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

Accurately quantifying and predicting the reactive transport of nitrate (NO3-) 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 NO3- export regimes in a tile-drained corn-soybean field. The natural abundance stable nitrogen and oxygen isotopes of tile drainage NO3- were also measured to provide constraints on biogeochemical NO3- 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 NO3- isotopes. Combining the water age estimates with NO3- isotope fingerprinting shed new light on NO3- export dynamics at the tile-drain scale, where a large mixing volume and the lack of a strong vertical contrast in NO3- concentration resulted in chemostatic export regimes. For the first time, NO3- isotopes were embedded into a water age-based transport model to model reactive NO3- transport under transient conditions. The results of this modeling study provided a proof-of-concept for the potential of coupling water age modeling with NO3- isotope analysis to elucidate the mechanisms driving reactive NO3- transport.