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Health problems are rare in tested Tehama County wells.

All 6 health checks came back rare here.

From 66 wells tested in Tehama County since 2007; your own well needs its own test. How to read this

Tested wells over the limit TehamaCalifornia
Arsenic5%3 of 66Nitrate0%0 of 66Manganese0%0 of 65Uranium0%0 of 66Fluoride0%0 of 66Lead0%0 of 43
GermsTest every year, whatever the chart says
Coliform bacteria14%4 of 29E. coli0%0 of 29
StainsNot a health risk
Iron stains2%1 of 65

Not charted, because fewer than 10 wells were tested: radon (8).

Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+

Lab results: Water Quality Portal (USGS and EPA)

Who drinks from a well here

6 in 10 people 38,995 people

Nobody else tests these wells. Only you do.

People on wells: EPA researchers' 2020 estimate

How deep wells go here

160 ft Near the surface: fertilizer, germs Deeper: arsenic, uranium from rock 400 ft

Deep wells: fewer surface germs, more minerals from the rock.

Well depths: US Groundwater Well Database

Radon risk (a gas from rock)

Less likely, but the EPA still says test your air.

Radon zones: EPA map

Here's how Tehama County compares with California and the counties around it.

Each result puts Tehama County next to the 6 counties that border it, on the chart's 0 to 50% scale.

All 6 health checks are rare in Tehama County.

Arsenic3 of 66 wells over

3 of 66 wells tested here from 2007 to 2022 were over the EPA limit of 10 µg/L, against about 1 in 7 across California. Half the results were under 1.6 µg/L, and the top 1 in 10 were 4.1 µg/L or more. That's a smaller share than in 36 of the 46 other California counties with 10 or more tested wells.

Tehama and its neighbors
Mendocino12%
Tehama5%
Butte2%
Glenn0%
Shasta0%
Trinity0%

Too few tested to show: Plumas.

Arsenic guide

Nitrate0 of 66 wells over

None of the 66 wells tested here from 2007 to 2022 were over the EPA limit of 10 mg/L, against about 1 in 10 across California. Half the results were under 0.93 mg/L, and the top 1 in 10 were 3.5 mg/L or more. Tehama County had none, while 28 of the 46 other California counties with 10 or more tested wells had at least one.

Tehama and its neighbors
Butte0%
Glenn0%
Mendocino0%
Shasta0%
Tehama0%
Trinity0%

Too few tested to show: Plumas.

Nitrate guide

Manganese0 of 65 wells over

None of the 65 wells tested here from 2007 to 2022 were over the EPA health advisory of 0.3 mg/L, against about 1 in 12 across California. At least half the results were too low for the lab to measure, and the top 1 in 10 were 0.0026 mg/L or more. Tehama County had none, while 42 of the 45 other California counties with 10 or more tested wells had at least one.

Tehama and its neighbors
Mendocino12%
Butte11%
Glenn6%
Shasta3%
Tehama0%

Too few tested to show: Plumas, Trinity.

Manganese guide

Uranium0 of 66 wells over

None of the 66 wells tested here from 2007 to 2022 were over the EPA limit of 30 µg/L, against about 1 in 13 across California. At least half the results were too low for the lab to measure, and the top 1 in 10 were 0.63 µg/L or more. Tehama County had none, while 22 of the 43 other California counties with 10 or more tested wells had at least one.

Tehama and its neighbors
Butte0%
Glenn0%
Mendocino0%
Shasta0%
Tehama0%
Trinity0%

Too few tested to show: Plumas.

Uranium guide

Fluoride0 of 66 wells over

None of the 66 wells tested here from 2007 to 2022 were over the EPA limit of 4 mg/L, against about 1 in 37 across California. Half the results were under 0.13 mg/L, and the top 1 in 10 were 0.26 mg/L or more. Tehama County had none, while 11 of the 44 other California counties with 10 or more tested wells had at least one.

Tehama and its neighbors
Butte0%
Glenn0%
Mendocino0%
Shasta0%
Tehama0%
Trinity0%

Too few tested to show: Plumas.

Lead0 of 43 wells over

None of the 43 wells tested here from 2007 to 2022 were over the EPA action level of 15 µg/L, against under 1 in 100 across California. At least half the results were too low for the lab to measure, and the top 1 in 10 were 0.92 µg/L or more. Tehama County had none, while 5 of the 42 other California counties with 10 or more tested wells had at least one.

Tehama and its neighbors
Butte0%
Glenn0%
Mendocino0%
Shasta0%
Tehama0%

Too few tested to show: Plumas, Trinity.

Lead guide

Too few Tehama County wells were tested for this one.

Radon in water8 wells tested

Only 8 wells here were tested from 2007 to 2010, too few to say how common it is. Across California it's about 1 in 30.

Radon guide

Coliform bacteria turned up in 4 of 29 tested Tehama County wells, so test for it every year.

Coliform bacteria4 of 29 wells had it

About 1 in 7 wells tested here from 2010 to 2019 had coliform bacteria, against about 2 in 10 across California.

Tehama and its neighbors
Mendocino43%
Trinity40%
Butte17%
Tehama14%
Shasta14%

Too few tested to show: Glenn, Plumas.

Bacteria guide

E. coli0 of 29 wells had it

None of the 29 wells tested here from 2010 to 2019 had E. coli, against about 1 in 23 across California.

Tehama and its neighbors
Mendocino13%
Shasta3%
Butte0%
Tehama0%
Trinity0%

Too few tested to show: Glenn, Plumas.

E. coli means human or animal waste is getting in, so if a test of your Tehama County well finds it, boil drinking water until the well is fixed.

Bacteria guide

Iron reached staining levels in 1 of 65 tested wells, and it isn't a health risk.

Iron stains1 of 65 wells had it

1 of 65 wells tested here from 2007 to 2022 had enough iron to stain (0.3 mg/L), against about 1 in 10 across California.

Tehama and its neighbors
Mendocino27%
Butte11%
Trinity10%
Shasta5%
Tehama2%
Glenn0%

Too few tested to show: Plumas.

Iron guide

Tehama County has 66 tested wells, and the busiest decade for testing was the 2000s.

38 wells had a test in the 2000s, 35 in the 2010s and 5 since 2020.

Only 5 of the 66 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.

Most Tehama County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).

One mail-in lab test covers the 3 things to check in Tehama County.

  • Bacteria
  • Nitrate
  • Arsenic

What California offers well owners

Half the household wells here are shallower than 160 feet.

That comes from 11,486 household wells in California's drilling records. Most wells of every kind here are 115 to 263 feet deep. Wells as deep as Tehama County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here are shallower than in 42 of the 56 other California counties with enough drilling records.

Look up your own well's depth in California's well record search, or see how to find it.

Well depths: US Groundwater Well Database

Here's what we know about radon and PFAS in Tehama County.

Tehama County is in the EPA's lowest radon zone, so radon from well water is unlikely to be a problem. Radon guide

Between 2023 and 2025 the EPA had 2 town water systems in Tehama County test for PFAS. 1 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 7.4 ng/L of PFOA. Only 5 wells in Tehama County have public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide

Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025

Next door to Tehama County
ButteMendocinoShasta
Elsewhere in California
RiversideFresnoLos AngelesSonomaSan Diego

All of California

Get a yearly reminder to test your Tehama County well

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Questions people ask about well water in Tehama County

Is well water in Tehama County safe to drink?

Most tested wells in Tehama County came in under the limits. Of 6 health checks with enough tests, arsenic came closest, at 3 of 66 wells over. Every well is different, so test yours for bacteria, nitrate and arsenic.

Can I get my well tested for free in Tehama County?

The State and Regional Water Boards fund or regulate 11 domestic well programs that offer free well testing in their service areas. If not, use a lab California has certified for drinking water. Ask for bacteria, nitrate and arsenic, the tests that matter most in Tehama County.

How deep are wells in Tehama County?

Half the household wells here are shallower than 160 feet. Typical household wells here are shallower than in 42 of the 56 other California counties with enough drilling records.

How does Tehama County compare with the counties around it?

For arsenic, the share of tested wells over the limit: 1 of the 5 neighboring counties with data had a higher share than Tehama County, at 5%. Mendocino had the most, at 12%. Groundwater doesn't stop at county lines, so Mendocino's 12% is worth knowing about if you live near it.

How often should I test my well in Tehama County?

In Tehama County, 4 of 29 tested wells had coliform bacteria, so the CDC's advice matters here: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.

By the Tested Water teamData from the USGS, EPA and State Water Resources Control BoardUpdated November 8, 2026

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Where the Tehama County numbers come from

  • People on wells: 38,995 in Tehama County, from EPA estimates built on the 2020 Census (Murray et al.).
  • Lab tests: 605 well water samples from 66 wells in the Water Quality Portal, run by the USGS and EPA, 2007 to 2022.
  • Well depths: 11,486 California drilling records for Tehama County, gathered in the US Groundwater Well Database (Lin and others, 2024).
  • Neighboring counties: the 6 counties that share a border with Tehama County, from the Census Bureau's county adjacency file.
  • PFAS: EPA tests of 2 town water systems in Tehama County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
  • Radon: Tehama County is zone 3 on the EPA Map of Radon Zones.
  • State programs and links: State Water Resources Control Board, checked November 1, 2026.
Download this county's data (CSV)How we read these numbersReport an error
Test for bacteria and nitrate every yearSee lab tests