Storm-Induced Solute Dynamics and Hysteresis Behaviour in an Eogenetic Karst Aquifer

Abstract

Eogenetic karst aquifers that maintain high carbonate bedrock permeability can have distinctive aquifer hydrodynamics that would be captured in hysteresis behaviour. Analysing the hysteresis behaviour of these systems can be a concise and efficient method to provide insight into the dominant recharge mechanisms, porosity integration and contributing land uses to spring discharge. The availability of deployable water quality sensors that can monitor spring water chemistry at high temporal resolution across multiple events allows us to capture delayed or attenuated signals that may occur in eogenetic karst. Additionally, analysing hysteresis across multiple events can identify patterns in the responses that occur. The Upper Floridan Aquifer (UFA) is an example of an eogenetic karst aquifer that has extensive phreatic conduits embedded in a high permeability carbonate matrix. We collected high temporal resolution (15 min) discharge, specific conductance and nitrate. We analysed storm-induced changes to nitrate and specific conductance and quantified hysteresis by calculating the hysteresis index (HI) and complementary flushing index (FI) at two major springs that drain agriculturally dense regions of the UFA. The combined analysis of HI and FI showed that 95% of all events had delayed connectivity of discrete feature recharge emerging at each spring. Mobilisation (FI > 0) of specific conductance occurred 95% of the time at both springs, but nitrate was diluted (FI < 0) 85% of the time at one spring and mobilised 95% of the time at the other. Changes to both specific conductance and nitrate were less than 15% of the pre-storm values, which illustrates how discrete recharged water mixes with substantial volumes of older, stored water in the aquifer. The hysteresis behaviour in the UFA was in contrast with a telogenetic setting, where higher variability in nitrate responses occurred between events, dilution was mostly observed for specific conductance and nitrate, and responses varied between delayed and rapid connectivity. Our work quantifies the storm-induced hydrodynamics of an eogenetic karst aquifer, which can help guide local water resource management decisions and advance our knowledge of karst aquifer hydrodynamics across a wide range of diagenetic and karstification stages.