Deterioration of water quality in a reservoir receiving pyrite-bearing rock drainage and its geochemical modeling

The investigation of water chemistry of a reservoir and the geochemical modeling were carried out to elucidate the deterioration of the water quality. The chemical properties showed that the sulfate ion concentration was high at the sampling locations in the reservoir where the pH value was less than 5, and that there was sufficient dissolved oxygen to keep the redox condition in the reservoir oxic. Reddish-brown iron precipitates were also observed on the surface of the excavated rocks used for the landfill constructed in the center of the reservoir, because the rocks contained an appreciable amount of pyrite. Accordingly, the cause of the reservoir acidification was expected to be the oxidation of pyrite contained in the excavated rocks. The geochemical code, PHRREQE, was applied to evaluate the behavior of aluminum detected in the reservoir. The calculated results showed that the aluminum concentration was restricted by the solubility products of aluminum hydroxides. This suggests that the acidified reservoir water leached aluminum from the surrounding soils, rocks and suspended solids.