Well water in Mariposa County, California
In Mariposa County, 2% of 42 tested well samples exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.
284 test results across 10 contaminants from at least 55 wells, 1991 to 2023; fluoride has the most at 59. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 12,533 people in Mariposa County get their drinking water from a private well, per USGS 2015 county water use. That is about 71% of residents.
Separately, the USGS national arsenic model estimates that 3% of the 12,533 people on private wells in Mariposa County draw water above 10 ug/L (the model's range is 3% to 5%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Mariposa 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 |
|---|---|---|---|---|---|---|
| Arsenic 1991 to 2023 | 42 | 50% | 0.11 | 27.6 | 10 ug/L EPA limit | |
| Fluoride 1991 to 2023 | 59 | 46% | 0 | 2.2 | 4 mg/L EPA limit | |
| Lead 1991 to 2023 | 42 | 17% | 0 | 2.71 | 15 ug/L EPA limit | |
| Copper 1991 to 2023 | 42 | 19% | 0 | 0.01 | 1.3 mg/L EPA limit | |
| Uranium 2008 to 2023 | 10 | 90% | 0.79 | 7.45 | 30 ug/L EPA limit | |
| Manganese 1991 to 2023 | no limit | 59 | 71% | 2 | 172 | 300 ug/L health guideline |
| Radon 1992 to 2008 | no limit | 16 | 100% | 4,300 | 12,000 | 2,000 pCi/L health guideline |
Sampled fewer than 10 times, too few to summarize: PFOS (6), PFOA (6), Radium (226 + 228) (2).
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 arsenic (2% of 42 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.
Manganese is widespread in local groundwater. It is mainly a staining and taste problem, but high manganese also carries a health advisory for infants.
The median radon reading (4,300 pCi/L) is above the 2,000 pCi/L level at which treatment is recommended. Radon in water raises indoor air radon when it is used for showers and laundry.
What to test for in Mariposa County
- 1Arsenic2% of 42 local samples were above the limit.
- 2Coliform bacteriaRecommended for every private well, every year.
- 3RadonMedian local reading is 4,300 pCi/L.
- 4NitrateRecommended for every private well, especially near farms or septic systems.
- 5ManganeseDetected in 71% of local samples.
- 6Volatile organic compounds (VOCs)103 leaking underground storage tank sites on record locally.
Test your Mariposa County well for arsenic, coliform bacteria and radon
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 Mariposa County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Mariposa County
Not a principal aquifer
The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.
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 Mariposa County
California GeoTracker and EnviroStor sites records 103 leaking underground storage tank sites in Mariposa County, among 122 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.
- 103
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 8
- lined landfills and transfer facilities
- 11
- other remediation projects
Treatment that fits the local profile
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Radon
Aeration (bubble-up or diffused) whole-house; Granular activated carbon whole-house (lower levels only)
Systems we would look at first
Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.
For arsenic
Under-sink reverse osmosis system
under-sink or countertop, about $300
Look for NSF/ANSI 58 certification listing the contaminant you need removed.
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 arsenic
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.
Free, no obligation
Get on the list for a free in-home water test in Mariposa County
No water treatment company serves Mariposa County through Well Water Nerd yet. We are signing licensed companies on county by county: they test at your tap for free, walk you through the numbers, and quote treatment only if something needs fixing. Leave your details and you are first in line. Nothing is shared until a company joins for Mariposa County, and you get an email naming it first.
Common questions
Is there arsenic in well water in Mariposa County?
Arsenic was detected in 50% of 42 public well samples in Mariposa County, and 2% 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 Mariposa County?
The median radon level across 16 public samples in Mariposa County was 4,300 pCi/L, with a high of 12,000 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.
How often should I test a private well in Mariposa 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 Mariposa 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
- USGS Water Quality Portal: 284 samples in Mariposa 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 12,533 people on private wells in Mariposa 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: 122 regulated sites located in Mariposa 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.