Well problems usually fall within predictable patterns, so the tendency is to jump immediately to a solution once you know what the problem is. Every now and again, though, we run into a problem that may appear to have a specific cause but turns out to be something else. That is a cautionary tale and it is good to remind us not to jump to conclusions, and to go through the process of diagnosing the cause of the problem, even if it seems obvious.
That was the case on a recent well project we assisted on. The client was a small water system in California where they had just conducted rehabilitation on a well. After the well rehabilitation was completed, they had problems disinfecting the well. They were following all the standard procedures for disinfection but kept getting Total Coliform detections, and that is where I got involved.
I usually like to start the diagnostic process with well construction information and a well log. Since Total Coliform can often be a surface water intrusion issue, I suspected an issue with the seal depth. It seemed possible that the rehabilitation process had opened up the screens in the upper part of the well to allow shallow groundwater to enter the well, where plugging may have reduced flow from the upper zone prior to the rehabilitation. I drew up a sketch of the well construction and the logged geology and, indeed, it appeared there was a potential for shallow water to enter the well and cause a possible Total Coliform issue.
However, before I recommended some expensive diagnostics to test this scenario, I proposed that they try disinfecting again using a buffering solution to stabilize the pH for the chlorine to work effectively. Chlorine is the most effective at around a neutral pH where it can form hypochlorous acid, however, the reactions of chlorine with some types of groundwater will often move the pH out of the range necessary for hypochlorous acid formation, rendering the disinfection process mostly ineffective.
This was indeed the case, and after performing another disinfection with the buffered chlorine solution, they got non-detect results on their Total Coliform test. It was a relatively inexpensive solution that solved the problem without diving into the expensive diagnostics that we were originally looking at. The end result was a happy client who solved a perplexing problem and got their well back online without a lot of additional expense.