Arsenic in Drinking Water: MCL Status

Last Update: July 25, 2007

Arsenic is ubiquitous in nature and is commonly found in drinking water sources in California.  Public health concerns about arsenic in drinking water related to its potential to cause advserse health effects are addressed through the adoption of drinking water standards, called maximum contaminant levels (MCLs).

Ingestion of arsenic can pose a risk of cancer, according to the Office of Environmental Health Hazard Assessment's (OEHHA's) Public Health Goal for Arsenic in Drinking Water (April 2004).  The PHG is 0.004 micrograms per liter (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.

What is the status of regulation of arsenic in drinking water?

The California Health & Safety Code directs development of a new state MCL for arsenic: §116361 requires CDPH to establish a revised arsenic MCL by June 2004.  It also requires OEHHA to establish 18 months earlier a PHG for arsenic. In addition,  §116365(a) requires CDHS, while placing primary emphasis on the protection of public health, to establish a contaminant's MCL at a level as close as is technically and economically feasible to its PHG.

CDPH is proceeding with the regulatory process.  Click here for its status.

The state MCL cannot be less restrictive than the 10-µg/L federal MCL for arsenic, which has been in effect since January 2006.  For more about the federal standard, see US EPA's "Arsenic Virtual Trade Show" and CDPH's FAQ about compliance with the federal rule. 

Where can I find out more?

Water systems' annual consumer confidence reports (CCRs) contain information about drinking water quality.  Many reports are on US EPA's CCR website.

Is there financial assistance for water systems with arsenic problems?

CDPH has funding opportunities for water systems, some of which may arsenic-related projects.

Where has arsenic been found?

 Monitoring results (2002-2005) reflect arsenic's natural, widespread occurrence and show about 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 CDPH is confident about the quantity being reported by analytical laboratories.  Results at or above the DLR are required to be reported to CDPH.

The results also show peak arsenic levels greater than the prior MCL of 50 µg/L in 72 sources, and peak levels greater than the 10-µg/L MCL in 598 sources.