Unraveling biogeochemical transformation of organic carbon and nitrogen compounds in groundwater along a hill slope transect

The origin and the fate of organic carbon and nitrogen compounds in groundwater play an important role in the global biogeochemical cycling of carbon and nutrients and have implications for drinking water production. While the input of these compounds into the subsurface is strongly driven by land use, their fate in subsurface environments such as fractured aquifers is controlled by a complex interplay between hydrological and biogeochemical processes at different temporal and spatial scales and is poorly understood yet. Determining the fate of these organic compounds in fractured aquifers is additionally challenging due to spatial heterogeneities at various scales down to centimeter scales, leading to a multitude of flow paths of different lengths and residence times.  This causes an overlap of solute residence times for compounds moving from the surface through the subsurface to surface waters or groundwater observation wells, stretching from days to many years, thus affecting the dynamics of the biogeochemical processes and the quantitative assessment of compound fluxes. To address this issue, a travel time-based modeling approach is employed to simulate the fate of carbon and nitrogen compounds in groundwater along a hill slope transect of the Hainich Critical Zone Exploratory (CZE), located northwest of Thuringia (central Germany). This transect is set up under the Collaborative Research Center AQUADIVA. It is subject to an intensive surface and subsurface monitoring program providing groundwater quality and quantity data. Travel time distributions obtained from a numerical groundwater flow model of the transect and its vicinity are combined with a set of numerical 1D simulations describing the biogeochemical transformations of carbon and nitrogen. The simulated complex reaction network describes the transformation of carbon and nitrogen along individual groundwater flow paths, which considers varying microbial functional groups such as aerobes and anaerobes, as well as key microbial life processes under different redox conditions, including aerobic, nitrate-reducing, ammonia-oxidizing, and sulfate-reducing conditions.  The model-predicted concentrations of reactive species at various observation wells are compared to measured concentrations to validate the approach. The results show that processes on the surface strongly shape the dynamics of resulting recharge zones represented by different land use areas, which have an impact on observed concentrations at wells. Travel time distributions combined with simulations of the biogeochemical transformation along a flow path can provide a model-based interpretation of measured observations and the factors controlling them. This allows predicting fluxes of chemical species through the entire sub-catchment and their dependency on the dynamics of surface conditions.