Assessing the Risk of Saltwater Intrusion on the Gulf Islands, BC

In coastal regions, the quality of groundwater can be compromised due to saltwater intrusion (SWI) caused by various natural (sea level rise and storm surge) and anthropogenic (pumping) hazards. The goal of this research was to distinguish groundwaters impacted by SWI in the bedrock aquifers of the Gulf Islands, BC and identify thresholds for select chemical parameters that can be used for monitoring purposes, as well as to develop and test an approach for assessing risk to groundwater quality in coastal aquifers. The most reliable indicators were Cl/(HCO3 + CO3), BEX (base exchange index), Cl vs. EC, depth vs TDS, and a quantile analysis, resulting in 138 well samples (out of 795) that appear to be impacted by SWI. Based the 95th percentiles, for which 100% of the samples graphically showed strong evidence of SWI, the recommended threshold for Cl is 480 mg/L, 2,090 µS/cm for EC, and 970 mg/L for TDS. These samples were collected from wells that predominantly fall along the coastline. The vulnerability of the bedrock aquifers to SWI was assessed spatially by mapping hazards in combination with the aquifer susceptibility. Hazards due to pumping have the greatest influence on the vulnerability. Risk was evaluated spatially using an economic valuation of loss – here replacement of a water supply. The combination of chemical indicators and risk assessment maps are useful tools for identifying areas vulnerable to SWI, and these tools can be used to improve decision-making related to monitoring and community development for coastal areas.