Well water testing in Merced County, California
What official data says about private wells in Merced County, what to test for, and the state-certified labs nearby.
About 21% of the county's 268,455 residents. Rank 8 of 58 in California.
95% range 6,248 to 8,969 private-well users (2010 base year). Rank 4 of 58 in California.
Low potential (average indoor radon may be less than 2 pCi/L). EPA says the map should not be used to decide whether an individual home needs testing.
Labs with an address in this county, from the official state lists on this site, matched by ZIP code.
What to test for in Merced County
Private wells are not regulated like public water: EPA advises owners to test their well every year for total coliform bacteria, nitrates, total dissolved solids and pH. Your state's own schedule:
- The State Water Board's well-owner guide recommends testing once a year. At a minimum: total coliform bacteria, nitrate and electrical conductivity every year. A minerals panel (arsenic, lead, manganese, iron and other metals) every 5 to 10 years, or sooner if conductivity, taste, color, odor or nearby land use changes. source
Arsenic. USGS estimates that about 7,499 people using private wells in Merced County had arsenic above 10 µg/L, EPA's limit for public water (2010 base year; 95% range 6,248 to 8,969). USGS describes this kind of estimate as useful "for broad screening of the population at risk", so it says nothing certain about any one well. Arsenic has no taste or smell: test for it by name.
Radon. EPA puts Merced County in Zone 3: low potential (average indoor radon may be less than 2 pCi/L). That map is about radon in indoor air, and EPA says it should not be used to decide whether an individual home needs testing. If radon is found in your air and you have a well, EPA suggests testing the water too: see radon in well water.
For anything specific to your home (farms nearby, old plumbing, a new baby, a smell or stain), use the which-tests checklist. When the results arrive, check your numbers against EPA's limits.
Show the numbers
| Item | Value | Note |
|---|---|---|
| Los Angeles County | 17,244 people (2010 base year) | |
| San Joaquin County | 9,497 people (2010 base year) | |
| Fresno County | 8,027 people (2010 base year) | |
| Merced County (this county) | 7,499 people (2010 base year) | |
| San Diego County | 6,352 people (2010 base year) | |
| Tulare County | 5,344 people (2010 base year) |
USGS model estimate for broad screening, not a measurement of any well. Only a test tells you about your own well. Source: USGS, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter (Ayotte, Medalie and Qi, 2017), accessed 2026-11-01.
PFAS in public water near Merced County (public supply, not your well)
This is public water data, not your well. A private well draws its own groundwater, which can have more or less PFAS than the public supply nearby. Only a test of your well tells you.
Under EPA’s fifth Unregulated Contaminant Monitoring Rule (UCMR 5), public water systems sampled their water for 29 PFAS between Jan 3, 2023 and Jun 17, 2026. 11 public systems that serve ZIP codes mainly in Merced County reported results. 5 of them had at least one sample with PFOA or PFOS at or above 4.0 ng/L, and 5 had at least one of the 29 PFAS measured at or above its reporting level.
| Public water system | System ID | Highest PFOA (ng/L) | Highest PFOS (ng/L) |
|---|---|---|---|
| City Of Atwater | CA2410001 | 33 | 45 |
| Planada Csd | CA2410007 | 14.7 | |
| City Of Merced | CA2410009 | 6.1 | 8.9 |
| Cal Am - Meadowbrook | CA2410008 | 8.3 | |
| Delhi Cwd | CA2410006 | 7.3 |
4.0 ng/L (parts per trillion) is both EPA’s UCMR 5 reporting level for PFOA and PFOS and the number in EPA’s 2024 limit for public water. That limit is judged on a running annual average, so one sample at that level is not a violation. EPA proposed changes to its PFAS rules in May 2026: see PFAS in well water for the current status and who should test a well. To test your own well, find a lab certified for PFAS.
Method: a system counts for this county when it reports serving a ZIP code whose largest land part is in this county, so a system serving several counties counts in each. Source: U.S. EPA, UCMR 5 occurrence data (by state), file dated Oct 15, 2026; downloaded Nov 1, 2026.
Testing programs and help in California
- Free well testing is offered in some areas through domestic well programs funded or regulated by the State Water Board or a Regional Water Board. The Board counts 11 such programs. Services and eligibility differ by program, and some also provide replacement drinking water. source
- The Board's Well Testing Program Directory (updated June 2026) lists well testing programs in Fresno, Kern, Kings, Madera, Mariposa, Merced, Monterey, San Benito, San Joaquin, San Luis Obispo, Santa Barbara, Santa Cruz, Stanislaus and Tulare counties, and in parts of Riverside (eastern Coachella Valley), Yolo (Yolo groundwater basin), San Mateo, Santa Clara and Ventura counties. Many cover only a groundwater basin or part of a county, so check the service area. source
- The state does not regulate water quality in most private domestic wells, but counties or local agencies may have their own rules. For example, some wells on farmland may have to be tested for nitrate. Local environmental health agencies may help you read your results. source
Buying a home with a well in Merced County
Loan test (FHA, VA, USDA) in Merced County, CA
- VA. Test results are valid for 90 days from the date the local health authority certifies them, unless the local authority says otherwise. VA’s chapter names no list of contaminants: the water must meet the local health authority’s requirements, or EPA guidelines where it has none. Your lender may ask for specific tests.
- FHA. A well test is required in the situations FHA’s handbook lists, among others, and the report may not be more than 180 days old at disbursement.
- USDA. Guaranteed loans need a water quality test for a private well, done by the local health authority or a state-certified lab, and no more than 180 days old at closing.
Lenders can ask for more (extra tests, a particular lab, who takes the sample): ask your lender before you book. Federal rules as read on Nov 3, 2026 source 1 source 2 source 3. Full comparison: VA, FHA and USDA well water test requirements. Loan test kits sold by mail, with price and days to results: kits compared.
State-certified labs in Merced County
Labs on the California list whose own websites say they take homeowner samples: Abalone Coast Analytical, Inc. (San Luis Obispo), Analytical Environmental Lab, LLC (San Diego), Antelope Valley-East Kern Water Agency (Palmdale), Brelje and Race Laboratories, Inc. (Santa Rosa), Caltest Analytical Laboratory (Napa), Clinical Laboratory of San Bernardino II (Lompoc), Clinical Laboratory of San Bernardino, Inc. (Grand Terrace), Coachella Valley Water District (Palm Desert) and 14 more. Use Check my ZIP to sort labs by distance.
See all California labs in the finder and filter by the test you need and by labs that take homeowner samples.
- USGS, Estimated use of water in the United States, county-level data for 2015 (Dieter et al., ver. 2.0, 2018)
- USGS, Estimated county level domestic well population with arsenic greater than 10 micrograms per liter (Ayotte, Medalie and Qi, 2017)
- U.S. EPA, Map of Radon Zones, county list (EPA 402/A-16/001)
- U.S. Census Bureau, 2020 ZCTA to county relationship file (lab ZIP codes to counties)
- California ELAP - Accredited Laboratories (map layer) + per-lab Fields of Accreditation (Map layer fetched 2026-11-15; scope PDF as of 6/8/2026)
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