A Multidisciplinary Approach to Studying the Nature and Occurrence of Saline Groundwater in the Gulf Islands

A multidisciplinary study involving hydrogeochemical sampling and borehole geophysics was undertaken to study the nature and occurrence of saline groundwater on two islands in British Columbia, Canada. Saturna and Hornby Islands are located at the south and north ends, respectively, of the Canadian Gulf Islands situated off the lower mainland of British Columbia. Two large-scale geochemical sampling programs were undertaken to investigate the spatial distribution of saline groundwater and to describe the geochemical evolution of groundwater in the fractured aquifer systems. Major ion chemistry indicates that cation exchange between calcium-rich waters and sodium-rich sites complicates a simple mixing trend, and saline waters appear to represent mixing either between fresh type (calcium bicarbonate) groundwater and seawater (recent saltwater intrusion) or sodium-rich groundwater and seawater (resident saline waters). Sodium-rich exchange sites are speculated to be a remnant of ocean water intrusion during the Pleistocene when the islands were submerged. As a result of its high mobility and conservative nature, chloride has been flushed from the shallow bedrock during a process of natural desalinization, but may remain trapped in the pores and fractures at depth, giving rise to a transitional saltwater wedge. Modern saltwater intrusion, brought about by increased development on the islands, is now competing with natural desalinization along the coast, and has left many drinking water supplies contaminated. Borehole geophysical logs from coastal wells confirm that saline groundwater is present at depth, and that its occurrence is related to fracture locations