Best Filter for Arsenic in Well Water

Why Arsenic in Well Water Is a Serious Concern

Arsenic is a Group 1 human carcinogen — the highest evidence tier. Long-term exposure is strongly linked to bladder, lung, and skin cancer. The EPA MCL is 10 ppb (parts per billion), but even at the legal limit, lifetime exposure carries meaningful cancer risk — some epidemiological models put bladder cancer risk at roughly 1 in 500 at exactly 10 ppb. Arsenic is tasteless and colorless, so you will not detect it without a lab test. See the full arsenic contaminant guide for the health and geographic context.

The Three Technologies That Actually Remove Arsenic

1. Reverse Osmosis

Reverse osmosis removes 90–95% of dissolved inorganic arsenic (As(V) and As(III) in oxidized form) by forcing water through a semi-permeable membrane. RO is the most accessible point-of-use solution for private well owners — under-sink systems are available for $150–$400, require no whole-house plumbing modification, and simultaneously address lead, nitrates, PFAS, and most other dissolved contaminants. This makes RO the default recommendation for well owners with arsenic plus other concerns. See the full reverse osmosis guide.

2. Adsorptive Media (Activated Alumina / Iron-Based Media)

Adsorptive media filters use alumina or iron-based granular media that arsenic bonds to as water passes through. They are highly effective for arsenic specifically — often exceeding 95% removal — and are commonly used in whole-house point-of-entry systems. Activated alumina is the most established; iron-based media (such as GFO — granular ferric oxide) is increasingly common. These systems require periodic media replacement. They do not remove nitrates, PFAS, or lead, so they work best when arsenic is the isolated concern.

3. Distillation

Distillation removes virtually all arsenic by boiling water and collecting the steam — arsenic and other dissolved solids stay behind. It is highly effective but slow (typically 1 gallon per hour), expensive to run (uses significant electricity), and impractical as a whole-house solution. Distillation is rarely the first choice compared to RO or adsorptive media, but is an option for households without access to RO installation.

What Does NOT Remove Arsenic

• Standard activated carbon filters (pitcher, faucet, under-sink) — minimal arsenic removal; not the right mechanism
• Water softeners — remove calcium and magnesium; not arsenic
• UV purification — kills bacteria and viruses; no effect on dissolved chemicals
• Sediment filters — remove particles; arsenic is dissolved, not particulate
• Boiling — concentrates arsenic as water evaporates; never use for arsenic treatment

Reverse Osmosis vs Adsorptive Media for Arsenic

An Important Note About Arsenic Chemistry

Arsenic exists in groundwater in two oxidation states: arsenate As(V) and arsenite As(III). Most filters perform best on As(V). If your well water is low in dissolved oxygen (common in deep wells), As(III) may dominate and some filters will perform less effectively. Pre-oxidation — injecting a small amount of air or using a chlorine contact tank — converts As(III) to As(V) before filtration. If your lab test reports a high proportion of As(III) or if your well is anaerobic (low oxygen), discuss pre-oxidation with a water treatment professional.

Certifications to Look For

• NSF/ANSI Standard 58 — for RO systems; look for arsenic reduction specifically in the system's certified claims
• NSF/ANSI Standard 53 — for carbon filters claiming arsenic reduction (limited options; verify explicitly)
• NSF/ANSI Standard 62 — for distillation units
• WQA Gold Seal — alternative third-party certification recognized for adsorptive media systems
• Verify that certification covers the full system at the rated flow rate — not just the media or membrane in isolation

Decision Framework by Arsenic Level and Situation

How to Know Your Arsenic Level

Testing is the only way to know your well's arsenic level — it is tasteless, colorless, and odorless. Use a state-certified laboratory, not home test strips. Home strips for arsenic are not accurate at concentrations near the 10 ppb EPA limit. A certified arsenic test typically costs $15–$40 as a standalone or is included in a comprehensive well water panel. See the full well water testing guide for step-by-step collection instructions and lab selection.

Geographic Areas with the Highest Arsenic Risk

Arsenic in well water is a nationwide concern, but risk is highest in specific geological formations. Well owners in the following areas should prioritize arsenic testing: the western United States (Arizona, Nevada, California's Central Valley), New England (Maine, New Hampshire, Vermont), the Midwest (Minnesota, Wisconsin, Michigan), and parts of the Great Plains (South Dakota, Wyoming). See the arsenic contaminant guide for state-level detail.