Highest-Risk Counties for Well Water Arsenic
These are the counties where the USGS model estimates the largest number of people rely on private wells with arsenic above the 10 µg/L EPA limit.
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.
| # | County | Est. well residents over arsenic limit | 90% range |
|---|---|---|---|
| 1 | Wayne County, OH | ~22,522 | 13,370–29,829 |
| 2 | Rockingham County, NH | ~21,388 | 16,663–27,013 |
| 3 | Mecklenburg County, NC | ~19,183 | 11,310–31,626 |
| 4 | Penobscot County, ME | ~18,708 | 15,590–22,028 |
| 5 | Los Angeles County, CA | ~17,244 | 12,915–22,850 |
| 6 | Hillsborough County, NH | ~17,240 | 12,990–22,650 |
| 7 | Kennebec County, ME | ~15,277 | 12,241–18,700 |
| 8 | Canyon County, ID | ~14,043 | 11,996–16,346 |
| 9 | Worcester County, MA | ~12,238 | 9,139–16,208 |
| 10 | Tolland County, CT | ~12,109 | 8,740–16,678 |
| 11 | Lubbock County, TX | ~11,642 | 10,023–13,432 |
| 12 | Wake County, NC | ~11,448 | 8,251–15,632 |
| 13 | Fairfield County, CT | ~11,418 | 7,738–16,791 |
| 14 | Tuscola County, MI | ~11,252 | 9,269–13,487 |
| 15 | York County, ME | ~10,409 | 7,720–13,730 |
| 16 | Licking County, OH | ~10,054 | 4,806–18,655 |
| 17 | Medina County, OH | ~9,736 | 5,251–16,134 |
| 18 | Androscoggin County, ME | ~9,698 | 7,590–12,313 |
| 19 | Aroostook County, ME | ~9,500 | 7,680–11,432 |
| 20 | San Joaquin County, CA | ~9,497 | 8,029–11,222 |
| 21 | Summit County, OH | ~9,197 | 5,310–15,038 |
| 22 | Fairfield County, OH | ~8,809 | 4,363–15,913 |
| 23 | Will County, IL | ~8,733 | 7,092–10,811 |
| 24 | Kosciusko County, IN | ~8,677 | 6,307–11,702 |
| 25 | Valencia County, NM | ~8,431 | 6,801–10,256 |
| 26 | Trumbull County, OH | ~8,323 | 4,295–14,666 |
| 27 | Clinton County, MI | ~8,229 | 6,935–9,738 |
| 28 | Guilford County, NC | ~8,187 | 5,061–13,358 |
| 29 | Fresno County, CA | ~8,027 | 6,705–9,627 |
| 30 | Twin Falls County, ID | ~7,858 | 6,339–9,411 |
| 31 | Fulton County, OH | ~7,840 | 6,249–9,380 |
| 32 | Rowan County, NC | ~7,764 | 4,633–12,434 |
| 33 | Livingston County, MI | ~7,681 | 6,123–9,693 |
| 34 | Allen County, IN | ~7,595 | 5,986–9,573 |
| 35 | Oakland County, MI | ~7,540 | 6,002–9,534 |
| 36 | Merced County, CA | ~7,499 | 6,248–8,969 |
| 37 | Midland County, TX | ~7,306 | 6,380–8,317 |
| 38 | Somerset County, ME | ~7,247 | 5,876–8,786 |
| 39 | Merrimack County, NH | ~7,219 | 5,172–9,917 |
| 40 | Ashland County, OH | ~7,053 | 3,813–10,727 |
| 41 | Genesee County, MI | ~6,942 | 5,661–8,501 |
| 42 | Yakima County, WA | ~6,940 | 5,534–8,761 |
| 43 | Madison County, IN | ~6,936 | 5,517–8,622 |
| 44 | Suffolk County, NY | ~6,834 | 4,337–10,721 |
| 45 | Geauga County, OH | ~6,792 | 3,548–12,472 |
| 46 | New Haven County, CT | ~6,752 | 4,609–9,786 |
| 47 | Wicomico County, MD | ~6,673 | 4,696–9,392 |
| 48 | York County, SC | ~6,446 | 3,755–10,693 |
| 49 | Harris County, TX | ~6,396 | 4,381–9,270 |
| 50 | San Diego County, CA | ~6,352 | 4,786–8,414 |
| 51 | Oxford County, ME | ~6,217 | 4,682–8,074 |
| 52 | Nye County, NV | ~6,192 | 4,997–7,471 |
| 53 | Middlesex County, MA | ~6,189 | 4,435–8,478 |
| 54 | Snohomish County, WA | ~6,178 | 4,661–8,142 |
| 55 | Lenawee County, MI | ~6,064 | 4,522–7,996 |
| 56 | Middlesex County, CT | ~6,056 | 4,092–8,780 |
| 57 | DeKalb County, IN | ~5,996 | 4,541–7,536 |
| 58 | Cumberland County, ME | ~5,972 | 4,526–7,827 |
| 59 | Crawford County, PA | ~5,954 | 3,022–11,042 |
| 60 | Sanilac County, MI | ~5,945 | 4,889–7,143 |
| 61 | Boone County, IN | ~5,896 | 4,407–7,572 |
| 62 | Erie County, PA | ~5,863 | 3,835–8,777 |
| 63 | Windham County, CT | ~5,826 | 4,239–7,888 |
| 64 | Sandoval County, NM | ~5,784 | 4,362–7,372 |
| 65 | Clark County, NV | ~5,783 | 4,832–6,888 |
| 66 | Gratiot County, MI | ~5,767 | 4,736–6,962 |
| 67 | Tippecanoe County, IN | ~5,755 | 4,305–7,520 |
| 68 | Morrow County, OH | ~5,738 | 2,953–9,887 |
| 69 | Midland County, MI | ~5,663 | 4,666–6,846 |
| 70 | Jasper County, IN | ~5,552 | 4,328–7,033 |
| 71 | Lapeer County, MI | ~5,544 | 4,626–6,637 |
| 72 | Ector County, TX | ~5,449 | 4,695–6,317 |
| 73 | Kent County, MI | ~5,442 | 4,440–6,746 |
| 74 | Defiance County, OH | ~5,434 | 4,354–6,433 |
| 75 | Monroe County, MI | ~5,370 | 4,071–7,667 |
| 76 | St. Joseph County, IN | ~5,356 | 4,096–7,061 |
| 77 | Tulare County, CA | ~5,344 | 4,469–6,420 |
| 78 | Gloucester County, NJ | ~5,321 | 3,498–7,840 |
| 79 | Washtenaw County, MI | ~5,310 | 3,937–7,157 |
| 80 | Benton County, WA | ~5,218 | 4,010–6,329 |
| 81 | Whitley County, IN | ~5,182 | 3,954–6,583 |
| 82 | Noble County, IN | ~5,162 | 3,721–6,965 |
| 83 | Eaton County, MI | ~5,128 | 4,365–6,075 |
| 84 | Waldo County, ME | ~5,118 | 3,992–6,436 |
| 85 | Richland County, OH | ~5,081 | 2,468–9,211 |
| 86 | Hancock County, IN | ~5,017 | 3,996–6,279 |
| 87 | Missoula County, MT | ~4,888 | 3,734–6,208 |
| 88 | Churchill County, NV | ~4,887 | 4,246–5,468 |
| 89 | Burlington County, NJ | ~4,874 | 3,195–7,254 |
| 90 | Sussex County, DE | ~4,839 | 3,298–7,053 |
| 91 | Atlantic County, NJ | ~4,783 | 3,138–7,207 |
| 92 | Hendricks County, IN | ~4,777 | 3,696–6,128 |
| 93 | Clark County, WA | ~4,768 | 3,493–6,387 |
| 94 | Durham County, NC | ~4,608 | 2,621–8,112 |
| 95 | Holmes County, OH | ~4,523 | 2,495–6,999 |
| 96 | Williams County, OH | ~4,417 | 3,468–5,424 |
| 97 | Cumberland County, NJ | ~4,410 | 2,956–6,568 |
| 98 | Saginaw County, MI | ~4,397 | 3,683–5,148 |
| 99 | Shiawassee County, MI | ~4,393 | 3,657–5,254 |
| 100 | Kankakee County, IL | ~4,374 | 3,361–5,596 |
Source: USGS domestic-well arsenic model (Lombard et al., 2021) and the associated county population product (2017). Methodology.