A multiscale multidisciplinary approach has been used to assess the water quality in the Potomac River Basin, a 38,000-km2 watershed that provides the major source of water to our Nation's capital at Washington D.C., and drains into the Chesapeake Bay estuary in northeastern United States. The Potomac River Basin is one of 60 study units of the U. S. Geological Survey's (USGS) National Water Quality Assessment Program (NAWQA). The Potomac River Basin contains a mixture of agricultural, forested, and urban land use as well as diverse lithology and topography. Three study scales were used for water quality assessment: 1) basinwide, 2) subunits defined by distinct lithology and physiography, and 3) a small stream basin in an important agricultural subunit. The primary focus of the study has been the occurrence and distribution of nitrogen, phosphorus, and hydrophilic pesticides. Ground-water and surface-water quality measurements have been combined with hydrological and biological community information at each scale of the study to provide a complete picture of the water quality conditions.
Eleven surface-water monitoring sites were established in the basin. Six of these sites have large watersheds with multiple land uses, types of bedrock, and physiographies, and were selected to integrate these combined effects in the basin. The five other sites were selected as "indicator" sites that specifically target important landuses such as agriculture or urban development. The purposes of the basinwide monitoring are to 1) establish the spatial distribution of surface-water quality and biological communities in the Potomac River Basin, 2) begin a database for trends analysis of nutrients and organic pesticides, and 3) calculate the spatial distribution of contaminant loads. Since 1993 to 1995 the 11 monitoring sites have been sampled bimonthly, and samples have been analyzed for pesticides, phosphorus, nitrogen, suspended sediment, and major ions. Biological stream communities and habitats at each site have been evaluated annually to establish a link between water quality and the community structure of the stream ecosystems. Additionally, in early June 1994, a basinwide synoptic survey was conducted at 23 sites with watersheds greater than or equal to 250 km2, targeting the period of most intensive applications of fertilizers and pesticides. The 1994 synoptic survey showed that cropland is the primary source of herbicides such as atrazine and simazine to surface and ground water, but that pastureland is only a minor contributor of these contaminants.
The Potomac Basin was divided into eight subunits, defined by unique combinations of physiography and lithology. In four of these subunits, synoptic surveys of small watersheds (approximately 25 km2) and ground-water surveys were conducted to assess the areal distribution of chemical contaminants and aquatic ecology within each subunit and to relate these parameters to land use. The five indicator monitoring sites mentioned earlier, which are located within these four subunits, provide a temporal and spatial link to the entire Potomac Basin. Datasets developed by the USGS using Geographic Information Systems (GIS), as well as statistical correlations, have been used to quantify the links between land use, biological communities, and water quality in the Potomac River Basin. For example, one synoptic survey targeted a predominantly agricultural subunit underlain by carbonate bedrock in a physiographic province called the Great Valley. The Great Valley Carbonate subunit has significantly higher concentrations of nitrate in surface and ground water than average values from the other subunits with less agricultural land use. Furthermore, cropland was again the predominant source of streamwater contamination.
An intensive ground-water flowpath study was conducted on a small section of a stream network, moving from the headwaters to about 1 km downstream. The stream, Muddy Creek, is in a small agricultural watershed in the Great Valley Carbonate subunit of the Potomac Basin. During 10 separate synoptic surveys, samples of surface and ground water were collected along the measured flowpath and analyzed for nutrients, pesticides, and major ions. Chemical results combined with a detailed hydrologic evaluation of the ground-water flowpath have provided a direct link between chemical applications on farm fields and ground-water contamination. This has allowed inference of the pathways and time scales to convey compounds such as atrazine and nitrate from the land surface, through the soil zone, to the shallow unconfined aquifers that supply streams.