Coliform Bacteria in Well Water: What a Positive Test Means
A lab report that says “total coliform: present” is one of the most common results private well owners get, and one of the most alarming. The good news is that most coliform bacteria do not make you sick, and the fix is usually simple.
This guide explains what total coliform and E. coli results mean, what to do today, and how to stop the bacteria coming back. Use the planner to get the steps for your exact result.
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Total coliform positive, E. coli negative: not an emergency. Inspect the well cap, shock chlorinate the well and retest within about a week, then again in 6 months.
E. coli positive: stop drinking the water untreated. Use bottled water or boil it for at least 1 minute, find the contamination source and disinfect the well.
Coliform Result Action Planner
Pick what your lab report says. The planner tells you what to do today, this week and long term.
General guidance based on state health department and extension advice. Follow any instructions from your local health department or lab.
What Coliform Bacteria Are
Coliforms are a large group of bacteria that live in soil, on plants and in the gut of people and animals. Most of them are harmless, but labs use them as an indicator: if coliforms got into your well, a path exists that harmful germs could also use.
E. coli is a type of coliform that lives in human and animal waste. Finding it means fecal contamination has reached your water, and that is when the risk of illness rises.
Coliform bacteria have no color, smell or taste, so clear water can still test positive. Only a lab or certified test kit can tell you (New Jersey Department of Health).
How to Read Your Result
| Result | What it means | What to do |
|---|---|---|
| Total coliform absent | No sign of bacterial contamination in this sample | Retest every year |
| Total coliform present, E. coli absent | A path exists for surface bacteria, but no fecal sign | Inspect, shock chlorinate, retest within about a week and again in 6 months |
| Total coliform present, E. coli present | Fecal contamination reached the water | Do not drink untreated. Boil or use bottled water, find the source, disinfect |
| Coliform back after shocking | A structural problem or ongoing source | Professional inspection, repair or continuous treatment |
Some labs report a number (colonies per 100 mL) instead of present or absent. For drinking water, the target is zero, so any count counts as a positive.
Why Bacteria Get Into Wells
- A loose, cracked or unvented well cap that lets insects, rodents or rain in.
- Flooding or heavy rain that pushes surface water down around the casing.
- Recent well or pump work, where tools and parts were not disinfected.
- A failed grout seal or cracked casing on older wells.
- Nearby septic systems or animal pens uphill from the well.
- Shallow or dug wells, which are more exposed to surface water than deep drilled wells.
Look at the well first. A new sanitary cap often fixes the problem for little money; see our guide to the best well caps and sanitary seals.
How to Fix It
- Protect your family first. If E. coli was found, or anyone at home is an infant, pregnant or has a weak immune system, use bottled or boiled water. Boiling at a rolling boil for at least 1 minute kills bacteria (New Jersey Department of Health).
- Inspect the wellhead. Check the cap, vent screen, wiring conduit and the ground slope around the casing. Water should drain away from the well.
- Shock chlorinate the well. A strong chlorine dose is circulated through the well and plumbing and left for at least 12 to 24 hours. Our step by step guide shows how to shock a well safely.
- Flush and retest. Run water until the chlorine smell is gone, then retest. Most guidance suggests retesting within about a week, then again in 6 months.
- If it comes back, find the source. Repeated positives point to a structural defect or a nearby source. A licensed well contractor can inspect and repair it.
When You Need a UV System
If coliform keeps returning and the source cannot be fixed, continuous disinfection is the answer. Ultraviolet (UV) light kills bacteria as water flows past a lamp, with no chemicals and no change in taste.
UV works only on clear water. Iron, manganese and sediment shade the bacteria, so most systems need a sediment prefilter and sometimes an iron filter first; see our iron filters for well water and sediment filter guides.
Whole House UV Water Purifier
A UV system mounts on the main line after your pressure tank and sediment filter. Size it to your peak flow rate in gallons per minute, not the house size.
- Size by gallons per minute, usually 8 to 15 GPM for a home
- Look for NSF/ANSI 55 certification where possible
- Replace the lamp every year
No chemicals and no taste change. Kills bacteria continuously.
Needs power and clear water, and does not remove nitrate, iron or chemicals.
How to Test and How Often
Test for bacteria every year, and again after flooding, well repairs, or any change in taste, color or smell (New Jersey Department of Health). Spring, after snowmelt and rain, is a good time.
A state certified lab is the most reliable choice, and many county health departments sell low cost sample bottles. Home kits are useful for a quick check between lab tests.
Safe Home Bacteria Test Kit
A simple presence or absence test you can run at home. It is useful between lab tests or after shocking, but use a certified lab for a result you can rely on.
- Tests for coliform bacteria
- Results in about 48 hours
- No lab shipping needed
Fast and simple. Good for retests after shocking.
Not a substitute for a certified lab test.
See every option compared in our best well water test kits guide, or check other results with our well water test results checker.
Frequently Asked Questions
Is it safe to drink well water with coliform bacteria?
If total coliform is positive but E. coli is negative, most coliforms are harmless, but the result shows a path for contamination. Use boiled or bottled water to be safe until you fix it and get a clean retest, especially for infants, pregnant women and people with weak immune systems.
What should I do if my well tests positive for E. coli?
Stop drinking the water untreated. Use bottled water or boil water for at least 1 minute, contact your local health department, find and fix the source and disinfect the well, then retest.
Can I shower in well water with coliform?
Showering is generally considered lower risk for healthy adults, but avoid swallowing the water. If E. coli is present, follow your health department’s advice, and use boiled or bottled water for brushing teeth.
How long after shocking a well should I retest?
Flush the system until you no longer smell chlorine, then retest. Many guides suggest about 7 to 10 days after shocking, and again in 6 months.
Why does coliform keep coming back in my well?
Usually a structural problem: a loose cap, a cracked casing, a failed grout seal or surface water draining toward the well. A nearby septic system or animal area can also be the source. A well contractor can inspect it.
Does a water softener or carbon filter remove bacteria?
No. Softeners and carbon filters are not disinfection systems. Use a UV system or chlorination for ongoing bacteria control.
Sources: New Jersey Department of Health private well guidance on total coliform and E. coli; Mississippi State University Extension; EPA private well guidance.
