WorldWide Drilling Resource

Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Developing Shallow Two-Inch Monitoring Wells Developing a small-diameter shallow monitoring well sounded to me like a routine task. That is what I thought about 30 years ago at a gasoline monitoring site where I asked a drilling contractor to perform this task on about a dozen wells for me. I asked him, “How do you normally develop these wells?” The response: “Well, we stick the suction side of a hose of a centrifugal pump and suck the well dry a few times.” I said, “That’s it?” He replied, “Yep!” Let’s back up a minute. Why do we develop a well? The purpose for developing wells are many, and related to the process during drilling when a well disturbs the natural sedimentary layering of the sands, silts, and clays. Drilling smears, compacts, and forces drilling fluids out into the formation that should be removed from the well to obtain a “representative” formation water sample from the “screened” zone for water quality analyses. Put another way, suppose 10 or 15 gallons of drilling water were “lost” into the borehole while drilling the well. This water (and turbidity) needs to be removed from the well to obtain a representative water sample. If a well, with a drive point, can be dr iven to the cor rect depth wi thout dr i l l ing f luids, the issue about the “induced” water is minimized; however, sediment samples will not be available from the driven well to locate the target sampling zone. Split spoon samples col- lected with driven hollow tubes are a good method for obtaining soi l and vapor samples of relatively undisturbed soil samples. After the split spoon sam- ples are obtained, a two- inch wel l screen can then be driven to the desired depth for future water sampling. One problem still exists with split spoon samples, and it is the “smearing” of the borehole wall from the spit spoons and inserting the well casing. To remove the fine sediments smeared along the borehole wall so a clear, nonturbid water sample can be obtained, it is recom- mended the casing screened area be “agitated” by bailing or surging (to move water back and forth) with a surge block. Simply removing/pumping water from a well creates flow in only one direction which will “trap” much of the sediment against the outside of the screen. It is recommended to “surge and pump out” to effectively remove the turbid water from the well until it is clear. There are a few commercial ly avai lable low-cost “surge blocks” which are effective in developing fine materials from the bore- hole that will produce more and “clearer” water than other methods of develop- ment. Tom Tom Kwader may be contacted via e-mail to michele@ worldwidedrillingresource.com ENV 31 WorldWide Drilling Resource ® DECEMBER 2018