Linking Water Age, Nitrate Export Regime, and Nitrate Isotope Biogeochemistry in a Tile‐Drained Agricultural Field

Abstract

Accurately quantifying and predicting the reactive transport of nitrate () in hydrologic systems continues to be a challenge, due to the complex hydrological and biogeochemical interactions that underlie this transport. Recent advances related to time‐variant water age have led to a new method that probes water mixing and selection behaviors using StorAge Selection (SAS) functions. In this study, SAS functions were applied to investigate storage, water selection behaviors, and export regimes in a tile‐drained corn‐soybean field. The natural abundance stable nitrogen and oxygen isotopes of tile drainage were also measured to provide constraints on biogeochemical transformations. The SAS functions, calibrated using chloride measurements at tile drain outlets, revealed a strong young water preference during tile discharge generation. The use of a time‐variant SAS function for tile discharge generated unique water age dynamics that reveal an inverse storage effect driven by the activation of preferential flow paths and mechanically explain the observed variations in isotopes. Combining the water age estimates with isotope fingerprinting shed new light on export dynamics at the tile‐drain scale, where a large mixing volume and the lack of a strong vertical contrast in concentration resulted in chemostatic export regimes. For the first time, isotopes were embedded into a water age‐based transport model to model reactive transport under transient conditions. The results of this modeling study provided a proof‐of‐concept for the potential of coupling water age modeling with isotope analysis to elucidate the mechanisms driving reactive transport.

The value of instream stable water isotope and nitrate concentration data for calibrating a travel time‐based water quality model

“…Overall, the accuracy of these values cannot be verified as the 'true' shape of the parameter's posterior density distribution is unknown. Nonetheless, values for the denitrification rate found in this study are within the ranges of other studies conducted in the same region (Nguyen et al, 2021;Yang, Heidbüchel, et al, 2018) and elsewhere Yu et al, 2023). The mass of denitrified N in the subsoil, along with its balance in the subsoil, is reported in Table S6, while for the N balance in the soil, we refer to the original studies of Nguyen et al (2021); .…”

“…However, these results may be site-specific and not readily apply to all catchments. For example, in a recent study by Yu et al (2023) conducted in a tile-drained agricultural field, SAS functions were calibrated using chlorides, revealing a significant preference for young water more pronounced during wetter conditions, as observed in our study. Instead, the NO 3 À export regime was classified as chemostatic (i.e., NO 3 À concentrations remain stable over time) based on C-Q relationships, which was attributed to a substantial mixing volume and the absence of a pronounced vertical contrast in NO 3 À levels.…”

Assessing impacts of cemeteries on water quality in an urban headwater watershed with mixed human‐built infrastructure

“…Van Meter et al (2017) showed that nitrogen older than 50 years accounts for approximately 33% and 5% of the nitrogen at the basin outlet for the Mississippi (3 000 000 km 2 ) and Susquehanna (70 000 km 2 ) rivers. Smaller rivers have faster biogeochemical cycling of nitrate (Basu et al, 2011) travel time was expected, we would expect to see coupled enrichment of nitrogen and oxygen isotopes due to subsurface denitrification (Yu et al, 2023), that is, N and O isotopes will be enriched following a 1:1 to 1:0.5 line, but that is not the case here (Figure 7). Nitrate loading is driven primarily by patterns of surfacegroundwater interaction, as loading corresponds to when the stream is a losing and gaining stream within the cemetery reach and is shown in Figure 8, where we predominantly see the stream is losing both discharge and nitrate load or gaining both discharge and nitrate load on a given day.…”

Nitrate and Water Isotopes as Tools to Resolve Nitrate Transit Times in a Mixed Land Use Catchment

“…The relevance of water transit times in relation to nitrate transport is pointed out in the study of van der Velde et al (2010) who investigated solute export at the Hupsel brook catchment and discovered the relationship between the dynamics of contact times of water and soil and the observed solute concentrations in stream. Nitrate transport in relation to water age was discussed by other studies (van der Velde et al, 2012;Molénat & Gascuel-Odoux, 2002;Kaandorp et al, 2021;Nguyen et al, 2021;Yu et al, 2023). Nitrate removal in relation to water age was explicitly pointed out by Benettin, Fovet, & Li (2020) who estimated water age based on chloride as tracer to analyze the relationship between water age and nitrate removal.…”