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National Water-Quality Assessment (NAWQA) Program

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Mapping arsenic in groundwater

Citation: Ryker, S.J., Nov. 2001, Mapping arsenic in groundwater-- A real need, but a hard problem: Geotimes Newsmagazine of the Earth Sciences, v. 46 no. 11, p. 34-36.

Updated maps

Equal-area map--
Arsenic concentrations found in at least 25% of samples within a moving 50km radius

Equal-area map

County map--
Arsenic concentrations found in at least 25% of samples in each county

County map
    > Original county map (Welch and others, 2000)

Data map--
31,350 ground-water samples collected in 1973-2001

Data map
    > Original data map (Welch and others, 2000)

Arsenic in ground water is largely the result of minerals dissolving from weathered rocks and soils. Several types of cancer have been linked to arsenic in water. In 2001 the US Environmental Protection Agency lowered the maximum level of arsenic permitted in drinking water from 50 micrograms per liter (ug/L) to 10 ug/L.

The USGS has developed maps that show where and to what extent arsenic occurs in ground water across the country. The current maps are based on samples from 31,350 wells. Widespread high concentrations were found in the West, the Midwest, and the Northeast.


USGS data-- The maps at right are based on 31,350 arsenic samples. Of those samples, 20,050 were collected by USGS. State data--
Several state agencies contributed data to these maps, including:

Related publications

Map of arsenic in groundwater of the United States, updated 2011-- Fact Sheet-- Detailed report-- Research--

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