Equilibrium constraints on buffer erosion based on the chemistry and chemical evolution of glacial meltwaters

Document available in extended abstract form only. Colloidal montmorillonite sols could form if the bentonite buffer in a KBS-3 repository for spent nuclear fuel should expand into a fracture and come into contact with dilute glacial meltwaters that have migrated to repository depths from the surface of a warm-based ice sheet. Transport of the sols away from deposition holes in flowing groundwater could conceivably result in significant erosional mass losses of the buffer, thus compromising one or more safety functions assigned to this key engineered barrier. Equilibrium constraints on sol formation were considered in the present study based on observations characterizing the chemistry of sub-glacial meltwaters and the possible chemical evolution of such solutions as they migrate through repository host rocks at Forsmark. Montmorillonite sols are stable under two general limiting conditions: 1) if the concentrations of divalent cations in the colloidal system are below a threshold 'critical coagulation concentration' (CCC) consistent with DLVO theory and the Shulze-Hardy rule, or 2) if monovalent cations occupy a significant fraction of ion-exchange sites. Regarding the first condition, the CCC for Ca2+, the dominant divalent cation in many Swedish groundwaters, is approximately 1 mM. Regarding the second condition, a limited number of experimental studies indicate that montmorillonite sols become unstable, forming gels, if the equivalent fraction, X, of Ca2+ on exchange sites ≥ 0.9. This behaviour results from the effects of ion-ion correlations, which are not accounted for in DLVO theory and which strongly increase the net attraction between colloidal particles having high surface charge and divalent counterions in the electrical double layer. Mass-action constraints for the reaction Ca2+(solution) + 2Na+(montmorillonite) = 2Na+(solution) + Ca2+(montmorillonite) assuming that 1) exchange sites are occupied predominantly by Na+ and/or Ca2+ and 2) the corresponding Gaines-Thomas selectivity coefficient, KGT, = 4.5 and is effectively independent of the density of montmorillonite-water systems, suggests that for XCa2+ ≥ 0.9, the activity ratio, a2Na+/aCa2+, ≤ 0.05. These conditions were evaluated in relation to the chemistry of meltwaters that were sampled in various studies of warm-based Alpine glaciers and continental ice sheets. The calculated aqueous speciation of the meltwater samples, evaluated at 0 C, indicated that although most were extremely dilute with Ca2+ concentrations well below the CCC, they also had very low a2Na+/aCa2+ values, which would tend to drive ion-exchange reactions toward the condition XCa2+ ≥ 0.9. The only exception to this observation is a joehkulhlaup sample from Casey Station, Antarctica, which is believed to be unrepresentative of sub-glacial environments due to the effects of CO2(g) degassing and concomitant precipitation of carbonate minerals. The available evidence thus suggests that glacial meltwaters would tend to inhibit, not promote, buffer erosion. A comprehensive analysis of the dynamic nature of the buffer-groundwater interface, taking into account advective/diffusional fluid mixing, spatial variations in bentonite density, and possible kinetic inhibition of ion-exchange and other reactions, could provide a framework for evaluating this possibility further. Equilibrium constraints on the chemical evolution of sub-glacial meltwaters that could migrate through host rocks at Forsmark were interpreted based on mineralogical information for fracture fillings at depth. The most recent generation is characterised by clay minerals (mainly corrensite) and calcite, but also minor amounts of goethite and pyrite, mainly associated with hydraulically-conductive fractures and fracture zones. Although smectite is reported to occur at all depth levels at Forsmark, it is recorded as being 'minor' in abundance in comparison with corrensite, illite, saponite, and mixed-layer smectite-illite. Calcite is also present at all depth levels, but gypsum, dolomite, and siderite are absent throughout the system. Therefore, site mineralogical data show that smectite and calcite occur at all depths in Forsmark fractures, with no evidence for removal/dissolution by previous glacial episodes. This natural analogue implies that these minerals may not have been eroded/dissolved during previous glacial episodes. Although SKB emphasise that groundwater compositions at Forsmark can be interpreted by simple mixing relationships alone, thermodynamic activity diagrams show that key parameters may be controlled by reactions involving montmorillonite and saponite clays and calcite present in fracture in-fills. These reactions may thus buffer these parameters in any future intruding glacial meltwaters. This conclusion would not be evident from approaches assuming that mixing is the only process responsible for major element variations. (Na+)2/Ca2+ activity ratios of most Forsmark groundwaters are < 0.05, implying that any clay exchanger in equilibrium with these waters would be > 90 % calcium end-member, and thus not in the stability field for sol formation