Groundwater Evolution and Mineral Alteration Reactions in the Basaltic Rock Sequence of Mt. Wasserkuppe, Germany: A Case Study |
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| Authors: | Florian Ludwig Ingrid Stober Kurt Bucher |
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| Institution: | (1) Wasser&Boden, Am Heidepark 6, 56154 Boppard-Buchholz, Germany;(2) Department of Environment, Schwedinstr. 12, 79102 Freiburg, Germany;(3) Institute of Mineralogy and Geochemistry, University of Freiburg, Albertstr. 23b, 79104 Freiburg, Germany |
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| Abstract: | Groundwater sampling was accomplished in the basaltic sequence of the Rh?n 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–HCO3 type with near-neutral pH and evolve to a Na–HCO3 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 Mg2+. Near-surface groundwaters do not indicate significant cation exchange. Initial cation exchange requires elevated pH values,
with Mg2+ removed from solution preferred to Ca2+. Na-alkalisation of the groundwaters at greater depth suggests the exchange of Na+ for Mg2+ and Ca2+ 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. |
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