The National Water-Quality Assessment Program of the U.S. Geological Survey has an ongoing Source Water-Quality Assessment program designed to characterize the quality of water in aquifers used as a source of drinking-water supply for some of the largest metropolitan areas in the Nation. In addition to the sampling of the source waters, sampling of finished or treated waters was done in the second year of local studies to evaluate if the organic compounds detected in the source waters also were present in the water supplied to the public.