Reports
Ground-water resources of the Southern Maryland Coastal Plain
1955, Otton, E.G.
Bulletin 15
Abstract
The Southern Maryland area, comprising the five counties of Anne Arundel, Prince Georges, Calvert, Charles, and St. Marys, includes more than 1,900 square miles, and had a population of about 375,000 in 1950. The area lies within the Coastal Plain physiographic province and has a maximum relief of only about 460 feet. Approximately 60 percent of Southern Maryland is farmland. Tobacco is the chief crop.
The sedimentary rocks of Southern Maryland consist of sand, gravel, clay, sandy clay, shell beds, and marl, which range in geologic age from Early Cretaceous to Recent. They are underlain by a “floor” or “basement” of hard crystalline rocks, chiefly pre-Cambrian. The crystalline-rock floor slopes gently southeast from the Fall Zone along the northwestern boundary of the Coastal Plain, or roughly along U.S. Route 1 between Baltimore and Washington, D.C. The sedimentary rocks thicken wedge-like southeastward to a thickness of more than 3,000 feet at Solomons Island at the mouth of the Patuxent River in Calvert County.
The Patuxent, Patapsco, Raritan, and Magothy formations of Cretaceous age are the most important aquifers in the area. The yields of wells tapping the sand and gravels in these formation are, in a few localities, greater than 1,000 gallons per minute. The Aquia greensand and the Nanjemoy formation of Eocene age are the chief aquifers in Calvert and St. Marys Counties, although the yields of wells are seldom more than 400 gallons a minute. Many of the dug wells supplying farms and rural residents yield 5 to 20 gallons a minute from the sands and gravels of Pliocene(?) and Pleistocene age. Local precipitation is the source of essentially all ground water in Southern Maryland. The water in dug and drilled wells occurs under both water-table and artesian conditions. Some ground water moves vertically between the essentially horizontal beds, indicating the existence of so-called “leaky” aquifers. In some of the tidewater areas flowing wells are obtained when the deeper artesian strata are penetrated. In a few localities the artesian head has declined so that wells which formerly flowed at the land surface must now be pumped.
Of the slightly more than 20 million gallons of ground water pumped, or discharged, daily during 1951 in Southern Maryland, about 11.5 million gallons (54 percent) was used for domestic and rural consumption, about 5.5 million gallons (24 percent) for military and institutional purposes, and the remainder, 4.5 million gallons (22 percent) was for public-supply and commercial purposes. The Patapsco and Raritan formations furnished about 6.75 million gallons (more than 30 percent) and the Aquia greensand, the next important aquifer, furnished a little less than 4 million gallons (about 18 percent). The remainder, about 11 million gallons a day, was withdrawn chiefly from the Patuxent, Magothy, and Nanjemoy formations, and from deposits of Pliocene(?) and Pleistocene age.
Chemical analyses of about 275 samples of water from the major aquifers show the quality of the ground water is satisfactory for most uses. In a few localities the iron content is excessive (as much as 30 parts per million), and treatment for iron removal is necessary even for domestic use. Some of the water in eastern Anne Arundel County and northern Prince Georges County contains free acid and has a low pH. The hardness of the water from most aquifers is below 100 parts per million. The chloride and nitrate contents of uncontaminated water are commonly below 25 parts per million. The relation of the chemical character of the water to the geology and hydrology of the area is discussed. Base exchange, or natural water softening, takes place in some of the water-bearing strata.
On the basis of hydrologic and geologic data from pumping tests, well records, sample studies and piezometric and geologic maps, the available ground-water supplies in four subareas of the Southern Maryland peninsula were estimated. The estimates indicate the untapped ground-water supplies are extensive and constitute one of the most valuable natural resources of the area.
The report contains records of representative wells, drillers’ logs, and sample-study logs of key wells.
