Well water by county: California
Across 53 counties in California, 1160 of 8314 sampled wells (14%) were above the arsenic limit and 1561 of 16344 (10%) above the nitrate limit. Updated 2026-10-17. Source: Water Quality Portal (USGS, EPA, NWQMC), siteType=Well.
Share of sampled wells whose latest result is above the EPA limit (arsenic 10 µg/L, nitrate 10 mg/L as N).
| County | Arsenic > 10 µg/L | Nitrate > 10 mg/L |
|---|---|---|
| Alameda County | 8% of 88 | 12% of 267 |
| Amador County | 5% of 21 | 10% of 21 |
| Butte County | 3% of 130 | 7% of 202 |
| Calaveras County | 7% of 15 | 0% of 15 |
| Colusa County | 8% of 136 | 2% of 164 |
| Contra Costa County | – | 11% of 70 |
| El Dorado County | 6% of 86 | 1% of 145 |
| Fresno County | 4% of 556 | 31% of 970 |
| Glenn County | 3% of 88 | 1% of 200 |
| Humboldt County | 5% of 22 | 3% of 38 |
| Imperial County | 20% of 81 | 3% of 224 |
| Inyo County | 17% of 87 | 5% of 143 |
| Kern County | 29% of 505 | 6% of 1,349 |
| Kings County | 36% of 149 | 15% of 228 |
| Lake County | 7% of 15 | 0% of 22 |
| Lassen County | 12% of 42 | 5% of 63 |
| Los Angeles County | 13% of 597 | 2% of 870 |
| Madera County | 8% of 118 | 6% of 245 |
| Mariposa County | 0% of 32 | 0% of 49 |
| Mendocino County | 13% of 71 | 1% of 128 |
| Merced County | 12% of 236 | 14% of 497 |
| Modoc County | 0% of 17 | 0% of 19 |
| Mono County | 57% of 63 | 2% of 52 |
| Monterey County | 6% of 290 | 16% of 293 |
| Napa County | 25% of 55 | 2% of 53 |
| Nevada County | 5% of 62 | 0% of 64 |
| Orange County | 8% of 66 | 3% of 486 |
| Placer County | 5% of 110 | 2% of 144 |
| Plumas County | 0% of 10 | 0% of 10 |
| Riverside County | 9% of 546 | 14% of 1,370 |
| Sacramento County | 19% of 234 | 0% of 270 |
| San Benito County | 0% of 30 | 16% of 37 |
| San Bernardino County | 25% of 1,441 | 10% of 2,260 |
| San Diego County | 8% of 279 | 8% of 843 |
| San Francisco County | – | 30% of 46 |
| San Joaquin County | 19% of 302 | 8% of 520 |
| San Luis Obispo County | 2% of 59 | 16% of 140 |
| San Mateo County | 17% of 12 | 2% of 45 |
| Santa Barbara County | 7% of 226 | 11% of 1,260 |
| Santa Clara County | 0% of 82 | 5% of 113 |
| Santa Cruz County | 2% of 115 | 9% of 178 |
| Shasta County | 3% of 99 | 0% of 151 |
| Siskiyou County | 7% of 29 | 0% of 45 |
| Solano County | 4% of 48 | 8% of 112 |
| Sonoma County | 8% of 123 | 1% of 133 |
| Stanislaus County | 5% of 238 | 13% of 411 |
| Sutter County | 32% of 94 | 3% of 158 |
| Tehama County | 3% of 144 | 1% of 222 |
| Trinity County | 0% of 12 | 0% of 12 |
| Tulare County | 19% of 182 | 19% of 353 |
| Ventura County | 1% of 76 | 7% of 292 |
| Yolo County | 6% of 136 | 3% of 231 |
| Yuba County | 2% of 59 | 2% of 111 |