privatewelldata / States / California / Nitrate
Nitrate in well water in California
10% of 16,344 sampled wells in California had nitrate above the EPA limit of 10 mg/L (as nitrogen) (latest result per well, 53 counties, samples 1908–2025). The highest county share was Fresno County: 31% of 970 wells. Data updated 2026-10-18.
of sampled wells above 10 mg/L (as nitrogen)
wells with a result
counties with data
counties where 1 in 4+ wells were above
Nitrate in wells most often comes from fertilizer, manure and septic systems, so shallow wells near fields or septic fields are more exposed, and levels can change with the seasons. Only a lab test shows it. Federal limits apply to public water systems; a private well is the owner's responsibility to test (EPA).
% of sampled wells above the EPA limit (latest result per well; counties with 10+ wells). Source: USGS/EPA Water Quality Portal, 2026-10-18 · privatewelldata.com
Nitrate by county in California
| County | Above 10 mg/L (as nitrogen) | Median | 90th pct. | Years |
|---|---|---|---|---|
| Fresno County | 31% of 970 | 2.196 mg/L as N | 63.025 | 1951–2023 |
| San Francisco County | 30% of 46 | 8.5 mg/L as N | 16 | 1987–2022 |
| Tulare County | 19% of 353 | 2.1 mg/L as N | 15 | 1949–2023 |
| San Luis Obispo County | 16% of 140 | 0.865 mg/L as N | 16.5 | 1927–2023 |
| San Benito County | 16% of 37 | 0.712 mg/L as N | 12.605 | 2008–2022 |
| Monterey County | 16% of 293 | 2.2 mg/L as N | 13 | 1971–2023 |
| Kings County | 15% of 228 | 0.31 mg/L as N | 18 | 1946–2021 |
| Riverside County | 14% of 1,370 | 1.64 mg/L as N | 13.531 | 1910–2025 |
| Merced County | 14% of 497 | 1.694 mg/L as N | 13.102 | 1925–2023 |
| Stanislaus County | 13% of 411 | 3.614 mg/L as N | 12.7 | 1947–2021 |
| Alameda County | 12% of 267 | 2.5 mg/L as N | 11 | 1954–2022 |
| Contra Costa County | 11% of 70 | 2.905 mg/L as N | 10.843 | 1957–1983 |
| Santa Barbara County | 11% of 1,260 | 0.904 mg/L as N | 11 | 1927–2023 |
| San Bernardino County | 10% of 2,260 | 1.422 mg/L as N | 10 | 1916–2023 |
| Amador County | 10% of 21 | 0.073 mg/L as N | 2.688 | 2008–2022 |
| Santa Cruz County | 9% of 178 | 0.055 mg/L as N | 8 | 1971–2023 |
| San Joaquin County | 8% of 520 | 1.804 mg/L as N | 9.3 | 1943–2020 |
| Solano County | 8% of 112 | 1.7 mg/L as N | 8.8 | 1958–2022 |
| San Diego County | 8% of 843 | 0.678 mg/L as N | 8.132 | 1939–2024 |
| Butte County | 7% of 202 | 1.31 mg/L as N | 8.742 | 1952–2021 |
| Ventura County | 7% of 292 | 0.124 mg/L as N | 5.81 | 1952–2022 |
| Kern County | 6% of 1,349 | 0.836 mg/L as N | 7.23 | 1917–2023 |
| Madera County | 6% of 245 | 1.99 mg/L as N | 7.68 | 1955–2023 |
| Santa Clara County | 5% of 113 | 2.779 mg/L as N | 6.65 | 1974–2022 |
| Inyo County | 5% of 143 | 0.25 mg/L as N | 2.3 | 1910–2024 |
| Lassen County | 5% of 63 | 0.429 mg/L as N | 3.55 | 1961–2020 |
| Yolo County | 3% of 231 | 1.8 mg/L as N | 6.6 | 1955–2021 |
| Orange County | 3% of 486 | 0.17 mg/L as N | 4.07 | 1937–2020 |
| Sutter County | 3% of 158 | 0.648 mg/L as N | 5.27 | 1957–2022 |
| Imperial County | 3% of 224 | 0.353 mg/L as N | 2.598 | 1917–2023 |
| Humboldt County | 3% of 38 | 0.04 mg/L as N | 2.507 | 1954–2020 |
| San Mateo County | 2% of 45 | 1.64 mg/L as N | 6.58 | 1981–2022 |
| Placer County | 2% of 144 | 1.105 mg/L as N | 3.5 | 1957–2022 |
| Los Angeles County | 2% of 870 | 0.452 mg/L as N | 4.495 | 1908–2023 |
| Mono County | 2% of 52 | 0.087 mg/L as N | 0.77 | 1973–2022 |
| Napa County | 2% of 53 | 0 mg/L as N | 1.534 | 2001–2022 |
| Colusa County | 2% of 164 | 1.3 mg/L as N | 4.744 | 1956–2021 |
| Yuba County | 2% of 111 | 0.61 mg/L as N | 3.2 | 1958–2021 |
| El Dorado County | 1% of 145 | 0.045 mg/L as N | 1.48 | 1986–2022 |
| Glenn County | 1% of 200 | 1.627 mg/L as N | 5 | 1957–2021 |
| Tehama County | 1% of 222 | 1.317 mg/L as N | 4.4 | 1957–2022 |
| Mendocino County | 1% of 128 | 0.145 mg/L as N | 2 | 1960–2025 |
| Sonoma County | 1% of 133 | 0.1 mg/L as N | 2.395 | 1971–2022 |
| Sacramento County | 0% of 270 | 0.923 mg/L as N | 3.39 | 1955–2022 |
| Shasta County | 0% of 151 | 0.59 mg/L as N | 2.349 | 1959–2022 |
| Nevada County | 0% of 64 | 0.13 mg/L as N | 1.16 | 1961–2022 |
| Mariposa County | 0% of 49 | 0.096 mg/L as N | 0.43 | 1991–2008 |
| Siskiyou County | 0% of 45 | 0.23 mg/L as N | 2.18 | 1958–2020 |
| Lake County | 0% of 22 | 0.08 mg/L as N | 0.61 | 1970–2025 |
| Modoc County | 0% of 19 | 0.23 mg/L as N | 2 | 1959–2020 |
| Calaveras County | 0% of 15 | 1.55 mg/L as N | 3.13 | 2008–2021 |
| Trinity County | 0% of 12 | 0.081 mg/L as N | 0.19 | 2010–2015 |
| Plumas County | 0% of 10 | 0.277 mg/L as N | 0.592 | 2008–2020 |
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Download this table (CSV) · public-domain source data: Water Quality Portal (USGS, EPA, NWQMC). How we calculate these numbers.
How California compares
Share of sampled wells above the EPA nitrate limit in each state we cover (states with 100+ wells).
| # | State | Above 10 mg/L (as nitrogen) |
|---|---|---|
| 1 | California | 10% of 16,344 |
| 2 | Pennsylvania | 8% of 6,478 |
| 3 | Minnesota | 6% of 7,259 |
| 4 | North Carolina | 5% of 1,964 |
| 5 | New York | 5% of 6,236 |
| 6 | Massachusetts | 4% of 2,556 |
| 7 | New Jersey | 4% of 3,915 |
| 8 | Wisconsin | 4% of 3,666 |
| 9 | Virginia | 3% of 1,394 |
| 10 | Maine | 2% of 726 |
| 11 | Michigan | 2% of 1,970 |
| 12 | Ohio | 1% of 1,239 |
| 13 | New Hampshire | 0% of 1,685 |
Where to test: official lab list for California
About this data
- Each well counts once, using its latest nitrate result in the Water Quality Portal (siteType = Well), downloaded 2026-10-18.
- Results reported as "not detected" count as below the limit. Results far outside the plausible range are withheld for review, not shown.
- Sampled wells include monitoring and research wells, not only household wells, and are not a random sample, so the share is a guide to local risk, not a rate for all homes.
- The limit is the EPA maximum contaminant level for public water systems (EPA).
Questions
Is nitrate common in California well water?
In our data, 1,561 of 16,344 sampled wells in California (10%) had a latest nitrate result above the EPA limit of 10 mg/L (as nitrogen), across 53 counties. That ranks California 1 of 13 states we cover for this share.
Which California counties have the most wells above the nitrate limit?
Among counties with at least 10 sampled wells: Fresno County (31%, 300 of 970 wells); San Francisco County (30%, 14 of 46 wells); Tulare County (19%, 68 of 353 wells).
Does this tell me whether my well is safe?
No. These are past results from wells sampled by USGS, EPA and state programs, including monitoring wells, and levels differ from well to well. The only way to know your water is to test your own well at a certified lab.
How do I test my well for nitrate in California?
Order a nitrate test from a lab on California's official certified-lab list (linked on this page) and ask for its bottles and sampling instructions. Our test helper suggests what else to test for at the same time.
What should I do if my nitrate result is above 10 mg/L (as nitrogen)?
Retest to confirm, use another water source for drinking and cooking while you sort it out, and look at certified treatment options. Our guide walks through each step with sources.