Coliform Bacteria in Well Water: What to Do After a Positive Test
Total coliform bacteria are an indicator: they show that surface water or soil is reaching the well or plumbing, which means disease-causing germs could too. A positive test isn't a reason to panic, but it is a reason to stop drinking the water untreated until the well is disinfected and retested.
The Minnesota Department of Health lays out the standard sequence: use bottled or boiled water, disinfect the well with a chlorine solution, then test again. If disinfection doesn't clear it, a licensed well contractor looks for the way contamination is getting in.
If this is unsafe or getting worse right now, don't wait — send an emergency request and it's flagged for priority routing to an independent local pro.
What to do in the first 5 minutes
- Switch to bottled water, or boiled water, for drinking, cooking, brushing teeth and making formula.
- If the test also found nitrate or arsenic, use bottled water; boiling concentrates those contaminants.
- Look at the wellhead: a missing or cracked cap, a broken seal, or ponding water around the casing are common entry points.
- Have the well shock-chlorinated by a licensed contractor, or follow your state's measured disinfection steps.
- Retest after the chlorine has been flushed out, and keep using bottled or boiled water until the result is negative.
Do not do this
- Don't drink or cook with untreated water while waiting for a clean retest.
- Use only unscented household bleach for disinfection, in the amount your state's instructions give.
- Don't flush heavily chlorinated water into a septic system all at once; run it to a spot away from the drain field and streams.
- If the test found E. coli, treat it as fecal contamination: don't use the water at all for drinking until the problem is fixed.
What's likely going on
Damaged or loose well cap
Insects, small animals and runoff get in through a cracked or poorly fitted cap. A sanitary, vermin-proof cap is a cheap fix.
Recent well or plumbing work
Any time a pump, pipe or tank is replaced, bacteria can be introduced. MDH recommends testing after any work on the well or water system.
Surface water reaching the casing
Flooding, a buried wellhead or ground sloping toward the casing lets runoff seep down the outside of the pipe.
Failed casing or grout
On older or shallow wells, a cracked casing or missing grout seal can let shallow groundwater in. If repeat disinfection fails, this is a contractor diagnosis.
Diagnose it now
What it typically costs
Shock chlorination itself uses a measured dose of household bleach (our shock chlorination calculator works out the amount for your well's depth and diameter). Repairs, from a new sanitary cap to casing work, depend on what a licensed well contractor finds, so get the cause identified before agreeing to bigger work.
Frequently Asked Questions
Can I shower if my well tested positive for coliform?
Most healthy adults can bathe while waiting for disinfection, as long as they don't swallow the water. Use bottled or boiled water for drinking, cooking and brushing teeth, and ask your health department about infants or anyone with a weak immune system.
How soon can I retest after shock chlorination?
Wait until the chlorine has been flushed out of the system; a chlorine residual can cause a false negative. Your lab or health department can tell you how long to wait.
What if my well keeps testing positive?
Repeated positives mean contamination is still getting in. A licensed well contractor can inspect the cap, casing and grout, and in some cases the fix is a repair or a new well.
How often should I test my well for bacteria?
MDH recommends a coliform test every year, plus any time the well or water system is worked on or the water changes in taste, smell or color.
The service that handles this
Guidance only — independent local providers assess and price their own work. Last reviewed: October 17, 2026.
Sources for the cost figures
Prefer to just talk to someone?
Send the short form — we'll route you to an independent local pro.