Well water in Modoc County, California
In Modoc County, 3% of 30 tested well samples exceeded the EPA limit for arsenic (10 ug/L), based on public data from USGS Water Quality Portal.
158 test results across 10 contaminants from at least 16 wells, 1959 to 2024; arsenic has the most at 30. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 4,533 people in Modoc County get their drinking water from a private well, per USGS 2015 county water use. That is about 51% of residents.
Separately, the USGS national arsenic model estimates that 7% of the 4,533 people on private wells in Modoc County draw water above 10 ug/L (the model's range is 5% to 9%), from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside Modoc 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 2003 to 2024 | 30 | 70% | 1.2 | 6,600 | 10 ug/L EPA limit | |
| Lead 2003 to 2020 | 28 | 54% | 0.064 | 0.821 | 15 ug/L EPA limit | |
| Copper 2003 to 2020 | 28 | 50% | 0 | 0.002 | 1.3 mg/L EPA limit | |
| Fluoride 1959 to 2020 | 22 | 100% | 0.205 | 0.53 | 4 mg/L EPA limit | |
| Uranium 2010 to 2020 | 16 | 88% | 0.661 | 2.36 | 30 ug/L EPA limit | |
| Manganese 2005 to 2020 | no limit | 19 | 68% | 14.1 | 570 | 300 ug/L health guideline |
Sampled fewer than 10 times, too few to summarize: E. coli (1), PFOA (3), PFOS (3), Radon (8).
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 |
|---|---|---|---|---|
| Arsenic | n/a | 0%12 samples | 7%14 samples | n/a |
| Lead | n/a | 0%12 samples | 0%13 samples | n/a |
| Copper | n/a | 0%12 samples | 0%13 samples | n/a |
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 (3% of 30 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.
What to test for in Modoc County
- 1Arsenic3% of 30 local samples were above the limit.
- 2RadonMedian local reading is 605 pCi/L.
- 3NitrateRecommended for every private well, especially near farms or septic systems.
- 4E. coliAbove the limit in 1 of 1 local samples; too few to quote a rate.
- 5FluorideDetected in 100% of local samples.
- 6ManganeseDetected in 68% of local samples.
- 7Volatile organic compounds (VOCs)19 leaking underground storage tank sites on record locally.
- 8Metals panel2 hazardous waste sites on record locally.
Test your Modoc County well for arsenic, radon and nitrate
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 Modoc County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under Modoc County
Pacific Northwest basaltic-rock aquifers
Igneous and metamorphic-rock. Granite and similar crystalline rock hold water only in fractures. Wells are deep and yields modest, and the rock itself is the source of arsenic, radon and uranium, which is why those three head the test list in bedrock country.
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 Modoc County
California GeoTracker and EnviroStor sites records 19 leaking underground storage tank sites and 2 hazardous waste sites in Modoc County, among 42 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.
- 19
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 2
- hazardous waste sitesSolvents, degreasers and metals from industrial sites can travel far in fractured bedrock.
- 7
- lined landfills and transfer facilities
- 14
- other remediation projects
Treatment that fits the local profile
Arsenic
Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange
Manganese
Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis
Systems we would look at first
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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.
For manganese
Iron and manganese oxidizing filter
whole-house, at the point of entry, about $1,800
Air-injection or greensand media; fixes staining at the source.
Free, no obligation
Get on the list for a free in-home water test in Modoc County
No water treatment company serves Modoc 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 Modoc County, and you get an email naming it first.
Common questions
Is there arsenic in well water in Modoc County?
Arsenic was detected in 70% of 30 public well samples in Modoc County, and 3% 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 often should I test a private well in Modoc 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 Modoc 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: 158 samples in Modoc 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 4,533 people on private wells in Modoc 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: 42 regulated sites located in Modoc 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.