Upscaling of Denitrification Rates from Point to Catchment Scales for Modeling of Nitrate Transport and Retention

Abstract

The spatial and temporal variability of denitrification makes it challenging to integrate conceptual, process-based understandings of nitrate transport and retention into numerical modeling at the catchment scale, although it is critical for the realism and predictive power of the model. In this study, we propose a novel approach where the conceptual understandings of the spatial structure of denitrification zones and the corresponding representative denitrification rates are transformed into a form that can be integrated into a multi-point statistical simulation framework. This is done by constructing a denitrification training image (TI) coupled to a geophysically based TI of the hydrogeological structure. The field observations and laboratory analyses of denitrification rates and the chemistry of water and sediment revealed that the study catchment's subsurface can be characterized by three zones: (1) the oxic zone with no nitrate reduction; (2) the slow-denitrification zone (mean of ln-transformed rate = −1.19 ± 0.52 mg N L −1 yr −1 ); and(3) the high-denitrification zone (mean of ln-transformed rate = 3.86 ± 1.96 mg N L −1 yr −1 ). The underlying controls on the spatial distribution of these zones and the representativeness of denitrification rates were investigated. Then, a TI illustrating the subsurface structure of the denitrification zone was constructed by synthesizing the results of these geochemical interpretations and the hydrogeology TI.

Nutrient delivery by groundwater discharge to headwater streams in agricultural catchments

“…These lower concentrations suggest a greater extent of denitrification associated with low-redox conditions in the subsurface at these sampling locations. Similar evidence for patchiness and "hotspots" of denitrification in the subsurface, including riparian settings, has been reported previously (e.g., Gold et al, 1998;Kim et al, 2021).…”

“…These isotope results also support the concept of short‐circuiting (“bypass”) via preferential pathways in the riparian zones. In addition to large variations in nutrient concentrations in shallow riparian groundwater (Table 3, Figure 9), we note that the shallow subsurface is characterized by heterogeneity of hydraulic properties (Cey et al, 1998; Van Stempvoort et al, 2021) and rates of biogeochemical processes (e.g., Carlyle & Hill, 2001; Kim et al, 2021). These considerations reinforce the reasonableness of our approach to use BF EE samples instead of sparse groundwater data to infer catchment‐wide fluxes of solutes from groundwater to our study streams.…”

“…The fact that the stream BF EE samples tended to have higher nitrate concentrations than some of the groundwater samples is likely related to variability of subsurface/groundwater conditions (Kim et al, 2021). Preferential pathways (tile drains and other macropores, sand layers) that deliver groundwater to the CC and NC streams may be characterized by higher redox conditions (in general) and less effective denitrification compared to the low redox/less permeable zones represented by some of the groundwater samples.…”

Influence of wetlands on nutrients in headwaters of agricultural catchments

“…However, the full range of rates reported for riparian sites in sand terrain (38%-99%) overlapped with the sparse available results for till terrain sites (50%-60%). In contrast, Kim et al (2022) reported complex spatial distribution of denitrification rates in an agricultural catchment in Denmark, where the shallow subsurface included various till units and meltwater sediments, largely sands and silts. They reported that denitrification rates in the saturated zone varied by more than four orders of magnitude, and decreased with increasing depth, but found no significant differences in denitrification rates related to redox conditions or lithology.…”

Assessing groundwater denitrification spatially is the key to targeted agricultural nitrogen regulation

“…The denitrification rates in the anoxic nitrate reducing zone and the reduced zone are determined in the laboratory using the core samples collected from the study catchment by the acetylene-blockage method 54 . In oxic zones, where denitrification is assumed to be negligible, the denitrification rate is set to zero 55 .…”