How often to test your well water in Sonoma County
Test for bacteria twice a year and after any flood. Test nitrates every five years. Use a state certified lab.
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The schedule
| Test | How often |
|---|---|
| Bacteria (total and fecal coliform) | Twice a year, and after a flood or other disaster |
| Nitrates | Every 5 years |
| Minerals | Every 5 years, or when taste or color changes |
| Petroleum, solvents, pesticides | Every 3 to 5 years |
| VOCs and SVOCs | After wildfire damage to the water system, or if you suspect industrial contamination |
Bacteria and nitrate are the most common problems in well water, so they get tested most.
If a result is high
- Nitrate above 45 mg/L as NO3, or 10 mg/L as nitrate-nitrogen: install a treatment device or find another water source.
- Total coliform present: look for the cause, fix it if you can, and retest.
After a wildfire
If plastic pipes or well parts got very hot, the system lost pressure, or ash and debris got in, test for VOCs and SVOCs.
State certified labs
The county lists these. It does not endorse any of them.
- Brelje and Race Laboratories, 425 S. E St., Santa Rosa, 707-544-8807
- Sonoma County Public Health Laboratory, 3313 Chanate Rd., Santa Rosa, 707-565-4711 (bacteria only)
- K Prime, Inc., 3621 Westwind Blvd., Santa Rosa, 707-527-7574
- Alpha Analytical Laboratories, 737 Southpoint Blvd., Petaluma, 707-769-3128
- Caltest Analytical Laboratory, Napa, 707-258-4000
Questions
Sonoma County Environmental Health, (707) 565-6565, 8 am to 4 pm, Monday to Friday. A single result is one point in time, so keep your results and compare them year to year.
Sources
Checked 2026-10-11. Rules and programs change. If something here is out of date, the source above wins. Spot an error? See our corrections policy.
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