Arsenic in California drinking water

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Why arsenic is common in California water

Arsenic is a naturally occurring element in rocks and soil. The State Water Board's arsenic page calls it ubiquitous in nature and commonly found in California drinking water sources. It dissolves into groundwater from volcanic and sedimentary deposits and into some surface water as well. LADWP, for example, notes in its 2025 report that the Owens Valley and Mono Basin watersheds feeding the Los Angeles Aqueduct release naturally occurring arsenic into creeks.

Industrial and agricultural history adds to the picture in places, but most arsenic in California tap water is geologic. That is why it shows up in wealthy suburbs and farm towns alike, and why the source of your water matters more than the age of your neighborhood.

California's arsenic limit and health goal

The gap here is huge. OEHHA based its arsenic Public Health Goal on lung and urinary bladder cancer risk, at a level corresponding to about one extra cancer case per million people drinking the water for 70 years. The legal limit is 2,500 times higher because it balances health risk against the cost and technology of removing arsenic. The board notes that arsenic can also cause non-cancer effects, such as skin and vascular effects, at higher exposures.

That balance may shift. The Division of Drinking Water says it is investigating the feasibility of lowering the MCL below 10 µg/L and closer to the PHG (arsenic MCL revision file), and it held a 2022 workshop on lowering the detection limit for reporting for several metals, including arsenic. Neither is a final rule yet.

What real California reports show

Here is how arsenic appeared in three large utilities' 2025 Consumer Confidence Reports:

• LADWP (Los Angeles): the highest level detected was 7.0 µg/L, with the Los Angeles Aqueduct Filtration Plant averaging 3.3 µg/L (range under 2 to 7.0). Because that is above 5 µg/L, half the MCL, the report includes California's required arsenic statement: the water meets the standard but contains low levels of arsenic, and the standard balances health effects against the cost of removal (LADWP report).
• Riverside Public Utilities: groundwater from the Bunker Hill and Riverside basins averaged 3.3 µg/L, range not detected to 4.6 µg/L (RPU report).
• City of San Diego: arsenic does not appear among the detected chemicals in the 2025 report tables (San Diego report).

All three met the MCL. All three detections are, like any detectable arsenic, above the PHG. Whether that gap matters enough to filter is a personal decision, and the numbers in your own report are what you need to make it.

How to read an arsenic result

1. Check the unit. Most reports use µg/L (ppb). 0.010 mg/L is the same as 10 µg/L.
2. Look at the highest value and the range, not just the average. Blended systems can vary by neighborhood and season.
3. Look for the 5 µg/L statement. California requires extra health language in the report when arsenic is above 5 µg/L but at or below 10 µg/L.
4. Note non-detects carefully. "ND" means below the lab's reporting limit (currently 2 µg/L for arsenic), not zero.
5. Confirm your utility on your water bill, then look it up in Drinking Water Watch for sampling history.

Private wells and arsenic

Private domestic wells are not monitored by the state. The Water Board's GAMA program publishes an arsenic groundwater information sheet with a statewide map of detections, which is a useful first look at whether your area is affected. If you suspect arsenic, the board advises not drinking the water until it is tested. Use an ELAP-accredited lab, and ask for total arsenic; if you plan to treat, ask whether the lab can speciate arsenic 3 and arsenic 5.

In parts of the Central Valley, programs connected to the Nitrate Control Program, such as the Valley Water Collaborative, now test eligible domestic wells for arsenic as well as nitrate, and state SAFER funding can pay for treatment for qualifying low-income or disadvantaged households.

Filters that reduce arsenic

Arsenic exists in two main forms. The State Water Board's water treatment device page explains that arsenic 3 is much more difficult to remove than arsenic 5, and that many devices are certified to reduce arsenic 5 but may reduce arsenic 3 only minimally. Public water that has been disinfected with chlorine is generally converted to arsenic 5; untreated well water can contain both.

• Reverse osmosis (RO) under-sink systems certified under NSF/ANSI 58 for pentavalent arsenic.
• Adsorptive media (iron- or aluminum-based) cartridges certified for arsenic reduction.
• Distillation units certified for arsenic.

Standard carbon pitchers and softeners are not arsenic treatment unless the product is certified for it. Look up the exact model in the NSF certified products listings, read the Performance Data Sheet California requires for registered devices, replace cartridges on schedule, and retest treated water periodically. The filter basics page compares RO and carbon.

Arsenic often travels with other groundwater issues. If your report lists it, also check nitrate, chromium 6 and PFAS.

Related guides

• Hard water in California
• PFAS in California drinking water
• Chromium 6 in California water
• Nitrate in California well and tap water
• Chlorine taste and smell in tap water
• How to check tap water by ZIP
• City water guides
• Regional water guides

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