Quantification of nitrate fate in a karst conduit using stable isotopes and numerical modeling

Nitrate (NO3-) fate estimates in turbulent karst pathways are lacking due, in part, to the difficulty of accessing remote subsurface environments. To address this knowledge and methodological gap, we collected NO3-, (delta N-15(NO3), and delta O-18(NO3) data for 65 consecutive days, during a low-flow period, from within a phreatic conduit and its terminal end-point, a spring used for drinking water. To simulate nitrogen (N) fate within the karst conduit, the authors developed a numerical model of NO3- isotope dynamics. During low-flow, data show an increase in NO3- (from 1.78 to 1.87 mg N L-1; p < 10(-4)) coincident with a decrease in delta N-15(NO3)(from 7.7 to 6.8 parts per thousand; p < 10(-3)) as material flows from within the conduit to the spring. Modeling results indicate that the nitrification of isotopically-lighter ammonium (delta N-15(NH4)) acts as a mechanism for an increase in NO3- that coincides with a decrease in delta N-15(NO3). Further, numerical modeling assists with quantifying isotopic overprinting of nitrification on denitrification (i.e., coincident NO3- production during removal) by constraining the rates of the two processes. Modeled denitrification fluxes within the karst conduit (67.0 +/- 19.0 mg N m(-2) d (-1)) are an order-of-magnitude greater than laminar ground water pathways (1-10 mg N m(-2) d(-1)) and an order-of-magnitude less than surface water systems (100-1000 mg N m(-2) d(-1)). In this way, karst conduits are a unique interface of the processes and gradients that control both surface and ground water end-points. This study shows the efficacy of ambient N stable isotope data to reflect N transformations in subsurface karst and highlights the usefulness of stable isotopes to assist with water quality numerical modeling in karst. Lastly, we provide a rare, if not unique, estimate of N fate in subsurface conduits and provide a counterpoint to the paradigm that karst conduits are conservative source-to-sink conveyors. (C) 2019 Elsevier Ltd. All rights reserved.