National Water-Quality Assessment
The USGS map provides a national scale snapshot of arsenic concentrations in the Nation's ground-water resource. This timely information is being used by the USEPA to augment their efforts in revising the drinking-water standard for arsenic, as mandated in the 1996 amendments to the Safe Drinking Water Act.
Use of the national map to identify contamination at a more detailed level, such as in an individual well or within a locality, is not intended or advised. The objective of the map, which statistically summarizes and displays information by county, is to provide a better understanding of where arsenic occurs across the Nation. Such information is useful to identify and prioritize regional and national water-management goals and strategies.
Although concentrations of arsenic are likely to be higher in certain geographic regions relative to others (as evident on the map), there can be a high degree of local variability because of well depth, aquifer type, geology, and other factors. Individual concentrations at a well do not necessarily conform to patterns shown at the county or national scale. The only way to be certain of the arsenic concentration in the water supplied by any given well is to have the water tested.
Public health departments can help in locating laboratories to have water tested and can direct you to additional information about arsenic in your geographic area. In addition, the USGS Water Science Center in your State might have information from ongoing or previous arsenic investigations.
Some of the wells sampled were individual homeowner wells, used without treatment for drinking water. Other wells supplied water for irrigation and other purposes. All wells were drawing water from aquifers used for drinking water supply, even if that specific well was not used for supplying drinking water. The intent of this study was not to directly measure the quality of drinking water, such as by monitoring water from taps and other "finished" water. Rather, we have described the quality of the untreated groundwater resource itself. Comparisons of these findings with those from previous drinking-water studies indicate that concentrations reported here are comparable on a national scale to concentrations in untreated ground water tapped by water utilities across the Nation. These findings also reflect the quality of the ground-water resource tapped without treatment by homeowners and small community water systems.
Arsenic is a naturally occurring element in the environment. Its presence in ground water largely is the result of arsenic-bearing minerals dissolving naturally over time as certain types of rocks and soils are weathered.
Several types of cancer have been linked to arsenic in water. In addition, arsenic has been reported to affect the vascular system in people and has been associated with the development of diabetes.
The USEPA has recently reduced (2001) the drinking-water maximum contaminant level from 50 micrograms per liter to 10 micrograms per liter. The new 10 ug/L standard matches the World Health Organization's international guideline for drinking water.
About 42,400,000 people (accounting for about 3,350 million gallons per day) are served through domestic supply; and about 91,200,000 people (accounting for about 15,100 million gallons per day) are served through public-water supply (Solley, W.B., Pierce, R.R., and Perlman, H.A., 1998, Estimated Use of Water in the United States in 1995, U.S. Geological Survey Circular 1200).
The paper version is out of print.
Dr. Alan Welch
U.S. Geological Survey
2730 N. Deer Run Road
Carson City, NV 89701
phone: (775) 887-7609
Go to Fact Sheet 063-00 report main page.
Go to WRI 99-4279 report main page.
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