Arsenic in well water

By the Tested Water teamBased on Penn State Extension, U.S. EPA, USGSFact-checked November 1, 2026

The short version

Arsenic dissolves into well water from natural rock and sediment. You can't taste or smell it, so a lab test is the only way to know. The EPA limit is 10 µg/L. Years of drinking it raise the risk of several cancers. A reverse osmosis filter at the kitchen sink removes most of it.

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What you need to know about arsenic

The limit	10 µg/L (micrograms per liter, the same as parts per billion). The EPA's health goal is 0.
Who it hurts	Anyone who drinks it for years, even at low levels.
Where it comes from	Mostly natural rock and sediment. Old orchards sprayed with arsenic pesticides and some mining or industry are less common sources.
How often to test	At least once. Retest if a new well came back between 5 and 20 µg/L.
What removes it	Reverse osmosis at the kitchen sink, activated alumina, or anion exchange, often with a step that converts the arsenic to an easier-to-remove form first.
What a test costs	About $15 to $50 at a certified lab, according to Penn State Extension.

These counties see the most arsenic in their wells

Ranked by the share of tested wells with a result over the limit. We only list counties with at least 25 tested wells, so a few bad results can't skew the ranking.

County	Wells tested	Wells over the limit
Malheur County, OR	48	94%^*
Kennebec County, ME	58	93%^*
Churchill County, NV	177	88%^*
Reynolds County, MO	26	69%^*
Eagle County, CO	34	62%^*
Yuma County, AZ	103	60%
Roosevelt County, MT	32	59%^*
Benson County, ND	59	59%
Owyhee County, ID	56	59%
St. Francois County, MO	27	56%^*

* Likely overstated, because of who tested these wells, when or where.

See the full rankings and download the data

Arsenic comes out of the rock around your well

Arsenic is a natural element in some rocks and sediments. When groundwater sits against them, some arsenic dissolves into it. Less often it comes from mining, industry, or old orchards where arsenic pesticides were used.

The USGS has found high arsenic in drinking-water wells in more than 25 states, and estimates about 2.1 million people use private wells with high arsenic.

The Southwest has the most. About 7% of wells USGS sampled nationally were over the limit, and about 16% in the Southwest's basin-fill aquifers, where the arsenic comes from volcanic and granite rock. Other problem areas include the glacial aquifers across the northern states, the Piedmont, Blue Ridge and Valley and Ridge rock of the East, and the Mississippi Embayment and Texas Coastal Uplands in the South.

Levels can change a lot over short distances. The Minnesota Department of Health says wells close together can test very differently. About 10% of private wells in Minnesota are over the limit.

Arsenic has no taste, smell or color. A lab test is the only way to know.

Years of drinking it raise your cancer risk

Arsenic's harm builds slowly. The Minnesota Department of Health says long-term drinking of even low levels is linked to diabetes and to cancers of the bladder, lungs, liver and other organs. It's also tied to heart and lung disease, skin changes, and lower intelligence in children.

The EPA's limit for public water is 10 µg/L. Its health goal is 0, because drinking arsenic below the limit for many years can still raise cancer risk.

If you've been drinking water over 10 µg/L, Minnesota suggests letting your doctor know.

Test once, and again if the number is close

Every well should be tested for arsenic at least once. If the lab finds none, Minnesota says you don't need to keep testing for it.

New wells can read differently once they settle. Minnesota suggests retesting 6 months or more after drilling if the first result was between 5 and 20 µg/L. Iowa recommends an arsenic test every 3 years.

Penn State Extension puts the cost of an arsenic test at about $15 to $50. Ask a state-certified lab.

Some states cover the cost. Iowa's Private Well Grants program tests private wells for arsenic, nitrate, bacteria and manganese for free.

Treat the tap you drink from

Arsenic is a risk when you swallow it. Penn State says showering and laundry aren't a concern, so most families treat only the kitchen tap.

Arsenic comes in 2 forms. Arsenite is harder to remove. Arsenate is easier. Penn State says removal often starts with a step that converts arsenite to arsenate, usually with chlorine, ozone or a greensand filter.

Option	What it treats	Typical equipment cost	Things to know
Reverse osmosis	1 tap	$300 to $2,000	Removes over 95% of arsenic at the right water pressure
Anion exchange	The whole house	$500 to $1,500	Sulfate, nitrate and other minerals compete with arsenic and cut removal
Activated alumina	1 tap or the whole house	Varies by size	The media is regenerated with chemicals and replaced every 1 to 2 years
Iron filter (oxidation plus filtration)	The whole house	$500 to $1,500	Can remove some arsenic along with iron; test to see how much

Costs are equipment ranges from Penn State Extension.

Test the treated water after the system goes in, and then on a schedule, so you know it's still working.

Questions people ask about arsenic

Is it OK to shower in water with arsenic?

Yes. Penn State Extension says arsenic is a health risk when you swallow it, not when you shower or do laundry.

My neighbor's well tested fine. Do I still need to test?

Yes. Arsenic levels can vary between wells even in a small area, according to the Minnesota Department of Health.

Will drilling a new well fix it?

Maybe not. Penn State warns that a new well may not be free of arsenic if the old one has it. Penn State suggests weighing treatment, or a hookup to public water if a line runs nearby.

What does "total arsenic" on my report mean?

It's both forms of arsenic added together. That's the number to compare with the 10 µg/L limit. If your report is in mg/L, 0.010 mg/L is the same as 10 µg/L.

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