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Inorganic

Arsenic in drinking water: a plain-English guide

Arsenic is a naturally occurring element that leaches into groundwater from rocks and soils in many U.S. regions, especially the Southwest, the upper Midwest, and parts of New England. It is a known human carcinogen. EPA's MCL is 0.010 mg/L (10 ppb); the MCLG is zero. Private wells, which are not regulated under the Safe Drinking Water Act, are the highest-risk population.

Where arsenic in drinking water comes from

Arsenic is a natural element in the earth’s crust. Groundwater dissolves arsenic out of certain rock formations as it flows through them. Regions with elevated background arsenic in U.S. drinking-water sources:

  • The Southwest: Arizona, New Mexico, parts of Nevada and California
  • The upper Midwest: Minnesota, Wisconsin, the Dakotas
  • New England: Maine, New Hampshire, parts of Massachusetts and Vermont
  • Parts of the Mississippi alluvial aquifer

Industrial sources (mining, historical pesticide residues, treated wood) contribute in localized areas but are not the dominant U.S. source. Most arsenic-contaminated water is contaminated by geology, not by industry.

Why it matters for health

Arsenic is a known human carcinogen. Long-term ingestion is associated with cancers of the skin, bladder, lung, and kidney. Non-cancer effects include cardiovascular disease, skin lesions (hyperpigmentation, hyperkeratosis), peripheral neuropathy, diabetes, and developmental effects in children.

The dose-response is roughly linear with no demonstrated safe level, which is why the MCLG is zero. EPA’s 0.010 mg/L MCL is a practical compliance threshold, not a safety threshold; lifetime cancer risk at the MCL is non-zero (on the order of 1 in 1,000 excess lifetime cancers).

EPA regulation

  • MCL: 0.010 mg/L (10 ppb), effective 2006.
  • MCLG: zero.

EPA initially proposed lowering the MCL to 5 ppb in the late 1990s. The final rule landed at 10 ppb after substantial industry pressure on treatment costs for small systems. Several states have adopted lower state-level limits; New Jersey’s MCL is 5 ppb.

Private wells are not covered

If your home is on a private well, your water is not regulated under the Safe Drinking Water Act, and no agency tests it for you. In high-arsenic geology, well testing every one to three years is the standard recommendation. Most state health departments offer low-cost arsenic test kits.

Treatment

Treatment that works at the point-of-use or whole-house level:

  • Reverse osmosis (most common household choice)
  • Activated alumina (whole-house, common in private wells)
  • Anion exchange (whole-house)
  • Iron-based adsorbents (specifically for arsenate, As(V))
  • Distillation (slow, effective)

Treatment that does NOT work:

  • Carbon filters alone
  • Boiling (concentrates arsenic by evaporating water)
  • Standard sediment filters

This page is general information and is not medical advice. If your drinking water has tested above the MCL, talk to your clinician about whether biological testing or further evaluation is appropriate.

Sources

Editorial review: reviewed 2026-05-11 by RK 2026-05-11. Editorial standards.