HYDROGEOLOGICAL DESCRIPTION OF ONE OF THE LARGEST DRINKING WATER RESERVOIRS IN CENTRAL EUROPE: THE UPPER RHINE VALLEY

Hydrogeological investigations in the quarternary and Pliocene of the Upper Rhine valley are presented. The Rhine valley is one of the most important drinking water reservoirs in Central Europe. Permeability data derived from several thousand analyzed pumping tests in Quarternary sediments of the Upper Rhine valley varying from of K = 4.5*10-7 m/s to K = 2.7*10-1 m/s (geometric mean: K = 2.25 * 10-3 m/s) show that the highest values are found in sections beneath recent and historic fluvial systems. Fossile fans of small rivers from the Black Forest could be localized in the Rhine valley on the basis of the permeability pattern. The average hydraulic conductivity decreases from south to north along the flow direction which corresponds to a decrease in grain-size of the fluvial sediments. The research borehole Marlen is situated in the central part of the Upper Rhine valley south of Kehl (near Strasbourg), SW-Germany. A total of 251 m was drilled in the Quaternary, Early Quaternary and Pliocene. A large number of hydraulic, hydrochemical and isotopic investigations were carried out. The thickness of the Quaternary is about 120 m and it's transmissivity T = 3.05 10-1 m2/s. Transmissivity of the Early Quaternary/Pliocene is on the order of T = 5.72 10-3 m2/s. Well logging data allowed for the calculation of permeability from transmissivity. The permeability varied significantly with depth. Surprisingly, the highest permeability was found in the mid Quaternary and not in the upper part of the Quaternary. The composition of water stored in the Quaternary, early Quaternary and Pliocene changes with depth. Near surface water is of the Ca-HCO3-type. This young water contains tritium. However, it is saturated with respect to calcite. In the deepest part of the research borehole we found tritium-free Na-Cl-water. The salinity is of natural origin. We also observed a general increase of TDS with depth.