Organic compounds studied in this U.S. Geological Survey (USGS) assessment generally are man-made, including, in part, pesticides, solvents, gasoline hydrocarbons, personal care and domestic-use products, and refrigerants and propellants. A total of 85 of 277 compounds were detected at least once among the 25 samples collected approximately monthly during 2003-05 at the intake of the Washington Aqueduct, one of several community water systems on the Potomac River upstream from Washington, D.C. The diversity of compounds detected indicate a variety of different sources and uses (including wastewater discharge, industrial, agricultural, domestic, and others) and different pathways (including treated wastewater outfalls located upstream, overland runoff, and ground-water discharge) to drinking-water supplies. Seven compounds were detected year-round in source-water intake samples, including selected herbicide compounds commonly used in the Potomac River Basin and in other agricultural areas across the United States. Two-thirds of the 26 compounds detected most commonly in source water (in at least 20 percent of the samples) also were detected most commonly in finished water (after treatment but prior to distribution). Concentrations for all detected compounds in source and finished water generally were less than 0.1 microgram per liter and always less than human-health benchmarks, which are available for about one-half of the detected compounds. On the basis of this screening-level assessment, adverse effects to human health are expected to be negligible (subject to limitations of available human-health benchmarks).