DIFFERENCES IN WATER TYPES IN WELLS AND SPRINGS OF THE OZARK PLATEAUS IN NORTHERN ARKANSAS

Physical (pH, temperature, conductance) and geochemical (major ions) characteristics of groundwater are used to define water types for aquifers within varying geologic regions. Water type can provide hydrogeologic information, such as differences in aquifer materials, rock/water interaction, and changes in redox conditions that occur along groundwater flow paths. This research will characterize groundwater geochemistry of three physiographic sections in the Ozark Plateaus of northern Arkansas: Boston Mountains, Springfield Plateau, and Salem Plateau. The region is one of the major karst landscapes in the United States. Large-scale groundwater geochemistry mapping can be used to assess spatial variation across the regions and quantify variation between wells and springs. Aqueous geochemical data were obtained from the National Water Quality Monitoring Council database for groundwater in 13 counties in northern Arkansas that encompass the Ozark Plateaus. Initial results revealed median pH values ranging from 7.3 to 7.4 for all three sections. The overall neutrality of groundwater in the Ozarks is due to the underlying carbonate formations acting as a buffer system. Chloride and sulfate concentrations varied among the sections and between wells and springs. Median chloride concentrations were 2.0 mg/L in Boston Mountains springs and 8.4 mg/L in wells, 5.2 mg/L in Springfield Plateau springs and 5.5 mg/L in wells, and 7.5 mg/L in springs and 2.8 mg/L in wells for the Salem Plateau. Differences in chloride concentrations are due to variability in lithologic and surface-derived sources. Median sulfate concentrations were 9.7 mg/L in Boston Mountains springs and 14.0 mg/L in wells, 5.2 mg/L in Springfield Plateau springs and 13.0 mg/L wells, and 5.3 mg/L in springs and 8.8 mg/L wells for the Salem Plateau. Sulfate concentration differences are likely due to numerous geochemical processes, such as redox reactions and dissolution of sulfate-bearing minerals. Lower spring concentrations compared to wells are due to shallower flow paths and less rock/water interaction. Spatial distributions of water types throughout the Ozarks, among different aquifers, and between wells and springs will be characterized with Geographic Information System techniques and statistical analyses.