The short answer: arsenic is a naturally occurring element that dissolves into groundwater from certain rock formations. It has no taste, odor, or color, and long-term exposure at levels above the EPA limit is linked to several cancers and other serious effects. The EPA's Maximum Contaminant Level for drinking water is 10 parts per billion (10 ppb, or 0.010 mg/L). Because you cannot detect it any other way, a laboratory test is the only way to know whether your well has it.
Of all the contaminants a private well can carry, arsenic is the one that most rewards testing, precisely because it gives you no warning. It does not change how your water looks, smells, or tastes. It is not tied to anything you did or any equipment you installed; it comes from the ground your well draws through. And unlike most well contaminants, its health risk is a cancer risk that accumulates over years of ordinary drinking and cooking.
Where arsenic in well water comes from
Most arsenic in private wells is geological, not industrial. It occurs naturally in many rock types and sediments, and as groundwater moves through those formations, arsenic can dissolve into it. This is why arsenic risk is strongly regional: it tracks the bedrock and aquifer your well draws from, not your household activity. Parts of New England, the upper Midwest, and the Southwest, among other areas, have geology that produces elevated arsenic in domestic wells.
The US Geological Survey, through its sampling of thousands of private wells, has mapped where arsenic tends to exceed the health benchmark. That regional pattern is useful for deciding whether arsenic belongs on your test list, but it is not a substitute for testing: two neighboring wells drawing from different depths can show very different arsenic levels.
The health risk
Arsenic is classified as a known human carcinogen. Long-term consumption of water with elevated arsenic is associated with cancers of the skin, bladder, and lung, and with non-cancer effects including skin changes, cardiovascular effects, and developmental concerns. The risk is a function of both concentration and years of exposure, which is why a contaminant that causes no immediate symptoms is still treated seriously.
This long-term, cumulative nature is the reason the EPA sets the limit where it does, and the reason testing matters even when your water seems perfectly fine.
Why the EPA limit is 10 ppb
The EPA's Maximum Contaminant Level for arsenic in drinking water is 10 parts per billion. This standard took effect in 2006, lowered from the previous limit of 50 ppb, on the basis of research linking lower arsenic levels to elevated cancer risk. The MCL is legally enforceable for public water systems.
Two honest caveats matter here. First, the EPA's own health goal (the Maximum Contaminant Level Goal) for arsenic is zero, reflecting that no exposure is considered risk-free for a carcinogen; the 10 ppb MCL is the enforceable level judged feasible, not a line below which arsenic is harmless. Second, a private well is not legally required to meet the 10 ppb limit at all. The number is still the right yardstick, because it is the level a public utility must not exceed, but on your own well, exceeding it is a decision you have to act on yourself.
Regional risk tells you to test; it does not measure your well
Living in an area with arsenic-bearing bedrock raises the odds that your well has arsenic, which is exactly why it should be on your test list. But the regional pattern cannot tell you your own concentration. Only a laboratory test of your water can. Our report uses the USGS regional data to tell you whether arsenic is worth prioritizing for your address.
How to test for arsenic
Arsenic is measured by a certified drinking-water laboratory, not by a taste, smell, or simple field strip. To test:
- Use a state-certified lab. Your county or state health department can run the test or point you to a certified lab. Ask specifically for total arsenic in drinking water.
- Follow the lab's collection instructions. The lab supplies the correct bottle. Arsenic sampling is less sensitive to technique than bacteria sampling, but following the instructions still matters for an accurate result.
- Compare the result to 10 ppb. A result at or above 10 ppb (0.010 mg/L) is above the EPA limit and warrants action. Because the health goal is zero, some households choose to treat below 10 ppb as well.
- Retest periodically. Arsenic levels are generally stable but can shift with water table changes, so include it in the every-few-years panel described in our guide on how often to test.
If your arsenic is high
Boiling water does not remove arsenic; in fact it concentrates it slightly as water evaporates. Effective treatment options include specific point-of-use and point-of-entry systems such as certain reverse-osmosis units, adsorptive media designed for arsenic, and anion exchange, with the right choice depending on your water chemistry and the arsenic form present. Because the effective method depends on your specific water, confirm the result with a certified lab and consult a water-treatment professional before buying equipment.
Sources
- US Environmental Protection Agency. Arsenic in Drinking Water and National Primary Drinking Water Regulations. Maximum Contaminant Level of 0.010 mg/L (10 ppb), effective 2006, with an MCLG of zero. epa.gov.
- US Geological Survey. Arsenic in Groundwater and Domestic Wells. Regional occurrence of naturally occurring arsenic in private well water. usgs.gov.
- US Centers for Disease Control and Prevention / ATSDR. Arsenic Toxicity and Health Effects. Association of long-term arsenic exposure with cancer and other effects. cdc.gov / atsdr.cdc.gov.
Related guides
Find out if arsenic is a priority for your well
Our report checks the USGS regional data for your address and tells you whether arsenic ranks high enough on your local panel to test for it first.
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