Maryland Department of Natural Resources

Reports

Hydrogeology and simulation of ground-water flow in the upper Wicomico River Basin and estimation of contributing areas of the City of Salisbury well fields, Wicomico County, Maryland


1997, Andreasen, D.C. and Smith, B.S.

Report of Investigations 65


Abstract

The entire water supply for the City of Salisbury, Maryland, located in the drainage basin of the Upper Wicomico River on the lower Delmarva Peninsula, is provided by two well fields that tap a shallow, unconfined Coastal Plain aquifer (Salisbury aquifer) . In 1993, average daily pumpage was 3.5 million gallons per day from the northern well field ("Paleochannel well field") and 1.7 million gallons per day from the southern well field ("Park well field"). Total pumpage from the Salisbury aquifer in the basin was approximately 8.7 million gallons per day in 1993. Industrial, residential , commercial, and agricultural land-use areas are located near the well fields. The aquifer is thus potentially vulnerable to both point-sources (such as disposal and storage sites) and non-point sources of pollution (such as pesticides and road salt). Data from hydrogeologic analyses were applied using the U.S. Geological Survey's three-dimensional ground-water-flow model (MODFLOW) and particle-tracking routine (MODPATH) to determine areas contributing recharge to the city's well fields under current and future pumping conditions and to identify possible sources of contamination. The study, conducted between July 1993 and June 1996, was a cooperative effort between the City of Salisbury Department of Public Works, the Maryland Geological Survey, and the U.S. Geological Survey.

The Salisbury aquifer in this area is composed of coarse sand and gravel and has a high transmissivity. Transmissivity values range between 8,200 feet squared per day and 57,000 feet squared per day with an average of about 22,000 feet squared per day in the Park well field to as much as 53,500 feet squared per day in the Paleochannel well field, where an unusually thick (greater that 200 feet) paleochannel deposit is present. In the paleochannel, the confining bed at the base of the Salisbury aquifer is breached, resulting in a hydraulic interconnection with the underlying Manokin aquifer.

The Salisbury aquifer receives recharge from precipitation in topographically high areas in the basin and discharges water to numerous small streams, man-made ponds, the tidal part of the Wicomico River, and public and private water-supply wells. Because the aquifer is unconfined, has a high permeability, and is recharged rapidly from percolating precipitation, water levels respond to seasonal changes in recharge but show only small local responses to pumping. The average net ground-water recharge rate is estimated at 13.4 inches per year from baseflow separation of long-term streamflow record in a sub-basin (Beaverdam Creek basin). The Salisbury aquifer shows good hydraulic connection to streams within the basin. Pumping from Paleochannel well 1 lowered baseflow by 0.1 cubic foot per second (approximately 60 percent of total stream flow) in Little Burnt Branch, a small tributary that passes near the well. Results of a three-dimensional, finite-difference ground-water flow model (MODFLOW) indicate that average outflow to the tidal parts of the Wicomico River and streams in the basin is approximately 4.7 and 44 million gallons per day, respectively.

Contributing areas of the city's wells extend upgradient from the wells approximately 1,000 feet for the 0- to I -year time zone; 6,000 feet for the 1- to lO-year time zone, and 10,000 feet for the 20-year time zone. Most of the water entering the water table within these time periods is captured by the top part of the well screens. The areal patterns of the contributing areas are irregular and are affected by streams and ponds. The position and shape of the contributing areas vary with slight changes in hydraulic properties input to the ground-water-flow model. For example, a 7-percent increase in recharge caused the westtrending contributing area for Paleochannel well I to become narrower and to shift northward slightly. The northeast-trending portion is approximately 500 feet longer than the contributing area based on the calibrated model. A l0-percent reduction and increase in aquifer porosity lengthens and shortens, respectively, the contributing areas for Paleochannel well 1 approximately by 500 feet. Contributing areas based on the estimated pumpage in 2010, which represents an increase of 15 percent from 1993 pumpage, show a general broadening of less than 500 feet for most of the city 's wells. The contributing areas of the Paleochannel well field are predominantly in agricultural and open areas, whereas the contributing areas of the Park well field are predominantly in commercial and residential areas.

Water particles tracked forward from sites with the potential to be sources of contamination to the Salisbury aquifer indicate that recharge entering the aquifer at two sites--the Conrail railroad and a site with a ground-water discharge permit located at Route 13 and Naylor Mill Road--would be captured by Paleochannel well 2 after 2 and 11 years of travel time, respectively.

Recharge dates for water sampled at various depths in the Salisbury aquifer near the well fields, estimated using tritium and chlorofluorocarbon analysis, ranged from pre-1963 to 1992. The older water was withdrawn from the deeper wells and the younger water was withdrawn from the shallower wells. The oldest water sampled was from the deep part of the paleochannel where mixing with artesian Manokin aquifer water occurs. The range of recharge ages determined by use of tritium and chlorofluorocarbons generally agree with the median recharge travel times calculated by the flow model.