Genetic geochemical model for mining affected groundwaters of the Lusatian post-mining district

Abstract

► Buffering processes due to pyrite oxidation in Lusatian mining dumps release Ca. ► Calcium release leads to water quality controlling gypsum precipitation. ► Gypsum precipitation explains decreasing c(Ca) along with increasing I above 60 mM. ► Carbonate buffer formation explains c(Fe)/c(SO 4 ) > 0.5 at pH > 5.0 and gypsum precipitation. Using groundwater quality data from the Lusatian post-mining district a hydrogeochemical model is derived for the evolution of mining affected groundwaters in pyrite-rich dumps which consist mainly of silicates and variable amounts of calcite. Pyrite oxidation paralleled by buffer processes leads to gypsum saturation in a significant portion of the water. Gypsum precipitation controls SO 4 and Ca concentrations in groundwaters above an ionic strength ( I ) of 60 mM. It has been found that there is always a clear relationship between I , SO 4 and Ca concentrations. In particular, there is a tendency that Ca concentrations decrease with increase in ionic strength above I = 60 mM and a striking rareness of samples with SO 4 concentrations between 20 and 30 mM above an ionic strength of 100 mM. These observations are explained by a genetic model. This model also explains the observed relationship between the c(Fe)/c(SO 4 )-ratio, the ionic strength, and the observed pH-values. Based on the field data and supported by geochemical equilibrium calculations, it is shown that silicate weathering along with calcite dissolution must be a significant buffering process at least in some areas.