Typical values from 5 sampled wells in Marin County · Full groundwater profile
Marin County Well Water at a Glance
No common nuisance problem in the typical well. The typical Marin well is moderately hard, with iron below the stain threshold and a pH of 8.3.
Safe to drink? Generally yes in the records, though only your own test covers your well.
Typical treatment, in plumbing order:
Usually nothing beyond routine testing. Most Marin wells need no softener or iron filter, so test before buying equipment.
Not sure what to install? Get a filter recommendation for Marin County homesNot on a well? Tap water from a utility is covered by its yearly water quality report (CCR), not by these well samples. Find your utility's report
On this page (8 sections)
Where to Test Well Water in Marin County
- Your county environmental health department handles private well questions in Marin County; for a water test, use a state-accredited lab, many of which accept mailed or dropped-off samples.
- Certified labs: State Water Board list of accredited labs (ELAP)
Ask the lab for sample bottles and how quickly the sample must reach it before you collect.
What Are You Noticing in Your Water?
Confirm with a lab test before buying equipment.
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Explore Marin County and nearby counties
Marin County has about 254,000 residents, and its largest communities are San Rafael, Novato and Mill Valley. Homes outside city and community water systems rely on private domestic wells. Wells here tap the California Coastal Basin aquifers, and the largest DWR groundwater basin in the county is Sand Point Area.
Across 5 sampled wells, groundwater is moderately hard (5.3 gpg in the typical well). Arsenic, nitrate and chromium-6 were below California limits in most sampled wells, but individual wells vary, so test yours.
In this guide
Groundwater Profile: Marin County
The largest California DWR Bulletin 118 groundwater basins in the county are Sand Point Area, Novato Valley and Ross Valley.
Another address? Check hardness by ZIP code.
No contaminant tracked here was above its EPA guideline in the typical (median) sampled well.
Arsenic, Nitrate, Chromium-6 and Salinity in Marin County
| Contaminant | Typical well | Wells above limit | California standard |
|---|---|---|---|
| Arsenic | 0.1 µg/L | Limited data (5 wells) | 10 µg/L (primary MCL) |
| Nitrate (as nitrogen) | 0 mg/L | Limited data (5 wells) | 10 mg/L (primary MCL) |
| Chromium-6 | 0.04 µg/L | Limited data (1 wells) | 10 µg/L (primary MCL since Oct. 1, 2024) |
| Salinity (dissolved solids) | 288 mg/L | Limited data (5 wells) | 500 mg/L recommended, 1,000 mg/L upper (secondary) |
| Hardness | 5.3 gpg | Limited data (5 wells) | No limit; above 7 gpg is hard |
Typical well = median. Standards: State Water Board; samples: Water Quality Portal, since 2000.
How Marin County Compares to California and the U.S.
See How Marin County Compares to California & the US
| Share of wells with… | Marin CountyMarin | CaliforniaCA | United StatesUS |
|---|---|---|---|
| Hard water (above 7 gpg) | Limited data | 64% | 60% |
| Iron above 0.3 mg/L | Limited data | 9% | 19% |
| Manganese above 0.05 mg/L | Limited data | 22% | 21% |
| Dissolved solids above 500 mg/L | 0% | 38% | 15% |
| Acidic water (pH below 6.5) | 0% | 2% | 14% |
| Nitrate above 10 mg/L (EPA limit) | Limited data | 9% | 4% |
| Arsenic above 10 µg/L (EPA limit) | Limited data | 14% | 7% |
| Chromium-6 above 10 µg/L (California limit) | Limited data | 6% | — |
| Median hardness | 5.3 gpg | 9.9 gpg | — |
| Median pH | 8.3 | 7.6 | 7.3 |
Sources: Water Quality Portal (since 2000); US: USGS Circular 1332. Highlighted: well above the US share.
See what a typical Marin County well needs
Pre-filled with the Marin medians. Swap in your own results.
Testing Your Well in Marin County
In California, county environmental health departments handle private well permits and can advise on testing. For bacteria, nitrate, arsenic or a full panel, use a laboratory accredited by California’s Environmental Laboratory Accreditation Program (ELAP); the State Water Board’s certified lab map lists them.
What to test for in Marin County:
- Bacteria (total coliform and E. coli), nitrate and arsenic, at least once a year (California State Water Resources Control Board recommendation)
- After any flood, new well work or a change in taste, color or smell
Which tests to order and how: how to test well water.
After a positive coliform test, work out the bleach dose and shock the well and retest 10 to 14 days later.
Nearby Counties
Marin County borders these counties with published reports:
Counties with similar well water
Farther away, these California counties share the same main water issues:
Frequently Asked Questions
Is well water in Marin County safe to drink?
Usually, as far as public records go: no health-based limit is commonly exceeded across 5 sampled Marin wells. Still test for bacteria and nitrate every year, because county data cannot vouch for a single well.
How hard is well water in Marin County?
Groundwater samples from 5 wells in Marin County have a median hardness of 5.3 grains per gallon (91 mg/L), which the USGS classes as moderately hard. Limited local data (fewer than 20 samples): individual well testing recommended. The California statewide median is 9.9 gpg.
Is iron a problem in Marin County well water?
The median iron level across 5 sampled wells is 0.01 mg/L, below the EPA aesthetic guideline of 0.3 mg/L (limited local data (fewer than 20 samples): individual well testing recommended). Most wells will not stain, but if yours does, a lab test tells you whether iron or manganese is the cause.
Why does my well water smell like rotten eggs in Marin County?
The data cannot say: just 1 Marin wells were tested for hydrogen sulfide. If your water smells, test for sulfide, and if the smell is only in hot water, check the water heater anode rod. It is unpleasant but rarely a health risk.
Is chromium-6 a concern in Marin County well water?
Only 1 wells in Marin County have been sampled for hexavalent chromium, too few to say how common it is. California’s limit is 10 µg/L, and only a lab test of your own well can show whether it is affected.
Does boiling well water make it safe to drink in Marin County?
Boiling handles bacteria and nothing else. It does not soften Marin water, remove iron or take out nitrate, which ends up more concentrated. Read when boiling helps and when it hurts.
How often should I test my well water in Marin County?
The California State Water Resources Control Board recommends testing domestic wells every year, including for bacteria, nitrate and arsenic, and again after flooding or well repairs.
Written by Well Water Advisor Editorial Team.