DEEP RECHARGE BY OXIDIZING WATER IN GRANITE: A GEOCHEMICAL MODELLING STUDY

Most deep ( > 200m) groundwaters are reducing, largely due to previous near-surface interactions with organic particles in soils and sediments that act as reducing agents. It is possible that oxidizing groundwaters were more common in the past during major glaciation cycles, when large volumes of low-salinity, oxidizing meltwaters may have entered recharge zones at the base of a thick ice sheet on scoured bedrock. Such a scenario is of interest for performance assessment (PA) modelling for a deep geologic repository for used nuclear fuel in granitic rock of the Canadian Shield. Based on a range of existing groundwater properties, PA modelling generally has assumed that groundwaters at repository depth would be reducing, a factor that would help to provide stable and largely unreactive geochemical conditions for engineered barriers and wasteforms.

If a future continental glaciation leads to deep recharge by oxidizing meltwaters, however, the geochemical evolution of the flow system may vary beyond the range of conditions that have been considered for PA modelling. Two stylized PHREEQC geochemical modelling approaches have been used to investigate how meltwater compositions might evolve with depth in such circumstances. One approach approximated the movement of oxidizing water to depth with little interaction along the flow path. The other approximated the evolution of meltwater by mixing with other groundwaters and by reaction with minerals. Hydrogeochemical data from groundwaters of the Lac du Bonnet batholith in southeastern Manitoba and representative mineral compositions were used in the simulations.

For the system modelled, both approaches indicated that reactions with biotite or chlorite would buffer the oxidizing capacity of the meltwater in the subsurface, provided that such reactions could proceed to equilibrium or to an advanced steady state. The insights provided by this geochemical modelling study have been used to design a field study of fracture minerals in the Lac du Bonnet batholith to look for evidence of previous reaction of biotite and chlorite with oxidizing groundwaters.