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

Admin Husic, James Fox, Ethan Adams, Erik Pollock, William Ford, Carmen Agouridis, Jason Backus

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

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⁻, δ15NNO3, and δ18ONO3 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 δ15NNO3 (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 (δ15NNH4) acts as a mechanism for an increase in NO3⁻ that coincides with a decrease in δ15NNO3. 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.

Original languageEnglish
Article number115348
JournalWater Research
Volume170
DOIs
StatePublished - Mar 1 2020

Bibliographical note

Funding Information:
The authors would like to thank the editor and two anonymous reviewers whose comments helped improve the manuscript. The authors would like to acknowledge funding from Kentucky Senate Bill 271 and National Science Foundation Award # 1632888 . The authors also thank the University of Arkansas Stable Isotope and Kentucky Geological Survey labs for isotopic and elemental analysis, respectively. Finally, the authors thank the University of Kansas Center for Research Computing for providing high-performance computing resources used to run millions of simulations during uncertainty analysis. Appendix A

Funding Information:
The authors would like to thank the editor and two anonymous reviewers whose comments helped improve the manuscript. The authors would like to acknowledge funding from Kentucky Senate Bill 271 and National Science Foundation Award #1632888. The authors also thank the University of Arkansas Stable Isotope and Kentucky Geological Survey labs for isotopic and elemental analysis, respectively. Finally, the authors thank the University of Kansas Center for Research Computing for providing high-performance computing resources used to run millions of simulations during uncertainty analysis.

Publisher Copyright:
© 2019

Keywords

  • Biogeochemistry
  • Denitrification
  • Karst
  • Nutrient cycling
  • Sediment
  • Water quality

ASJC Scopus subject areas

  • Ecological Modeling
  • Water Science and Technology
  • Waste Management and Disposal
  • Pollution

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