Arsenic in Well Water: Testing, Results and Treatment

Mostly from rock, not pollution

Arsenic is an element found naturally in the earth's crust. In private wells, USGS says it is presumed to come primarily from the rocks and minerals the water flows through. EPA also lists runoff from orchards and wastes from glass and electronics production as sources.

A 2017 USGS study, based on tests from 20,450 wells, estimated that about 2.1 million people in the U.S. may be getting their drinking water from private wells with high arsenic. USGS has found high arsenic in wells in more than 25 states. The largest numbers of people likely affected are in much of the West, including Washington, Oregon, Nevada, California, Arizona and New Mexico, with more in parts of the Northeast, the Midwest and the Atlantic southeast.

• Federal limit (MCL)0.010 mg/L, also written 10 ppb or 10 µg/L
• EPA health goal (MCLG)Zero
• Limit in force sinceAdopted January 22, 2001; effective January 23, 2006
• People likely affectedAbout 2.1 million private well users (USGS 2017 estimate)

What health agencies say

EPA's drinking water standards list the possible effects of long-term arsenic above the limit as skin damage, problems with the circulatory system, and an increased risk of cancer. ATSDR, the federal agency on toxic substances, says swallowing inorganic arsenic can increase the risk of skin cancer and of cancer in the liver, bladder and lungs. The U.S. Department of Health and Human Services and EPA classify inorganic arsenic as a known human carcinogen.

ATSDR also describes skin changes from long-term exposure: a darkening of the skin and small "corns" or "warts". These effects come from long-term exposure, so a well can have high arsenic while nobody in the home feels ill.

One test isn't always enough

Order an arsenic test from a certified laboratory; your state's certification program or local health department can point you to one. Arsenic is often not part of a basic bacteria test, so ask for it by name or choose a package that includes metals.

States give different advice on how often to test, so follow your own state's guidance:

• MinnesotaTest every well for arsenic at least once
• VermontTest for arsenic every five years
• WashingtonTest twice: once in late summer and once in early spring
• Lab price examples$30 (Penn State) to $38 (Wisconsin State Lab of Hygiene) for a single arsenic test

Compare the number against 0.010 mg/L

Labs report arsenic in different units. 0.010 mg/L, 10 µg/L and 10 ppb are the same amount, so convert before you compare. A result marked "ND" or "<" means none was found above the lab's reporting limit, which is not quite the same as zero.

• Read your test resultsEnter your lab numbers to compare them against EPA limits and standards.Read my results →
• What a non-detect really meansWhy "not detected" is not the same as zero.Read the explainer →

• If Your result is above 0.010 mg/LIt is above the federal limit for public systems. Use another drinking water source and look at treatment.
• If Your result is detected but under 0.010 mg/LIt meets the federal limit, but the health goal is zero. Retest on your state's schedule.
• If Arsenic was not detectedNone was found above the reporting limit in this sample. Keep testing on your state's schedule.

What removes arsenic at home

State health departments list several home treatments that reduce arsenic: reverse osmosis, distillation, adsorptive filters with special media such as iron oxide, and anion exchange, which works like a water softener but swaps arsenic for chloride. Vermont advises choosing devices certified for arsenic, such as reverse osmosis units certified to NSF/ANSI Standard 58.

Vermont also lists systems that add an oxidation step before anion exchange or a carbon block filter certified to NSF/ANSI Standard 53. A water treatment professional can help match a system to your lab result. After installing any system, test the treated water to confirm it works, and keep up with filter changes.

From your county to your tap

Start with your state's well water page to see the modelled arsenic for your county, then order a lab test. If you are already testing for bacteria and nitrate every year, ask the lab to add arsenic. For what a full well test costs, see our guide to well water testing prices.

• Private well water by stateCounty-level arsenic and nitrate estimates and testing guidance.Pick your state →
• How much does well water testing cost?Real 2026 prices from state and university labs.See lab prices →
• Nitrates in well waterThe other contaminant to test for every year.Read the guide →
• How reverse osmosis worksWhat an RO system removes and what it costs to run.Read the guide →