Reports
The water resources of St. Mary's County
1953, Martin, R.O.R. and Ferguson, H.F.
Bulletin 11
Abstract
St. Marys County forms the southernmost part of the Western Shore of Maryland. Tidal water of the Chesapeake Bay and adjoining estuaries surrounds the county on the northeast, east, south, and west; Charles County adjoins it on the northwest. The county lies entirely within the Coastal Plain province and is underlain by unconsolidated sediments of Early and Late Cretaceous, Paleocene, Eocene, Miocene, and Pleistocene ages. These sediments consist chiefly of sand, clay, and gravel. With the exception of the Pleistocene deposits, which are essentially flat lying and form a thin cover over the underlying formations, the Coastal Plain formations dip gently to the east and southeast. In general, ground water occurs under water-table conditions in the Pleistocene sediments and under artesian conditions in the underlying formations.
The Coastal Plain sediments are 2,000 to 3,000 feet thick in St. Marys County and contain many water-bearing formations; however, only the water-bearing formations in the Upper Cretaceous, Eocene, and Pleistocene deposits are utilized as sources of ground water. Most of the wells are less than 500 feet deep and draw water chiefly from one or another of three aquifers, the Aquia greensand of Eocene age, the Nanjemoy formation and sediments of Jackson age, also of Eocene age, and the Pleistocene sediments. The Aquia greensand yields water to domestic and farm wells in the northern and western parts of the county and to most of the public-supply wells. The communities of Leonardtown, Lexington Park, and St. Clement Shores and the Patuxent Naval Air Station and the Naval base at Piney Point obtain a total of 1,500,000 to 2,000,000 gallons of water a day from the Aquia greensand. The Nanjemoy formation and the sediments of Jackson age yield water to domestic, farm, and some public-supply wells in the central and southern parts of the county. Approximately 1,000,000 gallons a day is pumped from this aquifer. The Pleistocene sediments yield adequate supplies of water for most domestic and farm wells in the upland parts of the county; the total pumpage is estimated at 400,000 to 500,000 gallons a day.
Water-level measurements and old records of water levels indicate that pumping has caused the artesian head of the artesian aquifers to decline in areas of heavy pumping. Water-level fluctuations in the water-table wells are caused chiefly by the seasonal and annual variations in precipitation.
The chemical character of the ground water is satisfactory for most uses. The Aquia greensand yields moderately hard calcium bicarbonate water in the northern part of the county and soft sodium bicarbonate water in the central and southern parts. The aquifer comprising the Nanjemoy formation and sediments of Jackson age yields moderately hard calcium bicarbonate water in the central part of the county and soft sodium bicarbonate water in the southern part. The Pleistocene sediments contain water usually low in total mineral content, but locally excessive in iron.
Water-bearing sands are present in formations of Cretaceous age below the Eocene formations but are tapped by few wells at present because the Eocene aquifers have furnished adequate supplies to meet the needs so far. The Cretaceous aquifers constitute an important potential source of ground water in the county.
