Groundwater vulnerability indicator assessment of karst island water resources: Enhancement of the Freshwater Lens Assessment Protocol

Karst islands such as those found in the Philippine Archipelago present challenges for local stakeholders to manage their water resources sustainably. Anthropogenic climate change, an increasing population and changes in land use and industry have all combined to altering the water balance on these islands. The freshwater lens is susceptible to saltwater intrusion through human drivers (such as over-abstraction) and natural processes (including variable precipitation and storm surges). The dynamics of such freshwater lenses involve an interplay between physical, chemical and socio-economical processes; therefore, finding a solution necessitates an interdisciplinary approach and a range of data collection strategies. This approach was formalized in a Freshwater Lens Assessment Protocol (FLAP) and expanded through the application of a Vulnerability Indicator Assessment (VIA). The VIA provides an accessible framework for data integration and an improved knowledge base, from which it is possible to make better decisions and establish effective management programs to protect and maintain this vital resource. The FLAP approach and VIA analysis were developed and tested on Bantayan Island (Cebu Province) in the Philippines. The results of the analysis reveal areas of saltwater intrusion vulnerability located along coastal boundary margins, mapped structural discontinuities and zones of upconing due to over-pumping. We infer that the VIA forms an appropriate, cost-effective interdisciplinary tool that synthesizes data sets through hazard characterization, integrates stakeholder knowledge pertaining to water resource management and operational policy, assesses the risk and assigns a risk designation for groundwater vulnerability to saltwater intrusion.