Field and laboratory investigations were performed to identify the principal mechanisms of the hydrochemical groundwater evolution among low mineralised groundwater in the Triassic Bunter sandstone aquifer of the Odenwald low mountain range, central Germany. Hydrochemical composition comprises low pH, SO 4 -rich shallow groundwaters issued by springs (Ca-Mg-SO 4 -type) grading to SO 4 -poor deep groundwaters with near-neutral pH (Ca-HCO 3 -type). Batch experiments of the original rock were run to determine primary mineral alteration reactions and the origin of dissolved ions. Principal experimental reactions comprise the decomposition of anorthite, K-feldspar, biotite and jarosite as mineral components of the original sandstone rock and the formation of clay minerals of the smectite group (Ca-montmorillonite, beidellite), and iron hydroxides as secondary minerals. Mobilisation of fluid inclusion in quartz grains contributes to Na and Cl concentrations in the leachates. The evolution of deep groundwater circulation proceeds by mineral alteration reactions calculated by the inverse modelling of both primary and secondary minerals to produce low-T mineral phases. The dissolution of K-feldspar converts Ca-montmorillonite to illite (illitisation). The formation of Na-beidellite correlates with decreasing concentration of Na in solution. Mineral reactions further proceed to the formation of kaolinite as stable mineral phase. As indicated by modelled adsorption curves, the decrease of SO 4 concentrations during groundwater evolution relates to the adsorption of SO 4 on iron hydroxides. The leaching of calcite indicated for individual groundwaters relates to the distribution of loess in the appropriate catchment areas.

Groundwater sampling was accomplished in the basaltic sequence of the Rhön mountain range, Germany, in order to investigate hydrochemical groundwater evolution and to delineate mineral alteration reactions involved in natural weathering. The hydrochemical compositions of near-surface groundwaters indicate a Ca/Mg-HCO 3 type with near-neutral pH and evolve to a Na-HCO 3 type with high pH at greater depth. Column experiments were performed with basaltic and phonolitic rock samples to determine individual mineral alteration reactions. The basic reactions could be related to the alteration of olivine, Ca-pyroxene, plagioclase, pyrrhotite, and feldspathoids under formation of secondary clay minerals (smectites, illite) and goethite. The mineral alteration reactions deduced from the leaching experiments by inverse modelling were found to be consistent with the mineral reactions associated with the natural groundwaters. The reactions calculated for groundwater evolution involve the alteration of primary and secondary minerals to produce low-T mineral phase. The conversion of secondary Na-beidellite to illite occurs at a later stage of groundwater evolution, reducing the concentrations of K ? and Mg 2? . Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values, with Mg 2? removed from solution preferred to Ca 2? . Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na ? for Mg 2? and Ca 2? on Na-beidellite, supported by cation exchange on coatings of iron hydroxides as alteration products. Among the mature high-pH groundwater at greater depth, the dissolution of anorthite and albite has significant effect on groundwater composition.