California county well guide

Well Water in Marin County, CA: Testing, Contractors and Rules

About 1,165 households in Marin County, 1% of the county, rely on a private well. Nobody tests that water unless the owner does.

Labs within 40 milesLicensed contractors
1%of households on a private well
Share on wells, county vs state
Marin County1%
California average4%
20licensed contractors serve the county
16 mito the nearest certified lab, Cel Analytical, Inc.
Nostatewide test required when a home is sold

Wells are the exception in Marin County: about 1,165 households, under 1% of the county, are on a private well. Being in the minority means less local help, so this page gathers what applies.

Local rules for wells in Marin County

The rules that apply in Marin County are set at the state level in California. The state does not require a test on a new well, and nothing is required at sale. Some counties and townships add permits or setbacks of their own, so check locally before drilling. The state's own recommendations are below.

At a glance

Water test for a new wellNot required statewide

No statewide water test. State law (Water Code 13801) makes the county the permitting agency: you need a well permit from the county environmental health department before drilling, the well must be built to the California Well Standards in DWR Bulletin 74 (annular seal, setbacks from septic systems and property lines, disinfection) by a C-57 licensed contractor, and the driller files a Well Completion Report with the Department of Water Resources within 60 days. Most counties require a bacteriological sample, and some add nitrate, before they sign off the permit, so check your county's well ordinance.

Water test when a home is soldNot required statewide

No statewide requirement. The Transfer Disclosure Statement only says whether the property is on a well. FHA, VA and USDA loans normally require a bacteria, nitrate and lead sample from an accredited lab before closing, and a handful of counties and cities (Santa Cruz County among them) require a well inspection or test at transfer.

What the state recommends

The State Water Board recommends testing a domestic well every year. Bacteria and nitrate are the yearly minimum; a first test should add arsenic, uranium, hexavalent and total chromium, 1,2,3-TCP, fluoride, hardness, iron, manganese, sodium, boron and total dissolved solids, with perchlorate and PFAS where the county or a nearby public system has found them. Test again after a flood, a wildfire, well work or a change in taste, color or smell, and check the level in dry years.

Free or low-cost testingAvailable

In the San Joaquin and Sacramento valleys the nitrate management zones set up under the Central Valley Water Board's Nitrate Control Program test private wells for free and supply bottled water where nitrate is over the limit: the Valley Water Collaborative covers the Modesto, Turlock, Eastern San Joaquin, Delta-Mendota, Merced, Madera and Yolo basins ((209) 750-3867), and the Kings, Kaweah, Tule and Chowchilla zones run similar programs through Self-Help Enterprises. Several counties (Santa Clara Valley Water and Sonoma County among them) run free or reduced-cost domestic well testing days. Elsewhere, county public health labs run a bacteria test for roughly $30 to $60.

Getting your water tested near Marin County

The nearest certified lab is Cel Analytical, Inc. in San Francisco, about 16 miles from the county center. There are 12 more within 40 miles, listed below with what each tests for.

13 certified labs within 40 miles

Licensed well contractors serving Marin County

Licensed help in Marin County includes American Construction and Supply Inc and Benevent Building and 18 others. Every listing was matched to the California license list; the "verify" link on each profile opens the state lookup.

Well depth and water chemistry

Well depth and water chemistry in Marin County follow the rock underneath it. Typical wells here run 150 to 310 feet; the median of 216 domestic wells reported to DWR since 2000 is 250 feet. Contaminants worth testing for in this part of California include nitrate (san joaquin valley, salinas valley, dairy and orchard country), arsenic (tulare lake basin.

Typical well depth

150 to 310 feet; the median of 216 domestic wells reported to DWR since 2000 is 250 feet

Contaminants to test for
  • Nitrate (San Joaquin Valley
  • Salinas Valley
  • Dairy and orchard country)
  • Arsenic (Tulare Lake basin
  • Western San Joaquin Valley
  • The deserts)
  • Uranium (eastern San Joaquin Valley
  • Sierra foothill granite)
  • Hexavalent chromium (Coachella Valley
  • Parts of the Central Valley)
  • 1
  • 2
  • 3-TCP and DBCP (former fumigated farmland in the Central Valley)
  • Coliform bacteria (shallow foothill and coastal wells near septic systems)
  • Hardness and total dissolved solids (Coast Ranges
  • Desert basins)
  • Iron and manganese (Delta
  • Coastal alluvium)
  • Fluoride and boron (Kern
  • Imperial
  • Desert basins)
  • Perchlorate (Inland Empire)
  • PFAS near airfields and industrial areas

Neighboring counties: San Francisco County, Napa County, Contra Costa County, Solano County.

What well work costs in Marin County

Bacteria test (coliform and E. coli)$38 to $61published California lab fees
Nitrate test$66published California lab fees
Broad panel (metals and minerals)$445published California lab fees
Well inspection$250 to $550$350 to $900 with water testing
Submersible pump replacement$1,000 to $2,500installed; deep wells cost more
Pressure tank replacement$800 to $1,500installed
One treatment unit$700 to $3,500softener, iron filter or UV, installed
Multi-stage whole-house treatment$2,500 to $10,000+several units or whole-house RO, installed
New well, complete$6,000 to $16,000typical 200 ft well with pump; drilling alone about $20 to $30 per foot; deeper wells cost more
Shock chlorination$80 to $200by a contractor; under $30 to do yourself

Sonoma County Public Health Lab ($38 bacteria) and a commercial certified lab (Caltest: $61 bacteria, $66 nitrate, $445 homeowner panel). Installed costs are national 2026 ranges; local quotes vary with depth, access and materials. Sources, checked September 2026: Sonoma County Public Health Lab fee schedule, Caltest homeowner packages, HomeGuide cost guides, Angi: cost to shock a well.

Questions Marin County well owners ask

How many homes in Marin County are on a well?

About 1,165, or 1% of households, based on Census housing data and state well surveys. The share is highest outside the towns with public water.

Where is the closest certified lab to Marin County?

Cel Analytical, Inc. in San Francisco, about 16 miles away. Call first to ask whether they accept homeowner samples.

Do I need a permit to drill a well in Marin County?

No statewide water test. State law (Water Code 13801) makes the county the permitting agency: you need a well permit from the county environmental health department before drilling, the well must be built to the California Well Standards in DWR Bulletin 74 (annular seal, setbacks from septic systems and property lines, disinfection) by a C-57 licensed contractor, and the driller files a Well Completion Report with the Department of Water Resources within 60 days. Most counties require a bacteriological sample, and some add nitrate, before they sign off the permit, so check your county's well ordinance.

What does a well test cost near Marin County?

Published lab fees in California: $38 to $61 for a bacteria test and $66 for nitrate. Sonoma County Public Health Lab ($38 bacteria) and a commercial certified lab (Caltest: $61 bacteria, $66 nitrate, $445 homeowner panel). Mail-in kits cost about the same as walking in.