Aquatic Geochemistry 3: 43–60, 1997. 43 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Permeabilities and Chemical Properties of Water in Crystalline Rocks of the Black Forest, Germany INGRID STOBER Geologisches Landesamt Baden-W¨ urttemberg, Albertstr. 5, D-79104 Freiburg i.Br., Germany (Received: 12 October 1996; in final form: 18 March 1997) Abstract. Investigations were carried out to determine the hydraulic and hydrochemical properties of crystalline rocks in the Black Forest of Germany and neighbouring regions. Rock permeabilities ( ) were determined to a depth of 3500 m. These parameters range from = 3.5 10 10 ms 1 to = 8.7 10 5 ms 1 ; and can increase up to an order of magnitude which is typical for porous aquifers. It is shown that on an average, granites are more pervious than gneisses and only the permeabilities of gneisses decrease with depth. The geochemistry of natural waters in crystalline rocks is not constant, but varies with depth and location. The concentration increases with depth and the water-type changes from a Ca—Na—HCO3- type (or Na—Ca—HCO3—) at shallow depths to a Na—Cl-type at greater depths. Thermal springs are found only in granitic rocks with on average higher permeabilities than in gneisses. Thermal waters are welling up in valleys at the bottom of steep mountains. The chemical composition of thermal spring water is identical to that of water found at greater depths. Using geothermometers it is found, that the depth of the deposits of thermal spring water in the crystalline basement rocks of the Black Forest is some 1000 m below the surface. The topographic relief in the mountains induces a deep circulation of infiltrating rain-water with an upwelling as thermal springs in the valleys. Key words: crystalline rocks, hydraulic properties, hydrochemical properties, Black Forest. 1. Introduction Super deep wells, such as the Kola well – situated in the former U.S.S.R – with a depth of 12 500 m show, that there are open, waterfilled fissures in crystalline rocks. The object of this investigation was to explore the rock permeabilities and the chemical properties of water in the crystalline rocks of the Black Forest in Germany and neighbouring regions (Figure 1). The crystalline basement is being considered as a possible host rock for a waste repository, because of its assumed low hydraulic conductivity. However, hydrogeologic evidence shows, that there are several thermal and mineral water springs in crystalline rocks, possibly contradicting assumptions of generalized impermeable conditions. The area of investigation (200 70 km) the crystalline basement of the Black Forest was situated in Southwestern Germany. During the Variscan orogeny, the crystalline basement underwent strong structural and metamorphic/hydrothermal overprinting, accompanied by plutonic and volcanic activity. Since then, the lithol- ogy and structure of the basement have not changed significantly. During the