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Understanding the physics of nitrate contamination in surface and subsurface water is vital for mitigating downstream water quality impairment. Though high frequency sensor data have become readily available and computational models more accessible, the integration of these two methods for improved prediction is underdeveloped. The objective of this study was to utilize high-frequency data to advance our understanding and model representation of nitrate transport for an agricultural karst spring in Kentucky, USA. We collected two-years of 15-minute nitrate and specific conductance data and analyzed source-timing dynamics across dozens of events to develop a conceptual model for nitrate hysteresis in karst. Thereafter, we used the sensing data, specifically discharge-concentration indices, to constrain modeled nitrate prediction bounds as well as the uncertainty of hydrologic and nitrogen processes, such as soil percolation and biogeochemical transformation. Observed nitrate hysteresis behavior at the spring was complex and included clockwise (n = 11), counterclockwise (n = 13), and figure-eight (n = 10) shapes, which contrasts with surface systems that are often dominated by a single hysteresis shape. Sensing results highlight the importance of antecedent connectivity to nitrate-rich storages in determining the timing of nitrate delivery to the spring. After integrating hysteresis analysis into our numerical model evaluation, simulated nitrate prediction bounds were reduced by 43 ± 12% and parameter uncertainty by 36 ± 20%. Taken together, this study suggests that discharge-concentration indices derived from high-frequency sensor data can be successfully integrated into numerical models to improve process representation and reduce modeled uncertainty.

This OSF project is meant to serve as a long-term storage repository for presentations and workshop materials for the Aquatic Ecosystem Modeling-Junior (AEMON-J) and Virtual Summit: Incorporating Data Science and Open Science (DSOS) communities. Contributors in this repository include past presenters and workshop organizers. Contributors are only responsible for those individual presentations that are labeled with their surname.

Nitrogen removal rates can vary with time, space, and as a function of environmental drivers, such as climate and land use change, but are underreported for subsurface karst environments. We carried out a multi-year study of a karst conduit where we: (1) measured inputs and outputs of sediment nitrogen (SN and δ15NSed) and nitrate (NO3⁻ and δ15NNO3); (2) developed, calibrated, and applied a numerical model of nitrogen physics and biogeochemistry; and (3) forecasted the impacts of climate and land use changes on nitrate removal and export. Data results from conduit inputs (SN = 0.43±0.07%, δ15NSed = 5.07±1.01‰) and outputs (SN = 0.36±0.09%, δ15NSed = 6.45±0.71‰) indicate net-mineralization of SN and increase of δ15NSed (p < 10-2). However, stable nitrogen isotope enrichment cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically-heavier mineral nitrogen (δ15NNO3 = 11.25±6.96‰). SN and δ15NSed model calibration provided a boundary condition for DIN simulation and improve NO3⁻ model accuracy (from NSE = 0.06 to NSE = 0.68) and highlight the importance of the seasonality of conduit recharge. Spatial zones of removal occur in close proximity to conduit entrances, where deposition of labile organic matter during the wet-season promotes a three-fold increase in denitrification (~60 mg N m-2 d-1). Temporal gradients of nitrogen removal occur during the dry-season where longer residence times cause up to 90% removal of aquifer NO3⁻ inputs. Projected effects of environmental drivers suggest an increase in denitrification by +14.1%; however, this removal is largely offset by greater nitrate leaching from soil (+28.1%) because the wet-season is forecasted to be wetter. Our results suggest that the karst conduit is influenced by internal spatiotemporal removal gradients as well as external environmental drivers.

Sediment management is currently limited by uncertainties in the applicability of existing radionuclide tracers and the effects of urbanization. Here we use plutonium isotopes (239+240Pu) from weapons-testing fallout to trace sediment transport across five watersheds in an urbanizing landscape in Kansas, USA. Historic flooding in the region provided an opportunity to assess extreme connectivity of sediment sources. 239+240Pu activity of transported sediment decreased as catchments urbanized, indicating a greater contribution from subsurface bank sediment; Bayesian modeling predicted 50% (8–80%) bank sourcing in the most rural watershed, which rose to 93% (73–100%) in the most urban catchment. 239+240Pu activity provides explanatory information on the superposition of sediment sources, which is beyond that given by traditional organic and geochemical tracers that primarily infer vegetative and geologic sourcing, respectively. Our study demonstrates the utility of 239+240Pu as a sediment tracer for managing erosion under anthropogenic change.

Land use change threatens aquatic ecosystems through freshwater salinization and sediment pollution. Effective river management requires an understanding of the dominant hydrologic pathways of sediment and solute delivery. To address this, we applied hysteresis analysis, hydrograph separation, and linear regression to hundreds of events across a decade of specific conductance and turbidity data from three streams along a rural-to-urban gradient. Thereafter, we developed an index ("β") to quantify the relative influence of surface runoff to event-scale suspended sediment generation, where a value of ‘1’ indicates complete alignment of suspended sediment generation with the temporal structure of runoff whereas ‘0’ indicates total alignment with baseflow. Solute hysteresis results showed a predominance of dilution for the rural and mixed-use streams irrespective of road salt presence. On the other hand, urban stream behavior shifted from dilution to flushing following salt application, which was largely driven by greater runoff coefficients and the connectivity of distal solutes to the stream corridor. The newly developed index ("β") indicated that suspended sediment dynamics were more aligned with runoff in all three streams: rural stream ("β" = 0.70), mixed stream ("β" = 0.57), and urban stream ("β" = 0.64). The relative importance of baseflow to sediment generation grows slightly in urbanizing streams, as impervious surfaces disconnect upland sediment, which would otherwise transport with runoff, while piston-flow baseflow erodes exposed streambanks. Our findings emphasize the need to consider the impact of human modification of the landscape on solute and sediment transport in freshwater systems for effective water quality management. Further, our "β" index provides a useful tool for assessing the relative influence of surface runoff on event-scale solute or sediment generation in streams, supporting river management and conservation efforts.

This dissertation investigated the physical and biogeochemical processes affecting the source, fate, and transport of sediment, carbon, and nitrogen within a highly-coupled fluviokarst system. Elemental and isotopic datasets were collected at surface and subsurface locations for both dissolved and particulate contaminant phases, new methodology regarding data collection was presented to the karst research community, an in-cave sediment transport model coupling physical transport with elemental and isotopic mass balances of carbon and nitrogen was formulated, pathway and process control on nitrate leaching from agricultural karst watersheds was assessed, and nitrate mobilization and fractionation were modeled using high frequency storm sampling and long-term low-flow sampling. Data and modeling results indicate that phreatic karst conduits are transport-limited during hydrologic events and experience subsurface deposition of labile, storm-injected sediment which is subsequently decomposed by heterotrophic bacteria. An estimated 30% of the organic carbon associated with sediment is decomposed during transport in the subsurface karst. Concentrations of nitrate in subsurface waters are consistently 50% greater than surface inputs suggesting an additional source of subsurface nitrate. Further modeling of nitrate leaching indicates that quick-flow water sources dilute nitrate concentrations and slow-flow (epikarst and phreatic) sources account for approximately 90% of downstream nitrate delivery. Field sampling of extreme events highlights the physical transport and delayed release of high nitrate concentrations by intermediate karst pathways, which is likely associated with a transition from epikarst to soil drainage during storm recession. Modeling of sediment carbon and nitrogen within the karst SFGL supports the idea that the cave sediment bed experiences hot spots and hot moments of biogeochemical activity. Sediment nitrogen tracing data show a significant increase in δ15NSed at the spring outlet relative to karst inputs indicating the potential for isotope fractionation effects during dissolved N uptake by cave biota. Dissolved nitrogen stable isotopic composition shows a significant downstream decrease in δ15NNO3 within the conduit, likely associated with nitrification. Data and modeling results of sediment, carbon, and nitrogen emphasize the role of multiple pathways, turbulent transport, and in-conduit transformations in controlling contaminant flux from karst watersheds. Admin Husic's ORCID ID: https://orcid.org/0000-0002-4225-2252 Copyright: © Admin Husic 2018 Funding information: University of Kentucky College of Engineering, Kentucky Water Resources Research Institute, United States Geological Survey, Kentucky State Senate, National Science Foundation