Deep groundwater in the crystalline basement of the Black Forest region

摘要

Two major types of groundwater can be readily distinguished in the Variscian crystalline basement of the Black Forest in S-W Germany. Saline thermal water utilized in spas has its origin in 3-4 km deep reservoirs and developed its composition by 3 component mixing of surface freshwater, saltwater (of ultimately marine origin) and a water-rock reaction component. In contrast to the thermal water, CO_2-rich mineral water, tapped and bottled from many wells in the Black Forest, has low salinities but a TDS distribution similar to that of thermal water. It developed its chemical composition entirely by reaction of CO_2-rich water with the gneissic of granitic aquifer rock matrix. Particularly important is the contribution of various plagioclase dissolution and weathering reactions that may, at some locations, involve precipitation and dissolution of secondary calcite. Sodium/Ca ratios of water and of rock forming plagioclase in the basement rocks suggests that plagioclase 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; Ca-HCO_3 near the surface, Na-Ca-HCO_3-SO_4 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 is significantly lower in gneisses (gneiss: mean K = 5 × 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-surface flow system of CO_2-rich mineral waters are hydraulically and chemically unconnected.