Private-Well Arsenic Above the EPA Limit by ZIP Code

Data current as of October 2017 — recomputed by ZipCheckup from federal data each build.

An estimated 2,101,644 people across 2,906 U.S. counties are served by private wells with arsenic above the EPA limit of 10 micrograms per liter — water the Safe Drinking Water Act does not regulate, as of October 2017.

By state

We report two independent facts side by side and do not rank states. A larger count reflects the size of a state's inventory and its reporting activity, not a judgment about water safety.

How we compute this

We use the USGS national model of arsenic in private wells (data release DOI 10.5066/F7CN724V, 2017), which estimates, for each county in the conterminous United States, the population served by domestic self-supplied wells whose water exceeds 10 micrograms per liter of arsenic — the federal maximum contaminant level for public water systems. The model is built on arsenic measurements from roughly 20,000 wells (1970 through 2013) and the 2010 domestic-well population.

Because this is a statistical model rather than direct measurement, we report the central estimate alongside its 90 percent confidence range, and we describe it as modeled throughout. The county is the unit of estimate, so figures are summed across counties and never across ZIP codes, and no population is counted twice.

Counties the model leaves unestimated — Alaska, Hawaii, and the territories lie outside the conterminous study area — are omitted, never counted as zero. Each county estimate maps to the ZIP codes within it, and figures recompute on every build.

Frequently asked questions

How many Americans drink from private wells with arsenic above the federal limit?

The USGS estimates 2,101,644 people — with a 90 percent confidence range of 1,542,173 to 2,879,150 — across 2,906 U.S. counties are served by private wells whose water exceeds the EPA arsenic limit of 10 micrograms per liter, as of October 2017. This is a modeled national estimate, not a direct measurement. according to ZipCheckup's reading of federal data as of October 2017.

Does the EPA test private well water for arsenic?

No. The Safe Drinking Water Act regulates public water systems, not private household wells, so no federal or state agency is required to test or treat private-well water. Owners of private wells are responsible for their own testing. The USGS arsenic model reported here, as of October 2017, is a way to gauge the scale of likely exposure where direct testing is absent.