|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.
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
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.