Coliform Bacteria in Well Water: How to Test and What to Do

On occasion, we hear from customers who want to test their well water for coliform bacteria, usually because they're wondering whether their water is safe, or because a test they've already had done came back positive. A TDS meter is a quick way to check total dissolved solids, but it's limited: it won't tell you anything about bacteria or most chemical contaminants. This guide covers what coliform bacteria is, when and how to test for it, how to collect a sample correctly, and what to do if your results come back positive.

Where Coliform Bacteria Comes From

Coliform bacteria occur naturally in soil, on vegetation, and in the intestinal tract of warm-blooded animals. They're relatively easy to detect in a lab and tend to behave, in the environment and under water treatment, in a way that's similar to more dangerous pathogens, which is why they're used as an indicator rather than tested for directly. Fecal coliform and E. coli specifically point to possible contamination from septic systems, water table influences, or runoff from woodland, pasture, or feedlots.

What a Positive Coliform Result Means

Coliform bacteria themselves are largely harmless. Total coliform is a broad group of relatively harmless microorganisms; fecal coliform, the most common member being Escherichia coli, is a subgroup tied specifically to the waste of warm-blooded animals, and its presence is more serious because it points to possible sewage or animal-waste contamination. Coliform and E. coli are used as indicators because they're easy to culture and far more common than the pathogens themselves, not because they're dangerous on their own.

It's sometimes assumed that if coliform is present, larger disease-causing organisms (pathogens) must be present too. That assumption doesn't reliably hold. Pathogens vary widely in size (see the table further down), and the presence of smaller, non-disease-causing bacteria in a sample doesn't tell you whether larger pathogenic organisms are also present. This is exactly why coliform and E. coli results are treated as indicators of possible contamination, not direct proof of it, and why a positive result calls for follow-up rather than panic.

When to Test Your Well Water

The CDC recommends testing private well water at least once a year for total coliform bacteria, nitrates, total dissolved solids, and pH. Beyond that annual check, test again: every spring as part of a general well inspection, after any flooding or nearby land disturbance, after replacing or repairing any part of the well system, when a baby joins the household or during pregnancy, and any time you notice a change in your water's taste, color, or smell. If you don't have a specific reason to test sooner, late spring or early summer is a good default window, since coliform contamination is more likely to show up during wet weather. Contact your state or county health department to find a certified testing lab; fees are usually modest. If your well is near active oil or gas drilling, coliform is only one piece of a larger testing panel worth running. See our guide to testing well water near oil and gas activity for the full list.

How to Collect a Water Sample for Testing

Proper collection and handling matters as much as the test itself. Always get your sample container directly from the testing lab, since containers are sometimes specially prepared for a given contaminant, and bacteria samples must go in a sterile container.

1. Run the water for about one hour to flush out stale water and bring in fresh water.
2. Wipe down the sample point with an alcohol swab. Don't touch or contaminate the inside of the container, bottle, or cap.
3. Open the sample container carefully, holding only the outside of the cap.
4. If you're sampling from a Berkey system, place the container in the lower chamber directly under a filter and collect the sample from the drip water. Setting the container on top of an upside-down cup inside the chamber can make this easier.
5. Don't collect the sample from the spigot, since it may carry contamination from handling and everyday use.

Bacteria can double in number roughly every twenty minutes under the right conditions, so a single microbe could become over a million within about four hours. Refrigerate your sample right away and get it to the lab within six hours, ideally in a cooler with ice. Avoid dropping a sample off on a Friday, since it may sit untested until Monday, and don't mail a bacteria sample, since results from mailed samples are typically unreliable.

How Labs Test for Coliform Bacteria

Direct testing for pathogens themselves is expensive and impractical, since pathogens are rarely found in water bodies even when contamination is present. Approved methods for total coliform include membrane filtration, multiple-tube fermentation, MPN, and MMO-MUG (marketed as Colilert). The membrane filter method passes the sample through a fine filter that retains bacteria, incubates it on a growth medium for 24 hours at 35°C, and counts the resulting colonies directly; a positive result requires a confirmation test using more specific media, adding another 24 to 48 hours. (Source: EPA, Volunteer Estuary Monitoring Manual, Chapter 17: Bacteria, Indicators of Potential Pathogens; Wilkes University Center for Environmental Quality, GeoEnvironmental Science and Engineering Department.)

For scale, here's roughly how a few common pathogens compare in size to the coliform indicators labs actually test for:

If Your Test Comes Back Positive

Since the strength of a person's immune system affects whether contamination makes them sick, different family members can react differently to the same result, and the health effects, mainly diarrhea, cramps, nausea, and headaches from strains like E. coli, aren't always immediately obvious. Biological contaminants like coliform bacteria are most effectively addressed through chlorine disinfection (including shock chlorination of the well), filtration, ultraviolet irradiation, or ozonation. Some homeowners prefer to avoid ingesting disinfectants like chlorine directly and instead use a water filtration system, whether a whole-home system, an under-sink unit, or a countertop system like the Berkey Water Filter, which is effective at reducing many chemical contaminants and improving taste in treated water. A water filter is not a substitute for well testing or disinfection: if coliform bacteria is suspected or confirmed, treat the well first, such as with shock chlorination, before relying on any point-of-use filter.