Well water in San Francisco County, California
In San Francisco County, none of 96 tested well samples exceeded the EPA limit for fluoride, based on public data from USGS Water Quality Portal.
The strongest signal is coliform bacteria: Recommended for every private well, every year, too few samples to carry the headline but first on the test list below.
268 test results across 11 contaminants from at least 48 wells, 1987 to 2022; fluoride has the most at 96. Data refreshed September 15, 2026.
Compiled by Nick at Well Water Nerd from public records. How these pages are built
An estimated 4,158 people in San Francisco County get their drinking water from a private well, per USGS 2015 county water use. That is about <1% of residents.
Separately, the USGS national arsenic model estimates that 0% of the 4,158 people on private wells in San Francisco County draw water above 10 ug/L, from geology and well depth rather than local tests.
What individual samples found
Each row summarizes every public sample for that contaminant inside San Francisco 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 |
|---|---|---|---|---|---|---|
| Fluoride 1987 to 2022 | 96 | 96% | 0.1 | 0.4 | 4 mg/L EPA limit | |
| Sodium 1987 to 2022 | no limit | 96 | 100% | 43.5 | 110 | 20 mg/L health guideline |
| Iron 1989 to 2022 | no limit | 26 | 81% | 6 | 840 | 300 ug/L nuisance level |
| Manganese 1989 to 2022 | no limit | 20 | 85% | 56.5 | 3,500 | 300 ug/L health guideline |
Sampled fewer than 10 times, too few to summarize: Lead (6), Copper (6), Uranium (6), Arsenic (6), PFOA (1), PFOS (1), Radon (4).
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
Among contaminants with a federal limit, no tested sample in San Francisco County came back above it. That is reassuring, but sampling is uneven: wells that were never tested are not in this data.
Iron and Manganese 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 San Francisco County
- 1Coliform bacteriaRecommended for every private well, every year.
- 2RadonMedian local reading is 205 pCi/L.
- 3NitrateRecommended for every private well, especially near farms or septic systems.
- 4FluorideDetected in 96% of local samples.
- 5ManganeseDetected in 85% of local samples.
- 6IronDetected in 81% of local samples.
- 7Volatile organic compounds (VOCs)2,319 leaking underground storage tank sites on record locally.
- 8Metals panel48 hazardous waste sites on record locally.
Test your San Francisco County well for coliform bacteria, 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 San Francisco County numbers, $29. Every line explained, ranked by what matters, with the fix for each.
The rock under San Francisco County
California Coastal Basin aquifers
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 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 San Francisco County
California GeoTracker and EnviroStor sites records 2,319 leaking underground storage tank sites and 48 hazardous waste sites in San Francisco County, among 2,717 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.
- 2,319
- leaking underground storage tank sitesGasoline and heating oil that leaked from tanks move through groundwater as benzene, MTBE and other volatile organic compounds.
- 48
- hazardous waste sitesSolvents, degreasers and metals from industrial sites can travel far in fractured bedrock.
- 3
- lined landfills and transfer facilities
- 347
- other remediation projects
Treatment that fits the local profile
Manganese
Oxidizing filter (greensand, birm) whole-house; Water softener (low levels); Point-of-use reverse osmosis
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 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 San Francisco County
No water treatment company serves San Francisco 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 San Francisco County, and you get an email naming it first.
Common questions
How often should I test a private well in San Francisco 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 San Francisco 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: 268 samples in San Francisco 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 4,158 people on private wells in San Francisco 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: 2,717 regulated sites located in San Francisco 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.