This fact sheet provides information on where and to what extent natural concentrations of arsenic in ground water exceed possible new standards. Arsenic concentrations in ground water of the United States. These wells are used for irrigation, industrial purposes, and research, as well as for public and private water supply. The large number of samples, broad geographic coverage, and consistency of methods produce a more accurate and detailed picture of arsenic concentrations than provided by any previous studies. Arsenic concentrations in ground water generally are highest in the West.
Arsenic concentrations appear to be lower in the Southeast, based on a smaller amount of data.
Arsenic concentrations also could be high at locations not shown on figure 1 because data are not available everywhere. Even at sampled locations, concentrations might differ between shallow and deep waters. Nonetheless, these data illustrate how arsenic concentrations vary across broad regions of the country. As the concentration for a possible new MCL decreases, the likelihood of exceeding that standard increases. Public systems exceeding a new, lower MCL will be required to either treat their water or find alternative sources of supply.