Assessment of the alteration and acid-generating potential of deep boreholes for geological disposal

An integrated study, including mineralogy, geochemistry, lead isotopic composition, acid–base accounting, and factor analysis, was conducted on the rock core samples from two deep boreholes (AH1 and BH3) to assess the alteration and acid-generating potential of two representative lithologies of Korea (granite and geniss) for geological disposal. The alteration condition represents water circulation features in crystalline basement rocks, which is one of the key points to be assessed in repository sites. In addition, this study proposes that the acid-producing potential and acid-consuming minerals in the host rock are important for the long-term safety of a radioactive waste repository, given that sulfide oxidation may occur in artificial fractures created during construction. Fe³⁺ ions can then react with other sulfide minerals in reducing conditions and release H⁺ and SO₄ ²⁻, which can deteriorate cement-based grouting materials used for carven walls as well as the host rock. The results of the study show that the composition of Pb isotopes is a good indicator of the alteration of rock core samples compared to the chemical index of alteration and loss on ignition. The acid potential ratio and net acid production potential indicate that most of the core samples in both boreholes have a high alkali-generating potential. Factor analysis shows that the neutralizing capacity in AH1 is highly correlated with the alteration, while the neutralizing capacity in BH3 is correlated with secondary minerals. The alteration appears to be related to secondary minerals and the acid-producing capacity, while the changes in major elements are not related to the Pb isotopes in BH3 due to leaching along fractures.