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

CountyEst. well residents over arsenic limit90% range
Wayne County~22,52213,370–29,829
Licking County~10,0544,806–18,655
Medina County~9,7365,251–16,134
Summit County~9,1975,310–15,038
Fairfield County~8,8094,363–15,913
Trumbull County~8,3234,295–14,666
Fulton County~7,8406,249–9,380
Ashland County~7,0533,813–10,727
Geauga County~6,7923,548–12,472
Morrow County~5,7382,953–9,887
Defiance County~5,4344,354–6,433
Richland County~5,0812,468–9,211
Holmes County~4,5232,495–6,999
Williams County~4,4173,468–5,424
Henry County~4,2553,413–5,188
Wood County~4,2223,073–7,516
Mercer County~3,8312,807–5,106
Marion County~3,7432,692–5,632
Darke County~3,5812,761–4,609
Crawford County~3,2892,321–4,319
Knox County~3,1361,479–6,052
Hancock County~3,0272,212–4,887
Shelby County~2,8312,150–3,692
Stark County~2,8022,247–3,496
Ashtabula County~2,6641,678–4,031
Portage County~2,3741,722–3,257
Pickaway County~2,2471,477–3,287
Sandusky County~2,0181,427–3,763
Hocking County~1,832921–3,377
Paulding County~1,7721,314–2,915
Seneca County~1,7631,331–2,612
Lucas County~1,7441,366–2,296
Miami County~1,4641,188–1,817
Clinton County~1,3441,105–1,634
Auglaize County~1,322950–2,080
Ross County~1,229620–2,289
Clark County~1,2141,011–1,480
Columbiana County~1,146929–1,420
Greene County~1,037873–1,230
Coshocton County~932638–1,372
Fayette County~834622–1,252
Tuscarawas County~793645–979
Union County~787424–2,251
Warren County~629530–747
Preble County~629529–751
Perry County~561293–1,008
Lake County~551315–947
Hardin County~529340–1,266
Champaign County~524424–709
Carroll County~493400–615
Franklin County~478279–769
Logan County~464323–866
Vinton County~446244–788
Delaware County~443255–675
Huron County~427243–626
Wyandot County~411278–892
Madison County~376218–1,065
Muskingum County~375246–572
Ottawa County~368264–663
Putnam County~35769–2,128
Lorain County~294146–520
Van Wert County~27667–1,545
Mahoning County~228176–299
Montgomery County~199168–238
Clermont County~191150–242
Morgan County~137106–176
Jackson County~12975–220
Pike County~12765–235
Guernsey County~10179–130

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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