Well Water Testing: What to Test, How Often, How to Read It

Test a private well at least once a year for total coliform bacteria, nitrate, total dissolved solids (TDS) and pH, using a state-certified laboratory. That is the baseline both the CDC and the EPA set. Test again, outside the annual schedule, after flooding near the well, after any repair to the well or pump, when the water’s taste, color or smell changes, and when someone in the home becomes pregnant or a baby arrives.

No agency tests a private well for you. Public water systems are regulated; private wells are the owner’s job, which is why the schedule below matters.

The annual baseline

The EPA adds that households with small children, elderly adults, or a pregnant or nursing person should test more often, because those groups are more vulnerable to contaminants.

Ask your county or state health department which local contaminants belong on top of this list. In some regions that means arsenic, uranium, radon or manganese; near farms it means pesticides; near fuel storage it means volatile organic compounds.

When to test outside the annual schedule

The CDC and EPA name the same set of triggers:

• Flooding or land disturbance near the well. Minnesota’s health department uses a concrete threshold: retest for bacteria when floodwater comes within 50 feet of the well.
• Any repair or replacement of the well, pump, pressure tank or piping. Opening the well can introduce bacteria.
• A change in taste, color or odor.
• A pregnancy, a new baby or a child moving in. Minnesota recommends a nitrate test before bringing home a newborn.
• Known problems in your area, such as a neighbor’s contaminated well or a health-department notice.
• New construction, industry or waste disposal nearby.

Extra tests keyed to what is around your well

The EPA publishes a chart that matches conditions to tests. The common ones for private wells:

If stains or a smell are the reason you are testing, our guides to iron in well water and sulfur smell in well water cover which follow-up numbers matter.

Certified lab or home test kit?

Use a state-certified lab for anything health-related: bacteria, nitrate, arsenic, lead and the other primary contaminants. The CDC and EPA both say so, and many state and county health departments run their own low-cost or free well-testing programs. The EPA maintains a list of state certification programs (linked in the sources below) that leads to certified labs in each state. Penn State Extension puts a total coliform test at about $10 to $30.

Test strips and home kits are useful for screening and for checking a treatment system between lab tests: hardness before sizing a softener, pH before choosing an iron filter, chlorine residual after shock chlorination. They are not a substitute for a lab result when the question is whether the water is safe to drink. Our comparison of well water test kits separates mail-in lab kits from strips and colorimetric kits.

How to collect a bacteria sample

A bad sample produces a false positive, which leads to an unnecessary round of shock chlorination. Follow the lab’s instructions first; where they are silent, these steps come from the Tacoma-Pierce County Health Department’s sheet for well owners:

1. Get a sterile bottle from the lab. It may contain a small amount of liquid or powder that neutralizes chlorine. Do not rinse it out.
2. Pick a representative cold-water tap. Avoid swivel faucets, single-lever mixing faucets, leaky faucets, frost-free hose bibs and taps at or near ground level.
3. Remove the aerator, screen, hose or filter and disinfect the faucet tip with a chlorine solution.
4. Run the cold water for at least 5 minutes, then turn it down to a stream about the width of a pencil and run it 1 more minute.
5. Open the bottle without touching the inside of the cap or the rim, and do not set the cap face down.
6. Fill to the fill line without overflowing, keeping the bottle off the faucet. Cap it.
7. Fill out the lab slip and deliver the sample fast. Analysis must begin within 30 hours of collection; refrigerate the sample if you cannot deliver it right away. Check the lab’s drop-off days so the sample does not sit over a weekend.

Do not sample within days of shock chlorinating; residual chlorine can produce a false negative. Wait until a test strip shows no chlorine at the tap.

Results also swing with weather. Penn State Extension found coliform counts usually run highest after several weeks of rain and lowest in dry, cold winters, so a single clean winter result is not proof the well is sealed.

Reading your results

Labs report most results in milligrams per liter (mg/L), which equals parts per million (ppm). Bacteria come back as present/absent or as colonies per 100 mL. Compare each number with the right kind of limit.

Health-based limits (EPA MCLs)

Maximum contaminant levels are enforceable for public water systems. They are not legally binding on a private well, but they are the standard to judge your water against.

Nuisance limits (EPA SMCLs)

Secondary standards are non-enforceable guidelines for taste, odor, staining and scale. The EPA does not consider these contaminants a health risk at the SMCL. They decide whether you need a softener, an iron filter or a neutralizer.

Hardness has no EPA standard. If yours is high, see our water softener guide.

What to do after a bad result

1. Retest to confirm if the result is a surprise, using the same certified lab.
2. For bacteria or nitrate over the limit, switch to bottled water for drinking, cooking and infant formula until the problem is fixed. Do not boil water for nitrate; boiling concentrates it.
3. Call your county or state health department. They can help interpret the result and may recommend a testing schedule.
4. Find the cause before buying treatment. A cracked well cap or a failing septic system will defeat any filter.
5. Retest after treatment to confirm it works, then keep the annual schedule.