|Hydrogeology & Hydrology Program|
Wicomico River and
Rockawalking Creek basins
David Andreasen (firstname.lastname@example.org)
T. Brandon Fewster (email@example.com)
The Salisbury aquifer is a shallow, unconfined aquifer with relatively high transmissivity throughout central Wicomico County. The aquifer, recharged rapidly from precipitation, is potentially vulnerable to both point-sources, such as disposal and storage sites and non-point sources of pollution, such as pesticides, fertilizers and road salt. Delineation of contributing areas of recharge to wells serves as a basis for the development of a well-head protection plan. Contributing areas for public-supply wells in the Upper Wicomico River Basin and clustered domestic wells in the Rockawalking Creek study area were determined using two separate finite-difference, ground-water-flow models (MODFLOW) and a particle-tracking routine (MODPATH). The public-supply wells selected for analysis include supply wells for mobile-home parks, commercial wells serving 25 people or more, and City of Salisbury Park wells 17 and 18. Contributing areas for the remainder of the wells within the City of Salisbury's well fields were determined in an earlier study (Andreasen and Smith, 1997). Contributing areas were also determined for several hypothetical public-supply production wells.
The finite-difference ground-water-flow mode developed by Andreasen and Smith (1997) was used to determine contributing areas for existing and hypothetical public-supply wells within the Upper Wicomico River Basin. The model grid was refined from 80 columns and 84 rows to 115 columns and 136 rows. The finer grid size improved the overall accuracy of simulated ground-water flow in the Salisbury aquifer and allowed for a more precise discretization of the lateral no-flow basin-boundary. Pumpage applied to the model was updated using 1998 figures. Total 1998 pumpage from the Salisbury aquifer was about 9.1 Mgal/d.
Contributing areas for City of Salisbury Park wells 17 and 18, pumping at a simulated rate of 0.36 Mgal/d per well, form an elongated pattern that extends approximately 4 mi from the wells in an east-northeast direction. Particles were backtracked 50 years. Minimum travel time was 242 and 425 days for wells 17 and 18, respectively. Particles entering the water table immediately adjacent to the wells bypass the wells and flow to Beaverdam Creek and Beaglin Branch.
Contributing areas for public-supply wells (commercial, industrial, institutional and residential use such as mobile-home parks) were determined for periods ranging from up to 1, 1 to 10, 10 to 20, and 20 to 50 years. Contributing areas for wells pumped at higher rates (13,200 to 500,000 gal/d) are contiguous with the wells and range in length from 2.0 to 3.2 mi and width from 0.1 to 0.5 mi. Contributing areas for the lower yielding wells (1,000 to 8,000 gal/d) form narrow elongated shapes occurring at distances up to 1.8 mi from the wells. These wells essentially capture ambient ground-water flow.
Contributing areas for three hypothetical wells pumping at rates of 2.0 Mgal/d and located in the deepest part of the Salisbury Paleochannel form broad bands extending away from the pumping sites in easterly and westerly directions. The contributing areas range in width from 0.3 to 1 mi wide and 1.3 to 4 mi long.
Particles tracked forward from the West Road rubble landfill and an application area for waste-water sludge were captured by Paleochannel well 1 after 26 and 7 years, respectively. Particles tracked forward from two hazardous-waste sites, located south of Naylor Mill Road and east of the North Prong Wicomico River, were not captured by public-water supply wells.
The Rockawalking Creek study area shares similar hydrogeologic characteristics with the adjacent Upper Wicomico River Basin. The unconfined Salisbury aquifer has a relatively high transmissivity, ranging from 2,400 to 14,400 ft2/d. The direction of ground-water flow is generally southward toward the tidal Wicomico River. Water use in the Salisbury aquifer consists solely of agricultural (irrigation) and domestic supply from individual wells. A total of approximately 1.1 Mgal/d is withdrawn from the Salisbury aquifer. A finite-difference ground-water flow model (MODFLOW) was developed and calibrated to water levels measured in an observation-well network established during the study and to baseflow measured in Rockawalking Creek. The flow model consisted of one layer with 70 columns and 90 rows. Model cell size was uniform at 170 x 170 ft. Output from the model was used with a particle-tracking routine (MODPATH) to determine contributing areas for wells within the Rockawalking Creek study area.
Currently there are no public-supply wells pumping from the Salisbury aquifer in the Rockawalking Creek study area. Contributing areas were therefore determined for three hypothetical wells located in the south, central, and northern parts of the study area. The 50-year contributing area of the three hypothetical wells - pumped independently at 0.25 Mgal/d - range in length from 0.7 to 1.6 mi and width from 0.5 to 0.65 mi. Minimum particle travel times ranged from 62 to 95 days. When the pumping rate is increased two-fold (0.5 Mgal/d) in the northern well, the 20-year contributing area expands southward downgradient approximately 0.28 mi farther than when the well is pumped at 0.25 Mgal/d.
Contributing areas for clustered domestic wells within subdivisions are generally localized occurring within 500 to 2,500 ft from withdrawal sites determined by particles backtracked from the mid-point of the aquifer. The 20-year contributing areas cover broad areas mostly in the southwestern and central part of the study area. Domestic wells primarily capture ambient ground-water flow. The average particle travel time was 6.8 years.