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The Effect of Three Drilling Fluids on Ground Water Sample Chemistry |
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| Authors: | Robert B Brobst Paul M Buszka |
| Institution: | Robert B. Brobst is employed as city engineer for the city of Antigo (Director of Public Works, City of Antigo, Antigo, WI54409). Prior to employment with the city he was employed as a district engineer for Wisconsin Department of Natural Resources, responsible for municipal and industrial water and waste. He received a B. S. in engineering from Michigan Technological University, Houghton, Michigan, in 1977. In addition, he has a B.S. in environmental science from Lake Superior State College, Sault Ste. Marie, Michigan, in 1976. He is a registered professional engineer in Wisconsin.;Paul M. Buszka is a hydrologist with the U.S. Geological Survey, WRD in San Antonio, Texas. While researching this paper, he was a hydrogeologist-environmental geochemist with STS Consultants Ltd., Green Bay, Wisconsin. He earned his M.S. in earth science/row. the University of Waterloo in Waterloo, Ontario, Canada, in 1982andaB.S. in geology from Michigan State University in East Lansing, Michigan, in 1979. |
| Abstract: | Three monitoring wells were installed in borings that were constructed using water-based drilling fluids containing either (1) guar bean, (2) guar bean with breakdown additive, or (3) bentonite. These fluids were selected to observe their effect on the chemistry of subsequent water samples collected from the wells. The wells were installed to depths of 66 feet, 100.5 feet and 103 feet, respectively, in fine-to-medium sand and gravel outwash deposits near Antigo, Wisconsin. Drilling fluids were necessary to maintain an open borehole during well construction through strata containing cobbles and boulders. The bentonite and guar drilling fluids caused temporarily elevated concentrations of chemical oxygen demand (COD) in ground water samples collected from the monitoring wells. Using standard development, purging and sampling procedures, elevated COD concentrations persisted for about 50 days for the well bored with the guar-with-additive fluid, 140 days for the bentonite well and 320 days for the guar well. Unfiltered ground water samples for all wells had greater concentrations of COD than samples filtered through a 0.45 micron filter. Sulfate concentrations also decreased with time in the guar-with-additive well and bentonite well, but not in the guar well. The elevated COD concentrations are attributed to the large concentrations of oxidizable carbon present in the guar bean drilling fluid and in the organic polymers present in the bentonite drilling fluid. Well development and purging procedures, including borehole flushing, surging, bailing and/or chemically induced viscosity breakdown of the guar mud decreased the time before background conditions were achieved. Future research should evaluate the physical and geochemical interaction of different drilling fluid compositions with a variety of geologic matrices and drilling, well development and well purging techniques. |
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