Arsenic in California Well Water: Testing, Treatment Options and Costs
Arsenic is the most consequential contaminant most California well owners have never tested for. It has no taste, no smell, and no color; it doesn't stain fixtures like iron and manganese or announce itself like sulfur; and it occurs naturally in the state's geology, so it's present in wells that have never been near a farm or a factory. If your home is on a private well — especially in the Central Valley — an arsenic test belongs at the top of the list.
Where it comes from and who's at risk
Arsenic in groundwater here is mostly geologic: it desorbs from sediments into aquifers, with concentrations shaped by geochemistry, well depth, and pumping. In California, elevated arsenic is a known issue across parts of the San Joaquin Valley (especially the Tulare basin), in desert basins, and in scattered pockets statewide. The scale of the problem is quantified in peer-reviewed work: a study by Ayotte and colleagues at the U.S. Geological Survey, published in Environmental Science & Technology in 2017, estimated that about 2.1 million people in the U.S. rely on domestic wells with arsenic above the federal limit of 10 micrograms per liter, with the western states — California among them — prominent on the hazard maps (study on Google Scholar).
The regulatory catch: the 10 µg/L maximum contaminant level protects customers of public water systems. Private wells are unregulated in California — nobody tests your well but you, a point we develop in our well testing guide. Long-term arsenic exposure is associated with skin, bladder, and lung cancers and cardiovascular disease, which is why the limit exists at all.
Testing: cheap, and do it right
- Certified lab test: roughly $25–$60 for arsenic alone, $150–$400 as part of a broad well panel (bacteria, nitrate, metals). Use a state-certified lab; county environmental health departments keep lists and sometimes subsidize testing.
- Test before designing treatment — and ask the lab about arsenic speciation (arsenite As-III vs arsenate As-V) if levels are elevated, because the two forms treat differently: As-III often needs oxidation before it can be removed efficiently.
- Retest periodically. Arsenic levels can drift with drought, pumping, and season — one clean result years ago isn't a property of the well forever.
Treatment options and costs
The strategy question is the same one as for nitrate: treat the whole house, or just the water people drink?
- Point-of-use reverse osmosis (kitchen tap): about $300–$1,500 installed. RO removes arsenate reliably and is the standard answer for drinking and cooking water — full details in our under-sink RO guide. Since arsenic isn't absorbed through skin in meaningful amounts at these levels, showering isn't the exposure that matters; drinking is.
- Whole-house adsorptive media (iron-based media beds): roughly $2,000–$6,000+ installed, plus periodic media replacement. Chosen when levels are high, when the household wants every tap covered, or when As-III requires an oxidation stage (which can double as iron treatment).
- What doesn't work: standard water softeners, carbon filters, and boiling do not remove arsenic — boiling actually concentrates it. Softener-vs-filtration roles are untangled in our whole-house filtration guide.
- On a public system rather than a well? Your utility already treats to the MCL; the related worry there is chromium-6 or PFAS, each covered separately.
The practical sequence
Test first ($40 answers the whole question), speciate if elevated, then size treatment to the number and the household — an RO tap for a modest exceedance, whole-house media for a serious one. Installation of either is a normal job for a plumber who does water treatment work: request a free quote, bring the lab report, and you'll get a system priced to your actual water rather than to fear. For everything else your well should be screened for, start with our complete well testing guide.
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