High sulphate concentrations in squeezed Boom Clay pore water: Evidence of oxidation of clay cores

Pore water has been extracted from Boom Clay by mechanical squeezing. Clay cores were obtained from various boreholes, all drilled at the SCK·CEN domain (Mol, Belgium).In contrast to pore water collected from piezometers, high sulphate concentrations are measured in the squeezed pore water. The lowest sulphate concentrations (<60 mg/l) were measured in pore waters squeezed immediately after drilling. Higher sulphate concentrations were often measured in the pore water when the clay cores were preserved for some time (generally <500 mg/l SO₄^2-, but sometimes up to 20,000 mg/l SO₄^2-). Nevertheless, a relation between preservation time and sulphate content could not be retrieved. However, major ion concentrations were obviously correlated with the sulphate content in the squeezed waters. The observed evolution in chemical composition were explained by water-rock interactions considering the pyrite oxidation and the subsequent ion exchange and mineral dissolution reactions.