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Notes from the Groundwater Guy by Thomas E. Ballard, P.G., C.H.G. Southeast Hydrogeology, PLLC Common Causes of Well Failure - Contaminants As we continue our discussion of common causes of well failure, we are going to consider one of the more perplexing well problems: contaminants. There are really two ways to deal with contaminants in a well. The best and most cost-effective way is to identify the contaminants during the design and construction phase, then build the well accordingly - with appropriate seals and casing/grouting zones which contain higher levels of contaminants to reduce the contaminants to levels not requiring treatment. This must be balanced against water production from the various zones in the well. Proper zone testing can identify these zones during the test boring phase and the final well can be designed accordingly. Of course, there are situations where there is a relatively uniform distribution of contaminants in the aquifer, and selectively sealing zones may not be an effective strategy, so at that point, the decision must be made as to whether to complete the test well as a production well and commit to a treatment plant, or explore another location less likely to contain contaminants at levels requiring treatment. For existing wells with contaminant issues, vertical flow and contaminant profiling can often pinpoint intervals of higher contaminant concentrations which can be sealed off without substantially impacting well production through a well modification process involving sealing off screen zones in the areas of highest contaminant inflow to the well. The vertical profiling method- ology employs assessing flow contribution from specific zones under pumping conditions, utilizing a flow meter or other method, balanced against depth-specific sampling for contaminant concentrations to produce a vertical profile of the well. A mass bal- ance calculation is made to determine which zones can be sealed without substantially reducing well production capacity. Certain contaminants lend themselves better to the well profiling and modification approach more than others. In particular, arsenic is often associated with specific geologic horizons and rock types and, by identifying high arsenic zones via well profiling, well modifications can be made which may eliminate the requirement for expensive arsenic treatment plants in many cases. Chromium and radionuclides are other common well contaminants which often have a correlation to specific geologic conditions. Nitrates, on the other hand, are a shallow groundwater issue. Nitrate impacts in wells are often caused by an inadequate seal in areas of high-nitrate groundwater. This goes back to well construction decisions as repairing seals in an existing well can be challenging. Contaminants in wells do not necessarily mean a failed well or expensive treatment plants. With vertical profiling tech- niques, well modification can be a cost-effective alternative to the much more expensive options. Tom Tom Ballard may be contacted via e-mail to 21 WorldWide Drilling Resource ® FEBRUARY 2020 WTR