National Water-Quality Assessment (NAWQA) Program
Citation: Welch, A.H., and Ryker, S.J., 2000, Arsenic in ground-water resources of the United States--A new national-scale analysis, in: Fourth International Conference on Arsenic Exposure and Health Effects, San Diego, June 18-22nd, 2000.
The U.S. Geological Survey (USGS) has recently analyzed a database of about 20,000 measurements of arsenic in ground water across the United States. This database describes a broad population of ground-water supplies, including currently regulated community water supplies, non-community supplies, domestic wells, and potential drinking-water resources not currently used for supply. In the past, the maximum contaminant level (MCL) of 50 micrograms per liter (ug/L) of arsenic in drinking water regulated only community supplies. The new MCL will be applied to an additional class of non-community water supplies serving schools, office buildings, churches, and hospitals. In many states, arsenic concentrations have not been well documented for this new class of regulated supplies, making the USGS database a valuable repository.
The USGS database also provides a broad-scale, consistent view of arsenic in ground water across the Nation, facilitating national-scale forecasting of where ground-water supplies are likely to exceed possible new MCLs for arsenic. In about one-quarter of counties with sufficient data, at least 10% of ground-water supplies are likely to contain arsenic concentrations greater than 10ug/L. In general, ground water of the southeastern coastal plain has low concentrations (rarely exceeding 3ug/L), while many parts of the west, upper midwest, and northeast have ground water with arsenic concentrations greater than 10ug/L. The USGS, through the National Water-Quality Assessment Program and cooperative efforts with other agencies, continues to collect data and apply geologic, hydrologic, and agrochemical knowledge to expand our understanding of arsenic in ground water.