Arsenic in Well Water: How to Test for It and Which Filters Remove It
By SubcontractorHub Editorial Team·Published September 2026
Short version: arsenic has no taste, color or smell, so a certified lab test is the only way to know it is there. The EPA limit is 10 ppb (0.010 mg/L). Before choosing a filter, find out whether the arsenic is arsenic III or arsenic V. Reverse osmosis and anion exchange remove arsenic V well but not arsenic III, so arsenic III has to be oxidized first. Point-of-use RO is the most common fix; whole-house adsorptive media is the usual step up. Softeners, carbon filters and boiling do not work.
This guide covers where arsenic comes from, what the numbers mean, and which treatment methods reduce it. It is not medical advice. It is written for well owners and the water treatment dealers who design and service these systems.
1. Where Arsenic in Well Water Comes From
Most arsenic in well water is natural. It dissolves out of bedrock and sediments, and levels vary widely, even between neighboring wells. Elevated arsenic is found in parts of New England, the Upper Midwest, the Southwest and elsewhere in the West, but any well can have it. Because it is tasteless and odorless, a well can test high for years without anyone noticing.
2. The Numbers: EPA’s 10 ppb Limit
10 ppb = 10 µg/L = 0.010 mg/L: the EPA maximum contaminant level (MCL), enforceable for public water systems since 2006
0 ppb: the EPA health goal (MCLG), which is not enforceable
The limit is based on long-term exposure. Private wells are not covered by the Safe Drinking Water Act, so the owner is responsible for testing. Lab reports use ppb, µg/L or mg/L, so check the unit before comparing. Some states set lower guidance levels.
3. Arsenic III vs Arsenic V: Why the Form Decides the Treatment
Arsenic in groundwater shows up in two main forms. Arsenic V (arsenate) carries a negative charge at normal well water pH. Membranes reject it and anion resins and iron-based media grab it readily. Arsenic III (arsenite) carries no charge at that pH, so it slips through an RO membrane far more easily and anion exchange resin barely touches it. Arsenic III is common in low-oxygen water that also carries iron, manganese or a sulfur smell.
The fix is to oxidize arsenic III to arsenic V before the main treatment step. Chlorine, potassium permanganate, ozone and manganese dioxide media all do this. Aeration alone does not do it reliably. Ask the lab for arsenic speciation (the split between III and V). It usually needs a special sample kit, so arrange it before you take the sample.
4. Arsenic Water Filter Options Compared
Removal figures are typical, not guaranteed. Costs are rough residential ranges.
Point-of-use reverse osmosis
An under-sink RO system at the kitchen tap is the most common answer, typically rejecting 90 to 95 percent of arsenic V. NSF/ANSI 58 arsenic claims are normally for arsenic V, so arsenic III needs oxidation upstream; the carbon pre-filter then strips leftover chlorine before it reaches the membrane. The reverse osmosis system cost calculator estimates installed cost, and the water softener vs reverse osmosis guide explains how RO sits alongside a softener.
Whole-house adsorptive media
Iron-based media (granular ferric oxide or hydroxide) and titanium-based media bind arsenic to their surface. They treat every tap, need no salt and send little water to drain. The trade-off is that the media gets used up. Phosphate and silica compete for the same sites and capacity falls as pH rises, so media life varies a lot between homes. Spent media gives no warning, which is why the testing schedule matters most here. Look for NSF/ANSI 53 certification with an arsenic claim.
Anion exchange and oxidation/filtration
Anion exchange resin removes arsenic V and regenerates with salt, but it prefers sulfate. In high-sulfate water capacity drops, and if regeneration runs late, sulfate can push arsenic back off the resin at a higher level than came in. It suits low-sulfate water. Oxidation and filtration suits wells that also have iron: oxidized arsenic sticks to the iron particles the filter catches. The iron filter guide covers those systems. Chlorine injection is a common oxidation step, and the chlorine injection system sizing calculator sizes the feed pump and contact tank.
What does not work
Water softeners exchange hardness minerals and do not remove arsenic. Standard carbon filters and pitchers remove little or none. Sediment filters catch particles, and dissolved arsenic passes straight through. Boiling concentrates arsenic as water evaporates.
5. Worked Example: A 25 ppb Well With Arsenic III
A lab reports 25 ppb total arsenic: 20 ppb arsenic III and 5 ppb arsenic V, with 0.4 mg/L iron.
RO with no oxidation: arsenic V 5 × (1 − 0.95) = 0.25 ppb, plus arsenic III 20 × (1 − 0.60) = 8 ppb if the membrane catches 60 percent. About 8 ppb out, too close to the limit and could be worse.
Oxidize first, then RO: nearly all 25 ppb becomes arsenic V. 25 × (1 − 0.95) = 1.25 ppb; at 90 percent rejection it is 2.5 ppb. About 1 to 3 ppb out.
Whole-house option: oxidation followed by iron-based adsorptive media sized to the home’s peak flow. The iron here is too low (about 16 parts per part arsenic) for iron filtration alone to be dependable. Test the outlet every three to six months and replace the media before it passes 5 ppb.
The same RO unit gives very different results depending on the form of arsenic, which is why speciation comes before the quote. The well water treatment system selector helps put the whole test report, including iron, hardness and sulfate, into one system plan.
6. Testing and Retesting
- Test the raw water with a state-certified lab. An arsenic test typically costs about $30 to $100; speciation costs more. Home kits are for screening only.
- Test the whole picture. Iron, manganese, pH, sulfate, phosphate and silica all change which method works and how long media lasts. Wells with arsenic can carry other natural contaminants; the radon in water guide covers one of them.
- Test the treated water within a few weeks of installation, from the tap you drink from.
- Retest on a schedule: at least yearly for RO, every three to six months for adsorptive media, and the raw well water every year or two, since levels can shift.
For Water Treatment Dealers
Arsenic jobs start with the test: an in-home screen, then a certified lab sample with speciation, walked through with the homeowner using the math above. The quote is usually point-of-use RO with pre-oxidation or a whole-house oxidation and adsorptive media system, shown as good-better-best options with a monthly financing option beside the cash price. Installation is standard plumbing work.
The service plan carries the job: filter changes, a membrane every few years, media changes on schedule and a retest at every visit. SubcontractorHub is software for water treatment dealers that puts test results, system options, financing, e-signature and recurring service scheduling in one place.
Frequently Asked Questions
What is the safe level of arsenic in drinking water?
The EPA maximum contaminant level (MCL) for arsenic is 10 parts per billion (ppb), or 0.010 mg/L, for public water systems. Private wells are not regulated, so the owner has to test. The EPA health goal is zero, so treated water well below 10 ppb is the usual target.
Does reverse osmosis remove arsenic?
Yes, for arsenic V (arsenate), typically about 90 to 95 percent. Arsenic III (arsenite) is removed much less reliably, so it should be oxidized to arsenic V before the membrane. Look for NSF/ANSI 58 certification with arsenic reduction listed and test the RO faucet after installation.
Does boiling water remove arsenic?
No. Arsenic does not evaporate, so boiling concentrates it in the water that is left. Distillation is different because it collects the condensed steam and leaves arsenic behind.
Will a water softener or carbon filter remove arsenic?
No. A softener removes calcium and magnesium, not arsenic. Ordinary carbon filters, including pitchers and fridge filters, remove little or none. Rely only on a product with a certified arsenic reduction claim.
Is it safe to shower in water with arsenic?
Health agencies treat drinking and cooking as the main exposure routes, because little arsenic is absorbed through the skin, which is why kitchen-tap treatment is the usual answer. At high levels, or for specific health questions, ask your state health department or a healthcare provider.
How often should I test my well for arsenic?
Test the raw water with a state-certified lab before choosing treatment, with speciation if it is over the limit. Test treated water within a few weeks of installation, then at least yearly, or every three to six months for adsorptive media, which stops working without any visible sign.
Related Guides & Tools
- Well Water Treatment System Selector
- Reverse Osmosis System Cost Calculator
- Chlorine Injection System Sizing Calculator
- Iron Filter for Well Water
- Radon in Water
- Water Softener vs Reverse Osmosis
- → SubcontractorHub for Plumbing
- → Water Treatment and Water Purification Contractor Software
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