Changes in groundwater composition as a consequence of deglaciation – Implications for performance assessment

The behaviour of the deep geological repository planned by SKB will depend on the climatic evolution over a long term, such as the advance and retreat of ice sheets after glaciation and deglaciation periods. The melt water enriched in oxygen due to a deglaciation episode will infiltrate into the geological media and may compromise the reducing conditions of the repository. It has been established the importance of microbial activities and oxidation of minerals, such as Fe-bearing minerals to preserve the repository from the oxidant front. Former studies assessed the Fe-bearing mineral capacity to buffer against oxidant conditions due to the intrusion of melt water into the system. The objective of the present study is to update these works using the new data available from the site characterisation. The conceptual model considers a fast flow pathway, characteristic of a fracture into which is introduced melt water equilibrated with atmospheric O2 and CO2. The advance of the oxidant front is kinetically controlled by the dissolution-precipitation of annite (an Fe(II) mica). The alkalinity and the content of aluminium, silica and iron are controlled respectively by the calcite, kaolinite, chalcedony and iron precipitate present into the system. A sensitivity analysis is performed onto the transport and geochemical parameters of the system (flow velocity, porosity, the content of Fe-bearing mineral, the iron-precipitate, reactive surface of the minerals). In the reference case, the oxidant front affects mainly the first five metres of the system over 5,000 years. In the worst case where the flow velocity is important, it reaches 250 metres after 2,000 years. In all cases, the minerals contained in the system control the geochemical changes due to the buffering of the oxidant front.