UTILIZING KARST FEATURES TO ACCOUNT FOR DIFFERENCES IN WATER QUALITY IN THE OZARK PLATEAUS IN NORTHERN ARKANSAS

The occurrence of sinkholes and springs, in karst topography may produce local and regional differences in groundwater quality that challenge routine use and management. Within the Ozarks Plateau three physiographic sections (Boston Mountains, Springfield Plateau, and Salem Plateau) comprise one of the largest karst regions in the United States. A joint investigation by US Geological Survey and the University of Central Arkansas is evaluating differences in aqueous geochemistry (such as pH and major cations and anions) as a function of occurrence and density of karst features in the three sections in northern Arkansas. One objective of this study is to produce a large-scale hydrogeologic map that combines what is known about groundwater flow-paths with quality data for wells and springs in the study area. Initial summaries of previous data of the National Water Quality Database showed little difference in pH and a relatively neutral pH of 7.3 for wells and springs across the area. In comparison, sulfate concentration were consistently higher in wells than in springs across all sections and chloride concentrations differed by water source but less consistently. Within the Boston Mountains median sulfate concentrations were 14.0 mg/L for wells compared to 9.7 mg/L for springs, concentrations were lower within the Springfield Plateau but in the same order by source (13.0 mg/L for wells compared to 5.2 mg/L in Springfield Plateau springs), and were still lower within Salem Plateau (8.8 mg/L wells compared to 5.3 mg/L for springs). In contrast, median concentrations of chloride showed a similar order by source for Boston Mountains (8.4 mg/L for wells compared to 2.0 mg/L for springs), but were almost the same for wells and springs in Springfield Plateau (5.5 mg/L for wells and 5.2 mg/L for springs), and were in reversed order within Salem Plateau (2.8 mg/L in wells compared to 7.5 mg/L for springs). Observed patterns in constituent concentrations across the three sections and sample sources may be explained by a combination of lithologic properties, surface-source characteristics, flow-paths, and geochemical processes which are currently being evaluated. Future work will attempt to quantify the relation of water quality to sinkhole distribution throughout the Ozark Plateaus using Geographic Information System methods.