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Arsenic in Drinking Water: MCL Status

Last Update: December 1, 2008


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. 

Click on the following links for more information about the California MCL (additional documents are at the Office of Regulations and Hearings):

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, monitoring from 2002-2005 (Table 1) shows about 600 active and standby sources with peak detections of greater than 10 µg/L. 

Overall monitoring results (Excel)Opens in new window.for the same time period show 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.

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 that CDPH has funding opportunities for water systems, some of which may be available for such projects.


Table 1. Drinking water sources and systems with peak arsenic detections greater than 10 µg/L
(active and standby drinking water sources)*
(for all detections 2002-2005, see monitoring results (Excel)Opens in new window.)
County No. of 
No. of 
Butte 5 2
El Dorado 6 2
Fresno 9 4
Inyo  5 5
Kern 114 56
Kings 40 12
Los Angeles 48 21
Madera 13 7
Marin 2 2
Mariposa 3 3
Mendocino 1 1
Merced 15 12
Mono 11 3
Monterey 19 8
Napa 5 4
Nevada 4 2
Orange 4 3
Plumas 2 1
Riverside 26 8
Sacramento 22 8
San Benito 2 2
San Bernardino 64 22
San Diego 5 2
San Joaquin 49 18
San Luis Obispo 9 7
Santa Barbara 7 4
Santa Cruz 4 3
Shasta 1 1
Solano 6 3
Sonoma 23 18
Stanislaus 20 9
Sutter 34 19
Tehama 3 2
Tulare 16 11
Yuba 1 1
TOTAL 598 286
*Number of drinking water sources and systems with at least one detection in water sampled 2002-2005 (see monitoring results).  Numbers are considered draft; they will change with subsequent updates. 



Last modified on: 12/28/2010 10:37 AM