The Hydrogeology of Salt Spring Island, British Columbia

Groundwater on Salt Spring Island, British Columbia, flows through fractures in sedimentary and igneous rock aquifers. Recharge is dominantly by rainfall infiltration. At a local scale, groundwater discharges into lakes and streams; regional flow is toward the coast where the groundwater discharges to the ocean. Groundwater evolves from a Na-Cl rainwater to a Ca-HCO3 type through calcite dissolution. Cation exchange (Ca exchanges for Na) is a dominant process in the sedimentary rocks (but not in the igneous rocks), resulting in a Na-HCO3 type water. Mixing with a Cl-rich end-member is also a dominant process in both rock types. Some wells near the coast are known to be impacted by saltwater intrusion. Despite the differences in scale for testing, and rock type, pumping and tidal response tests yield similar averages across the Gulf Islands. Transmissivity is estimated on the order of 10-5 to 10-4 m2/s and hydraulic conductivity on the order of 10-7 to 10-6 m/s. Using Visual MODFLOW, a steady-state fresh groundwater simulation for the Swan Point area generated a representative flow system and established a reasonable range for the aquifer properties. A tidally-forced transient model simulated the tidal response and further constrained the aquifer properties. Next, a density-dependent flow and transport model was constructed in SEAWAT to simulate the current position of the saltwater interface, which was found to be near vertical at the coast. The lack of a wedge likely reflects the relatively steep topography at this site and suggests that submarine groundwater discharge may occur. However, the small model domain and imposed boundary conditions may over-estimate the amount of inflowing water. The SEAWAT model was then used to determine the sensitivity of the aquifer to climate change, encompassing an increase in mean annual recharge by ~1.5 % and SLR of 1.17 m by the end of this century, as well as pumping. Model results showed no significant impacts to the salinity distribution or saltwater wedge geometry at this particular site due to the steep topography. Areas with less steep topography may have greater impacts and merit further research