Biological, chemical, and physical data were collected from a network of 43 sites in Chester County, Pa., from 1981 to 1997. The goal of the network is to assess stream conditions and determine trends in benthic-macroinvertebrate and water-chemistry data. Most sites in the network were assessed as nonimpacted or slightly impacted on the basis of biological metric analysis of benthic-macroinvertebrate data. Sites assessed as impacted were affected by habitat degradation, organic pollution, or toxins. Analysis of calculated biological metrics indicates that most sites were assessed as nonimpacted or slightly impacted between 1981 and 1997. Sites in the Pigeon, French, Pickering, and upper East Branch Brandywine Creek Basins generally were assessed as nonimpacted with stable physical and chemical data through the 17-year period. Sites in the Darby, Crum, Ridley, Valley, and lower East Branch Brandywine Creek Basins are being degraded from the reference conditions, primarily because of habitat alteration. The areas with the poorest stream quality were sites with greater than 50 percent agricultural land use, sites with greater than 10 percent impervious surface, and sites affected by wastewater-treatment plant discharges. These sites are being degraded by erosion, sedimentation, and nutrient enrichment. Trend analysis indicates that calculated biological metrics at most sites improved or remained unchanged between 1981 and 1997. Trend analysis on chemical constituents indicated that concentrations of nitrate and chloride were increasing, and concentrations of phosphorus and ammonia were unchanged or declining across Chester County from 1981 to 1997. The highest concentrations of ammonia and chloride were at locations affected by wastewater-treatment plant discharges. Concentrations of pesticides and other organic compounds in stream-bottom sediments decreased between 1983 and 1997 but these compounds still are present at low concentrations throughout Chester County.
