Hydrogeochemical assessment of saltwater intrusion processes in the karst aquifer of Bantayan Island, Cebu, Philippines

<p dir="ltr">Globally, millions of people living on oceanic islands rely on freshwater lenses as their sole source of freshwater. These lenses are highly vulnerable to stressors such as sea-level rise from climate change, reduced recharge, and over-extraction, which can lead to saltwater intrusion. Although informed management and policy can help mitigate these risks, many countries with limited data and resources face monitoring and data collection challenges. In the Philippine archipelago, karst islands add further complexity to sustainable freshwater resource management. This study utilized a nimble methodology for assessing saltwater intrusion in settings where monitoring data and laboratory facilities are limited or non-existent, using hand-dug wells and hydrochemical analysis of select ions (chloride, sulfate, bicarbonate, potassium) and trace elements (boron and total bromine) and chloride to bicarbonate ratios. This methodology was applied to Bantayan Island where saltwater intrusion is affected by tidal forcing and groundwater overexploitation. The analysis revealed saltwater intrusion in selected zones in the terrace and depositional physiographic provinces, but not in the bedrock province. Groundwater salinization in Bantayan Island is driven not only by coastal saltwater intrusion via upconing and overtopping along the shoreline but also by geologic features and climate-related factors affecting recharge and freshwater lens dynamics.</p>