Sensitivity of ground water to contamination in Lawrence County, South Dakota

Ground-water supplies in Lawrence County, South Dakota, can be contaminated by agricultural, urban, suburban, commercial, and industrial land uses. To address this issue, the U.S. Geological Survey in cooperation with Lawrence County and the City of Spearfish mapped the sensitivity of ground water to contamination in Lawrence County.

Sensitivity of ground water to contamination was determined by delineating hydrogeologic settings with common hydrogeologic characteristics as described in the DRASTIC method, developed by the U.S. Environmental Protection Agency and the National Water Well Association. Within the framework of 11 hydrogeologic settings, sensitivity to contamination was ranked for six intrinsic hydrogeologic characteristics: (1) aquifer media, (2) unsaturated media (3) hydraulic conductivity, (4) recharge rate, (5) depth to water, and (6) land-surface slope. The rating conventions of DRASTIC were modified to provide a relative ranking of hydrogeologic characteristics without assignment of a combined numerical score. Soil characteristics were not included as a map layer because detailed digital data were not available; however, the general distribution of two soil characteristics were shown.

A total of 956 polygons were delineated and assigned a sensitivity-unit code that represented unique groups of sensitivity rank for the six intrinsic hydrogeologic characteristics. The polygons were created by overlaying and intersecting maps that describe the geology, precipitation, land-surface elevation, and depth to water using a geographic information system. Thirty drainage areas upstream from potential streamflow-loss zones were delineated to describe an additional mechanism of transport of potential contamination. The sensitivity of ground water to contamination was presented on a 1:100,000-scale map with code and label explanations. Limitations of the sensitivity map are described to facilitate appropriate use of the map as a screening tool to compare sensitivity to contamination.