Arsenic in Well Water: Testing and Removal Options
The water is clear. No rust ring in the toilet tank, no rotten-egg smell in the shower, no film on the glasses. Then a neighbor two streets over mentions the number on their well report — 24 µg/L — and suddenly the fact that your water looks perfect means nothing at all. Arsenic is geology, not plumbing: it dissolves out of bedrock into groundwater, and a well 300 feet away can read completely differently from yours.
How to find out whether you have it
No home test substitutes for a lab here. Arsenic strips run $15–30 for a handful, but they read in coarse bands — a strip that says "10" could be 6 or 18, and that gap decides whether you spend $2,000.
Start with your state health department or a state-certified laboratory. Arsenic alone is usually $20–100; a broader panel with coliform bacteria, nitrate, iron, manganese and hardness runs $100–300. The EPA and CDC recommend testing a private well annually for coliform and nitrate, and arsenic at least once — then every 3–5 years, or sooner after a new well, a pump change, or a drought.
One extra line item is worth asking about: speciation, a $30–80 add-on that splits the total into arsenite (As III) and arsenate (As V). It decides which equipment works. Low-oxygen groundwater — the kind that also carries dissolved iron and hydrogen sulfide — tends toward As(III), which holds no electrical charge at normal pH, and most treatment removes charged ions. That's why the same RO unit cuts arsenic 95% in one house and 50% in another.
| Result (total arsenic) | What it means | Reasonable next step |
|---|---|---|
| Below 5 ppb | Well under the federal limit for public systems | Retest in 3–5 years |
| 5–10 ppb | Under the MCL, but not by much | Retest within 12 months; check state guidance |
| Above 10 ppb (the EPA MCL) | Elevated; the regulatory line since 2006 | Confirm with a second sample, add speciation |
| Above 50 ppb | Above the pre-2006 standard | Bottled water for drinking and cooking until treatment is in |
The source is almost always the rock the well is drilled into. USGS mapping clusters elevated arsenic in New England bedrock wells, the Upper Midwest and Michigan, and the Southwest — Arizona, Nevada, central California — with roughly 2 million private-well users estimated above 10 ppb. Your county extension office knows whether you're in a hot zone.
What actually removes arsenic — and what it costs
The options split along one line: kitchen tap or whole house. Arsenic isn't meaningfully absorbed through the skin in a shower, so most households treat only drinking and cooking water — the cheap path.
| Method | Works on | How it works | Typical cost |
|---|---|---|---|
| Reverse osmosis (point-of-use) | As(V) well; As(III) poorly without pre-oxidation | Membrane rejects charged arsenate ions along with other dissolved solids | $150–500 installed; membranes $50–100 every 2–3 yrs |
| Adsorptive media (iron oxide / activated alumina) | As(V); some media handle both | Arsenic binds to media in a tank | $300–800 point-of-use; $1,500–4,000 whole-house; media every 1–3 yrs |
| Anion exchange | As(V) only | Resin swaps arsenate for chloride; high sulfate or TDS hurts it | $1,500–3,500 installed |
| Distillation | Both forms | Boils and recondenses; arsenic stays in the chamber | $100–400 countertop, slow (4–6 hrs/gal) |
| Oxidation + iron filtration | As(V), only where iron is present | Arsenic co-precipitates with iron floc | $1,200–2,500; needs roughly 20:1 iron-to-arsenic |
| Water softener | Nothing | Ion exchange targets calcium and magnesium | Not a solution for arsenic |
Read the certification line, not the marketing copy: NSF/ANSI 58 for RO systems, NSF/ANSI 53 for adsorptive point-of-use filters, with a claim that usually reads "arsenic pentavalent reduction" — that's As(V). A system with no arsenic claim, however many stages it lists, was never tested for this.
Why a reverse osmosis unit sometimes doesn't fix it
This is the trap that costs a second round of money. RO is the standard answer at the kitchen sink and handles arsenate well — 85–95% reduction. On arsenite it can drop to 50–70%, because an uncharged molecule slips past a membrane that works by rejecting charge. Start at 30 ppb As(III), take a 60% cut, and you're at 12 ppb: still over the EPA limit, with a brand-new system under the sink.
The fix isn't a better membrane, it's converting the arsenic first. Chlorination, potassium permanganate, or an air-injection oxidizing filter ahead of the RO turns As(III) into As(V), which the membrane then handles normally. Households already running a chlorine injector or an air-injection iron filter often have that step by accident.
The quieter failure is that nobody retests. A membrane has no warning light for arsenic. Pull a post-filter sample 2–4 weeks after installation, then repeat annually — that's your only evidence the money did anything.
Common mistakes
- Boiling the water first. The intuitive move makes it worse — water leaves as steam, arsenic stays, concentration rises. Boiling handles bacteria, never dissolved metals.
- Trusting a carbon pitcher or fridge filter. Standard activated carbon isn't an arsenic technology. Without a certified NSF/ANSI 53 arsenic claim, assume the number is unchanged.
- Buying equipment before speciation. Total arsenic says you have a problem; the As(III)/As(V) split says what to buy. A $60 test prevents a $2,000 mismatch.
- Skipping the confirmation test. One high sample deserves a second before you spend, and a new system deserves a post-treatment sample before you trust it.
FAQ
What is a safe level of arsenic in well water?
The EPA sets the Maximum Contaminant Level for public drinking water at 10 ppb (0.010 mg/L), lowered from 50 ppb in 2006. Private wells aren't covered by that rule, but 10 ppb is the benchmark nearly every state health department uses — and some advise action lower, so read your state's guidance alongside the lab result.
Does boiling water remove arsenic?
No — it concentrates it. Boiling drives off water as steam and leaves the arsenic behind, so a 15 ppb sample reduced by evaporation reads higher, not lower. Distillation works because it captures and recondenses the steam, which is a different process from boiling in a pot.
Does a reverse osmosis system remove arsenic?
It removes arsenate, As(V), well — typically 85–95% on a system certified to NSF/ANSI 58 with an arsenic pentavalent claim. Arsenite, As(III), is uncharged at normal pH and reduction can fall to 50–70%, which is why a speciation test and, if needed, an oxidation step ahead of the membrane matter so much.
Do I need to treat the whole house or just the kitchen tap?
For most households a point-of-use unit at the kitchen sink covers it, since arsenic is a concern through drinking and cooking rather than showering or laundry — a $150–500 decision instead of $1,500–4,000. Whole-house treatment makes sense at very high levels, or when iron, manganese, or sediment need a whole-house system anyway.
Related:
General information about home water quality and equipment, not medical advice. Arsenic, bacteria, nitrate, and lead can only be measured by a state-certified laboratory — no article, meter, or test strip replaces that result, and questions about health effects belong with your doctor or state health department. Prices and removal rates vary by model and by your water chemistry.