Methodology & Data Sources

Every risk figure on this site comes from published U.S. Geological Survey (USGS) groundwater models. We do not generate, simulate, or estimate any numbers ourselves. This page documents exactly what we use and what it means.

Arsenic (county level)

Source: USGS ScienceBase data release "Estimated county-level domestic-well population with arsenic greater than 10 µg/L", derived from the national logistic-regression arsenic model of Lombard et al. (2021). For each of 3,103 counties in the conterminous U.S., the USGS combined the modeled probability of arsenic exceeding 10 µg/L with the number of people on domestic (private) wells to estimate how many well users are likely above the limit. We report the mid estimate (field MIDpop2010) with its lower and upper 90% confidence bounds.

Nitrate (state level)

Source: USGS ScienceBase data release "Machine Learning Predictions of Nitrate in Groundwater" (Ransom et al., 2022). We use the published state-level summary for the domestic-well depth zone (Table S6): the percentage and count of each state's private-well population with modeled nitrate above the 10 mg/L EPA limit. 49 states and DC are covered.

Measured well chemistry (county level)

County pages also show real laboratory measurements from USGS well samples, aggregated by county. Manganese, pH and arsenic come from the USGS data release "Arsenic, manganese, and pH groundwater quality data… for wells in the conterminous U.S." (110,682 wells, 345,586 samples, 1988–2019; we collapse repeat samples to one median per well). Nitrate and iron come from the 43,334-well dataset published with McMahon et al. (2019), Environ. Sci. Technol. 53:29–38. Wells are assigned to counties by point-in-polygon against Census 2020 boundaries. For each parameter we report the number of wells, the median, the 90th percentile and the share of wells over the federal threshold (nitrate 10 mg/L MCL; manganese 300 µg/L health advisory and 50 µg/L secondary standard; iron 300 µg/L secondary standard; pH outside 6.5–8.5). A statistic is shown only where at least 5 wells were sampled; 2,115 counties qualify for at least one parameter. The sampled wells are a mix of domestic, public-supply, irrigation and monitoring wells, so they describe groundwater in the county — not any one household's well.

What the numbers are — and are not

• Arsenic and nitrate risk figures are modeled area estimates. They describe the statistical likelihood across a county or state, not the water in any specific well.
• Measured chemistry describes other wells. Real samples, but from wells that are not yours; sampling density reflects USGS effort, not risk.
• None of it is a test of your well. Two neighboring wells can differ dramatically. Only a lab test tells you your actual levels.
• Coverage is the conterminous U.S. The USGS models exclude Alaska and Hawaii, so we do not show estimates for them.
• County pages are published only where there is substantial, county-specific data: an estimated 100+ well residents above the arsenic limit (1,655 counties) or measured USGS chemistry from at least 5 wells. Together that is 2,444 county pages; the rest are summarised on their state page.

Health thresholds

EPA maximum contaminant levels: arsenic 10 µg/L, nitrate (as N) 10 mg/L, lead 15 µg/L action level. These are the federal standards for public water systems; private wells are unregulated, so owners are responsible for testing and treatment.

Product recommendations

Filter recommendations are matched to the contaminant's treatment requirements (e.g. arsenic and nitrate need reverse osmosis), not to paid placement. Product links are Amazon affiliate links; see our editorial guidelines and disclaimer.

Citations

Lombard, M.A., et al. (2021), Environmental Science & Technology. Ransom, K.M., et al. (2022), Environmental Science & Technology. Powell, D.E., Lombard, M.A., et al. (2020), USGS data release 5f36baa782cee144fb3873e8. McMahon, P.B., Belitz, K., Reddy, J.E., Johnson, T.D. (2019), Environmental Science & Technology 53:29–38, Supporting Information Data S1. U.S. Census Bureau cartographic boundary files (2020, 1:20m). Data accessed via USGS ScienceBase, 2026.