Well Water Testing in California: What to Test and How Often
A practical guide to well water testing in California. Learn what to test for, how often, certified labs, and when to test after drilling or repairs.
Key Takeaways
- Test annually for coliform, E. coli, and nitrates; every 2–5 years for arsenic, lead, uranium
- Use state-certified labs; basic panel $50–$150, comprehensive $300–$600+
- Required after new construction, repairs, deepening, and often for real estate
- Do not drink until results confirm safety after drilling or repairs
Private well owners are responsible for their own water quality. Unlike municipal water, well water isn't routinely tested by a utility—so it's up to you to ensure your water is safe to drink. Here's what California well owners should know about testing.
Required vs. Recommended Testing
When Testing Is Required
California doesn't mandate routine testing for existing domestic wells, but testing is typically required:
- After new well construction — Before use, to establish baseline quality
- After well repair or deepening — To confirm no contamination from work
- For real estate transactions — Lenders and buyers often require tests
- For some permit types — County or water district requirements may apply
Recommended Routine Testing
The CDC and state health agencies recommend testing private wells at least annually for:
- Total coliform bacteria — Indicates possible fecal or surface contamination
- E. coli — Indicates fecal contamination; do not drink if positive
- Nitrates — Especially important in agricultural areas; risk to infants
Additional tests every 2–5 years or when conditions change:
- Arsenic — Naturally occurring in some California aquifers
- Lead — From plumbing, especially in older homes
- Uranium — In certain geologic formations
- Manganese and iron — Aesthetic issues, staining
- pH, hardness, TDS — General water quality indicators
What to Test For by Concern
| Concern | Test | When |
|---|---|---|
| Bacterial contamination | Coliform, E. coli | Annually, after flooding, after repairs |
| Agricultural runoff | Nitrates, pesticides | Annually in ag areas |
| Natural contaminants | Arsenic, uranium, radon | Every 2–5 years or if geology suggests risk |
| Plumbing issues | Lead, copper | If plumbing is old or after changes |
| Taste, odor, staining | Iron, manganese, hardness | As needed |
Certified Labs
Use a state-certified laboratory for drinking water tests. The California State Water Resources Control Board maintains a list of certified labs. Your county environmental health department can also provide recommendations.
Typical Costs
- Basic bacteria + nitrates: $50–$150
- Expanded panel (arsenic, metals, etc.): $150–$400
- Full comprehensive test: $300–$600+
When to Test After Drilling or Repairs
After a new well is drilled or an existing well is deepened or repaired:
- Allow the well to develop — Your driller will typically pump and flush the well to clear drilling debris.
- Wait for water to stabilize — Run the well for at least 24–48 hours before sampling.
- Collect a sample — Follow lab instructions; often from a cold tap after flushing.
- Do not drink until results confirm the water is safe.
Your C-57 contractor may coordinate testing as part of the project. If not, schedule it before using the well for drinking or cooking.
Interpreting Results
- Coliform positive, E. coli negative — Possible surface contamination; shock chlorination and retest may resolve.
- E. coli positive — Do not drink; disinfect the well and plumbing, retest before use.
- Nitrates above 10 mg/L — Do not give to infants; install treatment or find alternative source.
- Arsenic above 10 µg/L — Install treatment (e.g., reverse osmosis) for drinking water.
When in doubt, consult your county environmental health department or a water quality professional.
Well Maintenance and Testing
Regular testing is part of well maintenance. Combine annual testing with visual inspections and pressure checks. If you notice changes in taste, odor, or appearance, test immediately.
For help finding a licensed driller for well repairs or new construction, use our California Well Driller Directory.