Simulating TrOCs concentrations along specific hyporheic flowpaths using an integral surface water-groundwater model

The hyporheic zone (HZ) describes an interfacial zone of permeable sediments, located in river beds, riparian and floodplain areas, where surface water mixes with groundwater. It exerts major control over the quality of river water by impacting the exchange processes between surface water and the sediment compartment through dynamically exchanging water, temperature and compounds as well as demonstrating an intensive nutrient turnover, providing the stream with a self-purification capability. Due to these properties, engineered hyporheic zones that aim to increase the hyporheic exchange flux are of great interest in the context of river management and restoration. The spatial and temporal scales on which the hyporheic exchange processes occur are manifold: Small topographical features of the streambed like ripples or burrows of aquatic organisms have to be considered as well as larger geomorphological features like meanders. In addition to steady-state-like streams and rivers, events like rapidly moving floods have to be taken into account when investigating the HZ. Numerical models are often utilised to gain a more comprehensive understanding of the interacting processes in the HZ. In contrast to widely used two-domain concepts, which are based on coupling surface flow and groundwater flow models,  integral one-domain modelling approaches to improve the resolution of the exchange processes and better account for feedback effects have recently attracted more attention. In this contribution, such an integral surface water – groundwater model, extended by a transport model, is validated against the results of a field experiment conducted in a side channel of the urban lowland River Erpe (Brandenburg-Berlin, Germany), which receives effluent from the wasterwater treatment plant Münchehofe, containing trace organic compounds (TrOCs) like Carbamazepine. The experiment consisted of a high-frequency sampling campaign to study the fate of the TrOCs along specific hyporheic flow path. The investigated flow path was dictated by a U-shaped pipe inserted into the sediment parallel to the surface water flow direction. To increase the hyporheic exchange, wooden debris was placed on the sediment in between the pipe openings. The model is set up using porousInter, which extends the multiphase flow solver interFoam of the computational fluid dynamics software package OpenFOAM, to allow flow through porous media. Flow is simulated by solving the three-dimensional Navier-Stokes equations extended by a porosity coefficient and additional drag terms to account for porous media flow. Further, the solver is extended by a transport model for a conservative tracer, which is used to simulate the spreading of TrOCs in the surface water and porous sediment. We expect to compute tracer  residence times and flow velocities inside the pipe in accordance with the results of the experiment.