The terrestrial subsurface functions as a reactor that regulates the fate of biogeochemically reactive compounds. The dynamics of these biogeochemical transformations and the associated changes in concentrations of nutrients and toxic substances at various scales are highly relevant for ecology and water management. However, a quantitative assessment of these transformations and of the export of compounds from groundwater to surface waters is challenging due to the complex interplay of various reactive processes and hydrological cycles.

We use a numerical modeling approach to study the fate of organic carbon and nitrogen species in a mesoscale catchment (~850 km²) upstream of the Nägelstedt gauge of the river Unstrut in central Germany. A travel-time-based model approach is employed, in which results from a spatially distributed groundwater model are coupled with biogeochemical simulations describing the microbial-driven transformation of carbon and nitrogen species along different flow paths. The approach is inititially applied to a hillslope transect within the catchment, where extensive datasets are used to constrain and validate the model. Based on the simulation results for the transect, the approach is extrapolated to describe the fate of compounds in the catchment and their impact on river water quality.

Results of the simulations of the groundwater transect show the validity of the approach to describe the fate of biogeochemical compounds in the subsurface and allow to determine the impact of surface variations (land use change, climate effects) on compound concentrations in the subsurface. Depending on the depth and location of observation wells along the transect, residence times of compounds vary between less than a year and several decades, resulting in different sensitivities of well concentrations to surface conditions and to potentially pronounced legacy effects. The results for the catchment indicate that the dynamics of flow, transport and reaction have only a negligible effect on the export of compounds to the river. Changes in species input from the surface into the subsurface, whether due to climate or anthropogenic effects, can have more pronounced influence on catchment scale budgets than changes of in situ process dynamics.