Transport and geochemical controls on the distribution of solutes and stable isotopes in a thick clay-rich till aquitard, Canada

A comprehensive understanding of the transport and geochemical processes controlling solutes in clay-rich aquitard confining units is needed to accurately predict the long-term migration of contaminants into the subsurface. To this end, the geochemical and stable isotopic composition of porewaters in the upper 22 m of a thick, unoxidized and nonfractured clay-rich, till aquitard (Sutherland Group) was examined in detail. The aquitard is overlain by about 8 m of oxidized and fractured till (Floral Fm). Concentrations of TDS, SO4(2-), HCO3-, Cl-, Na+, Mg2+, Ca2+ and porewater deuterium were greater in the Floral Fm and decreased with depth through the aquitard. The elevated and seasonably variable solute concentrations in the oxidized Floral Fm were attributed to geochemical weathering and dynamic water movement through fractures. Good fits between measured delta2H, TDS, SO4(2-), Cl- and HCO3- profiles through the aquitard and simulated solute transport profiles were obtained by diffusion (without advection) with transport times of 4-6 ka. The deficiency of geochemical reactions affecting HCO3- and SO4(2-) in the aquitard was supported by delta13C(DIC) and delta34S(SO4) analyses. Geochemical and isotope mass balance modelling (NETPATH) indicated that diffusive mixing with minor calcite dissolution and ion exchange could account for the distribution of Na+, Ca2+ and Mg2+ in the aquitard. Results of this study further suggested that microbiological activity in the aquitard was limited. With minor exceptions, the solute and isotopic profiles, their transport and the controlling geochemical reactions in the Sutherland are similar to those determined at another clay-rich till aquitard, 160 km south of this site, suggesting that geochemical and biological processes in some clay-rich aquitards may have a minimal effect on the migration of dissolved constituents.