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Arsenic in Drinking Water: MCL Status

Public health concerns about arsenic in drinking water related to its potential to cause adverse health effects are addressed through the adoption of state and federal drinking water standards, also called maximum contaminant level (MCLs).

California's revised arsenic MCL of 0.010 mg/L (equivalent to 10 micrograms per liter, μg/L) became effective on November 28, 2008. A 10-μg/L federal MCL for arsenic has been in effect since January 2006. Previous California and federal MCLs for arsenic were 50 μg/L.

Click on the following links for more information about the California MCL (additional documents are at the California Department of Public Health's (DPH's) Office of Regulations):

Ingestion of arsenic can pose a risk of cancer, according to OEHHA's PHG for Arsenic in Drinking Water (April 2004). The PHG is 0.004 μg/L, based on lung and urinary bladder cancer risk, corresponding to a de minimis cancer risk level (i.e., up to one excess case of cancer per million people per 70-year lifetime, if their drinking water contained arsenic at the concentration of the PHG). Arsenic can also result in a number of non-cancer effects at higher levels of exposure (e.g., vascular effects or skin effects), but the cancer risk is the most sensitive endpoint, and the basis for the PHG. Additional information on arsenic's health effects is available from the Agency for Toxic Substances and Disease Registry, the National Toxicology Program, and the World Health Organization.

Arsenic is ubiquitous in nature and is commonly found in drinking water sources in California. For example, an analysis by CDPH’s Drinking Water Program (now the Division of Drinking Water, DDW) of results of drinking water monitoring from 2002-2005 showed about 600 active and standby sources with peak detections of greater than 10 μg/L. Overall monitoring results (Excel) for the same time period showed 2,200 active and standby drinking water sources with arsenic present at concentrations greater than 2 μg/L, the detection limit for purposes of reporting (DLR). The DLR is the level at which the DDW is confident about the quantity being reported by analytical laboratories. Results at or above the DLR are required to be reported.

The Division of Water Quality’s GAMA Program, in its Groundwater Information Sheet for Arsenic (PDF), has maps of arsenic detections, based on monitoring information from DDW's water quality monitoring database. As of June 2010, 1,375 drinking water wells reported detections greater than 10 μg/L (of 10,425 wells sampled).

Information about drinking water quality (including arsenic and other contaminants) in specific drinking water supplies is available in public water systems' consumer confidence reports (CCRs), which are distributed annually. Many of these reports are on US EPA's CCR website.

Public water systems seeking assistance for arsenic-related projects should be aware of funding opportunities for water systems, some of which may be available for such projects.

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(Updated 9/22/16)