Using Stable Isotopes to Distinguish Sinkhole and Diffuse Storm Infiltration in Two Adjacent Springs

Two karst springs, Tippery Spring and Near Tippery Spring, have similar discharges (~0.1 m3/s, 5 cfs) and are only 30 meters apart, yet they show unique behaviors in terms of water chemistry and discharge response to storms. Near Tippery has higher Mg/Ca ratio and Tippery Spring has more variable temperature response to storm events. This contrast was further extended to differences in recharge pathways based on stable isotope analysis (δD & δ18O) of spring water samples collected using ISCO automated samplers during a May (3 cm, 1 inch) storm and June (8 cm, 3 inch) storm in 2017. Increased spring discharge preceded the arrival of storm water as conduits were purged of pre-storm water, indicated by no change in isotopic composition on the rising limb. The isotopic signature then became progressively more enriched at both springs, indicating storm water recharge. At Tippery, this enrichment began around peak flow, sooner than at Near Tippery where enrichment began during the descending limb. Thus, isotopes indicated a stronger surface connection at Tippery. Storm water recharge at both springs then progressed to a greater relative fraction of total discharge before recovering to pre-storm values within 24-36 hours. Storm intensity also affected the relative contribution of recharging water reaching both springs, with the June storm producing a larger recharge signature compared to the May storm. At Tippery, for a short time the majority of emerging water is storm water, with the absolute pre-storm contribution falling below its baseflow value. This reduction in pre-storm water may indicate a reversal in water exchange between the conduits and the surrounding matrix, an important consideration in karst contaminant transport. While both springs can be traced to sinks, their isotopic signatures reflect how storm water infiltrates and travels within each spring\u27s recharge area. Tippery is fed by a perennial sinking stream and more developed conduit network, while Near Tippery has a more diffuse recharge area with mixing of different surface inputs. As stable isotopes are unaffected by redox or dissolution processes, they can provide a conservative tracer with which to characterize how other parameters, such as temperature, alkalinity, and turbidity, are reflected in different spring recharge behaviors