Using Nutrients, Sediment, Ions, Isotopes, and Hydrograph Separation to Quantify Conduit-Dominated Recharge Processes in a Trinity Aquifer Site: Cave Without a Name
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Cave streams provide an ideal location for sampling waters transported through a karst system because they integrate basin-wide sources ranging from fast flow in conduits to slow flow through the bedrock matrix. Although numerous studies have monitored cave streams to characterize these process, most have infrequent sampling intervals and/or a limited number of measured parameters. This study used a large dataset that includes high frequency sampling and comprehensive stormwater analyses of surface and cave stream water from five storm events between July 2014 and July 2015 at Cave Without A Name (CWAN) in central Texas. The objectives were to 1) determine which environmental factors influence the timing and proportions of stormwater and pre-event water moving through the system and 2) to quantify relationships between discharge and sediment, nutrient, and ion concentrations within and across storm events. Results show that evapotranspiration (summed over prior 12 weeks to each storm), soil moisture (at 10-40cm), and cave-stream discharge prior to each storm affects the timing of the peak ratio of stormwater/pre-event water flow through the cave. As antecedent conditions became wetter from July 2014 to July 2015, peak stormwater arrival times dropped from days to hours. Progressively faster stormwater arrival times, heterogeneity within and across storm chemographs and sediment graphs, and water isotope data all indicate a flushing of accumulated solutes in the upper unsaturated zone during Events 1 and 2, and progressive wetting of unsaturated portions of the system from July 2014 to July 2015. Taken together, these data reveal complex hydrologic and mass transport dynamics, variable rainfall-runoff and rainfall-recharge relationships, and highlight that a single storm cannot be used to accurately describe how a karstic groundwater system responds to storm events under a wide range of hydrologic conditions. These data may also help explain inconstancies between findings from other studies in which contaminant or nutrient concentrations varied from storm-to-storm. This better understanding of recharge processes at CWAN will help guide future research on transport dynamics in karst systems, as well as contribute to better surface water/groundwater management in karst regions.