Arsenic occurs naturally in rock and soil and dissolves into groundwater — it’s most common in parts of the Southwest, Upper Midwest, and New England, and is more often a concern for private wells than treated public systems. EPA’s legal limit (MCL) is 0.010 mg/L (10 ppb), effective since 2006, with a health goal (MCLG) of zero. Long-term exposure is linked to skin damage, circulatory problems, and increased cancer risk.
What actually removes arsenic
- Reverse osmosis (NSF/ANSI 58) — a membrane fine enough to reject dissolved arsenic along with a broad range of other contaminants.
- Anion exchange (NSF/ANSI 53) — resin media that specifically targets and binds arsenic; commonly used in both point-of-use and whole-house arsenic treatment.
What doesn’t work
Arsenic exists in water in two forms — arsenic(III) and arsenic(V) — and treatment effectiveness can depend on which form (and what oxidation state) is present, which a standard consumer filter won’t distinguish on its own. Standard activated-carbon pitchers and faucet filters are not effective at removing arsenic; carbon adsorption isn’t the right mechanism for this contaminant, regardless of what the packaging implies about “reducing contaminants” generally. A water softener (cation exchange, designed for calcium/magnesium hardness) does not remove arsenic either — it’s the wrong exchange chemistry for an anion like arsenic.
How to choose
- Look specifically for NSF/ANSI 58 (reverse osmosis) or 53 (anion exchange) certification that names arsenic as a reduction claim.
- If your water’s arsenic is on the arsenic(III) form (more common in some groundwater, less charged and harder for some anion-exchange media to capture), a lab or treatment professional can advise whether pre-oxidation is needed before the filter stage — this is not something to guess at from a product label alone.
- Well owners in regions with documented naturally occurring arsenic (parts of the Southwest, upper Midwest, and New England) should treat testing as a priority even without symptoms — arsenic has no taste, odor, or color at these concentrations.
Test before you buy
Because arsenic occurs naturally and varies significantly by well and by depth — even between neighboring properties — a certified lab test is the only reliable way to know whether arsenic is present and at what level before choosing (and paying for) a treatment system.