Arsenic in Well Water: Arsenic III vs V, Reverse Osmosis vs Adsorptive Media, and What NSF Certifies

Arsenic is one of the well-water results people most often try to treat with the wrong equipment. You can’t see, taste, or smell it, a water softener and a carbon filter won’t remove it on their own, and boiling concentrates it. It also comes in two forms, and most certified filters are only certified for one of them. This guide covers how to read an arsenic result, what the Minnesota Department of Health tells well owners to do, how the treatment options compare, and what NSF’s own listings show about arsenic claims (checked Oct 29, 2026). Specs and prices come from official pages with the date we checked them, our math is labeled as ours, and we haven’t tested any of these systems.

This isn’t health advice. What an arsenic result means for your household is a question for your doctor and your local or state health department. We quote what health agencies say; we don’t add to it.

The short answer

1. Test your well for arsenic at least once at an accredited or state-certified lab. The federal limit for public water is 10 micrograms per liter (µg/L), which is the same as 10 parts per billion (ppb).
2. A softener, a carbon filter, a sediment filter, UV, and boiling don’t solve it. The Minnesota Department of Health (MDH) lists reverse osmosis, distillation, adsorptive media, anion exchange, and some oxidation-filtration setups as the options that reduce arsenic.
3. Arsenic comes as arsenic III and arsenic V. NSF-certified arsenic claims are for pentavalent arsenic (arsenic V) only. Arsenic III usually has to be converted to arsenic V first (pre-oxidation), and a lab can tell you which form you have.
4. “NSF/ANSI 58 certified” isn’t the same as arsenic certified. On Oct 29, 2026, 35 of the 56 companies on NSF’s reverse osmosis listing had an arsenic claim on at least one model, and each claim has a concentration cap (300 ppb or 50 ppb).
5. We found no whole-house (point-of-entry) arsenic claims on NSF’s 53 or 58 listings. Whole-house arsenic treatment relies on the media maker’s data, careful pre-treatment, and your own follow-up tests.
6. Test the treated water after installation and on a schedule.

Reading an arsenic result

The EPA’s National Primary Drinking Water Regulations (checked Oct 29, 2026) set the maximum contaminant level for arsenic at 0.010 mg/L, with a health goal of zero. The EPA lists the sources as erosion of natural deposits, orchard runoff, and glass and electronics production wastes. Public water systems have to meet that limit; private wells don’t, so testing is up to you.

Watch the units. 0.010 mg/L, 10 µg/L, and 10 ppb are all the same number. A lab report that says “0.012” is in mg/L and is above the limit; one that says “12” is probably in µg/L and is also above it. If the report doesn’t name the unit, ask the lab.

What MDH says, from its Arsenic in Well Water and Arsenic in Drinking Water pages (both checked Oct 29, 2026):

• Arsenic “occurs naturally in rocks and soil across Minnesota,” and “there is no way to know the arsenic level in water before a well is drilled.” Levels can vary between wells, even within a small area.
• About 10 percent of Minnesota private wells have arsenic above 10 µg/L. Some have been as high as 350 µg/L, but most results are below 50 µg/L.
• Since August 2008, Minnesota well contractors test each new well for arsenic. MDH notes the level can change in the first few months, and suggests retesting about six months or more after the well is drilled, “especially if the test results were between 5 and 20 µg/L.” Results for wells built since 2008 may be in the online Minnesota Well Index.
• If arsenic isn’t found, MDH says further testing isn’t necessary. If it’s found at any level, MDH suggests considering a treatment unit or a different drinking-water source, and it “highly recommends” taking action above 10 µg/L. Its drinking-water page adds that if a repeat sample confirms a result above 10 µg/L, you should use an alternate drinking-water source or install a treatment system that reduces arsenic, and let your medical provider know.

MDH also says drinking water with arsenic below the federal limit over many years can still raise health risks, which is why the EPA’s goal is zero. The health details are on MDH’s pages; ask your doctor or health department what your number means for you.

Where to test: In Minnesota, MDH keeps a list of accredited labs that accept samples from private well owners. Elsewhere, ask your state or county health department. Our guide to finding your water results covers the other yearly well tests. On city water, arsenic is in your utility’s Consumer Confidence Report.

Arsenic III vs arsenic V, and why it matters for buying

MDH’s information sheet Water Treatment Units for Arsenic Reduction (checked Oct 29, 2026) puts it simply: “There are two types of arsenic in Minnesota groundwater: arsenic III and arsenic V. Including preoxidation with your water treatment ensures the treatment reduces both types.”

The certified claims spell out the same limit. A.O. Smith’s AORO-50, a reverse osmosis system NSF lists with an arsenic claim, explains it in the “Arsenic Facts” section of its performance data sheet (dealer-hosted copy, checked Oct 29, 2026):

• Groundwater may contain trivalent arsenic (arsenic III, also called arsenite), pentavalent arsenic (arsenic V, arsenate), or both.
• The system “is designed to remove only pentavalent arsenic” and “has not been evaluated for its ability to remove trivalent arsenic.” Trivalent arsenic “is generally more difficult to remove.”
• Arsenic III converts to arsenic V with an effective oxidant such as free chlorine. Water with detectable free chlorine will have arsenic in the pentavalent form. Chloramine isn’t enough to ensure complete conversion.
• Private wells and water without a free chlorine residual “should be analyzed to determine the form(s) of arsenic present.” Some labs can “speciate” (measure the two forms separately) if you ask.

NSF’s listings carry the same caveat: arsenic claims are marked “This product is intended for use only on water supplies that have been shown to be at or below the referenced concentration and valency of arsenic.” Valency means the form, III or V.

What that means in practice:

• Most private wells aren’t chlorinated, so unless your lab result shows the arsenic is all arsenic V, plan on pre-oxidation (for example, a chlorine feed ahead of the treatment) or ask the seller and a licensed water treatment contractor how their system handles arsenic III.
• On city water, ask your utility whether it disinfects with free chlorine or chloramine. If it’s chloramine, the conversion isn’t assured. (Our chloramine guide explains how to find out.)

What doesn’t remove arsenic

MDH’s well page: “systems such as conventional water softeners and activated carbon filters alone will not alone remove arsenic. Also, boiling the water will only concentrate the arsenic, due to evaporation of some of the water.” Two more from the same logic:

• Sediment filters catch particles; dissolved arsenic passes through.
• UV only treats microorganisms (see our UV guide).

One partial exception: MDH lists oxidation filtration (the kind of filter used for iron and manganese) as reducing arsenic “only if level of iron is above 100 micrograms per liter.” That depends on your full test results, so ask the seller to work from them rather than assuming an iron filter will handle arsenic.

The options that do, with MDH’s cost estimates

From MDH’s arsenic treatment sheet (checked Oct 29, 2026). MDH’s costs are “estimates based on quotes obtained in 2017,” with the low end for a homeowner install and the high end for a professional one, so treat them as rough:

MDH also mentions non-treatment routes: a new well into a different water-bearing formation (with no guarantee it will be lower), or connecting to a public water system where possible. It says treatment costs “are often lower than bottled water costs over several years,” and its sheet lists loan and grant programs that may help Minnesota households pay for treatment.

Since arsenic exposure from a well is mostly about the water you drink and cook with, a point-of-use system at the kitchen tap is the most common setup, at a fraction of the whole-house cost. Whole-house treatment can make sense if every tap gets used for drinking or a health professional advises it. MDH says choose a unit certified for arsenic reduction by NSF, UL, or the Water Quality Association, and in Minnesota, treatment systems must be installed by a licensed contractor unless you’re the owner-occupant.

Reading an NSF arsenic claim

We read NSF’s full NSF/ANSI 58 listing (reverse osmosis) and NSF/ANSI 53 listing (health-effect filters, which covers adsorptive media cartridges), both “current as of Thursday, October 29, 2026.”

NSF/ANSI 58 (reverse osmosis):

• 35 of the 56 companies listed had an “Arsenic (Pentavalent)” claim on at least one model.
• The claims are written with a cap: most rows read “Arsenic (Pentavalent) ≤300 ppb Reduction,” and a smaller group reads “≤50 ppb.” If your well is above the cap on the model you’re considering, you’re outside what the certification covered.
• Some well-known RO brands are listed without an arsenic claim, including iSpring and Express Water, as of Oct 29. EcoWater, Pentair Residential Filtration, and Water-Right are listed with one on at least one model. Always check the exact model, not just the brand.
• US Water Systems doesn’t appear on the NSF/ANSI 58 listing.

The AORO-50 performance data sheet shows what a certified claim looks like: it was tested at 0.30 mg/L (300 ppb) arsenic V, reduced it to below 0.010 mg/L under standard test conditions, and lists 97% reduction. It says to have the treated water tested for total arsenic after installation, and that the RO membrane is the arsenic-removing part, with a “typical membrane life expectancy” of three years.

NSF/ANSI 53 (filters): Only 6 of the 109 companies listed carried an arsenic claim, all capped at ≤50 ppb and all for pentavalent arsenic. Every one we found was a point-of-use format (pour-through, countertop, or plumbed to a separate tap). We found no whole-house arsenic claim on either listing.

Our math, as an illustration only: a system that reduces 97% would bring 50 µg/L down to about 1.5 µg/L, and 200 µg/L to about 6 µg/L, but only if the arsenic is in the form and range the claim covers. Real performance depends on your water and maintenance, so treat this as a reason to test the treated water, not a prediction.

Whole-house arsenic treatment: what to ask

With no NSF point-of-entry claim to lean on, a whole-house system is a project to plan with your lab results in hand:

• Get the full picture first: arsenic (and its form, if the lab can speciate), iron, manganese, hydrogen sulfide, pH, hardness, sulfate, silica, and TDS. Iron, manganese, and sulfur usually need treating upstream, and high pH, silica, and sulfate cut the capacity of arsenic media (per the seller’s own notes below).
• Ask how arsenic III is handled. If there’s no oxidation step, the system may only be working on part of your arsenic.
• Ask what happens when the media is exhausted. MDH warns that poorly maintained anion exchange units can dump arsenic back into the water. Some adsorptive media is replaced rather than regenerated.
• Plan a testing schedule for the treated water, at the tap you drink from.

What US Water Systems sells for arsenic

Prices and specs from the official product pages (checked Oct 29, 2026); US Water Systems shows these as marked down from a higher listed price, and its sale pricing may change after Oct 31. Claims are the seller’s own.

What to know before buying from this line, as of Oct 29, 2026:

• None of these systems appear on NSF’s 53 or 58 listings. NSF lists no certified US Water Systems softener systems either (only NSF/ANSI 42 filter components), which matches what we’ve found for its other equipment. “NSF Certified components,” or media certified to NSF/ANSI 61 (a materials standard), isn’t the same as a system certified for arsenic reduction.
• The whole-house filter’s own page sets tight conditions. It says the system is “for applications where there is no iron, sulfur or manganese present,” lists ideal operating conditions of TDS below 1,000 ppm, hardness below 2 gpg, and pH 7.2 or below, and says “if Arsenic III is present, it should first be oxidized and converted to Arsenic V.” Its capacity claim (50 ppb reduced to under 10 ppb for more than 500,000 gallons per cubic foot) is “under ideal conditions,” limited by high pH, silica, and sulfate. In practice that can mean a softener and an iron filter ahead of it (size a softener with our calculator).
• For comparison: the 7 GPM model’s $4,133.95 sits within MDH’s $2,400 to $4,500 range for whole-house adsorptive media (2017 quotes, before installation and any pre-treatment).
• The RO’s replacement filters: its 5-stage arsenic filter pack is $109.95 (checked Oct 29), with changes suggested every 6 or 12 months depending on water quality.

If you want a third-party arsenic claim, look for a model that shows “Arsenic (Pentavalent)” on NSF’s own 58 or 53 listing, check that your level is under its cap, and read its performance data sheet.

After it’s installed

From MDH (checked Oct 29, 2026): follow the maker’s maintenance schedule, and “after installing treatment, retest the treated water to make sure the unit is working.” For arsenic in particular:

• Test the treated water for total arsenic at the tap you drink from, not just the well.
• Keep an eye on the raw well level, especially for a newer well, since MDH has seen levels change in the first months.
• Change cartridges, media, and RO membranes on the maker’s schedule. Adsorptive media and anion resin have a set capacity.
• If you rely on pre-oxidation, keep that part working too; without it, arsenic III may pass through.

Bottom line

• Test at least once at a certified lab, check the units, and confirm a high result with a second sample.
• Softeners, carbon, sediment filters, UV, and boiling don’t fix arsenic.
• Know your form: NSF arsenic claims cover arsenic V only, so most unchlorinated wells need pre-oxidation or a lab test showing the arsenic is already arsenic V.
• “NSF/ANSI 58” alone isn’t enough. Look for “Arsenic (Pentavalent)” on NSF’s listing for the exact model, and stay under its 300 ppb or 50 ppb cap.
• A point-of-use system at the kitchen tap is the usual setup; whole-house treatment needs careful pre-treatment and steady maintenance.

For any health question about arsenic, start with your local or state health department and your doctor. If you’re on city water, arsenic is in your utility’s Consumer Confidence Report; for a well, it takes a lab test.