Geohydrology of channel-fill deposits near Salisbury, Maryland
1972, Mack, F.K., Thomas, W.O., Jr., and Weigle, J.M.
The hydrologic characteristics of a very permeable sand and gravel aquifer filling a channel of uncertain origin were tested by pumping the aquifer at 4,000 gpm (gallons per minute) for 30 days. The aquifer 2.5 miles north of the City of Salisbury, Maryland, was first discovered in 1962 and was partly explored by test drilling in 1963 and 1964.
Sediments filling the ancient river valley are of Pleistocene age and are as much as 220 feet thick at the pumping-test site. Geologic data indicate that the sediments are in three distinct layers, which have been designated as zones A, B, and C with increasing depth. Zone C, grading from fine-grained sediments at the base to coarser grained sediments at its top, the oldest of the three, is less permeable than the overlying zones. It is 30 to 50 feet thick and occurs at altitudes from 130 to 160 feet below sea level.
Zone B, the major source of water in the three zones, ranges in thickness from 0 to 80 feet. It is a long, narrow deposit of coarse sand and gravel occurring at altitudes from 50 to 130 feet below sea level. Zone A ranges in thickness from 50 to 90 feet. It is composed of coarse sand but contains some clay layers and is generally finer in character than zone B. It ranges in altitude from 40 feet above sea level to 50 feet below sea level.
Water levels, measured in 18 observation wells, declined throughout the 30-day period of pumping but the cone of depression probably reached a steady condition. The pumped well, Wi-Ce 200, had a drawdown of 24 feet and a specific capacity of 165 gpm per foot of drawdown after 30 days of pumping. The aquifer transmissivity is 400,000 gpd (gallons per day) per foot in the immediate vicinity of the pumping well. Artesian conditions existed during the early minutes of the test, but water-table conditions probably existed throughout most of the test.
The North Prong Wicomico River would serve as a reliable source of substantial recharge to the aquifer in the vicinity of the test site, if the aquifer were to be developed by heavy pumping. Other sites on the aquifer farther from streams do not have this advantage and must be expected to have lower capabilities. Prediction of the long-term yield of the aquifer would be hazardous on the basis of the 30-day test but its gradual development, accompanied by careful monitoring of groundwater levels, streamflow, and pumpage will undoubtedly reveal the availability of many millions of gallons of water per day.
The water pumped from the aquifer is a sodium bicarbonate type having a dissolved-solids content ranging from 42 to 52 mg/l (milligrams per liter). The most significant change noted during the test was a gradual decrease in the iron concentration from 0.46 mg/l prior to the test to 0.27 mg/l at the end of the 30-day pumping period.
Water in the streams during the test was slightly colored and contained concentrations of dissolved solids slightly higher than the ground water.