A positive total coliform test means surface contamination has a pathway into your well, not that your water is necessarily making anyone sick. A positive E. coli test is more serious: it indicates fecal contamination and calls for safe water, disinfection, and a confirmed clean retest before drinking again.
What does a positive coliform test actually mean?
Coliforms are a large family of bacteria that live in soil, on vegetation, and in surface water. Most of them do not cause disease. Labs test for them anyway because they are superb indicators: coliforms are abundant on the surface and scarce in properly sealed groundwater, so finding them in your sample means the barrier between the surface world and your well has been breached somewhere, by a cracked cap, damaged casing, pooled rainwater, flooding, or work done on the well.
That framing should guide your emotional response. The EPA treats total coliform as an indicator of pathway, and under its Revised Total Coliform Rule a positive in a public system triggers assessment and repair, finding and closing the door, not an automatic public health emergency. Your private well deserves the same logic: a positive is a work order, and it is also genuinely urgent to understand which kind of positive you have, which is the next section.
Total coliform vs. E. coli: what is the difference?
Your lab report will distinguish two findings, and everything about your next steps depends on which boxes are checked:
| Result | What it indicates | Urgency |
|---|---|---|
| Total coliform present, E. coli absent | A contamination pathway exists; bacteria from soil or surface water are entering | Fix and disinfect soon; many households keep using water for washing while resolving it |
| E. coli present | Fecal contamination from human or animal waste; disease-causing organisms may be present | Stop drinking immediately; switch to bottled or boiled water until a clean retest |
E. coli lives in the intestines of warm-blooded animals, so its presence means waste from a septic system, livestock, or wildlife is reaching your water. The CDC's guidance during such an event is direct: use bottled water or bring water to a rolling boil for one minutefor drinking, cooking, ice and tooth brushing. The EPA's maximum contaminant level goal for E. coli in drinking water is zero, there is no acceptable amount.
What should you do first after a positive test?
The response is a sequence, and the order matters:
- Secure safe water. If E. coli is present, switch to bottled or boiled water for everything that goes in your mouth. If only total coliform is present, boiling for drinking is a sensible precaution while you work.
- Confirm with a second sample if the first was a routine test with no obvious cause. Sampling errors happen; a confirmed positive is worth acting on fully.
- Inspect the well. Check the cap for cracks and a tight seal, the casing for damage, the grading so rain flows away from the wellhead, and the distance from septic components. Florida DOH and UF/IFAS both put the inspection before disinfection, because chlorinating a well with an open door just schedules the next positive.
- Disinfect. Shock chlorination, described below, is the standard response for a one-time contamination event.
- Retest after the chlorine clears, and only return to normal use when the result is clean.
How does shock chlorination work?
Shock chlorination floods the entire water system, the well column, the pressure tank, the pipes and the water heater, with a high concentration of chlorine for several hours, far above the levels used in city water. Florida DOH and UF/IFAS publish detailed procedures; the shape of the job is consistent:
- A measured chlorine solution, based on well depth and diameter, is introduced into the well.
- Water is circulated with a hose back into the well until chlorine odor is present, then each indoor and outdoor tap is run until the chlorine smell arrives, loading the entire plumbing system.
- The system rests, typically overnight, while the chlorine does its work.
- The well is flushed through an outside hose, away from the septic drain field, lawns and water bodies, until the chlorine is gone, then indoor taps are flushed.
Two honest warnings. First, concentrated chlorine is hazardous to handle and the procedure involves electrical components at the wellhead, so many families hire a licensed well contractor rather than doing it themselves, a reasonable choice. Second, shock chlorination is a one-time disinfection, not a protection: it kills what is in the system today and does nothing about what enters tomorrow. The lasting fix is whatever repair closes the pathway.
When do you retest, and what if it stays positive?
Retest after the chlorine has completely left the system, since residual chlorine in the sample suppresses bacteria and produces a false clean. Run an outside tap until no chlorine odor remains, give the system days to return to its normal state, then sample. Florida DOH's guidance is to retest after disinfection and, if the well stays positive, to repeat the disinfection or move to a permanent solution. Many well owners also add a follow-up test a few months later, to confirm the fix held through a rain cycle.
If positives keep coming back, you are no longer dealing with an event but with a condition. The realistic options:
- Repair or upgrade the well: a new sanitary cap, casing repair, grout sealing, or in old shallow wells, replacement.
- Continuous disinfection: UV systems neutralize bacteria at the point of entry and are a common choice for Florida wells, usually with sediment prefiltration so the UV light reaches every organism.
- Continuous chlorination followed by carbon filtration, which also helps when iron and sulfur are part of the picture, common companions in Florida groundwater, as our complete Florida well water guide explains.
Bacteria are one piece of a bigger annual routine, the full schedule is in how often to test well water in Florida, and the broader safety question is covered in is Florida well water safe to drink without treatment. If you want a trained eye on your well water without spending anything, our well water treatment page shows how the free in-home test works.
Frequently asked questions about coliform in well water
Can I shower if my well tested positive for coliform?
For a total coliform positive without E. coli, bathing is generally considered low risk for healthy adults as long as you avoid swallowing water. With an E. coli positive, be more careful: use safe water for brushing teeth and bathing small children, who tend to swallow bath water, until disinfection and a clean retest.
How did bacteria get into my well in the first place?
The usual routes are a cracked or unsealed well cap, casing damage, a poorly graded wellhead where rain pools, flooding, recent repairs done without disinfection, or a nearby septic system failing. Coliform is an indicator that one of these doors is open, which is why inspecting the well matters as much as treating the water.
Does boiling water make it safe during a coliform event?
Yes for microbes: the CDC advises a rolling boil for one minute to kill bacteria, viruses and parasites, which covers drinking, cooking, ice and tooth brushing during the event. Boiling does not remove nitrate or chemicals, it only addresses the biological side.
Will a water filter remove coliform bacteria?
Standard sediment and carbon filters are not designed to make microbiologically unsafe water safe. The reliable residential tools are fixing the well defect, shock chlorination for one-time events, and continuous disinfection such as UV systems, sized and maintained correctly, for wells with recurring positives.
How often do Florida wells test positive for coliform?
Positives are common enough that Florida county health departments run year-round bacteriological testing programs and publish disinfection instructions. Frequency varies by region, well construction and season, with rainy season and post-hurricane periods producing more positives. That is why the CDC recommends every private well be tested at least annually.
Sources
- U.S. Environmental Protection Agency. Revised Total Coliform Rule and drinking water standards (E. coli MCLG of zero). epa.gov
- Centers for Disease Control and Prevention. Private Wells: coliform bacteria, E. coli and boil water guidance (rolling boil, one minute). cdc.gov
- Florida Department of Health. Private Well Testing: bacteriological sampling and well disinfection guidance. floridahealth.gov
- University of Florida IFAS Extension. EDIS publications on bacteria in private wells and shock chlorination procedures. edis.ifas.ufl.edu
- U.S. Geological Survey. Bacteria and microbial quality in domestic well water. usgs.gov