Arsenic
Also known as: As
A naturally occurring carcinogen in many groundwater systems; the fix is treatment, since you can't avoid the geology.
MCL 0.010 mg/L (10 ppb). EPA lowered the arsenic limit from 50 ppb to 10 ppb in 2001 (effective 2006).
A known human carcinogen linked to bladder, lung, and skin cancers, plus cardiovascular disease, diabetes, and skin lesions with long-term exposure.
Mostly natural — arsenic dissolves from rock and sediment into groundwater, especially in parts of the West, Upper Midwest, and New England. Mining and historic pesticides add to it locally.
Reverse osmosis, anion exchange, or adsorptive media (e.g., iron-based) remove arsenic. Whole-system treatment is common where geology is the source.
Source: EPA Safe Drinking Water Information System (SDWIS) · 2026 Q1
Arsenic is a naturally occurring element that leaches into groundwater from surrounding rock. Many small groundwater systems — particularly in the Southwest, Great Basin, and parts of the Midwest and Northeast — sit on geology that pushes arsenic above the 10-ppb limit.
Because the source is geological, arsenic violations cluster by region and disproportionately affect small rural systems that struggle to afford treatment. It's one of the most common health-based chemical violations in the country.
States with the most arsenic violations
Community water systems with a related health-based violation since 2016.
| # | State | Systems |
|---|---|---|
| 1 | California | 133 |
| 2 | Texas | 106 |
| 3 | Arizona | 76 |
| 4 | New Hampshire | 48 |
| 5 | Illinois | 28 |
| 6 | Washington | 27 |
| 7 | New Mexico | 20 |
| 8 | Nevada | 19 |
| 9 | Michigan | 19 |
| 10 | Oregon | 19 |
| 11 | Minnesota | 17 |
| 12 | Idaho | 15 |
Common questions
What is the EPA limit for arsenic in drinking water?
MCL 0.010 mg/L (10 ppb). EPA lowered the arsenic limit from 50 ppb to 10 ppb in 2001 (effective 2006).
Is arsenic in drinking water dangerous?
A known human carcinogen linked to bladder, lung, and skin cancers, plus cardiovascular disease, diabetes, and skin lesions with long-term exposure.
Where does arsenic in water come from?
Mostly natural — arsenic dissolves from rock and sediment into groundwater, especially in parts of the West, Upper Midwest, and New England. Mining and historic pesticides add to it locally.
How do I remove arsenic from my water?
Reverse osmosis, anion exchange, or adsorptive media (e.g., iron-based) remove arsenic. Whole-system treatment is common where geology is the source.
Which states have the most arsenic violations?
Across U.S. community water systems, 714 have had a related health-based violation since 2016, led by California with 133.