Chemical evolution of shallow and deep groundwaters in a Triassic sandstone aquifer (Buntsandstein), SW-Germany

Abstract

The widely extended Bunter sandstone (Lower Triassic) in SW-Germany mainly consists of quartz with minor portions of K-feldspar and illite. It represents an important aquifer system which can be subdivided into an unconfined zone (Black Forest) and a confined zone covered by carbonates with evaporites and continental beds. As a result of erosion, valley cutting and long-term significant groundwater circulation in the unconfined zone, most of the carbonate cement and formerly highly mineralized formation water has been leached. Thus, present-day groundwater is very low to low-mineralized, contains little hydrogen carbonate, and is devoid of geogenic sulfate and chloride. With respect to silicate minerals, very shallow groundwater is undersaturated, but somewhat deeper circulating water appears to be highly oversaturated. By contrast, slowly circulating groundwater from the confined zone is highly mineralized, ± saturated for calcite and dolomite, and moderately over saturated with respect to the silicate minerals. Chloride and sulfate are present in high concentrations. Whereas K-feldspar is being dissolved in the unconfined zone, it is well preserved in the confined zone.