Hydrogeologic framework and the distribution and movement of brackish water in the Ocean City-Manokin aquifer system at Ocean City, Maryland
1993, Achmad, G. and Wilson, J.M.
Report of Investigations 57
Ocean City, Maryland, is a coastal resort on the Atlantic Ocean. The town 's public-water supply is provided by the Ocean CityManokin aquifer system, which in 1990 supplied about 2,000 million gallons of water to the town. The uppermost aquifer at Ocean City is the unconfined to semiconfined Columbia aquifer. The Columbia aquifer is comprised of the Beaverdam Sand and overlying surficial units. Underlying the Columbia aquifer in order of increasing depth are the Pocomoke, Ocean City, Manokin, and Choptank aquifers. In the northern part of Ocean City, the Pocomoke aquifer is absent.
The Ocean City and Pocomoke aquifers contain only freshwater. At the Town of Ocean City's 44th Street well field, however, chlorides in the Ocean City aquifer rose from about 70 mg/L in 1975 to about 215 mg/L in 1988 due to upconing of brackish water from the underlying Manokin aquifer. Brackish water occurs in parts of the Columbia and Manokin aquifers at Ocean City although both these aquifers are predominantly fresh. The Choptank aquifer contains only brackish water. The confining unit between the Choptank and overlying Manokin aquifer, the St. Marys Formation, is brackish.
At Ocean City the upper Miocene Manokin formation (an informal unit) consists of a series of stacked coarsening upward sand sequences that comprise the Manokin aquifer. The Manokin formation is the product of a fluvial dominated, high-constructive delta system that was subject to some marine reworking along the delta front. Truncation and channelling within the Manokin formation have resulted in breached and discontinuous confining beds that influence the distribution of chlorides within the Manokin aquifer.
The upper Miocene "Ocean City beds" (an informal unit) overlie the Manokin formation at Ocean City and contain the Ocean City aquifer. The Ocean City beds are comprised predominantly of fine to coarse sand in the central and southern parts of Ocean City. In the northern section of Ocean City, the Ocean City beds are predominantly glauconite- and shell-bearing, clayey silts and fine clayey sands that contain discontinuous 10 to 25 foot thick beds of fine to medium sand. The lithofacies of the Ocean City beds have several hydrologic implications. The Ocean City aquifer has a higher transmissivity in the central and southern parts of the island than further north because of its coarser texture and greater thickness. Additionally, in the central part of Ocean City, infilled distributary channels have almost completely breached the confining unit between the Manokin and Ocean City aquifers and formed pathways for the upconing of brackish water from the Manokin aquifer.
Aspects of the hydrogeologic framework that influence the distribution of chlorides in the Manokin aquifer are: 1) the altitude of the contact between the basal sand facies of the Manokin aquifer and the underlying St. Marys Formation and 2) the effectiveness of the confining units within the Manokin aquifer. Chloride distribution in the Ocean City aquifer is controlled by 1) the effectiveness of the confining unit between the Manokin aquifer and the Ocean City aquifer, 2) the chloride distribution in the sands of the Manokin aquifer that underlie the Ocean City aquifer, and 3) head differences between the Ocean City aquifer and the Manokin aquifer caused by pumpage from the Ocean City aquifer.
A ground-water-flow model was constructed to determine the effects of increased pumpage on the ground-water-flow system at Ocean City. An annual average pumping rate of 9.1 Mgal/d was assigned to the model to simulate the expected amount of pumpage required from the Ocean City well fields in 2010. The increased pumpage, approximately 1.6 times the 1990 pumpage, expanded and deepened the cones of depression in the Manokin, Ocean City, and Pocomoke aquifers.
A particle-tracking program applied to the results of the flow model indicated that most of the recharge particles backtracked from the 44th Street and Gorman Avenue well fields originated inland of Ocean City in freshwater recharge areas of the Ocean City-Manokin aquifer system . However, some particles followed flowlines that went through the offshore part of the freshwater-saltwater mixing zone before arriving at the well fields. Mass balance calculations for the pumping cells indicate that lateral encroachment from the offshore part of the freshwater-saltwater mixing zone is the major source of brackish water in the Manokin aquifer at the Gorman Avenue well field. At the 44th Street well field, mass balance calculations for the pumping cell indicate that annual average pumping rates greater than 1.6 Mgal/d result in increasing chloride concentrations in the Ocean City aquifer because of greater upward leakage from the brackish Manokin aquifer.A cross-sectional solute-transport model was developed for the 44th Street and Gorman Avenue well fields in order to simulate chloride distributions in the coastal aquifers. The simulation produced an offshore plume of fresh to brackish water in the Ocean CityManokin aquifer system that extended over 13 miles Qffshore. Using that result as an initial condition, the model simulated annual average pumping rates at the 44th Street well field of 2.6, 3.3, and 4.4 Mgal/d; chloride concentrations in the pumping cell in 2010 were about 230, 235, and 243 mg/L respectively. Simulated annual average pumping rates of 4.5 and 9 Mgal/d at the Gorman Avenue well field resulted in chloride concentrations in the pumping cells of about 170 and 185 mg/L respectively in 2010.