Geochemistry at a Karst Spring Reveals Complex Stormflow Dynamics in an Eogenetic Karst Aquifer.

The Floridan Aquifer System (FAS) is a triple porosity, eogenetic karst aquifer that contains extensive phreatic cave networks embedded in a high permeability carbonate matrix. These unique characteristics create complex flow dynamics that impact residence time distributions within the FAS, which are important to constrain for implementing effective water resource strategies. The impacts of eogenetic karst characteristics on seasonal and longer term hydrological dynamics have been previously evaluated; however, stormflow remains understudied. Our study explores stormflow dynamics at a karst spring in the eogenetic FAS after major Hurricane Idalia made landfall in August 2023. We analyze data from in-situ sensors that collect NO₃-N, specific conductance, and discharge at 15-min intervals to capture potentially small changes in chemistry that could be significant. We coupled the sensor data with grab sample collection of water isotopes and major element chemistry to provide additional details on the stormflow dynamics. Our results show at least two stormflow pulses as evidenced by changes in NO₃-N and confirmed geochemically; albeit the absolute changes in NO₃-N for both stormflow pulses were small (<0.005 mmol). One stormflow pulse was diluted with respect to NO₃-N while the other mobilized NO₃-N. The stormflow pulse that is associated with mobilized NO₃-N was detected for at least 19 days after the rain began from Idalia, indicating long residence times before evacuation from the cave system. Both of the detected stormflow pulses were superimposed on seasonal trends in NO₃-N that are known to occur, whereby it appears storms could amplify NO₃-N seasonal effects. Our results have implications for understanding complex residence times in eogenetic karst aquifers and highlight the influence of the carbonate bedrock matrix on stormflow through the FAS.