Partner Blog

Information of use to those who serve private well owners.

Our Partners Speak! January Partner Poll

In our January Partner Newsletter we asked our subscribers to answer the following question:

“Should coliform bacteria sample results be provided to well owners as presence/absence or a quantified number of colonies?”

We received eight responses:

Quantify the results





Most coliform bacteria are not harmful, but they are an indicator for the potential presence of pathogens. When coliform bacteria are present in a private well water supply, there is a possible contamination pathway for a source of bacteria (surface water, septic system, animal waste, etc.) or other contaminant into your well. In other words, harmful bacteria may use this same pathway to enter your drinking water supply.

When coliform are present in a water supply, a follow-up test for E. coli is typically recommended. E. coli are the best indicator of potential health hazards, as they are only found in the gut and feces of warm-blooded animals. (To learn more about the basics of coliform bacteria, this guide from the New York Department of Health is helpful.)

The presence/absence test is simple, inexpensive, and goes by the theory that any coliform indicates a potential breach in the well. So if present, disinfect and don’t drink the water until the test comes back negative.

The camp that believes you should quantify the number of colonies thinks that understanding the magnitude of the contamination provides a better understanding of the possible causes. So, as one of our respondents noted, having one colony vs. hundreds certainly tells you something more than a presence/absence test.

The following response best sums up the argument from a public health perspective:

“There are some wells in our county that will continuously come back with a result of 1 (colony) test after test and shocking after shocking. There are also some wells that routinely come back over the maximum quantification level even after shocking. I feel like these two wells are completely different situations. The first well appears to have minor contamination while the second well has major contamination. Yes, they both pose a risk, but very few things are perfect in this world. I would be much more comfortable drinking out of the first well than the second well. Without quantification, there is no way to tell the difference between these two wells. If someone has repeated low levels of bacteria, I would recommend retesting more frequently to see how it changes; whereas the second well I would encourage them to talk to a contractor about rehabilitating the well or repairing the well system.”

The real take-home message from this exercise, at least based on the 8 responses we received, was that this is a confusing issue. Several responses mentioned bacteria being found miles below the earth for instance, and several assumed we were talking about fecal coliform in our question (E. coli), when we were not. We were asking about the total coliform test that is a requirement for community water supplies and considered the standard approach to judging the safety of a well from near-surface contamination.

Regarding the bacteria found at depth issue, bacteria are ubiquitous in our environment, indeed, but they are not all coliform bacteria. Iron bacteria, sulfate-reducing bacteria, and methanogenic bacteria, for instance, are all found in aquifers in Illinois. They actually help us understand other issues, like the reducing condition of the water in an aquifer, but they do not cause health issues. Several respondents seemed to think that all bacteria are a health risk; this is not the case.

Regarding the total coliform vs. fecal coliform vs. E. coli issue, testing for total coliform bacteria is a simple way to determine possible exposure to pathogens. The idea is to test for total coliform first because it’s easier and cheaper. Then, if there is a positive result, test for E. coli. We could have made this clearer, for sure, but for those who work with these results daily, the debate over a positive result versus a quantifiable result was the issue we were hoping to explore.

We were leaning more into the presence/absence camp when we started five years ago, to be honest. After all, any coliform means there could be a risk, and that is still true. But after hearing from many professionals, including at our conference last May, about the value of knowing the magnitude of the exposure, we are now leaning the other way. Both certainly have value, but, quantifying the test result does give you more information and provides value in making a decision.

If you would still like to chime in with your thoughts and opinions, we would love to hear from you. Take the poll