Buffering against intrusion of groundwater of undesirable composition

Favourable hydrogeochemical conditions in a geological repository for spent nuclear fuel might be unbalanced by intrusion of undesirable water compositions. Such conditions have been triggered by hydrogeological transients caused by glacial cycles in Fennoscandian Shield. Deglaciation and marine stages are such changes, which could have introduced either dilute, low pH and oxic melt water or marine source water for SO4 reduction producing dissolved sulphide, and have unbalanced favourable hydrogeochemical conditions in the geological past. It appears that groundwater system has a sufficient buffering capacity against these natural perturbations, and is able to stabilize chemical conditions. Current understanding of hydrogeochemical system does not support notable dilution and indicates strong buffering capacity against O2 infiltration or low pH water. However, the information cannot unambiguously prove that oxygen has not reached deep bedrock and one major uncertainty is associated with existence and rate of marine sulphate reduction. Therefore focussed fracture mineral studies and field investigations with modelling exercises are desirable to verify the stability of favourable hydrogeochemical conditions.