CA/ZR RATIOS AND REE PATTERNS AS TRACERS FOR KARST FLOW PATHS

Two new tracers, Ca/Zr ratios and rare earth elements (REEs), were assessed along with stable water isotopes to provide additional information on flow paths and recharge sources as they varied during a storm hydrograph. Two nearby springs in central Pennsylvania, Tippery Spring and Near Tippery Spring, were monitored using automatic samplers in response to a wet-weather storm and dry-weather storm. During baseflow, high Ca/Zr ratios reflect water having interacted dominantly with the carbonate host rock. As recharged stormwater reaches the springs, Ca/Zr ratios drop as the previously elevated calcium concentrations from matrix flow become diluted by stormwater while zirconium concentrations increased from the influx of stormwater carrying clastic surface sediments. Stable water isotopes were linearly related to the Ca/Zr ratio, making Ca/Zr ratios a useful substitute for stable water isotopes. Another advantage of the Ca/Zr signature is that Zr is typically analyzed with other trace elements, while stable isotopes require extensive sampling to develop a meteoric water line. Total REE concentrations were lowest (<0.15 ppb) at both springs during baseflow. With the arrival of stormwater, REE concentrations increased (rising to 1 to 1.4 ppb) at both springs. Furthermore, the REE patterns changed from negative cerium and europium anomalies during baseflow (indicating matrix interaction) to neutral cerium or europium anomalies with the arrival of storm flow, suggesting storm-recharged sediment derived from the clastic rocks along the ridges rather than the local carbonate bedrock. Elevated REE concentrations persisted after other parameters recovered suggesting slower flow paths recharging the two springs not indicated by other parameters such as stable water isotopes. These results illustrate the sensitivity of REEs in karst springs to storm events, recharge areas, and flow paths. Contrasting recharge behaviors were also observed, such as a stronger surface recharge signal for Tippery, and a stronger matrix water signal and buffered recharge arrival at Near Tippery. The relationships among these tracers (REE concentrations, stable water isotopes, and Ca/Zr ratios) ultimately provided a more complete picture of karst flow paths and contrasting recharge behavior between adjacent springs.