Nitrogen (N) contamination within agricultural-karst landscapes and aquifers is widely reported; however, the complex hydrological pathways of karst make N fate difficult to ascertain. We developed a hydrologic and N numerical model for agricultural-karst, including simulation of soil, epikarst, phreatic, and quick flow pathways as well as biochemical processes such as nitrification, mineralization, and denitrification. We tested the model on four years of nitrate (NO3−) data collected from a phreatic conduit and an overlying surface channel in the Cane Run watershed, Kentucky, USA. Model results indicate that slow to moderate flow pathways (phreatic and epikarst) dominate the N load and account for nearly 90% of downstream NO3− delivery. Further, quick flow pathways dilute NO3− concentrations relative to background aquifer levels. Net denitrification distributed across soil, epikarst, and phreatic water removes approximately 36% of the N inputs to the system at rates comparable to nonkarst systems. Evidence is provided by numerical modeling that NO3− accumulation via evapotranspiration in the soil followed by leaching through the epikarst acts as a control on spring NO3− concentration and loading. Compared to a fluvial-dominated immature karst system, mature-karst systems behave as natural detention basins for NO3−, temporarily delaying NO3− delivery to downstream waters and maintaining elevated NO3− concentrations for days to weeks after hydrologic activity ends. This study shows the efficacy of numerical modeling to elucidate complex pathways, processes, and timing of N in karst systems.
|Number of pages||25|
|Journal||Water Resources Research|
|State||Published - Mar 2019|
Bibliographical noteFunding Information:
The authors would like to thank the Associate Editor and three anonymous reviewers whose comments helped the authors to greatly improve the quality of this manuscript. The authors would like to acknowledge primary funding from the Kentucky Senate Bill 271B Water Quality program. Steve Workman and Charles Taylor are greatly acknowledged for their contributions and support. The researchers and staff at the Kentucky Geological Survey and the extensive field and laboratory work they performed in the Cane Run Watershed were instrumental in the completion of this work. The authors thank University of Kentucky Research Computing for making available the high‐performance computing resources used to perform uncertainty analysis in this study. Lastly, we gratefully acknowledge financial support of this research under National Science Foundation award 1632888, which provided partial support for three of the authors. Calibration data, computer code, and model results will be stored and publicly available at the following https://figshare.com/s/ 71d99b86998ff2c9144c. We have no conflict of interests to report.
©2019. American Geophysical Union. All Rights Reserved.
- nitrate leaching
- numerical model
- nutrient fate
ASJC Scopus subject areas
- Water Science and Technology