Well water in Tehama County, California
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Testing recommended
tested well samples in Tehama County exceeded the EPA limit for nitrate (10 mg/L), based on public data from California GAMA Domestic Well Project and USGS Water Quality Portal.
4,557 test results across 17 contaminants from at least 449 wells, 1957 to 2024; sodium has the most at 694. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 31,338 people in Tehama County get their drinking water from a private well, per USGS 2015 county water use. That is about 50% of residents.
Separately, the USGS national arsenic model estimates that 5% of the 31,338 people on private wells in Tehama County draw water above 10 ug/L (the model's range is 4% to 6%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Tehama County. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.
| Contaminant | Above limit | Samples | Detected | Median | Highest | Limit |
|---|---|---|---|---|---|---|
| Nitrate 1957 to 2024 | 584 | 94% | 1.9 | 45 | 10 mg/L EPA limit | |
| Arsenic 1959 to 2024 | 453 | 75% | 1 | 25 | 10 ug/L EPA limit | |
| Nitrite 2005 | 223 | <1% | 0 | 1.6 | 1 mg/L EPA limit | |
| Lead 1959 to 2024 | 372 | 26% | 0 | 66 | 15 ug/L EPA limit | |
| Gross alpha 2005 | 223 | <1% | 0 | 95 | 15 pCi/L EPA limit | |
| Copper 1959 to 2024 | 372 | 27% | 0 | 0.15 | 1.3 mg/L EPA limit | |
| Fluoride 1982 to 2005 | 232 | 51% | 0.1 | 0.34 | 4 mg/L EPA limit | |
| Uranium 2006 to 2024 | 110 | 90% | 0.227 | 7.01 | 30 ug/L EPA limit | |
| PFOS 2021 to 2024 | 16 | 6% | 0 | 1.2 | 4 ng/L EPA limit | |
| PFOA 2021 to 2024 | 16 | 0% | 0 | 0 | 4 ng/L EPA limit | |
| Radium (226 + 228) 2006 to 2007 | 16 | 44% | 0 | 0.3 | 5 pCi/L EPA limit | |
| Sodium 1957 to 2024 | no limit | 694 | 100% | 16.1 | 1,550 | 20 mg/L health guideline |
| Iron 1958 to 2024 | no limit | 528 | 44% | 0 | 9,700 | 300 ug/L nuisance level |
| Manganese 1959 to 2024 | no limit | 483 | 41% | 0 | 490 | 300 ug/L health guideline |
| Chloride 2005 | no limit | 223 | 100% | 14 | 310 | 250 mg/L nuisance level |
| Radon 2006 to 2010 | no limit | 11 | 100% | 380 | 970 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: E. coli (1).
Over time
Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.
| Contaminant | before 2000 | 2000s | 2010s | 2020s |
|---|---|---|---|---|
| Nitrate | 16%251 samples | 2%272 samples | 9%45 samples | 13%16 samples |
| Arsenic | 0%120 samples | 11%272 samples | 0%45 samples | 0%16 samples |
| Lead | 0%39 samples | <1%272 samples | 0%45 samples | 0%16 samples |
| Copper | 0%39 samples | 0%272 samples | 0%45 samples | 0%16 samples |
| Uranium | n/a | 0%49 samples | 0%45 samples | 0%16 samples |
Nitrate above the limit in Tehama County rose from 2% of 272 samples in the 2000s to 9% of 45 in the 2010s.
Arsenic above the limit in Tehama County fell from 11% of 272 samples in the 2000s to 0% of 45 in the 2010s.
Lead, copper, uranium have held within a few points between the two most recent periods with enough samples.
Closer than the county
See the sampled wells near your address
The same records, drawn around one point: which wells within a few miles were sampled and what they found, how deep the wells next to you were drilled, and any leaking tank or cleanup site within a mile.
Check my addressWhat this means
The contaminants most often above their limit here are nitrate (9% of 584 samples), arsenic (7% of 453 samples), and nitrite (<1% of 223 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.
Chloride is widespread in local groundwater. It is mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
What to test for in Tehama County
- 1Nitrate9% of 584 local samples were above the limit.
- 2Arsenic7% of 453 local samples were above the limit.
- 3Nitrite<1% of 223 local samples were above the limit.
- 4Lead<1% of 372 local samples were above the limit.
- 5Gross alpha<1% of 223 local samples were above the limit.
- 6Coliform bacteriaRecommended for every private well, every year.
- 7RadonMedian local reading is 380 pCi/L.
- 8Volatile organic compounds (VOCs)148 leaking underground storage tank sites on record locally.
- 9Metals panel4 hazardous waste sites on record locally.
Test your Tehama County well for nitrate, arsenic and nitrite
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.
Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.
Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.
Where to get a well tested in California: certified labs, state programs and prices.
Already have results back? Get your lab report read against the Tehama County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Tehama County
Central Valley aquifer system
Unconsolidated sand and gravel. Wells in sand and gravel are usually shallow and productive, and they recharge quickly from the surface. That makes them the most exposed to nitrate, bacteria and anything spilled nearby, and the least likely to carry arsenic or radon from the rock.
Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the county centroid. A single well can sit in a different unit, especially near a boundary.
Official help in California
Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.
- GAMA - Well Owners, State Water Resources Control Board
- Environmental Laboratory Accreditation Program (ELAP), State Water Resources Control Board
Contamination sources on record in Tehama County
California GeoTracker and EnviroStor sites records 148 leaking underground storage tank sites and 4 hazardous waste sites in Tehama County, among 234 regulated sites in total. Groundwater does not follow property lines, so a well within a few hundred feet of one of these is worth testing for what that site could release.
- 148
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 4
- hazardous waste sitesSolvents, degreasers and metals from industrial sites can travel far in fractured bedrock.
- 7
- lined landfills and transfer facilities
- 75
- other remediation projects
Treatment that fits the local profile
Nitrate
Point-of-use reverse osmosis; Anion exchange; Distillation
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Nitrite
Point-of-use reverse osmosis; Anion exchange; Distillation
Lead
Point-of-use reverse osmosis; Certified lead-removal carbon filter; Neutralizing filter to raise pH and stop leaching
Gross alpha
Point-of-use reverse osmosis; Water softener (cation exchange) for radium; Anion exchange for uranium
Systems we would look at first
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For nitrate
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
For nitrate
Aquasana SmartFlow reverse osmosis
under-sink or countertop, about $225
Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.
For arsenic
Whole-house arsenic and uranium filter
whole-house, at the point of entry, about $2,500
Adsorptive iron-based media; sized to your well yield.
For lead
Aquasana Claryum 3-Stage under-sink filter
under-sink or countertop, about $175
Certified to NSF/ANSI 53 to reduce 99% of lead and 99.7% of PFOA and PFOS at the tap. Not a fix for arsenic, nitrate or bacteria.
For lead
Acid neutralizing filter
whole-house, at the point of entry, about $1,200
Raises pH so plumbing stops leaching lead and copper.
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Common questions
Is there arsenic in well water in Tehama County?
Arsenic was detected in 75% of 453 public well samples in Tehama County, and 7% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.
How high is radon in well water in Tehama County?
The median radon level across 11 public samples in Tehama County was 380 pCi/L, with a high of 970 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Tehama County?
Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.
Counties near Tehama County
Closest first. Groundwater does not stop at a line on a map, so the next county over is a fair second opinion.
Want the numbers themselves? Download the California dataset as CSV, every county and town, with the sources named.
Sources
- California GAMA Domestic Well Project: 2,453 samples in Tehama County. Private well samples from the State Water Board GAMA Domestic Well Project county studies (El Dorado, Tehama, Tulare, San Diego, Yuba, Monterey and others), 2002 to 2011, from the GAMA results file on data.ca.gov. Retrieved September 15, 2026.
- USGS Water Quality Portal: 2,104 samples in Tehama County. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
- USGS national arsenic model (Ayotte 2017): estimated share of 31,338 people on private wells in Tehama County above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.
- California GeoTracker and EnviroStor sites: 234 regulated sites located in Tehama County. Leaking underground storage tank and cleanup sites from the State Water Board GeoTracker export on data.ca.gov, and DTSC EnviroStor cleanup sites. Retrieved September 15, 2026.
Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.