Summary-- Arsenic in Ground and Surface Water of the United States

Drinking Water

Arsenic causes skin, lung, bladder, liver, and kidney cancer in humans (National Research Council, 1999; U.S. Environmental Protection Agency, rev. July 28, 1999) . It also has been reported to adversely affect development; hearing; the vascular, gastrointestinal, and nervous systems; and to be associated with diabetes.

The present arsenic drinking water Maximum Contaminant Level (MCL) of 50 µg/L applies to the sum of all arsenic forms in finished drinking water. The MCL was established by the Safe Drinking Water Act Amendments of 1986 (U.S. Environmental Protection Agency, rev. July 28, 1999). A current Health Advisory (HA) of 2 µg/L, based on a lifetime cancer risk of 1:10,000, also is in effect (U.S. Environmental Protection Agency, 1996). The Safe Drinking Water Act Amendments of 1996 require the USEPA to review the current MCL and propose an arsenic rule by January 1, 2000. A final rule is due by January 1, 2001. The effective date for a revised MCL will be 3-5 years after the final rule is issued, thus, the USEPA plans to issue a revised HA for arsenic in early 2000. Because it seems likely that the USEPA review will result in a lowering of the arsenic MCL, Study Units should consider comparing arsenic concentrations to the current MCL of 50 µg/L and to lower values. The current HA of 2 µg/L and the World Health Organization (WHO) (1996) provisional guideline of 10 µg/L are logical choices.

Aquatic Life

National recommended aquatic life criteria for arsenic (U.S. Environmental Protection Agency, 1999) were derived from data for trivalent arsenic but are applied to the sum of all measured arsenic species, expressed as dissolved (filtered) concentrations.

Criterion	Arsenic Concentration (µg/L)
Freshwater (maximum concentration)	340
Freshwater (continuous concentration)	150
Saltwater (maximum concentration)	69
Saltwater (continuous concentration)	36

Occurrence and Distribution

More than 17,000 USGS analyses of arsenic in potable ground water from 595 U.S. counties were compared to data from the National Arsenic Occurrence Survey (Frey and Edwards, 1997). Results indicate that the USGS data are broadly representative of source (untreated) ground water commonly used by public supply systems (Focazio et al., 1999; Welch, Helsel, et al., 1999; Welch, Watkins, et al., 2000). Evaluation of the USGS data suggests that decreasing the MCL to 20 µg/L would increase the frequency of MCL exceedence in source ground water over that for the current MCL, but that less than 5% of public supply systems would be affected at either level. Decreasing the MCL below 10 µg/L likely would result in increased MCL exceedences for public water supplies, particularly those that are untreated. For example, at an MCL of 2 µg/L, about 20% exceedences would be expected. The USGS data also were evaluated to estimate the risk of withdrawing ground water with arsenic concentrations greater than certain levels, and the results have been displayed, by county, on a map of the United States (Welch, Watkins, et al., 2000; fig. 3). A tabular summary of the results (below) indicates that the percentage of counties with a 10% chance of exceeding the arsenic MCL increases rapidly as the proposed MCL decreases.

Arsenic Concentration (µg/L)	Percentage of U.S. Counties with at least a 10% Chance of Exceeding the Indicated Arsenic Concentration
2 (HA)	67
5	48
10 (WHO guideline)	30
20	14
50 (MCL)	4

In general, arsenic concentrations greater than 10 µg/L in ground water from the USGS data set occurred more frequently in the western United States than in the East. Studies in New England, the Upper Midwest, and Oklahoma during the past decade, however, indicate that values greater than 10 µg/L are more common than previously thought.

Several 1991 Study Units and one pilot Study Unit discussed arsenic concentrations in water.

• In the NVBR Study Unit, about 57% of dissolved arsenic concentrations in ground water from the Carson Desert exceeded the MCL. Values were as high as 120 µg/L in a stream affected by sulfide ore processing; 380 µg/L in surface water dominated by irrigation return flow; 1,400 µg/L in high pH terminal lakes affected by high evaporative concentration rates; and 4,000 µg/L in geothermal springs.
• In the RIOG Study Unit, elevated dissolved arsenic concentrations in ground water could be related to irrigation return flow, geothermal springs, or, possibly, urban runoff.
• In the SANJ Study Unit, dissolved arsenic concentrations in shallow ground water were as high as 870 µg/L (except for a value of 2,600 µg/L that may have resulted from a spill or herbicide application) (Fujii and Swain, 1995). Redox conditions were the most important geochemical control, with arsenate dominating under oxidizing conditions and arsenite dominating under reducing conditions.
• In the WILL Study Unit, current and historical (mostly unfiltered) analyses of arsenic in shallow ground water showed concentrations as high as 2,000 µg/L, with 8% of samples exceeding the MCL and 22% of samples exceeding the WHO guideline (Hinkle and Polette, 1999). High arsenic concentrations were associated with felsic volcanic rocks in the southern Willamette Basin and with alluvial deposits in the northwestern basin. Arsenic concentrations in 6 of 7 samples were greater than 60% arsenite, which is consistent with reducing conditions found in many of the wells.
• In the YAKI Study Unit, historical applications of lead arsenate to orchards from 1908 to 1947 may be an important source of arsenic to ground water beneath agricultural areas, especially those that are irrigated (Morace et al., 1999). Dissolved arsenic concentrations in agricultural drains were highest when the drains were fed primarily by shallow ground water, and concentrations in the Yakima River were highest when the river was fed primarily by irrigation return flow. Dissolved loads were 4-9 times suspended loads; the highest measured concentration in surface water was 9 µg/L.

Geochemistry

An evaluation of more than 20,000 USGS and other analyses of arsenic in potable and nonpotable ground water by Welch, Westjohn et al. (in press) indicates the following:

• Arsenate dominates in oxidizing waters, which are typical of most public supplies. Conversely, arsenite dominates in reducing (sulfidic) waters.
• Arsenate sorbs more strongly to iron oxide than does arsenite. Dissolution or desorption from iron oxide seems to be the most common cause of arsenic concentrations greater than 10 µg/L in ground water. Concentrations increase with increasing solution pH and with increasing phosphorus concentrations, as hydroxyl ions lower surface charge and phosphate ions compete for sorption sites.
• Impurities in pyrite and other metal sulfides are likely sources of arsenic in areas affected by coal or metal mine drainage or in areas underlain by glacial or alluvial deposits derived from sulfide-rich bedrock.
• Water with high organic carbon concentrations can reduce iron oxides and release arsenic. Concentrations of methylated arsenic compounds generally are low in ground water.

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 ground-water resources of the United States and limitations in drinking-water supply characterizations: U.S. Geological Survey Water-Resources Investigations Report 99-4279, 21 p.

Frey, M.M., and Edwards, M.A., 1997, Surveying arsenic occurrence: Journal of the American Water Works Association,
v. 89, p. 105-117.

Fujii, Roger, and Swain, W.C., 1995, Areal distribution of selected trace elements, salinity, and major ions in shallow ground water, Tulare Basin, southern San Joaquin Valley, California: U.S. Geological Survey, Water-Resources Investigations Report 95-4048, 67 p., 1 plate.

Hinkle, S.R., and Polette, D.J., 1999, Arsenic in ground water of the Willamette Basin, Oregon: U.S. Geological Survey, Water-Resources Investigations Report 98-4205, 28 p., 1 plate.
Abstract   PDF (1,200Kb)

Morace, J.L., Fuhrer, G.J., Rinella, J.F., McKenzie, S.W., and others, 1999, Surface-water-quality assessment of the Yakima River Basin in Washington--Overview of major findings, 1987-91: U.S. Geological Survey, Water-Resources Investigations Report 98-4113, 119 p.
Abstract   PDF

National Research Council, 1999, Arsenic in drinking water: Washington, D.C., National Academy Press, 310 p.

U.S. Environmental Protection Agency, 1996, Drinking water regulations and health advisories: U.S. Environmental Protection Agency rept. EPA 822-B-96-002 [variously paged].

U.S. Environmental Protection Agency, 1999, National recommended water quality criteria--Correction: U.S. Environmental Protection Agency rept. EPA 822-Z-99-001, 25 p. (URL http://www.epa.gov/ost/standards/wqcriteria.html).

U.S. Environmental Protection Agency, rev. July 28, 1999, Drinking water priority rulemaking--Arsenic: accessed September 1, 1999, at URL http://www.epa.gov/ogwdw/ars/arsenic.html (see also related internal links from this page).

Welch, A.H., Helsel, D.R., Focazio, M.J., and Watkins, S.A., 1999, Arsenic in ground water supplies of the United States, in: Arsenic exposure and health effects, W.R. Chappell, C.O. Abernathy and R.L. Calderon, Eds., Elsevier Science, New York, pp. 9-17.

Welch, A.H., Watkins, S.A., Helsel, D.R., and Focazio, M.J., 2000, Arsenic in ground-water resources of the United States: U.S. Geological Survey Fact Sheet 063-00, 4p.

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: Ground Water v.38, no.4.

World Health Organization, 1996, Guidelines for drinking-water quality--Volume 2, Health criteria and other supporting information (2d ed.): Geneva, World Health Organization, 973 p.