The Susquehanna River Basin Commission and the U.S. Environmental Protection Agency cooperated with the U.S. Geological Survey in a study to quantify nonpoint-source loadings from an agricultural area in Pennsylvania. Pequea Creek, a tributary to the Susquehanna River, drains a 154-square mile agricultural area in Lancaster County, Pennsylvania. Previous studies defined the Pequea Creek basin as a contributor of sediment, nutrients, and pesticides from nonpoint sources to the Susquehanna River. The purpose of this intensive watershed investigation was to determine the effects of various land uses on water quality of receiving streams. Streamflow was measured and monthly base-flow samples and water-weighted composite storm samples were analyzed for suspended sediment, nutrients, organic carbon, and triazine herbicides. Constituent loadings were calculated to quantify their discharge from the entire Pequea Creek basin and from four specific subbasins: forest, cornfield, rural residential, and pasture. Soil samples were analyzed for nutrients and selected herbicides, and land use and application data were collected to determine the source of loadings. Precipitation amounts and chemistry were also measured. Precipitation and streamflow were below average for much of the investigation period, May 1979 to December 1980. The annual precipitation for 1980 was 10 inches below normal, with drought conditions the last half of the year. During base flow, the highest concentrations of individual constituents generally observed were: 4.0 milligrams per liter total organic nitrogen and 1.4 milligrams per liter total phosphorus from the downstream pasture site; 24 milligrams per liter total nitrate nitrogen and 3.9 micrograms per liter total atrazine from the cornfield site; and 0.5 micrograms per liter total prometone, and 2.3 micrograms per liter total simazine from the residential site. Nearly all total nitrate nitrogen concentrations from the cornfield site during base flow were about double the U.S. Environmental Protection Agency (1977) criterion of 10 milligrams per liter. The highest constituent concentrations found in composite storm samples were nearly all in samples from the cornfield site. The highest concentrations found were: 16,000 milligrams per liter suspended sediment, 54 milligrams per liter total organic nitrogen, 41 milligrams per liter total nitrate nitrogen, 19 milligrams per liter total phosphorus, and 200 micrograms per liter total atrazine. The highest concentrations found in composite storm samples for total prometone was 6.4 micrograms per liter at the residential site and for total simazine was 4.8 micrograms per liter at the downstream pasture site. Generally, concentrations of all the constituents, except nitrate, were higher during storms than during base flow at all sites. Total concentrations of the constituents increase during storms predominantly due to increases in suspended concentrations. The highest storm concentrations of most constituents occurred at the cornfield site. Fertilizer and herbicide applications increase the available sources of nutrients and herbicides for transport to the stream. The highest phosphorus, atrazine, and suspended-sediment concentrations at the cornfield site occurred during intense storms soon after application and planting. Constituent yields (tons per square mile) from storms, about evenly distributed throughout the basin, were compared from the other specific land-use sites to the forest site, which represents a relatively undisturbed land use. During storms, yields for suspended sediment, total organic nitrogen, and total phosphorus were highest for the pasture and lowest for the forest site. Total nitrate-nitrogen yields were highest for the cornfield and about the same for the forest and residential sites. Yields of total organic carbon were about the same for the cornfield and residential sites, which were both slightly higher than the forest