Ohio Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Ohio. They describe the area's modeled risk — not any single well.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
0%
of the private-well population is modeled above the 10 mg/L nitrate limit
~0
people on wells estimated above the limit
~1,820,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Arsenic
Across Ohio, an estimated ~189,191 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Wayne County (~22,522 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Wayne County | ~22,522 | 13,370–29,829 |
| Licking County | ~10,054 | 4,806–18,655 |
| Medina County | ~9,736 | 5,251–16,134 |
| Summit County | ~9,197 | 5,310–15,038 |
| Fairfield County | ~8,809 | 4,363–15,913 |
| Trumbull County | ~8,323 | 4,295–14,666 |
| Fulton County | ~7,840 | 6,249–9,380 |
| Ashland County | ~7,053 | 3,813–10,727 |
| Geauga County | ~6,792 | 3,548–12,472 |
| Morrow County | ~5,738 | 2,953–9,887 |
| Defiance County | ~5,434 | 4,354–6,433 |
| Richland County | ~5,081 | 2,468–9,211 |
| Holmes County | ~4,523 | 2,495–6,999 |
| Williams County | ~4,417 | 3,468–5,424 |
| Henry County | ~4,255 | 3,413–5,188 |
| Wood County | ~4,222 | 3,073–7,516 |
| Mercer County | ~3,831 | 2,807–5,106 |
| Marion County | ~3,743 | 2,692–5,632 |
| Darke County | ~3,581 | 2,761–4,609 |
| Crawford County | ~3,289 | 2,321–4,319 |
| Knox County | ~3,136 | 1,479–6,052 |
| Hancock County | ~3,027 | 2,212–4,887 |
| Shelby County | ~2,831 | 2,150–3,692 |
| Stark County | ~2,802 | 2,247–3,496 |
| Ashtabula County | ~2,664 | 1,678–4,031 |
| Portage County | ~2,374 | 1,722–3,257 |
| Pickaway County | ~2,247 | 1,477–3,287 |
| Sandusky County | ~2,018 | 1,427–3,763 |
| Hocking County | ~1,832 | 921–3,377 |
| Paulding County | ~1,772 | 1,314–2,915 |
| Seneca County | ~1,763 | 1,331–2,612 |
| Lucas County | ~1,744 | 1,366–2,296 |
| Miami County | ~1,464 | 1,188–1,817 |
| Clinton County | ~1,344 | 1,105–1,634 |
| Auglaize County | ~1,322 | 950–2,080 |
| Ross County | ~1,229 | 620–2,289 |
| Clark County | ~1,214 | 1,011–1,480 |
| Columbiana County | ~1,146 | 929–1,420 |
| Greene County | ~1,037 | 873–1,230 |
| Coshocton County | ~932 | 638–1,372 |
| Fayette County | ~834 | 622–1,252 |
| Tuscarawas County | ~793 | 645–979 |
| Union County | ~787 | 424–2,251 |
| Warren County | ~629 | 530–747 |
| Preble County | ~629 | 529–751 |
| Perry County | ~561 | 293–1,008 |
| Lake County | ~551 | 315–947 |
| Hardin County | ~529 | 340–1,266 |
| Champaign County | ~524 | 424–709 |
| Carroll County | ~493 | 400–615 |
| Franklin County | ~478 | 279–769 |
| Logan County | ~464 | 323–866 |
| Vinton County | ~446 | 244–788 |
| Delaware County | ~443 | 255–675 |
| Huron County | ~427 | 243–626 |
| Wyandot County | ~411 | 278–892 |
| Madison County | ~376 | 218–1,065 |
| Muskingum County | ~375 | 246–572 |
| Ottawa County | ~368 | 264–663 |
| Putnam County | ~357 | 69–2,128 |
| Lorain County | ~294 | 146–520 |
| Van Wert County | ~276 | 67–1,545 |
| Mahoning County | ~228 | 176–299 |
| Montgomery County | ~199 | 168–238 |
| Clermont County | ~191 | 150–242 |
| Morgan County | ~137 | 106–176 |
| Jackson County | ~129 | 75–220 |
| Pike County | ~127 | 65–235 |
| Guernsey County | ~101 | 79–130 |
19 additional Ohio counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit).
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Treating arsenic in Ohio
Arsenic is not removed by carbon pitchers or faucet filters. Reverse osmosis is the treatment that brings it below the federal limit at your tap.
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