Regional hydraulic model of the Upper Rhine Graben

Abstract

Abstract. In this study we make use of 3-D hydraulic simulations to investigate the\nregional groundwater flow in the Upper Rhine Graben. The modeling is based\non an existing detailed 3-D structural model covering the whole Upper Rhine\nGraben from the surface down to 14\u2009km of depth. The overall goal of this study\nis to provide some quantitative analysis on the role of the hydraulic head\ntopology in shaping the underground hydrodynamics by taking into account\ninteractions with the heterogeneous subsurface sedimentary configuration of\nthe basin system. Therefore, the main question addressed by this study can\nbe summarized as follows: does the deep graben flow follow the topographic\ngradient and the flow direction of the river Rhine from the Alps northward\nto the northernmost area of the Upper Rhine Graben? Our results demonstrate the presence of a regional subsurface flow in the\nsedimentary rocks aligning from the graben flanks towards its center and in\nthe southern half of the graben from south to north. The graben-parallel\nflow velocity is found to be about 1\xa0order of magnitude lower than the velocity\npredicted perpendicular to the main graben axis. Besides these general\ntrends, the modeling highlights local heterogeneities in the shallow 3-D\nflow field. Those arise from the interaction between regional groundwater\nflow and the heterogeneous sedimentary configuration. Within the Cenozoic\nsediments forming the uppermost aquifer in the model, groundwater flows\nare 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 shallow\nand the deeper aquifer system (Muschelkalk, Buntsandstein, and Rotliegend) hinders hydraulic connection among the two aquifer systems. This\nis exemplified by the development of a flow system in the deeper aquifers,\nwhich shows a more continuous graben-parallel south–north direction.\nBased on these results we can conclude that both the hydraulic head topology\nand the level of structuration of the sedimentary sequence exert a 1st-order\nrole in shaping the regional flow system at depth. The regional model\npredicts a heterogeneous flow system within the upper 4\u2009km of the Upper\nRhine Valley, where flow velocities in the graben valley can reach up to 45\u2009mm\u2009yr −1 in the upper and lower aquifers. Back to the current conceptual\nhydrogeological model, the results question the presence of a graben through\nnorthward flow, being limited to the southern half of the graben. In the\nnorth, the groundwater dynamics turn out to be more complex, being\nstructurally linked to the local geology. This calls for additional studies\nwith a higher level of both structural and stratigraphic attributes in\norder to arrive at a better quantification of the local to the regional\ngroundwater dynamics in the area.