Arsenic in Well Water: Test, Treat, Re-Test

Signs of arsenic

There aren’t any you can notice at home. Arsenic has no taste, color or smell, and the only sign is a lab result. The CDC recommends asking your health or environmental department whether to test your well for arsenic, and using a state-certified laboratory [CDC: Guidelines for testing well water]. If you’ve never tested, or your neighbors have found arsenic, it’s worth doing.

The limit, and where it shows up in the Valley

The drinking water limit for arsenic, called the MCL (maximum contaminant level, the legal limit for public drinking water), is 0.010 mg/L, or 10 parts per billion, in both California and federal rules [State Water Board: California and federal MCLs (Sept 2024)]. Private wells aren’t regulated, but the MCL is the benchmark to compare your results to.

In the USGS study of the Northern San Joaquin Valley, arsenic was present at high concentrations in about 9% of the primary aquifers, and nearly all of the high results came from wells near the margin of the Sacramento–San Joaquin Delta in the Eastern San Joaquin study area [USGS Fact Sheet 2010-3079: Groundwater quality, Northern San Joaquin Valley]. That’s the west side of the Eastern San Joaquin area, closest to the Delta. An earlier USGS report found that between 1974 and 2001, up to one-third of wells sampled in the Eastern San Joaquin groundwater subbasin had arsenic above 10 µg/L [USGS Open-File Report 2008-1272: Arsenic, Eastern San Joaquin subbasin].

City water can be affected too. Hughson’s 2025 report shows arsenic averaging 12.3 ppb, above the 10 ppb limit, with 3 of 5 wells above it [City of Hughson 2025 Consumer Confidence Report]. See Hughson water and Stockton water.

How we test for arsenic

1. Free in-home water test. We come to you and check the basics at your tap: hardness, precipitation, chlorine and soap testing, and more, depending on what you’re noticing.
2. Lab test where it matters. For anything with a health-based limit, we arrange a state-certified lab test and walk you through the report in plain English.
3. A written, no-pressure estimate for only what your water calls for. Sometimes that’s nothing at all.
4. Tested before, tested after. Once your system is in, we test again, with a follow-up lab test for health-related contaminants. If the results don’t match what we showed you, we adjust it at no charge.

How we treat it

Depending on your situation and how high your results are, we use one of two approaches:

• An arsenic-specific whole-home filter that treats every tap.
• Reverse osmosis, either under-sink for drinking and cooking water or whole-home, depending on the situation and severity.

Your lab results and your home decide which one fits. After install we arrange another lab test, so you can see your results on paper. We’ll also remind you to keep testing on a regular schedule.

What treatment gives you

• Confidence in every glass, backed by a lab result after install, not a guess.
• A protected family and home, with treated water at every tap.
• No more bottled water for drinking and cooking.
• One local team that tests, installs, re-tests and services the system.
• Financing, so you don’t have to wait to act on a lab result.

Reading your arsenic result

Labs report arsenic in a few different units, which can be confusing. 0.010 mg/L, 10 µg/L and 10 parts per billion (ppb) are all the same number, and that’s the MCL in California and federal rules [State Water Board: California and federal MCLs (Sept 2024)]. If your result is close to the limit, or above it, we’ll talk through what it means and the options that fit your well and your budget, in plain English.

When whole-home treatment makes sense

Under-sink RO is a good fit for many homes, since arsenic is mainly a drinking-water concern. But your family uses water everywhere: the kitchen, the ice maker, the bathroom sink where kids brush their teeth. When levels are higher, whole-home treatment means you don’t have to think about which faucet is the treated one. It also means one system to service, from the same local team that tested your water.

What else to test for while you’re at it

If you’re already sending a sample to a lab, it’s a good time to check the basics too. The CDC recommends testing private wells at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH [CDC: Guidelines for testing well water]. Knowing the full picture lets us design one treatment plan instead of fixing problems one at a time.

Arsenic testing near you

We help homeowners across the corridor: Manteca · Lathrop · Stockton · Patterson · Hughson · Turlock · Modesto.