Overview of oxidation phenomena in Boom Clay around galleries of the HADES URF – Status 2019: In-situ and leachate pore water geochemistry, mineralogy and modelling assessment

The in-situ and leachate pore water geochemistry, mineralogy, (reactive) transport modelling and geochemical modelling data were used in this study in order to evaluate oxidation phenomenon in the Boom Clay around the galleries of the HADES URF. In general, the new data confirm the previously established conceptual model in that oxidation occurs immediately in the course of the gallery excavation due to the creation of the fractures. The extent of these fractures dictates the extent of the oxidation in the host rock, which is limited to a maximum of 1.2 m ahead of the galleries of the HADES URF. After selfsealing of these fractures, a combined diffusion-advection regime redistributes the oxidation products around the galleries. Indeed, the sulphate radial concentration profiles show the decrease of the sulphate concentrations in the deep filters and concomitant increase of their concentrations in the filters nearest to the gallery in the 6 year observation period. The results suggest that the concentrations of sulphates remain relatively high (2560 mg/L at a distance of 0.4 m) in the near field even 25 years after the excavation. This indicates that the sulphates are not easily degradable (reducible) in the course of the open drift phase. Low concentrations of thiosulphates are generally observed (<1 mg/L) and tend to decrease with time confirming their metastable character. The high sulphate concentrations are accompanied with the increased concentrations of Na, K, Ca and Mg. The geochemical model reproduced fairly well the pore water chemistry data and can be explained by mineral dissolution/precipitation and ion-exchange reactions. The gypsum and jarosite are the most common oxidation products based on the XRD, FTIR, Mossbauer spectroscopy and XANES investigation. In samples with jarosite, no carbonates were detected indicating partial or complete loss of pH buffering capacity of the clay. Nevertheless, these mineralogical changes are limited to first few cm from the concrete lining.