Reports
Availability of fresh ground water in northeastern Worcester County, Maryland with special emphasis on the Ocean City area
1974, Weigle, J.M.
Report of Investigations 24
Abstract
In the northern part of the area the Ocean City aquifer occurs between the Pocomoke and Manokin aquifers. Near Ocean City the Manokin and Ocean City aquifers are separated so poorly, hydraulically, that they function as a single hydrologic unit. As of June 1972, the entire public supply at Ocean City was obtained from wells in the Ocean City aquifer. The Manokin aquifer was virtually untapped there or elsewhere in northeastern Worcester County.
The Ocean City-Manokin aquifer system is potentially very productive. Its transmissivity averages about 10,000 ft2/day (feet squared per day) and may exceed 20,000 ft2/day in the northwestern part of the area. Beneath Ocean City, the transmissivity of the Ocean City-Manokin aquifer system averages about 14,000 ft2/day, or twice the value previously assumed for the Ocean City aquifer. Transmissivity of the Pleistocene and Pocomoke aquifers, by comparison, averages, respectively, 5,000 and 4,000 ft2/day in the area studied.
Potential short-term ground-water yields from the Ocean City-Manokin aquifer system (maintainable for several hours at a time) are estimated generally to exceed 1,000 gpm (gallons per minute), and may exceed 12,000 gpm in the northern part of the area. Yields from the Pleistocene and Pocomoke aquifers are estimated to range generally from 500 to 2,000 gpm each.
A cone of depression in the Ocean City-Manokin potentiometric surface results from ground-water withdrawals at Ocean City which averaged 3.04 million gallons per day in 1971. It changes cyclically in size and shape, principally in response to seasonal changes in the rate of withdrawal. In summer the cone attains a diameter of 12 miles or more; and in mid-summer of 1971, for example, the potentiometric surface was inferred to be below sea level approximately 3 miles offshore from Ocean City.
In autumn and early winter the cone recovers because of greatly reduced withdrawals. The rate of recovery is related principally to the amount and temporal distribution of nearby precipitation, which affects the Ocean City-Manokin water levels relatively quickly. More water is withdrawn from the Ocean City-Manokin aquifer system at Ocean City than is available from underflow in that system, yet the water levels in the Ocean City-Manokin system showed a net rise, indicating an increase in storage during the period of study (1970-72). This suggests that there is considerable recharge to the Ocean City-Manokin system by vertical leakage from the water-table deposits, probably on the nearby mainland.
Interaquifer leakage is important in northeastern Worcester County. At Ocean City, water not only from the Ocean City-Manokin system, but from the overlying units as well could probably be drained by long-continued large-scale withdrawals from the Ocean City-Manokin system.
In 1971, 1.1 billion gallons of water was pumped from the Ocean City-Manokin aquifer system. It is estimated that several times that rate of withdrawal could be sustained by the Ocean City-Manokin system indefinitely if the entire thickness of the aquifer were utilized effectively. However, there is a possibility of salt-water encroachment.
No evidence of contemporary salt-water intrusion was found in either the Ocean City-Manokin aquifer system or the Pocomoke aquifer at Ocean City or elsewhere in the area. Nevertheless, salty water from the adjacent ocean and bays or underlying salty aquifers could contaminate the Ocean City-Manokin system at Ocean City by vertical leakage to the extent permitted by the permeabilities of the intervening beds.
Salty water in the Manokin itself may be a more serious threat to the Ocean City water supply, especially if the salt-water/fresh-water interface in the aquifer is near shore. Water from seaward of the interface can move rather freely toward the well fields in response to head differences in the Manokin because under those circumstances lateral movement within the aquifer predominates.
The salt-water/fresh-water interface in the Manokin lies offshore from Ocean City, probably approximately parallel to the coastline although its proximity to shore is unknown at this time. If, by test drilling or by monitoring outpost observation wells the salt-water front in the Manokin is discovered to be dangerously close to the Ocean City well fields, the effects of the salt water on the public supply could be postponed by drilling new wells west of the present well fields.
