What states have the most arsenic in drinking water?

States with the highest arsenic in groundwater include Arizona, Nevada, Idaho, Montana, Wyoming, Utah, South Dakota, and parts of New England (Maine, New Hampshire). The western U.S. has naturally high arsenic due to volcanic and hydrothermal geology. Parts of Michigan, Wisconsin, and Minnesota also have elevated groundwater arsenic.

What is the best filter for arsenic?

Reverse osmosis (RO) systems are the most effective, removing 90–95% of arsenic. Activated alumina and iron-based media filters are also highly effective for arsenic specifically. Adsorptive media point-of-entry systems can treat whole-home water. Standard carbon block filters provide minimal arsenic removal.

Is 10 ppb arsenic safe to drink?

Technically legal, but not without risk. At exactly 10 ppb over a lifetime, some epidemiological models suggest a bladder cancer risk of roughly 1 in 500. The EPA set the limit at 10 ppb partly because of compliance cost concerns for small utilities. If your water tests near 10 ppb, a point-of-use RO filter is a prudent precaution.

Can arsenic be removed by boiling water?

No. Boiling does not remove arsenic. As water evaporates, the arsenic concentration actually increases. Never use boiling as a treatment for arsenic-contaminated water.

Does arsenic occur in well water?

Yes, and this is a major concern. An estimated 2.1 million Americans have private well water exceeding 10 ppb arsenic. Unlike public water systems, private wells are not federally regulated and are not required to be tested. Testing is strongly recommended for any private well, especially in the western U.S. and New England.

All Contaminants

moderate risk levelHeavy MetalsRelevant to well water

Arsenic in Drinking Water

Arsenic is a naturally occurring metalloid found in geological deposits across the western United States, New England, and the Midwest. It can also enter water through industrial processes. Long-term exposure is strongly linked to bladder, lung, and skin cancer. The EPA reduced the arsenic MCL from 50 ppb to 10 ppb in 2001, though some researchers advocate for an even lower limit.

Quick Answer

Arsenic (As) occurs naturally in rock and soil, dissolving into groundwater through natural weathering processes. Inorganic arsenic — the form found in drinking water — is a known human carcinogen. The western United States has particularly arsenic-rich geological formations, but elevated levels have been found in 48 states. Arsenic is tasteless and odorless.

Why Do People Care?

An estimated 2.1 million Americans drink water from private wells with arsenic above 10 ppb. Small public water systems in rural areas also struggle to meet the MCL. Even at the legal limit of 10 ppb, some epidemiological studies suggest meaningful cancer risk with lifetime exposure.

Rural residents relying on private wells in the western U.S., New England, and parts of the Midwest face the highest risk. Populations in areas with natural arsenic-rich geology (volcanic rock, marine sedimentary deposits) are disproportionately affected.

Known Health Effects

Bladder, lung, and skin cancer — the most well-established associations

Skin thickening (keratosis) and dark spots with chronic high exposure

Peripheral neuropathy and cardiovascular disease

Type 2 diabetes association at elevated levels

Developmental and cognitive effects in children

Immune system disruption

Common Sources

Natural geological weathering of arsenic-rich rock and sediment

Mining operations and smelter waste

Agricultural pesticides and herbicides (historical use)

Coal-fired power plant ash and waste

Industrial effluent discharge

Geothermal water in volcanic regions

Regulatory Limit

EPA Maximum Contaminant Level (MCL)

10 ppb

The EPA MCL for arsenic is 10 parts per billion (ppb), effective since January 2006 — reduced from the previous 50 ppb limit. The Maximum Contaminant Level Goal (MCLG) is zero, meaning no exposure is considered risk-free. Compliance is particularly challenging for small water systems in rural areas. The 10 ppb standard is 5× higher than WHO's guideline value of 10 µg/L for developed countries.

How to Test for It

Arsenic testing requires a certified laboratory — home test strips are unreliable for arsenic at regulatory levels. Private well owners in high-risk areas should test annually. Public water systems report arsenic levels in Consumer Confidence Reports. The EPA's EJScreen and ECHO tools allow you to look up monitoring data for public systems.

Effective Treatment Options

These treatment methods have demonstrated effectiveness for Arsenic.

Reverse Osmosis

Reverse osmosis (RO) is the most comprehensive point-of-use water treatment technology available for residential use. It removes 90–99% of dissolved contaminants including PFAS, lead, arsenic, nitrates, and disinfection byproducts by forcing water through a semi-permeable membrane with pores of approximately 0.0001 microns.

PFAS / PFOA / PFOS — 90–99% removalLead — 95–99% removal

Activated Carbon

Activated carbon is the most widely used residential water treatment technology. It removes chlorine, taste and odor compounds, disinfection byproducts, many volatile organic compounds (VOCs), and — with NSF/ANSI 53 certification — lead and some PFAS. It is available as pitcher filters, under-sink units, and whole-house systems.

Chlorine and chloramine — virtually complete removalTaste and odor compounds — highly effective

Frequently Asked Questions

Reverse Osmosis Filtration

Removes: Arsenic

Activated Carbon Filtration

Removes: Arsenic

Lead in Drinking Water

Heavy Metals

Data Sources & Provenance

All data on this page is sourced from official U.S. government or public datasets.

EPA Drinking Water Contaminant InformationView source

ATSDR ToxFAQs / Toxicological ProfilesView source

EPA SDWIS — violation and detection dataView source

Last updated: 2025-01-15

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