Hot Spots and Hot Moments of Nitrogen in a Riparian Corridor

Abstract

We use 3‐D high‐resolution reactive transport modeling to investigate whether the spatial distribution of organic‐carbon‐rich and chemically reduced sediments located in the riparian zone and temporal variability in groundwater flow direction impact the formation and distribution of nitrogen hot spots (regions that exhibit higher reaction rates when compared to other locations nearby) and hot moments (times that exhibit high reaction rates as compared to longer intervening time periods) within the Rifle floodplain in Colorado. Groundwater flows primarily toward the Colorado River from the floodplain but changes direction at times of high river stage. The result is that oxic river water infiltrates the Rifle floodplain during these relatively short‐term events. Simulation results indicate that episodic rainfall in the summer season leads to the formation of nitrogen hot moments associated with Colorado River rise and resulting river infiltration into the floodplain. The results further demonstrate that the naturally reduced zones (NRZs) present in sediments of the Rifle floodplain have a higher potential for nitrate removal, approximately 70% greater than non‐NRZs for typical hydrological conditions. During river water infiltration, nitrate reduction capacity remains the same within the NRZs, however, these conditions impact non‐NRZs to a greater extent (approximately 95% less nitrate removal). Model simulations indicate chemolithoautotrophs are primarily responsible for the removal of nitrate in the Rifle floodplain. These nitrogen hot spots and hot moments are sustained by microbial respiration and the chemolithoautotrophic oxidation of reduced minerals in the riparian zone.

Morphogenesis of a Floodplain as a Criterion for Assessing the Susceptibility to Water Pollution in an Agriculturally Rich Valley of a Lowland River

“…An investigation of temporal and biochemical changes of nitrogen as well as interactions between physical, biochemical, hydrological, and geological features controlling transport of considered pollutant would aid in proper interpretation of its fate. The study presented by Dwivedi et al [90] is a comprehensive example of integrating geophysical data, mineralogical analyses, monitoring of water levels, and water quality in the interpretation of the impact of several involved issues on nitrogen behavior. In the quoted study, 3-D simulations performed using the PFLOTRAN model were used in mapping spatial and temporal changes related to nitrogen fluxes.…”

Investigating the spatio‐temporal variations of nitrate leaching on a tea garden hillslope by combining HYDRUS‐3D and DNDC models

“…Though the biogeochemistry models have exhaustive processes for N transformation, but the soil hydrological processes are generally simulated based on simple cascade approach ( Deng et al, ; Zhang et al, ). Recently, some pathways were excavated to improve the modelling simulations such as revising the soil hydrological or N transformation modules of the existing models or constructing new models by coupling the sophisticate soil hydrological and biogeochemical processes ( e.g ., Dwivedi et al, ; Zhang et al, ). Even so, combing the existing soil hydrology‐ and biogeochemistry‐oriented models to improve the simulations might be an alternative with superiority in feasibility and time‐efficient, while this has seldom been considered in previous studies ( Zhu et al, ).…”

“…The spatio‐temporal variations of ‐N leaching flux have been widely reported across spatial scales in previous studies ( e.g ., Kurunc et al, ; Zhu et al, ; Baram et al, ; Dwivedi et al, ). Concepts of hot spots and moments (regions or times that exhibit disproportionately high reaction rates) were adopted to characterize the spatio‐temporal variations of ‐N leaching ( McClain et al, ; Kurunc et al, ; Dwivedi et al, ).…”

“…The spatio‐temporal variations of ‐N leaching flux have been widely reported across spatial scales in previous studies ( e.g ., Kurunc et al, ; Zhu et al, ; Baram et al, ; Dwivedi et al, ). Concepts of hot spots and moments (regions or times that exhibit disproportionately high reaction rates) were adopted to characterize the spatio‐temporal variations of ‐N leaching ( McClain et al, ; Kurunc et al, ; Dwivedi et al, ). Hot spots of ‐N leaching were commonly associated with coarse‐textured soils, shallow water table and converged topography, and hot moments were generally observed after the rainstorm events and the application of N fertilizer ( Harms and Grimm , ; Kurunc et al, ; Balestrini et al, ; Baram et al, ).…”

Estimation of Nitrate Trends in the Groundwater of the Zagreb Aquifer

“…Furthermore, it has been established that the definition of stream-aquifer interaction can be crucial for predicting hot spots of nitrogen that were microbially driven or flow related. Also, dynamic redox conditions in the riparian zone, related to intermittent groundwater flow reversal due to surface-aquifer interaction, can produce nitrogen hot spots [28]. It is evident that various statistical constraints and geochemical features can influence trend estimation of chemical parameters, in this case nitrates.…”