Reports
The water resources of Caroline, Dorchester, and Talbot Counties
1957, Rasmussen, W.C., Slaughter, T.H., Hulme, A.E., and Murphy, J.J.
Bulletin 18
Abstract
Caroline, Dorchester, and Talbot Counties, the middle three counties of the Eastern Shore, have abundant ground water. Not less than 100 million gallons a day are available, about 9 times the current use. The potential yield of the Pleistocene and Pliocene(?) series at depths of 30 to 100 feet is estimated to be 60 million gallons a day. Many million more gallons of mineralized water for restricted use are available from sands not yet penetrated at depths of 1,600 to 3,000 feet.
Water is produced from 10 aquifers which range in depth from the surface to more than 1,400 feet. Three of those aquifers are used extensively down to depths of 600 feet. Two potential aquifers lie at depths from 900 to 3,300 feet.
Caroline, Dorchester, and Talbot Counties lie in the Atlantic Coastal Plain province which is underlain by a huge wedgeshaped mass of sediments resting upon a sloping surface of hard crystalline rock of Precambrian and Paleozoic age, referred to as "the basement."
The Coastal Plain sediments range in age from Early Cretaceous to Recent. Beneath Caroline, Dorchester, and Talbot Counties, the Coastal Plain deposits range from 2,000 to 4,000 feet in thickness. They are composed of sands, greensands, gravels, silts, clays, shales, and shell beds. In general, the sands and gravels are porous and permeable and yield water freely; the finer-grained beds contain water but yield it slowly or not at all. The basement complex slopes generally to the southeast as do the overlying sediments, most of which thicken down the dip so that the slope of the upper formations is not as great as that of the lower formations.
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.
The lower Cretaceous rocks may contain good aquifers and good water but are so far below the surface that they have not been explored. The formations of the Upper Cretaceous series and of the Tertiary and Quaternary systems contain the principal aquifers. The significant aquifers in the area are in the Piney Point, Aquia, Choptank, and Calvert formations and in the Pleistocene and Pliocene(?) series. The most important of these on the basis of present development and probably potential yield are the Piney Point Formation and the Pleistocene and Pliocene(?) series.
Overlying the crystalline rocks is a series of sands, clays, and shales correlated with the Patuxent formation, part of the Lower Cretaceous series. The top of the Patuxent formation probably occurs about 1,600 feet below land surface on the northwestern margin and about 3,300 feet below land surface on the southeastern margin of the area. The formation increases in thickness from 600 feet to 800 feet in a southeasterly direction. The top of the formation dips to the southeast at an average rate of 50 feet to the mile. Regional geologic evidence warrants the assumption that the sands in the formation may yield large quantities of water, most of it, however, probably too highly mineralized for most purposes.
Overlying the Lower Cretaceous series is a thick group of shales and sands correlated with the Upper Cretaceous series. This series, divided into six formations, has an estimated thickness ranging from 1,000 feet along the southwest margin of Talbot County to 2,500 feet along the southeast margin of Dorchester County. The dip of the top of the series is about 8 feet per mile.
The lower three formations, the Arundel, Patapsco and Raritan, are composed of silty sands intercalated with silty clays. The cleaner sands are potential sources for large quantities of warm water at depths ranging from 1,000 to 3,500 feet. Water from a depth of 1,351 to 1,420 feet in a sand of Raritan(?) age, flowing from a test well at Wades Point, Talbot County, was irony but otherwise good. Water from the Raritan(?) from a flowing well at Church Creek was also reported “good.”
The Raritan formation is unconformably overlain by the Magothy formation, the most persistent aquifer of the Cretaceous system in Maryland. The formation consists of “sugary” sands and irregular lenses of lignitic clay. Thickness of the Magothy formation in this area ranges from 43 feet to 135 feet. The formation yields fairly large quantities of water of remarkably good quality to seven wells in Dorchester and Talbot Counties from depths of 800 to 1,100 feet below sea level.
The Matawan(?) formation unconformably overlies the Magothy formation. It consists of micaceous glauconitic clays and glauconitic sands encountered at depths from about 700 to 900 feet below sea level. The Matawan(?) formation functions as an aquifer in Caroline and Talbot Counties and as an aquiclude in Dorchester County. Lying conformably above the Matawan(?) formation is the Monmouth formation, which functions almost entirely as an aquiclude. The Monmouth is a glauconitic silty clay and clayey sand. The formation ranges from 34 feet to 230 feet thick. The top of the formation dips an average of 8 feet per mile in a southeasterly direction.
The Tertiary system includes the most important aquifers and thickest aquicludes.
The Paleocene series consist of alternate hard and soft beds of clay and sparsely glauconitic sand, with thin beds of shells and chalk(?). In general, it functions as an aquiclude, but it does yield water to a few wells. The Paleocene ranges in thickness from about 70 feet to more than 300 feet.
The major artesian water-bearing beds in use in Caroline, Dorchester, and Talbot Counties are in the Eocene series. The Eocene series is represented in ascending order by the Aquia greensand, the Nanjemoy formation, and the Piney Point formation.
The Aquia greensand is an important aquifer in the western half of Talbot County and northwestern Dorchester County. The aquifer is composed of green quartz sand, moderately glauconitic, with a few lenses of clay and occasional hard beds. The top of the formation dips from 255 feet below sea level at Claiborne, Talbot County, to 605 feet below sea level about 4 miles west of Cambridge, Dorchester County, where the formation presumably wedges out. The rate of dip is about 25 feet per mile. A maximum thickness of 231 feet is recorded at Wades Point, Talbot County. The average specific capacity for 99 wells producing water from the Aquia greensand is 2.0 gpm per foot of drawdown, indicating that high rates of yield will cause large drawdowns. The quality of water from the Aquia greensand is good for almost all purposes.
The Nanjemoy formation is primarily a leaky aquiclude, composed of blackish-green, highly glauconitic sand, silt, and clay. The formation is very irregular in thickness, apparently owing to erosion of its upper surface. The average recorded thickness is 166 feet. The formation slopes toward the southeast. So far the Nanjemoy formation yields water only to open holes in conjunction with formations above or below.
The Piney Point formation is the major aquifer in the area. It probably underlies all of Caroline, Dorchester, and Talbot Counties, although in places it is quite thin. It ranges from a few feet to more than 200 feet in thickness and averages 74 feet in logged wells. The Piney Point formation is a quartz sand, somewhat glauconitic, ranging in color from brown to olive-green to green. The top of the formation dips to the southeast at an average rate of 29 feet per mile. An estimated 1.9 billion gallons of water a year is produced from the Piney Point formation. Wells yielding more than 600 gpm have been developed at Cambridge. The water is of good quality.
The Miocene series contains relatively thin aquifers. The series is represented by its middle and upper parts, called the Chesapeake group, and is composed of the Calvert, Choptank. St. Marys formations of middle Miocene age and the Yorktown formation of late Miocene age. The Chesapeake group in the southern tip of Dorchester County is represented by a sand of late Miocene age, tentatively correlated with the Yorktown formation of Virginia and the Cohansey sand of New Jersey.
The Calvert formation is generally an aquiclude; however, in places it functions as an aquifer yielding small quantities of water to numerous wells, and moderate to large quantities of water at Cordova, Easton, Federalsburg, Hurlock, and Vienna. The Calvert formation is predominantly a silt, slightly glauconitic in the upper part and diatomaceous in the lower part, and contains several lenticular beds of sand. The top of the Calvert formations dips an average of 10 feet per mile toward the southeast. It ranges in thickness from 20 feet in western Talbot County to more than 300 feet in southeastern Dorchester County. The aquifers are of moderate to low productiveness. The average specific capacity is 2.7 gpm per foot of drawdown. The water from the Calvert formation is usable for most purposes.
Conformably overlying the Calvert formation is the Choptank formation. It functions as an aquifer, its principal development being in Caroline County. The formation consists of lenticular beds of sand and silt with shell marl. The top of the formation ranges from 55 feet above sea level in northern Caroline County to more than 200 feet below sea level along the Nanticoke River in the southeast margin of the area. The rate of dip is about 4 feet per mile. The Choptank formation ranges in thickness up to about 150 feet and averages about 80 feet. The average specific capacity is 2.4 gpm per foot of drawdown. The Choptank formation is capable of producing small to moderate yields beneath most of its area and moderately large yields at a few locations. In general, the quality of water is good; but in northern Caroline County it is very irony (12-13 ppm).
The St. Marys formation overlies the Choptank formation principally as a clayey silt and silty clay aquiclude with stringers and small lenses of sand. It blankets the southeastern half of the area. The top ranges from 58 feet above sea level at Goldsboro, Caroline County, to 83 feet below sea level at Elliott Island, Dorchester County. The thickness of the formation ranges up to about 120 feet and averages about 35 feet. Water is produced from local stringer sands, generally in rather small amounts per well.
The Yorktown and Cohansey formations(?) are sand containing a few shells. They are a small wedge of sediment at the low and marshy southern end of Dorchester County, resting upon the St. Marys formation. No wells were found in this area, presumably because the water is unpalatable. The dip on the formation is southeasterly at the rate of 8 to 10 feet per mile. The formation ranges in thickness from 20 to 62 feet.
A red and orange gravelly sand is tentatively correlated with the Brandywine and Bryn Mawr formations of Pliocene(?) age. The average thickness of the sand is estimated at 10 feet. It yields water to large-capacity wells at Ridgely, Preston, Hurlock, and Cordova. The red gravelly sand functions with the overlying formations of Pleistocene age as a single aquifer. The quality of water from the Pleistocene and Pliocene(?) series, in general, is the best of all the ground waters.
The Pleistocene series comprises all of the shallow yellow, buff, and tan deposits of sand, silt, and clay between soil zone and the Pliocene(?) red gravelly sands. The average thickness of the combined Pleistocene and Pliocene(?) series is 37 feet. The Pleistocene deposits yield water to more wells in Caroline, Dorchester, and Talbot Counties than any other series of sands. The specific capacities of wells in the Pleistocene and Pliocene(?) series are higher than those for any other formation, ranging up to 30 gpm per foot. The quality of ground water from the Pleistocene series is remarkably good.
Ground water satisfactory in quality for ordinary uses can be obtained at most places in Caroline, Dorchester, and Talbot Counties.
