Arsenic in Groundwater Maps

Mapping Arsenic Contamination in California's Groundwater Aquifers

These maps developed by CCKA, using U.S. Geological Survey (USGS) data, provide a picture of arsenic contamination in the aquifers of central California.  The wells mapped are domestic, private water supply wells, and the water quality data taken at these wells are intended to provide an indication of the drinking water quality of Californians that rely on private domestic wells for drinking water.  Data points are largely clustered in the Central Valley because this is where USGS studies have focused in California. 

Arsenic contamination in drinking water sources poses unacceptably high risks of cancer. While much of the arsenic contamination in California’s groundwater aquifers comes from natural geologic sources, some commercial fertilizers contain arsenic that can reach groundwater. In September 2000, the EPA conducted an extensive review of the MCL standard for arsenic (then at 50 micrograms per liter, or 50 µg/L), and proposed lowering it to 5 µg/L, a concentration that still produces an elevated cancer risk if allowed into drinking water. However, in 2001, the EPA standardized the MCL for arsenic at 10 µg/L. The California Department of Public Health officially adopted the federal MCL for arsenic of 10 µg/L, rather than the originally proposed 5 µg/L, effective November 28, 2008.  Visit CCKA’s Groundwater Page to learn more about pollution in California’s aquifers.

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Wells and Agricultural Irrigation
Wells and Domestic Groundwater Withdrawals
Wells and Land Cover