Analyzing Effects of Land Use on Ground-Water Quality in the Potomac River Basin

Abstract

The Potomac River Basin study unit of the U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) program is describing the water-quality conditions in ground water and relating these conditions to land use. Water-quality and land-use data are currently being collected in eight basin subunits representing various physiographic, geologic, ecological, and hydrologic settings. Ground-water-quality data collected in 1993 from the Great Valley Carbonate subunit of the Potomac River Basin were related to land-use information compiled and mapped from different sources to assess the primary natural and human factors affecting the quality of ground water. Compilation of new land-use data was necessitated by a lack of current, consistent, detailed land-use data that identified cropland and pasture areas as separate land-use categories.

Major land-use areas surrounding ground-water sampling sites were delineated to assess the relation of specific agricultural uses to ground-water quality. Thirty ground-water sampling sites randomly selected within the agricultural part of the subunit were located on enlarged black and white National Aerial Photography Program (NAPP) photographs. Mylar sheets were placed over the photographs and taken to the field, where major land-use categories were mapped within radii of one-quarter and one-half mile of each well. Crop types and patterns were noted and farming practices and potential point sources were identified. USGS 1:24,000 topographic maps were used to identify the upslope quarter-and half-circles for each sampled well. Boundaries were digitized for each major land-use area, and the percentage of each land use was calculated for the one-quarter and one- half mile circles and for the upslope quarter and upslope half of each circle.

Ground-water samples collected within the Great Valley Carbonate subunit were analyzed for concentrations of major ions, nutrients, and pesticides to assess the effects of land-use activity on the water quality of shallow ground-water systems. A comparison of calculated land-use statistics and ground-water-quality data indicated a strong correlation between nitrate concentrations and cropland within the subunit. Similar analyses are being planned for additional subunits of the Potomac River Basin, each requiring an analogous compilation of land-use data.