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⁻, δ¹⁵N_NO3, and δ¹⁸O_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 δ¹⁵N_NO3 (from 7.7 to 6.8‰; p < 10^-3) as material flows from within the conduit to the spring. Modeling results indicate that the nitrification of isotopically-lighter ammonium (δ¹⁵N_NH4) acts as a mechanism for an increase in NO₃⁻ that coincides with a decrease in δ¹⁵N_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.