Arsenic in Drinking Water in California
What residents of California need to know about arsenic in drinking water — including natural geological sources, private well risk, which utilities have documented violations, and how to remove arsenic from tap water.
Source: EPA SDWIS, State Water Resources Control Board, USGS · Last reviewed: 2025-01-01
Quick Answer
Is arsenic in drinking water a concern in California?
Yes — California has significant arsenic occurrence across its diverse geology. The San Joaquin Valley, Central Valley, Mojave Desert, and areas with geothermal and volcanic geology in northeastern California and the Coast Ranges all have documented elevated arsenic in groundwater. Multiple California public water systems have historically exceeded the 10 ppb MCL, particularly in the Central Valley and desert communities.
Where does arsenic come from in California's water?
Naturally occurring arsenic from volcanic, geothermal, and sedimentary formations is the primary pathway in California. The San Joaquin Valley's deep alluvial aquifers are affected by arsenic from volcanic ash deposits and reducing geochemical conditions. Desert communities in San Bernardino, Riverside, and Imperial counties face arsenic from geothermal and volcanic sources. Northeastern California (Modoc and Lassen counties) has elevated arsenic from volcanic geology.
What should California residents know?
Central Valley communities relying on deep groundwater wells, desert communities in Southern California, and small water systems in northeastern California face the highest documented risk. California has an active arsenic compliance program and has funded treatment for many small systems through the State Water Board's small community water system programs. Private well owners in these regions should test.
Key Facts
| EPA MCL | 10 µg/L (10 ppb) |
| MCLG | Zero |
| Primary source | Volcanic/geothermal geology; deep reducing aquifer conditions in Central Valley; desert geothermal areas |
| Highest-risk regions | San Joaquin Valley deep groundwater; Mojave/desert communities; NE California volcanic terrain |
| State regulator | State Water Resources Control Board (SWRCB) |
| Small system program | California has funded arsenic treatment for many small disadvantaged community systems |
| Health effects | Bladder, lung, skin cancer; cardiovascular disease; diabetes risk |
| Effective treatment | Reverse osmosis or activated alumina; both in use at California utility and household scale |
Why Arsenic Matters in California
California's Central Valley arsenic problem is tied to the same deep alluvial aquifer system that supplies water to much of the valley's agricultural and municipal users. As surface water allocations have been reduced, some communities have relied more heavily on deep groundwater — which has higher arsenic concentrations due to reducing geochemical conditions and volcanic ash inputs. Desert communities in Southern California face arsenic from geothermal vents and volcanic geology. California has one of the most active small system arsenic compliance programs in the country, but the problem remains significant for rural and disadvantaged communities.
California Arsenic Program
California SWRCB has enforced arsenic compliance since 2006, when the tighter 10 ppb MCL took effect. Many small California water systems — particularly in the Central Valley, the Mojave Desert, and the Central Coast — serve groundwater with naturally elevated arsenic and required treatment upgrades. Private well owners in these regions face ongoing arsenic risk.
Largest California Water Utilities
No arsenic violations on record in EPA SDWIS for California utilities in our database. Browse the largest utilities to review their full water quality record.
How Does Arsenic Get Into Drinking Water?
Arsenic in drinking water is almost always naturally occurring — it leaches from arsenic-bearing rocks and minerals into groundwater over time. New England granite, Southwest volcanic geology, and Upper Midwest glacial aquifers are the primary high-risk formations. It has no taste, odor, or color.
Full arsenic overview — geology maps, health effects, all 50 statesWho Should Pay Closest Attention
San Joaquin Valley communities relying on deep groundwater (Tulare, Kings, Fresno, Kern counties), desert communities in San Bernardino and Riverside counties, small water systems in Modoc and Lassen counties, and private well owners throughout California's volcanic and geothermal regions should prioritize arsenic monitoring.
Private well owners near mining districts or agricultural areas
Residents in states with documented volcanic or geothermal geology
Long-term consumers of water from small groundwater systems
Households in homes built before 1960 with older well casings
Residents whose well water has never been tested for arsenic
Anyone living in a state where bedrock wells are common
How to Check Your Situation in California
- 1
Identify your water source. If you use a public utility, use the ZIP lookup on this page to find your system and check its compliance record.
- 2
If on public water, review your utility's most recent Consumer Confidence Report (CCR) for arsenic monitoring data. The MCL is 10 ppb — your report should show recent test results.
- 3
If on a private well, order an arsenic test from a State Water Resources Control Board-certified laboratory. A basic arsenic test costs $15–$40. The state agency website maintains a certified lab list.
- 4
Test your well at the tap — not just at the wellhead. The entire water distribution system within your home can affect water quality.
- 5
If your test shows arsenic above 5 ppb, install certified treatment immediately. If above 10 ppb, do not use the water for drinking or cooking until treatment is installed.
- 6
Retest after installing treatment to confirm it is working as certified. Replace filter media on the manufacturer's schedule — an exhausted filter may not perform as rated.
Treatment Options for Arsenic
Boiling does not remove arsenic — it concentrates it. Standard activated carbon filters (Brita, etc.) do not effectively remove arsenic. Certified treatment is required.
NSF/ANSI Standard 58 — Reverse Osmosis
RO systems certified to NSF/ANSI 58 remove 85–95% of arsenic. Under-sink installation. Most effective for removing multiple contaminants simultaneously. Replace membranes and pre-filters on schedule.
Activated Alumina Filters
Activated alumina is specifically designed for arsenic and fluoride removal. Point-of-use or whole-house options available. Must be certified by NSF International or WQA for arsenic reduction. Requires periodic media regeneration or replacement.
Iron/Manganese Oxidation Filters
Effective for arsenic in iron-rich well water, which is common in the Midwest and New England. Oxidation converts dissolved iron and arsenic to a form that can be filtered out. Best when arsenic is co-occurring with high iron levels.
What does NOT work for arsenic
Standard activated carbon filters (Brita, refrigerator filters, most pitcher filters) do NOT effectively remove arsenic. Boiling concentrates arsenic. Water softeners do not remove arsenic. Only use products with NSF certification specifically for arsenic reduction.
Take Action Now
If you use a private well in California, test for arsenic — especially if you are in a region with granite, volcanic, or sedimentary geology. A basic arsenic test costs $15–$40 at a state-certified lab.
Public water users: check your utility's Consumer Confidence Report (CCR) for arsenic results. The EPA MCL is 10 ppb — any detection warrants attention.
If arsenic is detected above 10 ppb (or even below it, given MCLG is zero), install an NSF/ANSI 58-certified reverse osmosis system for drinking and cooking water.
Standard carbon filters do NOT remove arsenic — do not rely on a Brita or refrigerator filter for arsenic protection.
Frequently Asked Questions
Related Pages
Arsenic — National Overview
All U.S. utilities with arsenic records
California State Overview
All utilities and water quality data
California Well Water Guide
Testing, risks & certified labs for private wells
Reverse Osmosis Guide
Removes 85–95% of arsenic
Best Arsenic Filter Guide
What actually works (carbon doesn't)
PFAS in Drinking Water
The 'forever chemical' contamination overview
All Contaminants
Complete reference library
Data Sources & Provenance
All data on this page is sourced from official U.S. government or public datasets.
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