Summary

Description

This image shows national-scale patterns of naturally occurring arsenic in potable ground-water resources of the continental United States. The image was generated from the most recent arsenic measurement available for each of 31,350 wells and springs across the United States. Over 20,000 of the water samples were collected by the U.S. Geological Survey between 1973 and 2001. The remainder of the samples were collected by State agencies and analyzed by comparable laboratory techniques. The data set shows a moving 75th percentile, which can also be described as the maximum arsenic concentration found in 75% of samples within a moving 50 km radius (the median size of a U.S. county). In other words, for any given 50-km-radius region in the data, lower concentrations of arsenic were found in 75% of sampled wells, while higher concentrations of arsenic were found in 25% of sampled wells. This is a revised version of the August 2002 map layer, with a corrected shoreline for Greenland.