What Are Coliform Bacteria? What a Positive Well Test Means

Coliform bacteria are the first thing a well test looks for, not because they make you sick, but because they show whether germs can get into your water.

What does “coliform present” mean?

Coliforms are a large group of bacteria that occur naturally in soil, on vegetation and in surface water, according to the Wisconsin DNR. The Minnesota Department of Health notes that most don’t cause disease, but their presence suggests disease-causing microorganisms may be there too. Penn State Extension puts it as evidence that “a contamination pathway exists” between a source such as surface water, a septic system or animal waste and the water supply.

Lab reports often split the result into three levels:

• Total coliform: the whole group, the indicator.
• Fecal coliform: the coliforms that live in the intestines of warm-blooded animals.
• E. coli: a specific fecal coliform. Penn State calls a positive E. coli result much more serious than coliform alone, because it means human or animal waste is reaching the water.

What should you do about it?

On a private well, it’s your job. The EPA recommends testing for total coliform every year, and after flooding, well repairs or any change in taste, color or smell. If a test comes back positive, use boiled or bottled water, retest, and look for how the bacteria got in; our guide to bacteria in well water walks through retesting, shock chlorination and when UV makes sense.