Yolo County Well Water Testing
Private-well guidance for Yolo County, California: USGS area-risk estimates for arsenic and nitrate, a recommended test panel, and how to get your own well tested at a California-certified lab.
Yolo County groundwater risk (area estimates)
These are modeled USGS estimates for the county area — not a measurement of your specific well.
Arsenic
area estimate11%
modeled chance a well in this area exceeds 10 µg/L (the EPA limit).
16% chance of exceeding 5 µg/L.
Most-probable concentration category: <=5 ug/L.
Nitrate
area estimate1.36 mg/L
predicted nitrate (as N) for domestic-supply depth.
This estimate is below the 10 mg/L EPA limit, but individual wells can still exceed it.
Private wells
area estimate15,743
people on private wells in Yolo County.
Roughly 6,297 households, estimated from the USGS modeled domestic-supply population.
Area context
Additional state-only or optional layers for Yolo County, shown where the data exists.
Uranium
area estimateA state-only uranium model is not available for California. See observed samples and test your own well to learn its uranium level.
Radon
area estimateA state-only radon model is not available for California. See observed samples and test your own well to learn its radon level.
Primary aquifer
area contextWells in Yolo County most likely draw from the Central Valley aquifer system (USGS national aquifer code 106). This is the dominant principal aquifer mapped under the county area, not a determination for any single well.
Agricultural land use
area contextAbout 16% of the Yolo County area is row-crop farmland (USDA Cropland Data Layer). Intensively farmed row-crop land raises the likelihood of elevated nitrate in nearby groundwater, so it is a reason to include nitrate in your test panel. This is land-use context for the county area — not a measurement of nitrate in any well.
PFAS
public-system proxyPFAS were detected in 1 of 6 public water systems tested in Yolo County under the EPA's national PFAS monitoring program (UCMR5).
This reflects PFAS detections in public water systems near Yolo County — a regional proxy, not a measurement of private wells. If you rely on a private well, only a lab test of your own water reveals its PFAS level.
These figures are USGS area estimates: statistical groundwater models describing how likely elevated contaminant levels are across a county. They are not designed to predict the concentration in any single well. Only testing your own well reveals its water quality.
Recommended test panel for Yolo County
- coliform
- arsenic
Coliform bacteria is recommended for every private well as the universal baseline test. Arsenic is flagged because the USGS area model estimates a 11% chance of exceeding 10 µg/L. The federal loan minimum (FHA, VA, USDA) tests for coliform, nitrate, nitrite, lead.
Federal FHA, VA, and USDA home loans require testing for coliform, nitrate, nitrite, lead at the point of a federally-backed loan. See the program-specific rules: FHA well test, VA well test, USDA well test.
Already have lab results? Interpret your well water test results to see how your own numbers compare to EPA limits.
How Yolo County compares across California
Where Yolo County's modeled area estimates sit relative to the other California counties in our analysis.
- Arsenicabove the California median
Yolo County: 11% · California median: 7% · flagged in 17 of 58 counties.
See all California arsenic data → - Nitrateabove the California median
Yolo County: 1.36 mg/L · California median: 0.60 mg/L · flagged in 1 of 58 counties.
See all California nitrate data →
Municipal (public) water in Yolo County
Most Yolo County residents are served by a public water system, not a private well. If that's you, open your city's tap-water quality report:
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Find a state-certified lab
Test your Yolo County well through a lab certified by the California State Water Resources Control Board — find one in the official directory (Interactive map—click dots to see laboratory details).
Recognize & research well-water problems
Notice a problem? Diagnose it by symptom
Learn about these contaminants in drinking water
Data sources
Yolo County's estimates are modeled from public federal datasets. See the data & methodology for how we build, source, and validate them.
- USGS arsenic probability-of-exceedance model
- USGS nitrate predicted concentration, domestic-supply depth
- USGS domestic (private) well population density
- USGS principal aquifers
- USDA NASS Cropland Data Layer (CDL) 30m
- EPA UCMR5 PFAS occurrence in public water systems (Fifth Unregulated Contaminant Monitoring Rule)
- EPA/USGS Water Quality Portal (waterqualitydata.us)
- US Census TIGER/Line county polygons
By TapWaterData Editorial · Last updated June 26, 2026.
Yolo County well water FAQ
How do I test my well water in Yolo County?
Order a sample kit covering the recommended panel for Yolo County (coliform, arsenic), then send it to a California-certified drinking-water laboratory. Certified labs use EPA-approved methods, so the results are defensible for a federally-backed home loan. You can also use a mail-in test kit for a convenient EPA-certified analysis.
What does the USGS arsenic estimate mean for my well?
It is an area estimate, not a prediction for your specific well. The USGS model describes how likely elevated arsenic is across Yolo County as a whole; an individual well in the county can be much higher or much lower. Only testing your own well reveals its actual arsenic level.
How often should a private well be tested?
The CDC recommends testing private wells at least once a year for total coliform bacteria and nitrate, and more often if you notice a change in taste, color, or odor, or after flooding or nearby construction.
Does California require well testing?
Federal FHA, VA, and USDA home-loan programs require a private-well water test at the point of a federally-backed loan. A state sale, rental, or recurring testing mandate was not independently verified for this state — consult your state's program.
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