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Map Layer Info

Arsenic in Ground Water of the United States

What this map layer shows:

Naturally occurring arsenic in potable ground water resources, using water samples from 31,350 wells and springs.
opens the U.S. Geological Survey home page
Background Information
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Arsenic is a naturally occurring trace element found in rocks, soils, and the waters in contact with them. Arsenic has long been recognized as a toxic element and is also considered a human health concern because it can contribute to skin, bladder, and other cancers. Arsenic concentrations are measured in units of micrograms per liter (μg/L), which is equivalent to parts per billion. This map layer was compiled by the U.S. Geological Survey, National Water-Quality Assessment Program (NAWQA), which is responsible for developing long-term, consistent, and comparable information on streams, ground water, and aquatic ecosystems. This information supports national, regional, State, and local water-management and policy decisions that protect drinking water and other water resources, as well as public health. NAWQA information on arsenic in ground water is used by the U.S. Environmental Protection Agency (USEPA) to help set national standards for arsenic in drinking water, as mandated by the Safe Drinking Water Act. In 2001, the USEPA lowered the maximum level of arsenic permitted in drinking water from 50 μg/L to 10 μg/L.

The Arsenic in Ground Water of the United States image was generated from the most recent arsenic measurement available for each of 31,350 wells and springs across the United States. It shows national-scale patterns of naturally occurring arsenic in potable ground-water resources of the continental United States and Puerto Rico. The sampled wells are used for irrigation, industrial purposes, and research, as well as for public and private water supply. The map layer shows a moving 75th percentile, which can also be described as the maximum arsenic concentration found in 75 percent 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 percent of sampled wells, and higher concentrations of arsenic were found in 25 percent of sampled wells.

Further information on arsenic in ground water is available from the USGS Water Resources Discipline, Arsenic in Ground Water of the United States page and from the USEPA Arsenic in Drinking Water page. Further information on the creation of this image is available in the article Mapping Arsenic in Ground Water.


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Arsenic in Ground Water