Silica solubility in groundwater from Permian volcanogenic rocks (the Sudetes Mts., SW Poland) : the role of reversible aluminosilicate solids

The chemistry of groundwater in Permian volcanic rocks (rhyolitic tuffs, trachyandesites) in two small headwater catchments (the SudetesMts., SW Poland) has been investigated. The calculated transfer of silica released during weathering is greater than the real concentration in groundwater, and indicates that part of the silica is immobilized in secondary aluminosilicates. Silica solubility is interpreted in terms of the solubility of reversible secondary aluminosilicates (allophanes, hydroxyaluminosilicates). Partial chemical equilibrium of these phases is sometimes locally achieved. In groundwaters with pH above 6.7 from trachyandesites of the BukowiecMt. type, chemical equilibrium with respect to the hydroxyaluminosilicate HASB is probably maintained. Its solubility constant, calculated accordingly to the method given by Schneider et al. (2004), is lgK HASB = –44.74 (۪.52), and indicates that this "field" HASB has a solubility lower than the published experimental value (lgK HASB = –40.6 ۪.15) for synthetic HASB (op.cit.). Non-equilibrium with respect to aluminosilicates in groundwaters from rhyolitic tuffs and from trachyandesites of the Stożek Wielki Mt. type is caused by the short turn-over time of groundwater in the systemand by the poor neutralisation capacity of the bedrock. The silica concentration in these groundwaters results from steady-state conditions achieved between weathering release, hydrological removal and precipitation of secondary phases. The hydrogeochemical data are conducive to the presence of allophane minerals in the weathered rhyolitic tuffs.