Arsenic and nitrate in well water

Two problems a well owner has to find

No utility tests a private well, so no one else will catch these. Nitrate and arsenic both get into groundwater, both carry health limits, and each needs its own fix. Boiling removes neither: CDC says boiling "will not remove chemicals from water."

The well testing guide covers the full yearly checklist. This guide covers these two.

Nitrate

The limit, and who it matters most for

EPA's limit for nitrate is 10 ppm, measured as nitrogen. Some labs report nitrate itself instead, and on that scale the same limit is 45 mg/L, Penn State Extension says. Check which one your report uses before you compare. Nitrite, a related compound, has a lower limit of its own, and EPA suggests testing for both if you live near farmland.

The limit exists for babies. EPA says infants under six months who drink water with nitrate above the limit "could become seriously ill and, if untreated, may die," with symptoms that include shortness of breath and blue-baby syndrome. CDC explains why: high nitrate cuts the blood's ability to carry oxygen.

So the most important rule is simple. Don't mix formula with well water until you know its nitrate level. If it is high, the Minnesota Department of Health says babies under six months should not drink it even after home treatment, in case the treatment fails. Penn State Extension suggests bottled water for babies and pregnant women, and says nursing mothers do not pass nitrite to infants through breast milk.

Where it comes from

CDC lists fertilizers, septic systems that are not working right, animal feedlots, and industrial and food processing waste. EPA's list of reasons to test includes intensive farming nearby.

How often to test

EPA puts nitrate on its list of four things to test a well for every year, along with bacteria, total dissolved solids and pH. EPA also suggests testing more often if a pregnant or nursing person, small children or older adults live in the house.

A nitrate test costs $10 to $35 at the state and university labs that publish prices. What a water test costs lists them, and every state's certified labs are on TapBuff.

What removes it

CDC names three treatments for nitrate: ion exchange, distillation and reverse osmosis. A carbon filter certified for lead or PFAS is not one of them. Penn State Extension says boiling raises nitrate, because some of the water boils off.

Each treatment has a catch, Penn State says:

• Reverse osmosis removes about 83 to 92 percent of the nitrate coming in. With a very high starting level, what is left can still be above the limit, so know your number and test the treated water.
• Ion exchange works less well in water high in sulfate. Once the resin is full, it can release nitrate into the water it is supposed to be treating.
• Distillation is slow. A home unit makes about a gallon every four to five hours.

The certification claim to look for is nitrate and nitrite reduction under NSF/ANSI 58, the reverse osmosis standard. IAPMO also lists a nitrate claim under NSF/ANSI 53 for Culligan's ZeroWater pitchers, tested with water at 30 ppm. NSF lists no nitrate claim under 53. The nitrate page has each claim's exact wording.

Filters with a nitrate claim on their listing

Filter	Listing	Five years
AquaTru Classic (AT2000)Countertop reverse osmosis	IAPMO 58	$1,344
Aquasana SmartFlow Reverse Osmosis (AQ-SFRO2)Under-sink reverse osmosis	WQA 58	$2,917
Culligan 10-Cup Pitcher with ZeroWater Technology (ZEROP10)Pitcher	IAPMO 53	$5,103
Ranked by five-year cost: a household of 3 at the makers’ list prices, cartridges included. “No price” means the maker publishes none. A plus sign means a part due in those five years has no published price. Listings checked between Oct. 4, 2026 and Oct. 15, 2026.

Arsenic

The limit

EPA's limit for arsenic is 10 ppb, and its health goal is zero. EPA lists skin damage, circulatory problems and a higher risk of cancer from drinking water above the limit for years.

The Minnesota Department of Health says even levels below the limit raise the risk of cancer when the water is drunk for many years. It "highly recommends" acting on anything above 10 ppb, and suggests considering treatment or another source if arsenic is found at any level.

How common it is

Arsenic gets into wells from natural deposits in rock and soil. In 2021, the U.S. Geological Survey estimated that 2.7 million people use private wells with arsenic above EPA's limit, rising to 4.1 million during drought. Under normal conditions, the largest numbers of people were in Ohio, Michigan, Indiana, California and Maine.

It also varies from well to well. Minnesota's health department says levels can differ between wells even within a small area, and there is no way to know the level before a well is drilled. You can't taste, see or smell it.

How often to test

Test for arsenic at least once, the Minnesota Department of Health says, beyond the test done when the well is drilled. Penn State Extension says an arsenic test typically costs $15 to $50. Ask for a lab certified to test for metals.

Two forms, and why it matters

Arsenic shows up in water in two forms, and a filter's certification may cover only one.

• Arsenic (V), also called arsenate or pentavalent arsenic, is the easier one to remove.
• Arsenic (III), also called arsenite or trivalent arsenic, is harder to remove. Penn State Extension says it is more common in groundwater.

Penn State says that because arsenic (V) is easier to remove, treatment often starts by converting all of it to that form with an oxidizer, usually chlorine, ozone or a greensand filter. That also settles the form when, as with most test results, you know only the total.

That matters when you read a listing. The arsenic claims under NSF/ANSI 58 cover only arsenic (V), at one of two levels: 50 ppb or 300 ppb. NSF's listings say a system with the claim is meant only for water at or below that level, in that form. NSF's listings also carry an arsenic (V) claim under NSF/ANSI 53, on some of Multipure's systems, and WQA's include an arsenic (III) claim under 53, but no filter in TapBuff's catalog carries either.

So before you buy, ask your lab whether it can report the two forms separately. If it can't, or you have arsenic (III), plan on oxidizing the water first. Minnesota's health department suggests a water treatment specialist to help choose a system.

What removes it

CDC names reverse osmosis, ultrafiltration, distillation and ion exchange. Minnesota's health department adds adsorptive media and several oxidation-and-filtration systems. It also warns that conventional water softeners and activated carbon filters "alone will not remove arsenic," and that boiling only concentrates it.

Penn State Extension says reverse osmosis, usually an under-sink unit, removes more than 95% of arsenic under ideal pressure. Whatever you install, test the treated water now and then to make sure it is still working, as Minnesota's health department advises.

The kitchen tap is usually enough

Arsenic is a problem when you swallow it. Penn State Extension says it is only a health concern when water is ingested, and Minnesota's health department says skin does not easily absorb it, so showering, bathing and washing are safe unless arsenic is above 500 ppb. Below that, treating the water you drink and cook with, usually at the kitchen sink, covers the risk. Above it, ask a water treatment specialist about treating all the water.

Filters with an arsenic claim on their listing

Filter	Listing	Five years
APEC Ultimate RO-90Under-sink reverse osmosis	WQA 58	$549
AquaTru Classic (AT2000)Countertop reverse osmosis	IAPMO 58	$1,344
Brondell Capella RC250 Reverse Osmosis SystemUnder-sink reverse osmosis	WQA 58	$1,950
Aquasana SmartFlow Reverse Osmosis (AQ-SFRO2)Under-sink reverse osmosis	WQA 58	$2,917
Ranked by five-year cost: a household of 3 at the makers’ list prices, cartridges included. “No price” means the maker publishes none. A plus sign means a part due in those five years has no published price. Listings checked between Oct. 4, 2026 and Oct. 9, 2026.

The reverse osmosis guide compares these systems on what else their listings cover and what each costs over five years. The arsenic page has every claim's exact wording.

If you're on city water

Utilities test for both and publish the results each year. Look yours up instead: