Knowledge of groundwater flow is of high relevance for groundwater management or the planning of different subsurface utilizations such as deep geothermal facilities. While numerical models can help to understand the hydrodynamics of the targeted reservoir, their predictive capabilities are limited by the assumptions made in their setup. Among others, the choice of appropriate hydraulic boundary conditions, adopted to represent the regional to local flow dynamics in the simulation run, is of crucial importance for the final modelling result. In this work, we systematically address this problematic in the area of the central part of the Upper Rhine Graben. We quantify how and to which degree different upper boundary conditions and vertical cross-boundary fluid movement influence the calculated deep fluid flow conditions in the area under study. Robust results, which are insensitive to the choice of boundary condition, are: (i) a regional groundwater flow component descending from the graben shoulders to rise at its centre and (ii) the presence of heterogeneous hydraulic potentials at the rift shoulders. Contrarily, results affected by the chosen boundary conditions are: (i) calculated flow velocities, (ii) the absolute position of the upflow axis, and (iii) the evolving local flow dynamics. If, in general, the investigated area is part of a supra-regional flow system—like the central Upper Rhine Graben is part of the entire Upper Rhine Graben—the inflow and outflow across vertical model boundaries need to be considered.

To mitigate lake eutrophication, phosphorus (P) availability can be managed by iron (Fe) amendments, which bind P in the water column and settle as Fe oxy-hydroxides. In the fluvial-lacustrine system Spree, Fe oxy-hydroxides enter lakes due to lignite mining in the Lusatian Area (NE Germany). We hypothesized that the amount of P that can be retained from the water column by sediments is positively correlated with their iron content. Column experiments were used to investigate uptake and release of P in the sediments under oxic and anoxic conditions in three downstream lakes (Lake Neuendorfer See, Lake Glower See, Lake Müggelsee) with decreasing mining influence and thus iron loads, and one nearby non-mining-affected lake (Lake Schwielochsee). In lakes interconnected by River Spree, the cumulative P uptake in sediments increased significantly with increasing sedimentary Fe concentrations under both oxic and anoxic conditions. Only the sediments of Lake Glower See had higher P uptake under anoxic than oxic conditions, most likely due to vivianite formation. The net P sedimentation was higher with higher Fe concentration and higher under oxic than anoxic conditions. However, the lakes are classified as eutrophic because although the sediments of the Spree lakes can store further P, this additional P uptake is of little relevance for the P budget in highly P-loaded lakes with short water residence times (10–100 d), as is typical for fluvial-lacustrine systems.