Arsenic in Drinking Water: Where It's Found and How to Remove It

Natural arsenic affects millions. How to test and what filters work.

Where Found

Arsenic is a naturally occurring element found in rock formations across the United States, and it dissolves into groundwater that millions of people drink. Unlike most water contaminants, arsenic is not primarily an industrial pollution problem. It is geology. Regions with arsenic-bearing volcanic rock, sedimentary deposits, or geothermal activity have elevated levels in their water supplies.

The highest concentrations in the US are found in the Southwest and West: Arizona, Nevada, New Mexico, California, and parts of the Pacific Northwest. But arsenic is not limited to these areas. Pockets of elevated arsenic exist in the Midwest, New England, and the Appalachian region. USGS mapping shows that roughly 2.1 million Americans rely on domestic wells with arsenic above the EPA's MCL of 10 ppb.

Public water systems are required to treat for arsenic if levels exceed the MCL, and most do. The greater risk falls on private well owners, who are responsible for their own testing and treatment. Many wells in arsenic-prone areas have never been tested, and arsenic has no taste, color, or smell at concentrations found in drinking water. You cannot detect it without a test. Check your city's data for municipal arsenic levels, or test your well directly if you are on a private supply.

Health Effects

Arsenic is a confirmed human carcinogen. The EPA, WHO, and International Agency for Research on Cancer all classify inorganic arsenic as a Group 1 carcinogen, the highest designation. Long-term ingestion of arsenic in drinking water has been directly linked to cancers of the bladder, lungs, skin, kidneys, and liver.

The cancer risk is dose-dependent but there is no known safe threshold. The EPA set the MCL at 10 ppb in 2006, lowering it from the previous standard of 50 ppb. The MCLG (Maximum Contaminant Level Goal, the health-based target with no enforcement) is zero, reflecting the science that any exposure carries some risk. At 10 ppb over a lifetime, the EPA estimates an excess cancer risk of approximately 1 in 300 to 1 in 500, which is significantly higher than the 1-in-10,000 risk level the agency typically uses to set MCLs for other carcinogens.

Beyond cancer, chronic arsenic exposure causes a range of non-cancer health effects. These include cardiovascular disease (increased risk of heart attack and stroke), diabetes, neurological damage (reduced cognitive function in children), and characteristic skin changes including darkening and thickening. Studies in Bangladesh, where arsenic contamination is severe, have documented these effects at concentrations as low as 10-50 ppb.

Children and pregnant women are at heightened risk. Prenatal arsenic exposure has been associated with lower birth weight, immune system impairment, and increased infant mortality. If you are on a private well in an arsenic-prone region and have children or are pregnant, testing is not optional. It is urgent. Take our quiz for guidance on testing and treatment options matched to your situation.

Testing

Arsenic has no taste, color, or odor at the levels found in drinking water. The only way to know your exposure is to test. If you are on a municipal system, check your city's data on CheckMyTap for reported arsenic levels. If you are on a private well in the Southwest, West, New England, or any area with known arsenic geology, a home test is essential.

DIY test strips ($10-20): Basic screening strips cover hardness, chlorine, pH, and some metals, but most do not test for arsenic specifically. The 17-in-1 test strips are useful for general screening but should not be relied on for arsenic alone.

Certified lab test ($100-300): For arsenic, a lab test is the right call. Mail-in kits from EPA-certified labs measure arsenic to single-digit ppb precision, which matters when the MCL is 10 ppb and health effects start below that. SimpleLab Tap Score specifically tests for arsenic and reports results against both the EPA MCL and health-based guidelines.

If your well test comes back above 5 ppb, install treatment immediately while you investigate long-term solutions. If it is above the MCL of 10 ppb, contact your state health department, as some states offer assistance programs for arsenic-affected wells.

Removal

Standard carbon filters like Brita and PUR do not remove arsenic. Arsenic is an inorganic dissolved contaminant that requires specific treatment technologies. If arsenic is your concern, you need one of the following approaches.

Reverse osmosis is the gold standard for arsenic removal at home, reducing both arsenic-3 (arsenite) and arsenic-5 (arsenate) by 90-95% or more. Under-sink RO systems like the Waterdrop G3P800 are the most practical solution for drinking and cooking water. This is the recommended treatment for any home with arsenic above 5 ppb.

Adsorptive media filters using iron-based media (such as granular ferric hydroxide) are specifically designed for arsenic removal. These are available as point-of-use cartridges and whole-house systems. They are particularly effective for arsenic-5. If your water contains primarily arsenic-3, you may need a pre-oxidation step (adding chlorine or using an oxidation filter) to convert it to arsenic-5 before the adsorptive media can capture it.

Distillation also removes arsenic effectively, though countertop distillers are slow and impractical for large volumes. Boiling water does not remove arsenic and actually concentrates it, similar to nitrate. For well owners with arsenic above the MCL, a point-of-entry treatment system may be warranted to protect the entire household, but at minimum, an under-sink RO at the kitchen tap protects the water you drink and cook with.