USING STORMFLOW-HYDROGRAPH SEPARATION TO UNDERSTAND INFILTRATION AND RECHARGE PROCESSES IN THE TRINITY AQUIFER

Stormflow-hydrograph separation analysis is a commonly utilized technique for characterizing infiltration and recharge processes in karst settings. Previous studies have used spring discharge and cave stream data and are often limited to a few parameters and/or infrequent sampling intervals. To gain a more detailed understanding of how water, nutrients, and sediment move through a cave system, we analyze a large dataset that includes high frequency sampling and comprehensive stormwater analyses of four storm events from July 2014 to July 2015 at Cave Without A Name (CWAN) in central Texas.

An ISCO auto sampler collected discrete water samples in the cave stream during each storm event (about 100 samples per event) with higher frequency sampling in the rising limb and peak of the hydrograph. Discrete water samples were also collected at the three cave drip sites, at a surface runoff/direct recharge site where water directly enters a small cave entrance, and from a precipitation collector. In addition, we utilized continuous (10-minute intervals) hydrologic data at the cave stream and the three drip sites, as well as atmospheric data from a surface site above CWAN.

Cave stream, cave drip, surface runoff, and precipitation samples from each storm event were analyzed for δ¹⁸O and δ²D stable isotopes, major ions, dissolved and total nutrients (P, N, and C), and organic and inorganic sediment loads. Stable isotope and ion data were analyzed using end-member mixing model analysis to determine the proportion of formation water versus precipitation water reaching the various sites in CWAN across each storm event. Analysis of hydrochemographs graphs within a single event and across storm events give insights into how storm intensity, duration, and antecedent conditions affect the movement of event water and pre-event water through the system.

Nutrient and sediment dynamics in response to storm events in karst systems are poorly understood. Sediment and nutrient data were used to characterize nutrient cycling in response to rapid storm pulses. Ratios of carbon, nitrogen, and phosphorus across the stream hydrograph and at the different drip sites can be used to quantify temporal and spatial changes in nutrient limitations and uptake rates in response to a rapid storm pulse in a karst setting.