National Water-Quality Assessment--Potomac River Basin: Applications of a Geographic Information System in Water-Quality Assessment, Potomac River Basin Study Unit, National Water-Quality Assessment Program

Abstract

A Geographic Information System (GIS) was used to create, combine, and display geographic-data layers from various sources to facilitate interpretation of water-quality data collected in the Great Valley of the Potomac River Basin, as part of the U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) Program. A synoptic survey was conducted with samples that were collected from 25 small carbonate streams that drained a range of geographical areas and land uses. The samples were analyzed for major ions, nutrients, and pesticides. Ecological samples of benthic macroinvertebrates and algae also were collected as part of the survey.

By using a GIS, the natural and man-made influences on water quality in the watersheds were identified and compiled. Drainage boundaries for the watersheds above the 25 sampling sites were delineated and digitized. Areas of cropland, pasture, and non agricultural land use were determined with data from the Maryland Office of State Planning, the Virginia Department of Conservation and Recreation--Division of Soil and Water Conservation, the USGS Geographic Information Retrieval and Analysis System (GIRAS), and National Aerial Photography Program (NAPP) aerial photographs. Watershed areas that lacked recent (1990 or later) land-use data for cropland and pasture were plotted with the GIRAS polygons at the approximate scale of the NAPP photographs, overlaid onto the photographs, and used as a guide to delineate cropland and pasture areas. The new land-use areas were then digitized. Ratios of cropland and pasture in each watershed were then calculated by combining the land-use data with the drainage boundaries and dividing their areas by the total drainage area. County-based pesticide-usage and crop data from Resources For the Future (RFF) and the U.S. Department of Agriculture-- Census of Agriculture were used to associate particular pesticide usage with crop types grown in the watershed areas. USGS digital elevation models (DEM) also were combined with the drainage boundaries in the GIS to calculate basin relief.

The U.S. Environmental Protection Agency (EPA) River Reach 3 (RF3) files in the GIS were used to generate stream-segment information near the 25 sampling sites. Segment characteristics included: the EPA RF3 code, the segment length, the stream order, and the downstream link. Similarities in stream-segment characteristics will be related to water-quality data.

Citation

Brakebill, J. W., 1994, Applications of a geographic information system in water-quality assessment of the Potomac River Basin study unit, National Water-Quality Assessment Program [abs.] : Abstract Booklet, Mid-Atlantic Highlands Area Environmental Monitoring and Assessment Conference, Hershey, Pennsylvania, February 23-25, 1994, p. 63.