Silica origin and solubility in groundwater from the weathered zone of sedimentary rocks of the Intra- Sudetic Basin, SW Poland

Abstract

The groundwaters studied in the Intra-Sudetic Basin, SW Poland, occur in the regolith and in the fissured bedrock. Silica concentration in groundwaters ranges from 0.7 to 24.1 mg/L SiO₂. Mineral phases that are the source of silica solute were pointed out on bedrock mineralogy and saturation state analysis. Interpretation of aqueous chemistry in terms of mineral stability confirms that partial geochemical equilibrium is maintained. In groundwater with pH values below 4.8 from the fissured aquifer in Carboniferous siliciclastic rocks, kaolinite controls silica solubility. Congruent dissolution of the allophane phase and/or incongruent reaction between the Al(OH)₃ form and kaolinite/ halloysite or allophane can be responsible for silica activity in groundwater, with pH values between 5.5 and 6.7, from the regolith on Carboniferous rocks and from the fissured aquifer in Cretaceous mudstones. Groundwaters with pH values above 6.7 occur in the Permian and Cretaceous fissured aquifers which contain calcite/dolomite minerals. Silica and aluminium concentrations in these aquifers are clearly maintained by equilibrium with the hydroxyaluminosilicate HASB. The solubility constant of HASB estimated on groundwater chemistry is lgK HASB = -44.88 (±0.48), which is lower than the recently published experimental value (SCHNEIDER & al. 2004). In groundwater with pH values between 4.8 and 5.5 from the regolith sampled during snow melt, equilibrium, even with reversible secondary aluminosilicates, is not maintained. Silicon and aluminium concentrations result from kinetics or steady-state between weathering release and hydrological removal.