Nitrate in private wells: where it runs highest, and whether it is getting better

Nitrate is the one common well contaminant that comes from what happens on the land above the well: fertilizer, manure and septic systems. Across the 45 states with at least 2,000 private well samples, the public record holds 542,787 nitrate results. In Colorado, 40% of 5,831 samples were above the EPA limit, the highest share of any state with a large record.

Where nitrate runs highest

Is it getting better?

34 states have at least 100 nitrate samples in each of the 2000s, 2010s and 2020s. In 22 of them the share above the limit moved less than five points across those three decades: California, Idaho, Nebraska, Florida, Arizona. 7 fell by five points or more. 5 rose by five points or more. California, with the largest record, went from 13% of 9,158 samples in the 2000s to 13% of 3,694 in the 2020s.

Across most states the record does not show the problem fading. That fits how nitrate behaves: it moves slowly through soil and groundwater, so what is applied at the surface can take years to reach a well and years more to leave.

Homeowner tests

A few states also publish the nitrate results homeowners send to certified labs, which are closer to an ordinary well than federal samples.

What it means for one well

Nitrate is the reason every private well should be tested every year, alongside bacteria. It rises with shallow wells, sandy soil, nearby fields and septic systems, and it changes with the seasons, so one clean result does not settle it. Water above 10 mg/L should not be used to mix infant formula. Reverse osmosis at the kitchen tap or an anion exchange system removes it. Each state's page lists certified labs and free programs: well water testing by state.