To ensure the safe, long-term management of Canada’s used nuclear fuel, the Nuclear Waste Management Organization plans to construct a deep geological repository (DGR) at a depth of at least 500 m within crystalline host rock. The engineered barrier system (EBS) will contain copper-coated carbon steel used fuel containers (UFCs), surrounded by highly compacted bentonite (HCB) and lower density gap fill materials (GFM). Under anoxic conditions, corrosive anionic species such as bisulfide (HS⁻) may diffuse through bentonite barriers, reach the UFC surface, and promote corrosion of the copper coating. Following DGR closure, the bentonite buffer gradually saturates and density begins to homogenize. Saturation is expected within 2000–3000 years, whereas full density homogenization may never complete. When anoxic conditions are established and HS⁻ is transport to the UFC surface, GFM and HCB may retain their own physical and transport properties, which influence sulfide fluxes to the container surface. Accordingly, understanding anion transport through each material, but particularly the lower density GFM is essential for predicting copper corrosion. Low permeability in bentonite causes anion transport within bentonite to be predominantly diffusion limited. Therefore, the effective diffusion coefficient is a critical parameter for accurately predicting copper corrosion. This study investigates HS⁻ transport in bentonite GFM using through-diffusion experiments under simulated DGR environmental conditions. The use of HS⁻ enables evaluation of reactive transport processes anticipated as the DGR evolves over long timescales. Overall, these results provide insights into EBS performance and strengthen confidence in the safety of the Canadian DGR design.