USING ION RATIOS AND STORM HYSTERESIS TO TRACK KARST FLOW PATHS

Discharge from karst springs contains a mixture of conduit and matrix water, but the variations in groundwater mixing are poorly known, making it difficult to estimate contaminant travel times and to define source areas for protection. Stormwater sampling of springs has been used in a number of studies to try to improve understanding of the conduit and matrix contributions. Timing of hydrograph response can vary for different geochemical signatures, and these differences can be assessed by plotting ion ratios. For example, Mg/Ca ratios can trace water-rock interactions and residence time, with higher ratios indicating diffuse rather than conduit pathways. The timing along the rising and falling limb of storm hydrographs can be mapped more carefully using storm hysteresis plots, which show variation of the ion ratio with discharge. We examined Mg/Ca ratios in storm hydrograph plots in the literature and our own studies to look at the direction of hysteresis. We observed complex patterns in Mg/Ca ratios in karst systems, including both clockwise and counterclockwise hysteresis, variations between storms at the same site, and variations within storms. An increase in the Mg/Ca ratio at the beginning of some storms suggested diffuse recharge through the epikarst (clockwise loop). A late increase in Mg/Ca ratios may indicate activation of longer flow paths (counterclockwise loop). When two storms occurred in rapid succession, the second storms did not typically exhibit hysteresis in Mg/Ca suggesting pathways have been flushed. Rapid changes in Mg/Ca ratios can occur as rainfall waxes and wanes, and suggest a variety of flow paths are available resulting in additional flushing of Mg (hysteresis with small clockwise and counterclockwise loops). It is important to monitor multiple storms to determine the conditions that lead to different types of spring behavior. Storm hysteresis offers opportunities to identify variations in source areas and flow paths over time.