Buying a California home with a private well
A California home on a private well comes with water the state does not regulate: the owner tests it and the owner fixes it. Before you buy, read the public records: the well's completion report, what tested wells in the county show for nitrate and arsenic, whether wells there have been reported dry, and which groundwater basin the property sits in. Every county's figures are below.
Key findings
Households reported 6,190 dry or failing wells to the Department of Water Resources; Tulare County alone reported 1,874, peaking in 2016.
Voluntary reports: DWR says counts are under-representative and no data for a county does not mean no shortages.
Data: reports through 2026-11-03. Link to this finding
In Monterey County, 44.2% of the 626 domestic wells tested since 2016 had nitrate above the 10 mg/L drinking-water limit, the highest share of any county with at least 30 tested domestic wells.
GAMA untreated ambient samples, latest result per well; tested wells are not a random sample and a county share says nothing about a particular well.
Data: 2016-11 to 2026-11. Link to this finding
Source: State Water Resources Control Board, GAMA Groundwater Information System. Computed from our dataset “Well water nitrate and arsenic by county (GAMA)” (version 2026-11-04); download the CSV. Free to reuse under CC BY 4.0 with a link to this page. Findings update when the data does.
Public records to check before you buy
- The well completion report. Drillers file one with the Department of Water Resources for every well (depth, construction, yield). Ask the seller for it or search DWR's database; see well depth in California for typical depths by county.
- The county well permit file. County environmental health departments issue well permits; the file can show the well's history and any repairs.
- A water test of this well. Private wells are not regulated by the state's Division of Drinking Water, so a test is the only answer for the property. The county figures below show what to test for; see what the State Water Board recommends testing.
- Dry-well reports in the county (table below) and the groundwater basin and its SGMA priority, which decides the local groundwater agency and plan.
- Septic, if the home is not on sewer: see septic suitability by county and perc tests and septic system cost.
Testing the well: what the State Water Board recommends
“The State of California does not regulate water quality in most private domestic wells, Counties or local agencies may have their own requirements.” The State Water Board's Guide for Private Domestic Well Owners (June 2025) recommends:
- “At a minimum, it is recommended that private domestic well owners test for total coliform bacteria, nitrate, and electrical conductivity annually.”
- “It is recommended that private well owners test their well once a year to ensure their water is safe to consume.”
- Cost: “Basic sample analysis can range from $100-$400.” Accredited labs are listed in the state's ELAP laboratory search.
A buyer can ask for a test before closing; arsenic and other chemicals common in the county (table below) are worth adding to the minimum list.
How much water the well gives: county yield and water-test rules
Counties set their own rules for a well that serves a house, mostly when a building permit is issued: a pump test showing a minimum sustained yield, and in San Diego a potability test. The minimums run from 1 gallon per minute per dwelling (Riverside, Sonoma, Humboldt) to 3 (Nevada; Santa Cruz from November through July), with exceptions: Humboldt accepts 0.5 gallon per minute with at least 5,000 gallons of storage, Santa Cruz asks 2 in August through October, and Napa requires a storage tank below 5 gallons per minute (1 at the least). In their own words:
- Humboldt County: “the minimum required water supply per residence from the source shall be 1.0 gallons per minute (gpm) per dwelling unit. This quantity may be reduced to a minimum of 0.5 gpm per dwelling unit if a minimum of 5,000 gallons of domestic water storage is provided … The dry season testing period is August 1 through September 30.” (county page)
- Monterey County: “All wells that are proposed to supply water for domestic use or to be connected to a water distribution system shall first undergo a continuous source-capacity (pumping) test to determine the yield of the well.” (county page)
- Napa County: “Existing supply-provide a yield test report (conducted within the past 5 years) by a licensed well driller, pump installer, geologist, engineer or Registered Environmental Health Specialist (REHS). … Yield result less than 5 gallons per minute (gpm) but at least 1 gpm requires installation of a domestic storage tank.” (county page)
- Nevada County: “A well yield test is a four-hour pump test to determine if the well can produce at least 3 gallons/minute sustained production rate.” (county page)
- Placer County: “The four hour well yield test is used to determine the production or yield of the well in gallons per minute. … When a building permit involves plumbing, it is necessary to demonstrate that the water supply meets potable requirements.” (county page)
- Riverside County: “minimum two hour period until three or more wetted bore volumes of water have been discharged from the well. … Water production shall be kept at a constant rate of no less than 1 gallon per minute per residence or unit.” (county page)
- San Diego County: “The County of San Diego requires that all building permit applicants demonstrate that their private water well supply is potable prior to occupancy or change of use. … At a minimum, testing for bacteria and nitrates is required.” (county page)
- Santa Cruz County: “November through July: for each connection a sustained yield of 3 gallons per minute for 24 hours of continuous pumping or, until 4,320 gallons have been achieved … August through October: for each connection a sustained yield of 2 gallons per minute for 24 hours.” (county page)
- Sonoma County: “The testing must demonstrate a sustained yield of one gallon per minute per dwelling from a well, spring, or multiple wells or springs combined.” (county page)
These rules apply to building permits and new water systems. We found no county rule that requires a well test when a home is sold (San Luis Obispo and Kern pages could not be opened; El Dorado, Marin, Mendocino and Tulare were only partly confirmed), so in the counties we checked, a yield and water test at purchase is something the buyer arranges or negotiates in the contract. Sources accessed November 4, 2026.
Nitrate and arsenic in tested wells, by county
Share of wells tested since November 2016 whose latest result was above the drinking-water limit: nitrate 10 mg/L as nitrogen, arsenic 10 µg/L. “Domestic” wells are household wells as GAMA classifies them; “public supply” wells serve water systems. A share appears only where at least 30 wells were tested for that chemical; “few tested” shows the count where fewer were.
| County | Domestic: nitrate over limit | Domestic: arsenic over limit | Public supply: nitrate | Public supply: arsenic |
|---|---|---|---|---|
| Alameda | — | — | 0% of 76 | 1.5% of 65 |
| Alpine | — | — | 0% of 46 | few tested (13) |
| Amador | few tested (20) | few tested (16) | 0% of 86 | few tested (27) |
| Butte | 0% of 122 | 0% of 56 | 0.4% of 228 | 1.6% of 191 |
| Calaveras | few tested (11) | few tested (10) | 0% of 72 | few tested (29) |
| Colusa | 11.8% of 68 | — | 1.5% of 67 | 4.3% of 46 |
| Contra Costa | few tested (14) | — | 2.9% of 140 | 13.9% of 79 |
| Del Norte | — | — | 0% of 36 | few tested (18) |
| El Dorado | 9.3% of 43 | 6.1% of 33 | 1.5% of 204 | 9.2% of 76 |
| Fresno | 29.6% of 1,468 | few tested (26) | 3.5% of 980 | 4.1% of 732 |
| Glenn | 0.8% of 131 | few tested (<5) | 2.6% of 76 | 1.7% of 59 |
| Humboldt | few tested (6) | few tested (6) | 0% of 124 | 1.2% of 81 |
| Imperial | few tested (8) | few tested (<5) | 0% of 60 | 3.1% of 32 |
| Inyo | — | — | 0% of 191 | 5.7% of 123 |
| Kern | 19.3% of 233 | 15.6% of 64 | 4.8% of 932 | 13.7% of 820 |
| Kings | 7.1% of 170 | few tested (8) | 0% of 103 | 42.3% of 78 |
| Lake | few tested (20) | — | 0.7% of 135 | 2.3% of 87 |
| Lassen | few tested (<5) | — | 0% of 99 | 8.2% of 61 |
| Los Angeles | few tested (5) | few tested (5) | 3.4% of 1,249 | 4.6% of 1,177 |
| Madera | 8.5% of 284 | few tested (14) | 1.3% of 450 | 14.4% of 333 |
| Marin | few tested (<5) | few tested (<5) | 1.2% of 85 | 3.7% of 54 |
| Mariposa | few tested (<5) | — | 0.7% of 140 | 8.9% of 79 |
| Mendocino | few tested (6) | few tested (6) | 0.4% of 263 | 2% of 149 |
| Merced | 26% of 746 | 17.1% of 70 | 6.6% of 272 | 11.7% of 206 |
| Modoc | few tested (20) | — | 0% of 38 | few tested (21) |
| Mono | — | — | 0% of 154 | 32.8% of 64 |
| Monterey | 44.2% of 626 | 6% of 134 | 15.4% of 826 | 5.4% of 744 |
| Napa | few tested (10) | few tested (8) | 0% of 301 | 15.2% of 178 |
| Nevada | 0% of 40 | few tested (20) | 0% of 119 | 16.1% of 56 |
| Orange | — | — | 1.1% of 270 | 1.1% of 265 |
| Placer | 5.9% of 85 | 5.3% of 38 | 0% of 195 | 1.6% of 124 |
| Plumas | few tested (<5) | — | 0% of 241 | 2.4% of 83 |
| Riverside | 1.2% of 80 | 28.2% of 78 | 6.3% of 869 | 6% of 739 |
| Sacramento | 4.2% of 167 | 6.2% of 64 | 0.3% of 605 | 9% of 500 |
| San Benito | 23.6% of 199 | 8.2% of 61 | 9.8% of 183 | 8.5% of 153 |
| San Bernardino | 8.3% of 108 | 29.2% of 113 | 6.3% of 1,354 | 7.3% of 1,150 |
| San Diego | few tested (<5) | few tested (<5) | 7.5% of 468 | 0.6% of 323 |
| San Francisco | — | — | few tested (8) | few tested (8) |
| San Joaquin | 22.3% of 815 | 11% of 73 | 4.6% of 612 | 14.4% of 439 |
| San Luis Obispo | 10.6% of 615 | 17% of 464 | 4.9% of 468 | 5.9% of 391 |
| San Mateo | — | — | 5.1% of 98 | 3.6% of 84 |
| Santa Barbara | 28.6% of 196 | 0% of 33 | 4.2% of 403 | 5.8% of 312 |
| Santa Clara | 28% of 118 | few tested (20) | 1.9% of 413 | 0% of 368 |
| Santa Cruz | 9.5% of 378 | 4.5% of 247 | 1.8% of 274 | 2.1% of 242 |
| Shasta | 0% of 83 | 0% of 36 | 0.4% of 280 | 0.6% of 166 |
| Sierra | few tested (<5) | — | 0% of 42 | few tested (13) |
| Siskiyou | — | — | 0% of 116 | 5.6% of 72 |
| Solano | few tested (10) | few tested (<5) | 7.9% of 126 | 7% of 100 |
| Sonoma | few tested (19) | few tested (19) | 0.3% of 773 | 8.5% of 505 |
| Stanislaus | 37.2% of 596 | 4.8% of 84 | 7.7% of 482 | 6.3% of 367 |
| Sutter | 4.2% of 72 | 33.3% of 30 | 4.7% of 64 | 51.1% of 45 |
| Tehama | 0.7% of 138 | 0% of 64 | 0% of 193 | 1.3% of 150 |
| Trinity | — | — | 0% of 51 | few tested (26) |
| Tulare | 36.5% of 490 | 0% of 31 | 13.9% of 793 | 5.3% of 533 |
| Tuolumne | few tested (<5) | — | 0.5% of 222 | 2.1% of 145 |
| Ventura | — | — | 5.5% of 237 | 0.9% of 223 |
| Yolo | 5.3% of 38 | — | 9.7% of 176 | 4.5% of 111 |
| Yuba | 6.1% of 49 | few tested (16) | 0% of 138 | 2.1% of 95 |
How to read this
These are untreated ambient samples from wells that programs and owners chose to test, not a random sample, and the latest result per well. A county share says nothing about a particular well. Limits from the State Water Board's nitrate and arsenic pages. Download: well-water-nitrate-arsenic-by-county.csv.
Dry-well reports by county
Households reported 6,190 dry or failing wells to the Department of Water Resources. DWR's own caution: “Even where data is collected, it is entirely voluntary”, the counts “are undoubtedly under-representative of all shortages”, and “missing information or no data for a given county does not necessarily mean that there are no household water shortages in the county.”
| County | Reports, all years | 2022–2026 | Peak year |
|---|---|---|---|
| Tulare | 1,874 | 344 | 2016 (1,263) |
| Madera | 949 | 453 | 2022 (216) |
| Fresno | 821 | 610 | 2022 (433) |
| San Luis Obispo | 440 | 150 | 2015 (55) |
| Tehama | 410 | 221 | 2021 (139) |
| Mariposa | 209 | 58 | 2015 (110) |
| Merced | 204 | 101 | 2022 (63) |
| Kings | 164 | 61 | 2014 (50) |
| Stanislaus | 159 | 71 | 2022 (50) |
| San Joaquin | 133 | 81 | 2022 (60) |
| Butte | 125 | 61 | 2022 (46) |
| Glenn | 115 | 15 | 2021 (71) |
| Tuolumne | 93 | 6 | 2014 (80) |
| Kern | 80 | 52 | 2022 (24) |
| Siskiyou | 66 | 45 | 2022 (26) |
| Shasta | 62 | 53 | 2022 (47) |
| Mendocino | 39 | 6 | 2021 (31) |
| Lake | 30 | <5 | 2021 (29) |
| Nevada | 20 | 6 | 2021 (9) |
| San Bernardino | 20 | <5 | 2018 (6) |
| Trinity | 19 | <5 | 2014 (10) |
| Yolo | 19 | 7 | 2015 (6) |
| Napa | 17 | <5 | 2021 (12) |
| Colusa | 14 | <5 | 2014 (6) |
| Los Angeles | 14 | <5 | 2015 (11) |
| Sacramento | 13 | 10 | 2022 (10) |
| Riverside | 12 | 8 | 2022 (<5) |
| Santa Clara | 10 | 8 | 2022 (7) |
| El Dorado | 8 | <5 | 2022 (<5) |
| Modoc | 8 | <5 | 2014 (<5) |
| San Diego | 8 | <5 | 2015 (<5) |
| Sonoma | 8 | 5 | 2022 (5) |
| Lassen | 7 | <5 | 2014 (<5) |
| Placer | 7 | <5 | 2022 (<5) |
| Solano | 7 | <5 | 2021 (<5) |
| Contra Costa | 6 | 0 | 2015 (6) |
| Alameda | <5 | <5 | 2025 (<5) |
| Amador | <5 | <5 | 2021 (<5) |
| Calaveras | <5 | <5 | 2026 (<5) |
| Del Norte | <5 | 0 | 2015 (<5) |
| Humboldt | <5 | <5 | 2022 (<5) |
| Inyo | <5 | <5 | 2022 (<5) |
| Mono | <5 | <5 | 2023 (<5) |
| Monterey | <5 | <5 | 2025 (<5) |
| Plumas | <5 | <5 | 2022 (<5) |
| San Mateo | <5 | 0 | 2014 (<5) |
| Santa Barbara | <5 | <5 | 2023 (<5) |
| Santa Cruz | <5 | <5 | 2026 (<5) |
| Sutter | <5 | <5 | 2021 (<5) |
| Yuba | <5 | 0 | 2021 (<5) |
Statewide reports by year: 2013: 18, 2014: 334, 2015: 526, 2016: 1,395, 2017: 215, 2018: 50, 2019: 39, 2020: 76, 2021: 922, 2022: 1,489, 2023: 452, 2024: 170, 2025: 127, 2026: 113 (2026 partial; years with fewer than 5 reports in a county are not in these sums). A household with a dry well can file a report through DWR's dry-well reporting form. Counties not listed have no reports. Download: dry-well-reports-by-county.csv.
Frequently Asked Questions
Is private well water regulated in California?
Not like a public water system. The State Water Board's Division of Drinking Water says: "Private wells are not subject to drinking water regulation by DDW … The MCLs can be used for guidance." So the owner (and a buyer) is responsible for testing a private well; the drinking-water limits are the yardstick.
How often should a private well be tested in California, and for what?
The State Water Board's guide for private well owners says: "It is recommended that private well owners test their well once a year to ensure their water is safe to consume" and "At a minimum, it is recommended that private domestic well owners test for total coliform bacteria, nitrate, and electrical conductivity annually." Add the chemicals that are common in the county (the nitrate and arsenic table on this page shows where).
Is a well test required when you sell a house in California?
Not by the state, and we found no county that requires one at sale. Of the 15 counties checked, Santa Cruz, Sonoma, Nevada, Placer, Napa, Humboldt, Monterey, Riverside and San Diego set well yield or water-test rules, but for building permits and new water systems, not for a sale. A buyer can still ask for a yield (pump) test and a water test before closing.
How much does a well water test cost in California?
The State Water Board's guide says "Basic sample analysis can range from $100-$400." It also notes that it "provides a list of certified laboratories. However, it is not always required to use a certified laboratory." The list is the state's ELAP search of accredited labs.
What are the drinking-water limits for nitrate and arsenic in California?
Nitrate: 10 mg/L as nitrogen (22 CCR §64431, as the State Water Board's nitrate page states). Arsenic: 0.010 mg/L, the same as 10 micrograms per liter, effective November 28, 2008. These are the limits for public water systems and the guidance for private wells.
How do I find the records for a well on a property?
California drillers file a Well Completion Report with the Department of Water Resources for every well, recording its depth, construction and yield; DWR publishes them in a searchable online database. Ask the seller for the report, check the county's well-permit file, or search DWR's map by location.
What if a well goes dry?
Households can report a dry or failing well to the Department of Water Resources through its dry-well reporting system. Reports are voluntary, so the county counts on this page show where shortages were reported, not every dry well.
Do the county water-quality figures tell me about a specific well?
No. They are untreated ambient samples from wells that programs and owners chose to test, not a random sample, and a county share says nothing about a particular well. Only a test of that well does.
Sources: State Water Resources Control Board, GAMA Groundwater Information System (data.ca.gov: Ground Water - Water Quality Results), accessed 2026-11-03. California Department of Water Resources, Dry Well Reporting System (Household Water Supply Shortage Reporting System data), accessed 2026-11-03. Figures are derived from the public data; no well IDs, locations or individual results are republished.