Maryland Department of Natural Resources


Geohydrology of the fresh-water aquifer system in the vicinity of Ocean City, Maryland, with a section on simulated water-level changes

1982, Weigle, J.M. and Achmad, G.

Report of Investigations 37


Fresh ground water in northeastern Worcester County, Maryland, is restricted to a series of sediments about 450 feet thick between the land surface and the top of the St. Marys Formation. Water occurs in the Pleistocene aquifer primarily under water-table conditions but locally under semiconfined conditions. In the Pocomoke, Ocean City, and Manokin aquifers, water occurs under confined conditions.

Recharge originates as precipitation which is intercepted by the Pleistocene deposits throughout the area. The underlying aquifers are recharged by vertical leakage from the overlying Pleistocene deposits in the western part of the area, where heads are lower in successively deeper aquifers. Under natural conditions, most discharge occurs by upward leakage primarily in the eastern part of the area and offshore, where the relative head conditions are reversed. Lateral movement of the ground water is generally east-southeastward.

A cone of depression in the potentiometric surface of the Ocean City and Manokin aquifers is the result of pumping at Ocean City. Pumpage averaged 3.45 million gallons per day in 1976 and 8.1 million gallons per day in August of that year. The cone attained a diameter of more than 12 miles during the summer of 1976; in mid-summer of 1976, the potentiometric surface was inferred to be below sea level as far as 6 miles offshore from Ocean City. In autumn and early winter, the cone decreases considerably in size because of reduction in withdrawal.

Water needs for Ocean City and the nearby mainland are expected to substantially increase and most of this water will probably be withdrawn from the fresh-water aquifers underlying the area. Simulation of alternate ground-water development plans was made using a multi-layer aquifer model calibrated against six years (1971-1976) of historical pump age and water level data. The model showed that dispersal of pumping centers will reduce the amount of drawdown by spreading out the cones-of-depression. Simulated withdrawal of additional water needs from well fields at Gorman Avenue, the Isle of Wight, 100th Street, and 66th Street appears to produce more moderate drawdowns than the other pumping schemes that were run on the model. The results are considered only approximations, however, because some of the natural flow boundaries lie outside of the model area.

Excessive hydraulic stress in the aquifer system at Ocean City is of special significance because of the presence of salty water in the nearby ocean and back bays, and the possible occurrence of salty water offshore in the Manokin aquifer. During the past pumping conditions, no hard evidence of increasing saltiness was found in the Ocean City, Pocomoke, or Pleistocene aquifer. However, slightly salty water occurs in the basal part of the Manokin aquifer in the vicinity of Gorman Avenue, near the northern end of Ocean City. Salty water has apparently entered the basal part of the Manokin from the underlying St. Marys Formation, a clayey confining bed; however, the source of salty water could also be associated with an offshore salt-water front in the Manokin aquifer.