Freshwater lens assessment of karst island water resources: towards an interdisciplinary protocol

Abstract

Fresh groundwater lenses on karstic oceanic islands form a vital resource sustaining local populations. However, this resource is susceptible to saltwater intrusion through human drivers (over-abstraction) and natural processes (variable precipitation and storm surges). There is a paucity of means to assess the risks that freshwater lenses are exposed to. This is partly driven by a poor understanding of the root causes of saltwater intrusion, which leads to potentially inappropriate freshwater management strategies. Thus, effective management of these freshwater lenses requires a baseline understanding of the processes that drive saltwater intrusion and the degradation of freshwater lenses, and the temporal and spatial variability of these processes. Dynamics of such freshwater lenses involve an interplay between physical, chemical, and socio-economic 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). Results from the research developed and tested on Bantayan Island in the Philippines reveals a sufficient freshwater lens to support the current and projected population; however, local officials are operating abstraction wells from the wrong locations on the island. Such locations are utilized due to ease of access to existing infrastructure and government boundaries, but do not consider technical factors that influence saltwater intrusion. FLAP is an appropriate, cost-effective, interdisciplinary tool that uses a pragmatic approach to data collection, interpretation, and integration into an observational model. Continuous adjustments are possible through ongoing monitoring of the model, offering opportunities to evaluate the efficacy of resource management strategies.