Arsenic is a naturally occurring element found in rock formations, soil, and groundwater across the United States — particularly in the western states, New England, parts of the Midwest, and Texas. It has no taste, color, or odor in water. Long-term exposure to arsenic in drinking water above EPA limits is associated with bladder, lung, and skin cancers, cardiovascular disease, and neurological effects. Reverse osmosis is one of the most effective and accessible technologies for removing arsenic from drinking water at the residential and commercial scale.
Arsenic in Drinking Water: Sources and Prevalence
Arsenic enters drinking water from two main sources:
- Natural geological sources — arsenic-bearing rock formations leach arsenic into groundwater over time. This is the most common source for private well users in affected regions. States with the highest naturally occurring arsenic in groundwater include Arizona, Nevada, Utah, Idaho, Montana, New Mexico, Michigan, and Maine.
- Industrial and agricultural sources — historical use of arsenic-based pesticides in orchards, mining operations, and industrial processes can contaminate both surface water and groundwater near affected sites.
The EPA’s Maximum Contaminant Level (MCL) for arsenic in public drinking water is 10 parts per billion (ppb), established in 2001. Private well owners are not regulated — they must test and treat their own water. The WHO guideline is also 10 ppb. Some states (California, New Jersey) have set even lower limits: California’s MCLG for arsenic is 0.004 ppb, reflecting the carcinogenic risk at very low concentrations.
Health Effects of Arsenic in Drinking Water
Arsenic is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC). Health effects from chronic low-level arsenic exposure through drinking water:
- Cancer: Bladder cancer, lung cancer, and skin cancer are the strongest associations. Arsenic is estimated to cause 50–200 excess cancer cases per million people in the US drinking water at the current 10 ppb MCL.
- Cardiovascular disease: Arsenic interferes with vascular function; epidemiological studies link chronic arsenic exposure to increased risk of hypertension, coronary artery disease, and peripheral vascular disease.
- Diabetes: Evidence suggests arsenic impairs insulin signaling and glucose metabolism.
- Neurological effects: In children, arsenic exposure above 10 ppb is associated with reduced IQ and cognitive development impairment.
- Dermal effects: Characteristic skin lesions (keratosis, hyperpigmentation) develop from chronic high-dose exposure, common in populations relying on highly contaminated groundwater.
How Reverse Osmosis Removes Arsenic
Arsenic in drinking water exists in two primary forms: arsenate (As(V), the pentavalent form) and arsenite (As(III), the trivalent form). RO membranes reject both, but removal efficiency varies by speciation:
| Arsenic Form | Charge at pH 7 | Typical RO Rejection | Notes |
|---|---|---|---|
| Arsenate As(V) | Negative (anionic) | 92–96% | Dominant form in oxygenated (aerobic) groundwater and surface water. RO rejects efficiently. |
| Arsenite As(III) | Neutral at pH 7 | 60–80% | Dominant in anoxic (low-oxygen) groundwater. Neutral charge reduces RO rejection. Pre-oxidation to As(V) recommended. |
Key point: If your water is from a deep anaerobic well (common in rural areas), arsenic may be predominantly in the As(III) form, where RO achieves lower rejection. Adding a chlorine pre-oxidation step (or potassium permanganate) converts As(III) to As(V) before the RO membrane, restoring full removal efficiency. Your water test should specify total arsenic, As(III), and As(V) if arsenic is present above 5 ppb.
Arsenic Treatment Options: Comparison
| Treatment Method | Arsenic Removal | Other Contaminants Removed | Best For |
|---|---|---|---|
| Reverse osmosis | 92–96% As(V); 60–80% As(III) | TDS, heavy metals, nitrates, fluoride, PFAS | Households + light commercial; addresses multiple contaminants |
| Adsorptive media (iron oxide, activated alumina) | 90–99% As(V) and As(III) | Limited (media-specific) | Point-of-entry arsenic-specific treatment; high flow rate |
| Anion exchange resin | 90–95% As(V) only | Some other anions | Municipal-scale or large commercial; As(V) dominant water |
| Coagulation / filtration | Variable (50–90%) | Turbidity, some metals | Municipal treatment; not practical at residential scale |
For residential and commercial point-of-use applications, reverse osmosis is the most versatile choice because it addresses arsenic plus a full spectrum of co-occurring contaminants (fluoride, nitrates, heavy metals, TDS) that are common in the same groundwater sources that contain arsenic.
Testing Your Water for Arsenic
Municipal water systems test for arsenic and are required to report results in their annual Consumer Confidence Report (CCR), available from your utility. Private well owners should test annually for arsenic if you are in a high-risk region, or if a neighbor’s well has tested positive. Test through a state-certified laboratory — not a home test kit, which are too imprecise for arsenic at low ppb concentrations.
The EPA’s Safe Drinking Water Hotline (800-426-4791) can provide state laboratory referrals. Many state health departments offer free or subsidized well water testing programs for arsenic and other common groundwater contaminants.
AMPAC USA RO Systems for Arsenic Removal
AMPAC USA residential reverse osmosis systems using FILMTEC™ thin-film composite membranes achieve 92–96% rejection of arsenate (As(V)) under standard operating conditions (65°F, 50 PSI feed pressure, 500 ppm TDS feed water). For water testing positive for As(III) (trivalent arsenic from anaerobic wells), we recommend a pre-oxidation stage — contact our engineering team for the appropriate pre-treatment configuration based on your water test results.
Systems are available in under-sink point-of-use configurations (50–100 GPD) for drinking and cooking water treatment, and whole-house point-of-entry configurations (200–1,000 GPD) for comprehensive household arsenic protection including shower and bath water.
Have a water test showing elevated arsenic? Share the results with our team — including total arsenic, pH, and TDS — and we’ll specify the correct system and pre-treatment configuration. Contact AMPAC USA for a free technical consultation.
Related: Residential RO Systems | Whole House RO Systems | Free Water Quality Assessment