Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 3
Prev Page--Discussion
Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 258, part 3
Prev Page--Discussion
The results of this study reflect the lateral variability in the chemical quality of the water within the Ozark aquifer transition zone described in Macfarlane and Hathaway (1987) and provide documentation of the effects of pumping on produced-water quality. The chemical character of water produced from the Ozark aquifer ranges from mixed cation-bicarbonate type for Pittsburg wells 8 and 10 and Crawford RWD 4 well 3 to a sodium-chloride type for Cherokee RWD 3 well 1 and other wells yielding even higher chloride concentration. As a group, water from the multi-aquifer wells with a chloride content less than 80 mg/L (Weir city well, Crawford RWD 5 well1, and about half of the Columbus well 4 samples) had a higher bicarbonate percentage and a lower sulfate and chloride percentage of total anions than water only from the Ozark aquifer. However, the much lower sodium and potassium percentage and higher calcium and magnesium percentage of total cations for the multi-aquifer wells than for samples from the Springfield Plateau aquifer suggest that the contribution of water from the Springfield Plateau aquifer is minor relative to the contribution produced from the Ozark aquifer. With the exception of Columbus well 4 and the Weir city well, variability in the chemistry of the monthly samples collected during 2006-08 was small for each site relative to the range among the sample sites.
Data for the chemical-quality pumping tests indicate that different zones within the Ozark aquifer produce waters of differing chemistries at variable rates to Pittsburg well 8 as it pumps. The general decrease in TDS and chloride concentrations relative to bicarbonate concentration indicates mixing of a relatively greater amount of ground water of low TDS concentration and of calcium, magnesium-bicarbonate type than of sodium-chloride type. Differences in the manner in which the bicarbonate/chloride ratio changed over time between the June and September tests suggest that vertical head gradients, both under pumping and nonpumping conditions, play an important role in the temporal pattern of changes in salinity during these pumping tests. Variability in the water chemistry of the monthly samples from most of the sites seems to be primarily related to the range in water quality within the Ozark aquifer. This may also be the main cause of changes observed in the water quality for the Columbus 4 and Weir city wells.
The results of this study compared with the results of the 1979-1980 sampling of Macfarlane and Hathaway (1987) indicate that Pittsburg well 10 and Crawford Consolidated RWD 1 well 2 have experienced long-term increases in chloride. Changes for the other sites from 1979-1980 to 2006-08 were either small or insignificant. Chloride concentration increase in water from wells in the Pittsburg area most likely has resulted from pumping stress associated with the higher density of high-yielding supply wells in southeastern Crawford County and the adjacent area of Missouri. It is possible that the changes in quality are influenced by local upconing of higher chloride, higher TDS, transition-zone water from deeper zones within the Ozark or the St. Francois aquifers. If the long-term increases in chloride concentration are linear with time, the earliest dates that Pittsburg wells 8 and 10 would produce water greater than the recommended drinking water limit of 250 mg/L are the years 2045 and 2060, respectively.
The variability in the chemical data from monthly sampling and the Pittsburg well 8 pump tests indicates that detection of long-term water-quality changes in the produced water from the wells within the transition zone will be difficult unless its variability over shorter time scales is factored into the analysis. To be successful, 1) water sampling events over the entire network and at individual well sites need to be more frequent to assess decadal changes and monthly variability and 2) the changes in water chemistry that occur under different levels of pumping stress during a single extended pumping event need to be considered in the analysis of trend. However, for these recommendations to more fully contribute to our understanding of the Ozark Plateaus aquifer system as a whole, attention needs also to focus on gathering vertical hydraulic-head profiles and water-quality pumping tests like those conducted in Pittsburg well 10 in existing and future wells. Data from these tests are a requisite for fully assessing the mechanisms that cause water-quality degradation in individual, open borehole wells.
This work was made possible by grant funds from the U.S. Geological Survey managed by the Kansas Water Resources Research Institute at Kansas State University, Manhattan, Kansas. The author wishes to acknowledge the assistance of Ed Reboulet with the fieldwork for this project and Rudy Ghijsen, Elizabeth Petroske, and Mike Magnuson of the KGS Analytical Services Laboratory for their careful analysis of the water samples collected for this project. The author also wishes to express appreciation to the operators of the water supplies that were sampled in this project. The author especially wishes to express his appreciation to Steve Gillis, City of Pittsburg, for his assistance that enabled the KGS to conduct the pumping tests on Pittsburg well 8. While working on this project, the author benefited from discussions with Walt Aucott, Mike Pope, and John Czarnecki of the U.S. Geological Survey on the work they were conducting on the Ozark aquifer in the Tri-state region. Margaret Townsend, Jim Butler, and Don Whittemore of the Kansas Geological Survey; Jim Vandyke of the Division of Geology and Land Survey, Missouri Department of Natural Resources; John Schumacher, Supervisory Hydrologist of the U.S. Geological Survey's Missouri Water Science Center; and Ralph Davis, professor and chair of the Department of Geosciences, University of Arkansas-Fayetteville, critically edited and reviewed the manuscript and provided helpful suggestions that solidified the ideas expressed in the manuscript and strengthened its overall message.
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