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. In this study we make use of 3-D hydraulic simulations to investigate theregional groundwater flow in the Upper Rhine Graben. The modeling is basedon an existing detailed 3-D structural model covering the whole Upper RhineGraben from the surface down to 14 km of depth. The overall goal of this studyis to provide some quantitative analysis on the role of the hydraulic headtopology in shaping the underground hydrodynamics by taking into accountinteractions with the heterogeneous subsurface sedimentary configuration ofthe basin system. Therefore, the main question addressed by this study canbe summarized as follows: does the deep graben flow follow the topographicgradient and the flow direction of the river Rhine from the Alps northwardto the northernmost area of the Upper Rhine Graben? Our results demonstrate the presence of a regional subsurface flow in thesedimentary rocks aligning from the graben flanks towards its center and inthe southern half of the graben from south to north. The graben-parallelflow velocity is found to be about 1 order of magnitude lower than the velocitypredicted perpendicular to the main graben axis. Besides these generaltrends, the modeling highlights local heterogeneities in the shallow 3-Dflow field. Those arise from the interaction between regional groundwaterflow and the heterogeneous sedimentary configuration. Within the Cenozoicsediments forming the uppermost aquifer in the model, groundwater flowsare driven by imposed hydraulic gradients from recharge areas located at higher elevations in the Black Forest and Vosges Mountains to the discharge region at a lower elevation in the Rhine valley. The presence of a regional aquitard (Keuper) separating the shallowand the deeper aquifer system (Muschelkalk, Buntsandstein, and Rotliegend) hinders hydraulic connection among the two aquifer systems. Thisis exemplified by the development of a flow system in the deeper aquifers,which shows a more continuous graben-parallel south–north direction.Based on these results we can conclude that both the hydraulic head topologyand the level of structuration of the sedimentary sequence exert a 1st-orderrole in shaping the regional flow system at depth. The regional modelpredicts a heterogeneous flow system within the upper 4 km of the UpperRhine Valley, where flow velocities in the graben valley can reach up to 45 mm yr −1 in the upper and lower aquifers. Back to the current conceptualhydrogeological model, the results question the presence of a graben throughnorthward flow, being limited to the southern half of the graben. In thenorth, the groundwater dynamics turn out to be more complex, beingstructurally linked to the local geology. This calls for additional studieswith a higher level of both structural and stratigraphic attributes inorder to arrive at a better quantification of the local to the regionalgroundwater dynamics in the area.

Regional hydraulic model of the Upper Rhine Graben