WorldWide Drilling Resource

Notes from the Groundwater Guy by Thomas E. Ballard, P.G., C.H.G. Southeast Hydrogeology, PLLC A Cautionary Tale Well problems usually fall within predictable patterns, so the tendency is to immediately jump to a solution once you know what the problem is. Every now and again, though, we run into a problem which may appear to have a specific cause, but turns out to be something else. This 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. Such was the case on a recent well project we assisted on. The client had a small water system in California, where they had just conducted rehabilitation on a well. After the well rehabilitation was complete, 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 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 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 nondetect results on their total coliform test. It was a relatively inexpensive solution which solved the problem without diving into the ex- pensive diagnostics 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. Tom Tom Ballard may be contacted via e-mail to michele@worldwidedrillingresource.com 31 WorldWide Drilling Resource ® NOVEMBER 2020 WTR

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