The EPA limit for arsenic in drinking water is 10 micrograms per liter (10 ppb). Arsenic is tasteless, odorless, invisible, and naturally present in bedrock groundwater across large parts of the U.S. — which is why every private well needs to be tested for it at least once, and why boiling makes it worse, not better. Here's how to read your number, where it comes from, and the short list of treatments that actually remove it.
Reading your result
Labs report arsenic as µg/L (= ppb) or as mg/L — 10 ppb equals 0.010 mg/L. A result of "0.015 mg/L" is 15 ppb: over the limit. Unlike nitrate, arsenic is a chronic exposure question (long-term cancer and cardiovascular risk), so the response is measured rather than emergency — but "under 10 for drinking and cooking, permanently" is the target, and lower is better.
Where it comes from (and why your well isn't "dirty")
Mostly rock. Arsenic-bearing minerals in bedrock dissolve into groundwater in well-mapped regions — Maine and New Hampshire granite country, the desert Southwest, parts of Michigan, Minnesota, and Wisconsin, and pockets nationwide. Legacy orchard pesticides add local hotspots. This isn't a contamination pathway you can seal like a cracked casing; it's the aquifer's chemistry, which is why treatment (or a different source) is the durable answer.
What removes arsenic — and what doesn't
| Method | Removes arsenic? | Notes |
|---|---|---|
| Boiling | 🚫 Concentrates it | Never |
| Carbon pitcher / fridge filter | 🚫 No | Taste only |
| Water softener | 🚫 No | Hardness only |
| Reverse osmosis (kitchen tap) | ✅ Effective for arsenic V | $300–$1,500; removes 95–99% of arsenic V but only ~60–80% of arsenic III — add a pre-oxidation cartridge if speciation shows III |
| Iron-based adsorptive media (whole house) | ✅ Yes (best on arsenic V) | $1,500–$4,000 plus $1,000–$1,800 per media change every 1–3 years; capacity drops sharply with arsenic III — pre-oxidize |
| Anion exchange (whole house) | ✅ Yes for arsenic V | $1,500–$4,000; competes with sulfate — needs the lab panel |
| New/deeper well or connection | ✅ Source change | Sometimes the aquifer below is cleaner; sometimes not — costs here |
The detail that trips people: arsenic exists in two forms, and arsenic III — which dominates in many aquifers — is only partly removed by RO, anion exchange, or adsorptive media unless it's oxidized to arsenic V first. Ask the lab for speciation or let a treatment professional design around it — then retest after installation, the only proof a tasteless contaminant allows.
The sensible response, by result
- Under 10 ppb: file it, retest every 3–5 years. Keep drinking.
- 10–50 ppb: switch drinking/cooking to bottled or a treated tap now; install RO at the kitchen (with pre-oxidation if III); retest. Showering and laundry are fine — arsenic risk is ingestion.
- Higher, or several fixtures matter: whole-house adsorptive media or a source change; involve a treatment professional who works from your lab panel, not a showroom demo (how to buy treatment without getting oversold).
Frequently Asked Questions
What level of arsenic in well water is safe?
The EPA standard is 10 micrograms per liter (10 µg/L, also written 10 ppb or 0.010 mg/L). Labs sometimes report in mg/L, so 0.010 is the number to watch there. Arsenic is a long-term (chronic) risk rather than an acute one — the goal is getting drinking and cooking water reliably under 10 ppb, and lower is better.
Where does arsenic in well water come from?
Mostly geology: arsenic occurs naturally in bedrock and dissolves into groundwater in parts of New England, the Southwest, the upper Midwest, and scattered areas nationwide — it has nothing to do with how clean your well is. Old orchard pesticides and industrial sources contribute locally. Neighbors' results are a strong hint; your own lab test is the only answer.
Does boiling or a water filter pitcher remove arsenic?
No — boiling concentrates arsenic (water leaves, arsenic stays), and standard carbon pitchers and softeners don't remove it. Reverse osmosis at the kitchen tap, iron-based adsorptive media, and anion exchange do. Because arsenic is tasteless and odorless, the only way to know a treatment works is a follow-up lab test.
How often should I test for arsenic?
At least once for every private well — many owners never have, and arsenic country is wider than people assume — then every 3–5 years, or yearly if you've had a detection or run treatment (to confirm it's still working). Test before installing any treatment: the arsenic form (III vs V) changes which equipment succeeds.