Arsenic in well water: testing, safe levels and treatment

Arsenic is a natural element in many rocks and sediments, and groundwater can dissolve it on the way to a well⁵. It is not a sign that anyone did anything wrong, and a clean-looking, good-tasting well can have it: you cannot taste, see or smell arsenic in water¹.

The U.S. Geological Survey has estimated that about 2.1 million people in the United States use domestic wells with high arsenic⁵. It found arsenic more often, and at higher levels, in the West, with the Southwest the area of greatest concern, but it also names aquifers across the northern states and parts of the Southeast⁵. State figures give a sense of scale: about 1 in 10 private wells in Minnesota¹ and 1 in 10 wells in Maine⁶ are over the limit.

Why it matters

The risk comes from drinking it for years. Minnesota’s health department links long-term exposure to low levels with diabetes and with a higher risk of cancers of the bladder, lungs, liver and other organs, and says it can contribute to heart and lung disease and reduce intelligence in children¹. The effects may take many years to appear¹.

Oregon State University Extension adds risks in pregnancy, including miscarriage, premature birth and low birth weight³.

The limit, and the units on your report

EPA’s limit for public water systems is 0.010 mg/L, with a health goal of zero². Minnesota explains the gap: lower levels over many years can still raise cancer risk¹. Private wells aren’t regulated, so the limit is a guide for you, not a rule.

Lab reports use three units for the same thing³:

• 1 microgram per liter (µg/L) = 1 part per billion (ppb)
• 10 µg/L = 10 ppb = 0.010 mg/L

So a result of “0.012 mg/L” is 12 ppb, and over the limit. Our result checker converts the units for you.

What to do at each level

Oregon State publishes the most detailed table of any agency we checked. It is more cautious than the legal limit, because it starts advising care at 5 ppb³.

Result	Guidance
Under 5 ppb	Safe for all uses. Retest every 1 to 3 years
5 to 9.9 ppb	Not for infant formula or baby food. Pregnant people, young children and people with weakened immune systems should avoid drinking or cooking with it untreated
10 to 99.9 ppb	Not safe for people, pets or livestock to drink or to use for food. Treat the water. Fine for bathing, laundry and dishes
100 to 499.9 ppb	As above, and not safe for garden irrigation
500 ppb or more	Not safe for any use

Source: Oregon State University Extension Service, EM 9401. Other states use 10 ppb as their only action level.

Minnesota’s position is simpler: it “highly recommends” taking action above 10 µg/L¹. Wisconsin advises retesting first to confirm a high result⁴.

If your result is over the limit

1. Stop drinking it and cooking with it. Use bottled water or another safe source for drinking and for foods that absorb a lot of water⁴. Oregon State adds infant formula, washing produce and mixing drinks to that list³.
2. Don’t boil it. Boiling concentrates arsenic as the water evaporates¹.
3. Keep washing as normal. Showering, bathing, laundry and dishes are generally fine, because skin doesn’t readily absorb arsenic¹³.
4. Retest to confirm, at a certified lab⁴.
5. Then choose between treatment and a new well.

One small disagreement: Wisconsin says the water is fine for brushing teeth⁴, while Oregon State suggests using treated or bottled water for that and supervising young children so they don’t swallow bath water³. With small children in the house, the cautious version costs nothing.

Treatment that removes arsenic

The concern is the water you swallow, so treatment is often a “point-of-use” unit at a single tap. Maine’s health agency calls reverse osmosis the most cost-effective method for household supplies⁶. Whole-house (“point-of-entry”) systems exist and cost far more³.

Method	Where	Typical cost	Notes
Reverse osmosis	One tap	$300 to $1,500	Makes wastewater; may need pretreatment
Reverse osmosis	Whole house	$5,000 to $12,000	May overload a septic system
Distillation	One tap	$300 to $1,200	Uses energy; water can taste flat
Anion exchange	Whole house	$1,500 to $2,500	Uses salt; can make water more corrosive
Adsorptive media (such as iron oxide filters)	Varies	Not given	The media holds the arsenic and has to be replaced regularly

Costs are Oregon State's 2023 estimates, except where noted. Sources: Oregon State University Extension, Minnesota Department of Health, Maine CDC.

Costs and the first four rows are from Oregon State³; adsorptive media is described by Minnesota and Maine¹⁶. Maine puts a point-of-use reverse osmosis unit at $800 to $1,500 and says reverse osmosis is up to 95% effective on the most common form of arsenic in its wells⁶.

What doesn’t work: conventional water softeners and activated carbon filters alone¹, including pitcher and refrigerator filters³. Chlorine doesn’t make arsenic safe either³.

The two forms of arsenic

Arsenic in groundwater comes in two forms, often written arsenic III and arsenic V. It matters because they don’t behave the same in treatment: Maine notes that reverse osmosis needs an added oxidation step to deal with the +3 form, while iron oxide filters work on both⁶. Standard lab tests report total arsenic, and Oregon State says a separate test for the two forms is costly and often unnecessary³. It is a good question to put to an installer: how does this system handle arsenic III?

Or a new well

Minnesota notes that a well drilled into a different water-bearing formation may have less arsenic, and may cost less over time than maintaining treatment, but there are no guarantees¹. Levels can vary between nearby wells¹, so a neighbor’s clean result tells you little about yours. A licensed well contractor who knows the local geology is the person to ask.

How often to test

The agencies don’t agree, partly because local geology differs.

• Minnesota: at least once. If a new well shows arsenic, test again about six months after construction, because levels can rise or fall in the first few months¹.
• Wisconsin: at least every five years, and every year if arsenic has been found before⁴.
• Oregon State: every one to three years. If you’ve never tested, test once in the wet season and once in the dry season³.

If you treat the water, Oregon State suggests testing the treated water at least yearly and the untreated water at least every three years³. Oregon labs charge about $30 to $45 for the test³.

Buying a house with a well? Minnesota recommends testing for arsenic at least once, and our home buyer’s checklist lists the other tests worth running before closing. For everything else, see what to test well water for.

Well water test planner Tick what applies to your home and get the tests public health agencies suggest, and why.