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.
What the numbers are — and are not
- They are modeled area estimates. They describe the statistical likelihood across a county or state, not the water in any specific well.
- They are not 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 the finding is substantial (an estimated 100+ well residents above the arsenic limit), so we don't create thin pages with negligible data. That is 1,655 counties.
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. Data accessed via USGS ScienceBase, 2026.