Arsenic in Well Water: Why It's Often Invisible
An estimated 2 million U.S. private wells exceed the EPA limit for arsenic — and there are no acute symptoms.
Where arsenic comes from
Arsenic is a naturally occurring element in rocks and soil. It enters groundwater when underground formations weather and release it into the aquifer. Industrial sources exist — old wood-treatment plants, glass manufacturing, mining — but the dominant exposure for most U.S. households is geologic, not industrial.
That's why arsenic is overwhelmingly a private-well issue. Public water systems are required to treat for it; private wells aren't tested unless the owner orders the test.
Regional risk
The USGS has mapped arsenic risk extensively. Higher-risk regions include parts of New England (especially New Hampshire and Maine), the Upper Midwest (Michigan, Minnesota, Wisconsin), the Mountain West, the Southwest (Nevada, Arizona, parts of California), and parts of the Southeast.
But "low-risk region" doesn't mean zero risk — individual wells can deviate significantly from the regional pattern based on the specific aquifer your well draws from.
Why it's invisible
Arsenic at levels that affect health is completely undetectable to your senses. There is no taste, no smell, no color. Acute symptoms only appear at concentrations thousands of times above the EPA limit — concentrations that would only occur from deliberate poisoning, not from environmental exposure.
Chronic low-dose exposure is the real concern. The health effects develop over years to decades.
Health effects
Long-term low-level arsenic exposure is associated with increased risk of skin cancer, lung cancer, bladder cancer, cardiovascular disease, and skin lesions (a thickening or pigmentation on hands and feet). Pregnant people and young children are higher-risk; arsenic crosses the placenta and is associated with reduced infant immune function.
The numbers
EPA MCL: 10 parts per billion (ppb), set in 2006. Some states (notably New Jersey and New Hampshire) have set lower state-level limits of 5 ppb.
WHO guideline: 10 ppb.
MCLG: zero. As with lead, the EPA's health-based goal is zero — the enforceable limit reflects what's achievable.
How to test
Arsenic is straightforward to test. A single sample collected in a lab-provided bottle is sufficient for most homes. Labs use ICP-MS to detect levels well below the EPA limit.
Test once if you've never tested your well. Re-test every 3-5 years, or sooner if you notice any change in the well's depth, pump, or surrounding land use.
Treatment options
Reverse osmosis (RO): Effective for most forms of arsenic, typically used as point-of-use under the kitchen sink.
Anion exchange or adsorption media: Effective whole-house options, but the choice depends on whether your arsenic is in the more common arsenate (As V) form or the harder-to-remove arsenite (As III) form. A speciation test can determine which.
Distillation: Removes essentially all arsenic. Practical for drinking water in small volumes.
Activated carbon does NOT remove arsenic. Boiling does NOT remove arsenic — it concentrates it.
If you have a well
Test for arsenic once. If results are below 5 ppb, retest every 3-5 years. If results are 5-10 ppb, retest yearly and consider point-of-use RO. If results are over 10 ppb, install treatment and retest after installation to verify.
General education, not advice. This article is for information only — not medical or legal advice. Lab results should be interpreted in the context of your specific home and situation. See our full Disclaimer.