Coliform in well water: what a positive test means and what to do
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Short answer
Total coliform means surface water or soil is getting into your well. E. coli means the contamination includes animal or human waste. If E. coli is positive, stop drinking the water untreated today: use bottled water or boil it, and call your local health department. Then shock-chlorinate the well, fix the entry point, and retest. If bacteria keep coming back, a UV system treats every gallon entering the house.
If your test shows E. coli: don't drink the water, cook with it or brush teeth with it until it's treated. Bring water to a rolling boil for one minute, or use bottled water, and contact your local health department.
What the results mean
| Result | What it means | How urgent |
|---|---|---|
| Total coliform negative | No sign of surface contamination | Retest yearly |
| Total coliform positive, E. coli negative | Something is letting surface water or soil into the well, like a loose cap or cracked casing | Act within days |
| E. coli positive | Fecal contamination from animals or a septic system | Act today |
EPA recommends testing private wells every year for total coliform, plus right away after flooding, well repairs, or any change in taste, color or smell.
What to do, in order
- Protect your household now. Bottled or boiled water until a retest comes back clean.
- Find the entry point. Check the well cap (is it sealed and vermin-proof?), the casing for cracks, and the ground around the well. Water should drain away from it. Check the distance to septic systems and animal areas.
- Shock-chlorinate the well. Many county health departments publish instructions, or a well contractor can do it.
- Retest 1 to 2 weeks later, after the chlorine has flushed out.
- If bacteria return, install a UV system as permanent protection, and keep testing yearly.
UV systems for well bacteria
-
Best UV for most wells: 15 GPM
SpringWell UV Water Purification System
Water passes a UV-C lamp that scrambles the DNA of bacteria and viruses so they can't reproduce. It adds nothing to the water and removes no minerals, so the water must be clear first.
- Kills 99.9% of bacteria and viruses (maker's claim)
- 15 GPM, 1–7 baths
- Needs hardness < 7 gpg, iron < 0.3 ppm
- Turbidity < 1 NTU, tannins < 0.1 ppm
- Lifetime warranty, 6-month money-back
About $1,100lamp replacement about $119/year · checked October 2026 Check price at SpringWell -
Best for clean-mineral wells that also want taste filtration
Aquasana Rhino Well Water with UV
A whole-house carbon filter for taste, odor and chemicals, followed by a UV lamp for bacteria and viruses. It is not an iron filter.
- UV: up to 99.8% of bacteria and viruses (maker's claim)
- 500,000 gallons or 5 years
- 7 GPM rated, 14.7 GPM peak
- Needs iron < 0.3 ppm, manganese < 0.05 ppm, hardness < 7 gpg
- 5-year warranty, 90-day guarantee
About $1,750sale price at last check · checked October 2026 Check price at Aquasana
UV only works on clear water. Both systems above require iron under 0.3 ppm and hardness under 7 gpg, because minerals coat the lamp sleeve and shadow bacteria. If your well has iron or hard water, those get treated first. The system matcher puts everything in the right order.
Questions well owners ask
Can I drink well water with coliform bacteria?
Total coliform itself rarely causes illness, but it signals a pathway for germs that can. E. coli is a direct warning. Treat both as a reason to use safe water until you've fixed and retested.
Does a water filter remove bacteria?
Standard sediment and carbon filters don't reliably kill bacteria. UV, chlorination or boiling do. Some point-of-use filters are certified for microbial reduction; check the certification.
How often should I test my well for bacteria?
EPA recommends at least once a year, plus after floods, well work, or any change in your water. Test more often if infants, pregnant women or older adults live in the home.