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Manganese in California well water, by county

By the Tested Water teamLab data from the USGS and EPA Water Quality Portal, 2000 to 2025Updated November 8, 2026

Manganese is widespread, over the limit in 15% or more of tested wells, in 15 of the 46 California counties with 10 or more tested wells. A county's share tells you how likely a problem is nearby. Only a test of your own well tells you what's in it.

Find a manganese test See every county

How California compares with the rest of the country

Across California, about 1 in 12 tested wells had manganese over the EPA health advisory of 0.3 mg/L (386 of 4,639). Across all tested wells in the US it's about 1 in 7. That's less often than in the country as a whole. Counties still differ, so check yours below.

Why it matters: High levels over many years may affect the nervous system, and babies are more sensitive to it. It also leaves black stains.

Manganese dissolves out of rock and soil, especially in water with little oxygen.

Manganese in tested wells, county by county

Share of each county's tested wells with at least one result over the EPA health advisory of 0.3 mg/L. Counties need 10 or more tested wells to be listed. Widespread means 15% or more, worth checking 5 to 15%, rare under 5%.

CountyWells testedWells over the limitLevel
Humboldt County 19
32% 6 of 19
 Widespread
Sutter County 19
32% 6 of 19
 Widespread
Amador County 16
31% 5 of 16
 Widespread
Napa County 49
29% 14 of 49
 Widespread
San Mateo County 12
25% 3 of 12
 Widespread
San Diego County 203
22% 45 of 203
 Widespread
Orange County 80
21% 17 of 80
 Widespread
Sonoma County 109
21% 23 of 109
 Widespread
San Benito County 35
20% 7 of 35
 Widespread
Santa Barbara County 228
20% 45 of 228
 Widespread
El Dorado County 68
19% 13 of 68
 Widespread
Mono County 35
17% 6 of 35
 Widespread
Santa Cruz County 38
16% 6 of 38
 Widespread
Calaveras County 13
15% 2 of 13
 Widespread
San Joaquin County 137
15% 21 of 137
 Widespread
Lake County 14
14% 2 of 14
 Worth checking
Merced County 86
14% 12 of 86
 Worth checking
Alameda County 58
14% 8 of 58
 Worth checking
Mendocino County 32
12% 4 of 32
 Worth checking
Kings County 33
12% 4 of 33
 Worth checking
Butte County 45
11% 5 of 45
 Worth checking
Ventura County 65
11% 7 of 65
 Worth checking
Kern County 199
10% 19 of 199
 Worth checking
Modoc County 12
8% 1 of 12
 Worth checking
Sacramento County 87
8% 7 of 87
 Worth checking
Yolo County 13
8% 1 of 13
 Worth checking
Fresno County 147
7% 10 of 147
 Worth checking
Lassen County 16
6% 1 of 16
 Worth checking
Glenn County 17
6% 1 of 17
 Worth checking
Nevada County 58
5% 3 of 58
 Worth checking
Los Angeles County 470
5% 24 of 470
 Worth checking
Monterey County 80
5% 4 of 80
 Worth checking
Yuba County 21
5% 1 of 21
 Rare
Siskiyou County 22
5% 1 of 22
 Rare
San Luis Obispo County 53
4% 2 of 53
 Rare
Stanislaus County 141
4% 5 of 141
 Rare
Shasta County 58
3% 2 of 58
 Rare
Riverside County 347
3% 10 of 347
 Rare
San Bernardino County 977
3% 26 of 977
 Rare
Tulare County 120
2% 2 of 120
 Rare
Santa Clara County 60
2% 1 of 60
 Rare
Imperial County 69
1% 1 of 69
 Rare
Madera County 87
0% 0 of 87
 Rare
Tehama County 65
0% 0 of 65
 Rare
Placer County 41
0% 0 of 41
 Rare
Inyo County 23
0% 0 of 23
 Rare

10 more California counties have 1 to 9 tested wells for manganese, too few to rate.

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Where to get your well tested for manganese in California

1

Free testing, with limits

Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. Check which tests it includes before you count on it for manganese.

California's private well page
3

A mail-in kit

Fill the bottles at your kitchen tap and mail them back. We mark the kits that include manganese.

Compare kits

Keep reading about wells in California

Questions people ask about manganese in California

Is there manganese in California well water?

Manganese is widespread, over the limit in 15% or more of tested wells, in 15 of the 46 California counties with 10 or more tested wells. Across California, about 1 in 12 tested wells had manganese over the EPA health advisory of 0.3 mg/L (386 of 4,639). Across all tested wells in the US it's about 1 in 7. That's less often than in the country as a whole. Counties still differ, so check yours below.

Where can I get my well tested for manganese in California?

Free testing in California depends on where you live: 11 local domestic well programs offer it in their areas. If that doesn't cover you, use a lab certified by California for drinking water, or a mail-in kit that includes manganese from a certified lab.

See California's list of certified labs, or compare mail-in kits.

Where these numbers come from

  • Lab tests: Water Quality Portal (USGS and EPA), well samples since 2000. A well counts as over if any of its tests was over the EPA health advisory of 0.3 mg/L. Wells tested for studies and monitoring, not a random sample of homes.
  • Likely overstated: our own check of who tested the wells, when and where. How we check.
  • State programs and links: State Water Resources Control Board.