Hydrogeology and ground-water resources of Somerset County, Maryland
1990, Werkheiser, W.H.
Somerset County, Maryland, an area of about 597 square miles, relies on ground water for 84 percent of its water supply. Development in the county is expected to substantially increase the demand for water; hence, an assessment of the ground-water resources was conducted to collect baseline information against which future change can be measured. Specific goals were to (1) refine the understanding of the hydrogeologic framework; (2) describe the quality of ground water; and (3) evaluate the effects on the ground-water-flow system of projected ground-water withdrawals at Princess Anne and Crisfield.
Somerset County is in the Atlantic Coastal Plain physiographic province, and is underlain by a wedge of unconsolidated sediments that forms a series of aquifers and confining units. The aquifers and aquifer systems that supply water are (1) the surficial aquifer system; (2) the Pocomoke aquifer; (3) the Manokin aquifer; (4) the Choptank aquifer; (5) the Piney Point aquifer; (6) the Paleocene aquifer system; and (7) the Potomac aquifer system.
The surficial aquifer system consists of fine-grained sediments, is relatively thin throughout much of the county, and is used primarily for domestic water supply. The aquifer system may produce more water in the northeastern part of the county, where it is coarser-grained and thicker. Chemical analyses of four samples suggest the water is soft to moderately hard and slightly acidic. In areas containing anoxic water, dissolved iron may be present in elevated concentrations. In areas of oxygenated water, nitrate concentrations may be elevated where water comes into contact with nitrogen sources.
The Pocomoke aquifer is present only in the southeastern part of the county. The median reported specific capacity of 68 wells tapping the unit is 10 gallons per minute per foot [(gal/min)/ft]. Specific capacity from five 1-hour tests performed during the investigation ranges from 2.0 to 17.3 (gal/min)/ft. The most common water-quality problems associated with the Pocomoke aquifer are elevated concentrations of iron and manganese. All but 2 of 24 samples exceeded the U.S. Environmental Protection Agency (USEPA) secondary maximum contaminant level (SMCL) for iron and 18 of 24 exceeded the SMCL for manganese.
The principal aquifer of use in Somerset County is the Manokin aquifer. The median reported specific capacity of wells in the unit is 5.2 (gal/min)/ft. Hydraulic conductivity, calculated from five aquifer tests near Princess Anne, averages 13.2 feet per day (ft/d), and storage coefficients range from 0.0002 to 0.001. Throughout much of the county, water-level altitudes in the aquifer are below sea level, with the lowest near Princess Anne. Because ground-water flow is chiefly toward the pumping centers at Princess Anne, poor-quality water from the southern part of the county, or the Chesapeake Bay, could migrate in that direction.
There is marked areal variation in the quality of water in the Manokin aquifer. North of Westover, water is soft to moderately hard, relatively low in dissolved solids, and of the sodium bicarbonate type. South of Westover, water is moderately hard to hard, contains higher concentrations of dissolved solids, and is of the sodium chloride type. In the northeastern corner of the county, iron concentrations generally exceed the USEPA SMCL of 0.3 milligrams per liter (mg/L). In the southern part of the county, chloride concentrations generally exceed the USEPA SMCL of 250 mg/L. The 250-mg/L isochlor trends roughly from Pocomoke City to Deal Island and passes about 1 mile (mi) north of Westover. The 500-mg/L isochlor is located about 2 mi southwest of the 250-mg/L isochlor.
Few wells produce water from the Choptank or Piney Point aquifers. Water from the Choptank aquifer is reported to contain chloride concentrations in excess of 900 mg/L, and water from the Piney Point aquifer contains dissolved solids in excess of 1,000 mg/L.
The Paleocene aquifer system is used only by the town of Crisfield as a source of water supply. Reported specific capacity of two wells screened entirely in the aquifer system is nearly 2 (gal/min)/ft. The Potomac aquifer system supplies water to several municipalities along the Chesapeake Bay. Reported specific capacities of four wells range from 1 to 7 (gal/min)/ft. Transmissivity and storage coefficient, estimated from a multiple-well aquifer test, are 2,140 ft2/d (feet squared per day) and 0.0002, respectively. Transmissivity, estimated from a single-well aquifer test, is 1,280 ft2/d.
Water from the Paleocene and Potomac aquifer systems is soft, has concentrations of dissolved solids ranging from 475 to 1,070 mg/L, and has the highest pH of any ground water in the county. The water is of the sodium-bicarbonate type, with sodium comprising more than 95 percent of the cations. Seven of 10 water-quality samples contained fluoride concentrations above the SMCL and two of these exceeded the USEPA primary maximum contaminant level (MCL). The extent and water quality of the two aquifer systems east of Crisfield are not known.
A digital, steady-state, ground-water-flow model was used to evaluate the effects of projected increases in pumpage of 600,000 gallons per day (gal/d) from the Manokin aquifer near Princess Anne. Simulated water levels in the Manokin aquifer ranged from 15 to 70 feet (ft) below those measured in November 1986. Using model-derived ground-water velocities, it would take about 50 years for ground water to move from the vicinity of the 250-mg/L isochlor to the nearest simulated pumped well at Princess Anne. A travel time of 300 years was estimated for the distance from the 500-mg/L isochlor to the same well. The analysis did not include the effects of dispersion, which could hasten the first arrival of brackish water (chloride concentration greater than 250 mg/L).
A non-steady analytical solution was used to estimate additional water-level declines that may result from projected pumpage from the Paleocene and Potomac aquifer systems in the Crisfield area. Using a range of transmissivity and a storage coefficient from aquifer tests, additional water-level declines were estimated to range from 7 to 31 ft. It is not known if poor-quality water is migrating toward the pumping centers, because the nature of the aquifer systems east of Crisfield is unknown.