Analyses of multi-point electric logs suggest that brackish water is present in the Potomac Group at the Coast Guard Station on Stillpond Neck and the towns of Fairlee, Kennedyville, and Massey. Subsequent chemical analyses from Stillpond Neck and Fairlee showed chloride and dissolved-solids concentrations of 1,000 and 1,800 milligrams per liter, and 950 and 1,990 milligrams per liter, respectively. The relatively high chloride concentration in the aquifers of the Potomac Group is attributed to sea-water intrusion.
A geohydrologic cross-section shows that water enters the aquifers in the Potomac Group west of Chesapeake Bay and discharges to the Bay and its saline estuaries . There does not seem to be a significiant component of flow under Chesapeake Bay to the Delmarva Peninsula, except perhaps in the deepest aquifers. On the Delmarva Peninsula, water largely recharges the aquifers of the Potomac Group through overlying formations on the upper end of the Peninsula. It then appears to move laterally through the aquifers, then upward through overlying formations and discharges to the Chesapeake Bay and Delaware Bay.
This report presents geohydrologic data which can serve as a basis for modeling studies of aquifers in the Potomac Group. Such data probably will need to be supplemented with additional test drilling, however, because of the variable character of the strata of the Potomac Group.
