Reports
The water resources of Somerset, Wicomico, and Worcester Counties
1955, Rasmussen, W.C., Slaughter, T.H., Meyer, R.R., Bennett, R.R., and Hulme, A.E.
Bulletin 16
Abstract
Somerset, Wicomico, and Worcester Counties, the lower three counties of the Eastern Shore, have abundant ground water available for development. A conservative estimate indicates 360 million gallons per day of water suitable for most purposes available for an indefinite period from water-bearing beds within the uppermost 500 feet of the sedimentary sequence. This is about 30 times as much as the current use, estimated at 12.4 million gallons a day. Many million more gallons of somewhat mineralized water are available for restricted uses or for general purposes after treatment.
The water occurs in 14 aquifers, which range in depth from the surface to more than, 7,700 feet below the surface. Four of these aquifers are used extensively down to depths of 300 feet. Eight of the aquifers are used to a slight extent in most of the area but to an important extent locally, and wells in them produce from depths as great as 1,706 feet. Two of the aquifers lie at depths of several thousand feet and have not been tapped for water.
Somerset, Wicomico, and Worcester Counties are part of the Atlantic Coastal Plain. The land forms of the Coastal Plain have an important effect upon the retention and infiltration of rainfall, the retardation of runoff, and the discharge of ground water by evapotranspiration. Remnants of six coastal marine terraces account for the flatness of the landscape and the low stream gradients. Poorly drained oval-shaped depressions, ranging in size from 7 acres to over 17,000 acres, bounded by sandy rims of low relief are the most important minor land form. Meandering tidal streams, rejuvenated headwaters, older remnant barrier beaches, dunes, and periglacial soils are other land forms that control the entrance and discharge of ground water.
Above the basement, at depths ranging from 4,000 to 7,850 feet, brown shales, intercalated gray sands and shales, red and bottle-green sandstones, and an indurated basal conglomerate comprise 135 to 585 feet of rock which forms a doubtful aquifer, probably containing warm, highly mineralized water. It is correlated with the Triassic system. Overlying the Triassic rocks is a series of thick sands and thin shales, 600 to 2,300 feet in thickness, correlated with the Patuxent formation of Early Cretaceous age. These are overlain by more than 3,000 feet of thick sands and shales of the Patapsco, Arundel, and Raritan formations of Late Cretaceous age, which generally contain salty and brackish water. One well yields a large flow of useable water from the Raritan formation at Smith Island, Somerset County.
The Raritan formation is overlain by the Magothy formation, also of Late Cretaceous age, consisting of lignitic sand and shale, 30 to 120 feet thick, representing a transition from underlying non-marine sediments to overlying marine sediments. The Magothy formation is a persistent aquifer, encountered at depths ranging from 760 feet below sea level on the west to 2,400 feet below sea level on the east. Large to moderate yields of usable water are obtained from flowing wells at Crisfield and Smith Island in southwestern Somerset County.
Overlying the Raritan formation are the Matawan and Monmouth formations, the uppermost units of the Upper Cretaceous series. They are dark-green glauconitic sands and lead-gray clays, containing marine shells and Foraminifera. They function as an aquiclude.
The Cretaceous system is succeeded by the Tertiary system, predominantly marine sands and clays, divided from oldest to youngest into the Paleocene, Eocene, Miocene, and Pliocene series.
The Paleocene series consists of alternate beds of gray, green, and brown clay and gray glauconitic sand. The sand yields water to a few wells of moderate to large capacity at depths of about 1,000 feet at Crisfield in Somerset County.
The Eocene series represented chiefly by a white quartz sand and glauconitic greensand, equivalent of the Jackson group, yields moderate quantities of slightly saline water to wells on Deal Island and Rumbley, Somerset County, at depths of 588 and 726 feet, respectively, and to a well on the Isle of Wight, Worcester County, which has been flowing for 40 years, yielding a highly mineralized warm water from 1,706 feet depth. A deep city well at Crisfield derives a large quantity of potable water in part from this group.
The Miocene series contain the important artesian aquifers which are, in general, reached within 1,000 feet of the land surface in Somerset, Wicomico, and Worcester Counties.
The lowermost formation of the Miocene series is the Calvert, composed of gray diatomaceous silts and clays, containing lenses of gray sand, shell beds and Foraminifera. It is generally an aquiclude, about 400 feet thick, but it does contain the Nanticoke aquifer, named for production of water in the tributary area of the Nanticoke River, at Sharptown and Mardela Springs in Wicomico County, and Vienna in Dorchester County. The aquifer is a gray sand, about 40 feet thick, at the top of the formation, between 200 and 500 feet below land surface. According to a short aquifer test at Fruitland, central Wicomico County, it has a coefficient of transmissibility of 5,500 gpd/ft, and a coefficient of storage of .00011. The water is soft, high in sodium bicarbonate, and low in iron.
Overlying the Calvert formation is the Choptank formation, a gray and brown sand and clay containing shell marl and Foraminifera. The Choptank averages about 120 feet thick, and functions as an extensive aquifer, but it yields water high in dissolved solids.
The St. Marys formation, overlying the Choptank formation, is an extensive clayey-silt aquiclude. It is not known to yield water from wells, but it performs a useful function by preventing the brackish waters of the underlying Choptank formation from contaminating the waters of the overlying Yorktown and Cohansey formations(?).
The Yorktown and Cohansey formations(?) contain two important aquifers and two aquicludes. The basal unit, the Manokin aquifer, is extensively developed in the environs of the Manokin River, Somerset County. It is overlain by a clayey silt, called the lower aquiclude. The Pocomoke aquifer, extensively developed in the tributary area of the Pocomoke River, is a persistent sand above the lower aquiclude. The Pocomoke aquifer is overlain by a bed of sandy clay called the upper aquiclude.
The Manokin aquifer is the principal water-bearing source for Princess Anne, Snow Hill, and Ocean City and provides large to small quantities of water to many wells over much of the tri-county area. Its intake belt is 6 to 8 miles wide, lying beneath a relative thin mantle of the formations of Pleistocene and Pliocene(?) age in western Wicomico County. It dips southeast about 10 feet to the mile to depths of more than 300 feet below sea level in the southeast corner of the area. It is a gray, coarse to fine sand, about 80 feet thick. The water is suitable for most purposes in the northern three-quarters of the area, but it has a high chloride content, over 250 ppm, and high dissolved solids in the southern fourth of the area.
The Pocomoke aquifer is the principal aquifer at Pocomoke City, and an important source for Ocean City. It is a gray, predominantly medium-grained sand, with an average thickness of 45 feet, which yields fairly large quantities of water to a few wells and moderate to small quantities to many wells, chiefly in Worcester County. The quality of water is suitable for most purposes. The intake zone, covered by a permeable mantle of Pleistocene and Pliocene(?) deposits, crosses Somerset, Wicomico, and Worcester Counties as a diagonal belt, 6 to 7 miles wide, from the mouth of the Big Annemessex River through Pittsville into the State of Delaware. The aquifer slopes southeasterly to a depth of more than 200 feet below sea level beneath Assateague Island.
The Miocene series is overlain by a red gravelly sand, deposited as a valley fill, and found in wells beneath most of the area. It does not contain fossils but is correlated by lithology with the Brandywine and Bryn Mawr formations of Pliocene(?) age, which occupy high-level terraces on the edge of the Piedmont. The red gravelly sand is the aquifer of highest permeability and locally of highest yield in the tri-county area. It is the principal aquifer for the city of Salisbury where tests show it to have an average coefficient of transmissibility of 100,000 gpd/ft. and a coefficient of storage of 0.15. The red gravelly sand functions with overlying sands of Pleistocene age as a single aquifer under water-table conditions. The waters are slightly irony and low in pH (6.3), but in other respects are the purest waters in the area.
The Quaternary system overlies the Tertiary system, with deposits of the Pleistocene and Recent epochs. The Pleistocene deposits form the most important aquifer in Somerset, Wicomico, and Worcester Counties. They are capable of large yields of ground water and supply about 72 percent of the wells inventoried. Berlin, Worcester County, has large municipal wells in the Pleistocene series. The Pleistocene deposits are predominantly sand, have an average thickness of 50 feet, and cover the entire area. The water is slightly irony to irony, but otherwise good.
In general the quality of the waters is good, but iron is a problem. There is the possibility of salt-water contamination in the coastal areas, particularly bordering Chesapeake Bay. The deeper aquifers are brackish to highly mineralized.
The problems of well construction, particularly the high screen loss which increases the pumping lift and thereby reduces the yield of wells, are described and illustrated, because it appears to place a special limitation on high capacity wells which have only 40 or 50 feet of available drawdown, such as those in the city of Salisbury. A hydrograph analysis of 14 years of record on Beaverdam Creek enabled determination of an average ground-water runoff of 602,000 gallons a day per square mile.
