Reports
Water resources and estimated effects of ground-water development, Cecil County, Maryland
1988, Otton, E.G., Willey, R.E., McGregor, R.A., Achmad, G., Hiortdahl, S.N., and Gerhart, J.M.
Bulletin 34
Abstract
This report describes the results of a study of the ground- and surface-water resources of Cecil County, northeastern Maryland. Because of geologic differences, the county has two distinct types of terrane- the Piedmont and the Coastal Plain- that affect its water resources.
The crystalline rock in the Piedmont of Cecil County, Maryland, is highly indurated and contains free water only where the rock has been fractured or decomposed by weathering. Permeability of fractured rock depends on the number of fractures, the size of the fracture openings, and the interconnection of the fractures. Weathering increases the size of fracture openings, but is most significant in Cecil County because it has produced a mantle of unconsolidated, weathered rock at the land surface. The major significance of the weathered mantle is as a storage reservoir that supplies water to the fracture systems that supply water to wells. The median yield of wells in crystalline rock is 10 gallons per minute (gal/min). Median well yields for various topographic positions in the Piedmont are: flood plain and valley flat, 20 gal/min; upland draw, 14 gal/min; hilltop, 9 gal/min; and hillside, 8 gal/min.
The Coastal Plain sediments of Cecil County consist of unconsolidated stratified layers of clay, silt, sand, and gravel. The maximum thickness of Coastal Plain sediments is in the extreme southeastern corner of the county and is estimated to be about 1,600 feet (ft). Water occurs between grains in the sediments, and saturated sand and gravel constitute the aquifers. Interspersed in and grading laterally into the sand are clay and silt that act chiefly as confining and semiconfining layers. The major aquifers in Cecil County are the upper and lower Potomac aquifers. Yields of wells in the Potomac aquifers range from 0.5 to 703 gal/min and the median is 30 gal/min. The Magothy aquifer is the second most productive waterbearing unit in the county. Reported yields of 50 wells range from 7 to 270 gal/min; the median is 30 gal/min. The Monmouth is a major aquifer east of the Elk River and south of the Chesapeake and Delaware Canal. Reported yields of 25 wells producing from the Monmouth aquifer range from 8 to 42 gal/min; the median is 20 gal/min.
Decline in the water table in the Piedmont area caused by ground-water withdrawals tends to be local. By contrast, pumping from the Potomac aquifers causes widespread lowering of water level. Pumping of the Potomac aquifers has caused water levels to gradually decline; about 10 ft of decline has been measured in an observation well since 1967. A pronounced decline since 1983 has occurred near Elkton, Maryland, where about 20 ft of decline was measured in less than 3 years. Water levels near Elkton show effects of the intensive pumping from the Elkton well field and from another well field a few miles east in Delaware.
Generally, ground water in Cecil County is suitable for most uses except where it is contaminated . Dissolved-solids concentrations are generally low; only three ground-water samples had concentrations above 500 milligrams per liter (mg/L). Common chemicalquality problems are excessive iron concentrations and low pH. Iron concentrations range from less than 3 to 24,000 micrograms per liter (ug/L) . The median for crystalline rock is 12 ug/L; for the Potomac aquifers, 120 ug/L; and for other Coastal Plain aquifers, 87 ug/L. The pH ranges from 4.2 to 8.1. The median for crystalline rock is 6.0; for the Potomac aquifers, 5.6; and for other Coastal Plain aquifers, 5.8.
Streamflow data were measured at 10 continuous-record and 27 partial-record stations. Flow duration for five unregulated streams in Cecil County ranges from 3.1 to 4.8 cubic feet per second per square mile [(ft3/s)/mi2] at the 5-percent exceedance level and from 0.31 to 0.39 (ft3/s)/mi2 at the 95-percent exceedance level. The 7-day, 10-year, low-flow frequency for 31 continuous- or partial-record sites ranges from 0.01 to 0.44 (ft3/s)/mi2. The 7-day, 2-year, low-flow frequency for the same sites ranges from 0.02 to 0.68 (ft3/s)/mi2.
Stream water-column samples were collected at 29 sites during base-flow periods, in either August or November 1982. Dissolved-solids concentration of these base-flow samples ranges from a minimum of 39 mg/L to a maximum of 256 mg/L; the median is 92 mg/L. The pH ranges from a minimum of 5.8 to a maximum of 9.1; the median is 7.3.
Synthetic organic compounds were detected at 6 of the 10 streambed-sediment sampling sites that were analyzed for these compounds. The most frequently detected compounds were the organochlorine insecticides; none of the more soluble pesticides were detected.
Water-budget estimates for Piedmont basins show about 10 inches per year (in/yr) of ground-water runoff, 10 in/yr of storm runoff, and 22 in/yr of evapotranspiration. Total runoff is about 20 in/yr.
Ground-water flow models were constructed for three areas in Cecil County. Maximum drawdowns of more than 30 ft were projected under unsewered, non-drought conditions in the Elkton-Chesapeake City area; maximum drawdowns of 5 to 10 ft were simulated for the same conditions in the Rising Sun and Highlands-Meadow View areas . Maximum drawdowns of more than 40 ft were projected under sewered, drought conditions in the ElktonChesapeake City area; maximum drawdowns of more than 20 ft were simulated in the other two areas for the same conditions.
