Arsenic and Drinking Water

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Arsenic is a naturally occurring element, but long-term exposure can cause cancer in people. There has been a substantial amount of research done to address arsenic in groundwater and drinking-water supplies around the country. The USGS studies local and national sources of arsenic to help health officials better manage our water resources.


    Arsenic occurs naturally as a trace component in many rocks and sediments. Whether the arsenic is released from these geologic sources into groundwater depends on the chemical form of the arsenic, the geochemical conditions in the aquifer, and the biogeochemical processes that occur. Arsenic also can be released into groundwater as a result of human activities, such as mining, and from its various uses in industry, in animal feed, as a wood preservative, and as a pesticide. In drinking-water supplies, arsenic poses a problem because it is toxic at low levels and is a known carcinogen. In 2001, the USEPA lowered the MCL for arsenic in public-water supplies to 10 micrograms per liter (µg/L) from 50 µg/L.



    The USGS plays an active role in protecting human health from potential issues related to our Nation's natural resources. One vital aspect is assessing the water quality of groundwater supplies. Whether across the U.S. or around the world, the USGS helps measure and monitor drinking water supplies for contaminants like arsenic. For example, dangerously high levels of arsenic have been found in drinking water wells in more than 25 states in the United States, potentally exposing 2.1 million people1 to drinking water high in arsenic. Possibly the worst case ever of arsenic poisoning occurred in Bangladesh, where over 100 million people2 were poisoned by arsenic in groundwater supplies.

    Estimates of how many private domestic well users in each county may be drinking water with high levels of arsenic

    This map shows estimates of how many private domestic well users in each county may be drinking water with levels of arsenic of possible concern for human health.(µg/L, micrograms per liter)

    U.S. map showing arsenic concentrations in groundwater wells

    Arsenic was detected more frequently and at higher concentrations in wells in the West than in the East. Symbols represent median concentration in networks of 20-30 wells. Figure from USGS Circular 1360, Water Quality in Principal Aquifers of the United States, 1991-2010.

    In a national study of groundwater quality, the USGS found that arsenic was detected in nearly half of the wells sampled in parts of aquifers used for drinking-water supply at a concentration of 1 µg/L or greater. Detections were more common and concentrations generally were higher in the west than in the east. About 7 percent of the wells sampled contained arsenic at a concentration that exceeded the MCL of 10 µg/L, indicating a potential health risk. The greatest concern was in the Southwest, where concentrations of arsenic exceeded the MCL in about 16 percent of drinking-water wells sampled.  Other Principal Aquifers with concerns for arsenic included the Glacial aquifer system (northern U.S.), the crystalline rock aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Aquifers (northern U.S.), and the Mississipppi Embayment–Texas Coastal Uplands Aquifer System and Mississippi River Valley Alluvial Aquifer (southeastern U.S.).

    In the Southwest basin-fill aquifers, arsenic concentrations in drinking-water wells exceeded the MCL more than twice as frequently as in drinking-water wells nationwide. The source is the volcanic and granitic rocks through which that groundwater moves. [maybe use photo from p. 18]  Factors that contribute to elevated concentrations of arsenic in these aquifers include long groundwater residence times, rock type, high pH, arid climate, and irrigation practices. 

    The USEPA estimated in 2001 that the annual cost to reduce arsenic concentrations to below the MCL would range from $0.86 to $32 per household for customers of large public water systems (more than 10,000 people) to $165 to $327 per household for very small systems (25–500 people). Understanding the factors that affect concentrations of arsenic and other contaminants with geologic sources in groundwater can help water suppliers prioritize areas for new groundwater development and reduce treatment costs.



    The unique hydrogeologic character of this health hazard makes USGS research crucial for understanding this hazard and helping water-supply and public-health authorities implement strategies to mitigate this risk. The USGS investigates the presence and effect of arsenic in drinking water supplies, monitors the worldwide distribution of arsenic in groundwater, and assesses the impact of arsenic on local stream sediment chemistry.