Arsenic in private wells: what homeowner tests show, state by state

9 states publish summaries of the arsenic tests homeowners send to certified labs, together covering 374,421 private wells. In New Hampshire, 25% of 18,685 wells tested were above the state level of 5 ug/L, the highest share of any state with a program. Arsenic has no taste or smell and comes from the rock, not from anything on the surface, so a well can look and taste perfect and still carry it.

Homeowner tests, by state

Read the level column before comparing rows. New Hampshire and New Jersey set their guidance at 5 ug/L, half the federal limit, on the view that 10 ug/L is too lenient for a lifetime of drinking water. Against the federal 10 ug/L their shares would be lower. Of the states that use 10 ug/L, the highest is Maine at 17% of 27,616 wells.

The pattern follows the rock. The states at the top of the list sit on crystalline and volcanic bedrock and glacial deposits derived from it, where arsenic occurs naturally in the minerals that deep drilled wells draw from. States on sandy coastal plains and young sediments report far lower shares.

States without a homeowner program

Most states do not publish homeowner results. For them, the federal record is the best available guide: every private well arsenic sample in the USGS Water Quality Portal, against the EPA limit. These are the states with at least 500 samples and the highest shares.

Federal sampling often goes where a problem was already suspected, so these shares describe the wells that were sampled more than a typical one. They still mark the states where an arsenic test belongs on the first order for any private well.

What it means for one well

A state share is the odds before you test, not a result. Arsenic varies from one well to the next on the same road, with depth and with which fractures a well happens to cross. The only way to know is a lab test, once, and again every few years or after work on the well. Where results come back above the limit, reverse osmosis at the kitchen tap or a whole-house adsorption system brings it down reliably. Each state's page lists its certified labs and any free testing program: well water testing by state.