A Reactive Transport Modelling Approach for Biogeochemical Transformation in an Agricultural Catchment

The export of nutrients from terrestrial ecosystems is characterized by complex interactions between hydrological transport and biogeochemical transformations, posing considerable challenges to the effective management of water quality and the prediction of climate change. Excess nutrient inputs from agricultural activities into freshwater systems have the potential to impact the safe functioning of ecosystem services, while contributing to greenhouse gas emissions. However, quantification of these coupled processes remains challenging due to the variability in residence time of multiple flow paths and the limited ability to observe hydrological and biogeochemical processes in situ because of the involved long timescales and the inaccessibility of the subsurface. In this study, the catchment-scale hydro-biogeochemical reactive transport model (BioRT-HBV), which is based on a parsimonious structure and has minimal data requirements, is employed to explore the role of different processes in nitrogen reaction rates and concentrations in subsurface waters and rivers. The model uses hydrometeorology, discharge, and stream chemistry data from 2008 to 2023, as well as geological conditions from the Nägelstedt catchment, which is located in central Germany and is recognized as one of its most intensively used agricultural regions. The hydrological model demonstrates a good correlation between simulated and observed stream discharge, with high model efficiency. Snowmelt appears to be an important hydrological factor in regulating the discharge flows at the Nägelstedt catchment, leading to additional surface flow and shallow subsurface flow occur during brief periods corresponding with snowmelt events, while the deep subsurface flow contributes almost 80 percent of the annual discharge. Preliminary results show that high stream nitrate concentrations occur when shallow flowpaths connect the shallower soils to the stream, while low nitrate stream concentrations occur during baseflow and lower-flow conditions, when the stream is predominantly fed by deeper flowpaths, resulting in a flushing concentration-discharge pattern. This suggests that nitrate loss processes are driven by the long-term retention or depletion of nitrogen in soils and groundwater within this catchment. The model provides a foundation for comprehending the interdependence of complex nonlinear biogeochemical and hydrological processes and serves as a step toward the prediction of the impact of climatic perturbation and land use changes on chemical exports to river.