Deep groundwater in the crystalline basement of the Black Forest region

Two major types of groundwater can be readily distinguished in the Variscia n crystalline basement of the Black Forest in S-W Germany. Saline thermal w ater utilized in spas has its origin in 3-4 km deep reservoirs and develope d its composition by 3 component mixing of surface freshwater, saltwater (o f ultimately marine origin) and a water-rock reaction component. In contras t to the thermal water, CO2-rich mineral water, tapped and bottled from man y wells in the Black Forest, has low salinities but a TDS distribution simi lar to that of thermal water. It developed its chemical composition entirel y by reaction of CO2-rich water with the gneissic or granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase d issolution and weathering reactions that may, at some locations, involve pr ecipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagio clase weathering is strongly incongruent. Calcium is released to the water, whereas Na remains fixed to the albite feldspar component. The major element composition of 192 water samples used in this study also indicates a clear vertical stratification of the type of water chemistry; C a-HCO3 near the surface, Na-Ca-HCO3-SO4 at intermediate depth and Na-Ca-Cl at great depth. The mean permeability of Black Forest granite is about K = 10(-6) m/s; it i s significantly lower in gneisses (gneiss: mean K = 5 x 10(-8) m/s) leading to focused flow through granite. Highly permeable fracture and fault zones , particularly in granite, are utilized by high-TDS saline deep groundwater as ascent channels and flow paths. Although spatially closely associated, the topography driven upwelling system of saline deep water and the near-su rface flow system of CO2-rich mineral waters are hydraulically and chemical ly unconnected. (C) 1999 Elsevier Science Ltd. All rights reserved.