Well water in California

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Testing recommended

tested well samples in California exceeded the EPA limit for nitrate (10 mg/L), based on public data from USGS Water Quality Portal and California GAMA Domestic Well Project.

280,356 test results across 17 contaminants from at least 15,858 wells, 1908 to 2026; nitrate has the most at 57,577. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 1,435,510 people in California get their drinking water from a private well, per USGS 2015 county water use.

Separately, the USGS national arsenic model estimates that 8% of the 1,435,510 people on private wells in California draw water above 10 ug/L (the model's range is 6% to 10%), from geology and well depth rather than local tests.

What individual samples found

Each row summarizes every public sample for that contaminant inside California. 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 1908 to 2026	16%	57,577	86%	1.55	2,030	10 mg/L EPA limit
Arsenic 1945 to 2026	15%	18,251	86%	2	7,500	10 ug/L EPA limit
Uranium 1980 to 2026	8%	7,553	91%	2.04	5,600	30 ug/L EPA limit
PFOS 2019 to 2024	7%	898	16%	0	160	4 ng/L EPA limit
Gross alpha 2003 to 2011	6%	382	24%	0	602	15 pCi/L EPA limit
PFOA 2019 to 2024	6%	901	14%	0	300	4 ng/L EPA limit
E. coli 2003 to 2025	3%	658	3%	0	1,300	0 MPN/100mL EPA limit
Fluoride 1932 to 2026	3%	39,319	95%	0.4	170	4 mg/L EPA limit
Radium (226 + 228) 1980 to 2022	2%	2,468	70%	0.075	640	5 pCi/L EPA limit
Nitrite 2002 to 2011	1%	746	14%	0	4.08	1 mg/L EPA limit
Lead 1954 to 2026	<1%	10,600	39%	0	6,000	15 ug/L EPA limit
Copper 1951 to 2026	<1%	10,550	47%	0	130	1.3 mg/L EPA limit
Sodium 1908 to 2026	no limit	44,379	100%	75.9	90,000	20 mg/L health guideline
Iron 1908 to 2026	no limit	31,109	66%	9.7	6,600,000	300 ug/L nuisance level
Manganese 1932 to 2026	no limit	27,758	71%	2	540,000	300 ug/L health guideline
Chloride 1908 to 2026	no limit	25,586	100%	66	280,000	250 mg/L nuisance level
Radon 1988 to 2024	no limit	1,621	100%	510	40,700	2,000 pCi/L health guideline

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	17%36,502 samples	13%9,158 samples	16%8,223 samples	13%3,694 samples
Fluoride	3%26,078 samples	2%5,697 samples	3%5,486 samples	2%2,058 samples
Arsenic	16%5,747 samples	12%5,311 samples	17%5,246 samples	18%1,947 samples
Lead	3%2,579 samples	<1%3,249 samples	<1%3,401 samples	<1%1,371 samples
Copper	2%3,132 samples	0%3,237 samples	0%2,919 samples	<1%1,262 samples
Uranium	31%466 samples	5%2,152 samples	7%3,455 samples	5%1,480 samples

Nitrate, fluoride, arsenic, lead, copper, uranium have held within a few points between the two most recent periods with enough samples.

What this means

The contaminants most often above their limit here are nitrate (16% of 57,577 samples), arsenic (15% of 18,251 samples), and uranium (8% of 7,553 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.

Iron, Manganese, and Chloride are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

What to test for in California

1
Nitrate
16% of 57,577 local samples were above the limit.
2
Arsenic
15% of 18,251 local samples were above the limit.
3
Uranium
8% of 7,553 local samples were above the limit.
4
PFOS
7% of 898 local samples were above the limit.
5
Gross alpha
6% of 382 local samples were above the limit.
6
PFOA
6% of 901 local samples were above the limit.
7
E. coli
3% of 658 local samples were above the limit.
8
Volatile organic compounds (VOCs)
46,290 leaking underground storage tank sites on record locally.
9
Metals panel
1,226 hazardous waste sites on record locally.

Test your California well for nitrate, arsenic and uranium

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 $209 Already 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 California numbers, $29. Every line explained, ranked by what matters, with the fix for each.

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Send me the California report

The numbers above, which tests to order and where, and the treatment that fits the local profile, in one email.

The rock under California

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 state 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 California

California GeoTracker and EnviroStor sites records 46,290 leaking underground storage tank sites and 1,226 hazardous waste sites in California, among 79,173 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.

46,290: leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
1,226: hazardous waste sitesSolvents, degreasers and metals from industrial sites can travel far in fractured bedrock.
1,865: lined landfills and transfer facilities
29,792: 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

Uranium

Point-of-use reverse osmosis; Anion exchange; Whole-house anion exchange

PFOS

Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange

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 PFOS

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 PFOS

Whole-house PFAS carbon system

whole-house, at the point of entry, about $3,000

Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.

For PFOS

Aquasana HealthShield whole-house filter

whole-house, at the point of entry, about $2,099

Whole-house carbon system the maker certifies to reduce PFAS (PFOA and PFOS) and VOCs at every tap; a UV option adds bacteria protection.

Common questions

Is there arsenic in well water in California?

Arsenic was detected in 86% of 18,251 public well samples in California, and 15% 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 California?

The median radon level across 1,621 public samples in California was 510 pCi/L, with a high of 40,700 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in California?

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 in California

Want the numbers themselves? Download the California dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 268,833 samples in California. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
California GAMA Domestic Well Project: 11,523 samples in California. 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 national arsenic model (Ayotte 2017): estimated share of 1,435,510 people on private wells in California 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: 79,173 regulated sites located in California. 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.