Arsenic, in ug/L

EXPLANATION

>50

10-50

5-10

<5

Hawaii

Alaska

Figure 1. Arsenic concentrations in ground water of the United States.

Arsenic in Ground-Water Resources

of the United States

Fact Sheet FS–063–00

May 2000

U.S. Department of the Interior

U.S. Geological Survey

Arsenic is a naturally occurring element in rocks, soils, and the waters in contact with them. Recognized as a toxic

element for centuries, arsenic today also is a human health concern because it can contribute to skin, bladder, and other

cancers (National Research Council, 1999). Recently, the National Research Council (1999) recommended lowering the

current maximum contaminant level (MCL) allowed for arsenic in drinking water of 50 μg/L (micrograms per liter), citing

risks for developing bladder and other cancers. The U.S. Environmental Protection Agency (USEPA) will propose a new,

and likely lower, arsenic MCL during 2000 (U.S. Environmental Protection Agency, 2000). This fact sheet provides information on where and to what extent natural concentrations of arsenic in ground water exceed possible new standards.
The U.S. Geological Survey

(USGS) has collected and analyzed

arsenic in potable (drinkable) water

from 18,850 wells in 595 counties

across the United States during the

past two decades. These wells are

used for irrigation, industrial purposes, and research, as well as for

public and private water supply.

Arsenic concentrations in samples

from these wells are similar to

those found in nearby public supplies

(see Focazio and others, 1999). 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.

Where do high

concentrations of arsenic

in ground water occur in

the United States?

Arsenic concentrations in

ground water generally are highest

in the West. Parts of the Midwest

and Northeast also have arsenic

concentrations that exceed 10 μg/L,

the World Health Organization’s

(WHO) provisional guideline for

arsenic in drinking water (World

Health Organization, 1999). 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.

How frequently are arsenic

concentrations in ground

water likely to exceed

possible new maximum

contaminant levels?

To look at the Nation as a

whole, arsenic data were grouped

by county and linked to the number

of public-supply systems withdrawing

ground water in each county (Focazio

and others, 1999). Estimates of the

percentage of small public watersupply systems which exceed six

targeted arsenic concentrations in

their ground-water resource are

shown in figure 2. Systems were

called “small” if they served between

1,000 and 10,000 persons. Focazio

and others (1999) provide similar

information for both smaller and

larger sized systems. The highest

concentration evaluated is at the

current MCL of 50 μg/L, along

with several lower concentrations,

one of which may become the new

MCL.

As the concentration for a

possible new MCL decreases, the

likelihood of exceeding that standard

increases. Just over 13 percent of

small systems used water with arsenic

concentrations greater than 5 μg/L,

compared to fewer than 1 percent

exceeding the current 50 μg/L MCL.

Public systems exceeding a new,

lower MCL will be required to

either treat their water or find

alternative sources of supply. This

choice undoubtedly will increase

costs for consumers while decreasing

their exposure to arsenic. Although

homeowners with private wells

are not regulated, a lower drinkingwater standard would mean that

more homeowners will be consuming

water with concentrations that exceed

a standard.

USGS information provides a

broad picture of arsenic concentrations in ground water throughout

the United States. In 24 percent of

the U.S. counties where data were

available, at least 10 percent of samples had arsenic concentrations

exceeding 10 μg/L, the WHO provisional guideline for arsenic. Water

users in these counties (colored darkest brown in fig. 3) are the most likely

to have ground water exceeding new

standards for arsenic.

ARSENIC CONCENTRATION, IN MICROGRAMS PER LITER (μg/L)

Figure 2. Percentage of small public water-supply systems estimated to

exceed targeted arsenic concentrations in their ground-water resource

(μg/L, micrograms per liter).
Alaska

Hawaii

EXPLANATION

Generally highest arsenic concentrations

Insufficient data

Generally lowest arsenic concentrations

Figure 3. Counties with arsenic concentrations exceeding possible new MCLs in 10 percent or more of ground-water samples.

Counties with arsenic concentrations exceeding 10 μg/L in 10 percent or more of samples.

Counties with arsenic concentrations exceeding 5 μg/L in 10 percent or more of samples.

Counties with arsenic concentrations exceeding 3 μg/L in 10 percent or more of samples.

Counties with fewer than 10 percent of samples exceeding 3 μg/L, representing areas of

lowest concentration.

Counties with insufficient data in the USGS data base to make estimates.
References

Focazio, M.J., Welch, A.H., Watkins,

S.A., Helsel, D.R., and Horn, M.A.,

1999, A retrospective analysis on the

occurrence of arsenic in groundwater resources of the United States

and limitations in drinking-watersupply characterizations: U.S. Geological Survey Water-Resources

Investigations Report 99–4279,

21 p.

National Research Council, 1999, Arsenic

in drinking water: Washington, D.C.,

National Academy Press, 273 p.

U.S. Environmental Protection Agency,

Drinking water priority rulemaking—

Arsenic: accessed on 4/5/00 at URL

http://www.epa.gov/OGWDW/ars/

arsenic.html

Welch, A.H., Westjohn, D.B., Helsel,

D.R., and Wanty, R.B., in press,

Arsenic in ground water of the

United States—Occurrence and

geochemistry. (To be published

in the journal Ground Water,

v. 38, no. 4, in July/August 2000.)

World Health Organization, 1999,

Arsenic in drinking water: Fact

Sheet 210, accessed on 4/5/00 at

URL http://www.who.int/inf-fs/en/

fact210.html

—Alan H. Welch, Sharon A. Watkins,

Dennis R. Helsel, and Michael J. Focazio

For more information, please contact:

Dennis Helsel

U.S. Geological Survey

Box 25046, MS 415

Denver Federal Center

Denver, CO 80225–0046

dhelsel@usgs.gov

http://co.water.usgs.gov/trace