An inter-disciplinary freshwater lens assessment protocol for karst islands

Abstract

Karst islands such as those found in the Philippine Archipelago present challenges for local stakeholders to manage their water resources in a sustainable manner. Anthropogenic climate change, an increasing population and changes in land use and industry have all combined to alter the water balance on these islands. Bantayan Island (Cebu Province, Philippines) was selected as the study area. The single freshwater lens of this island is the sole source of drinking water for the burgeoning population and is vulnerable to saltwater intrusion due to several factors, including karst geology, coastal overtopping and abstraction well exploitation. The aim of the research was to utilise thorough non-invasive geologic research methods, coupled with social research methods, which could link the community, researcher and government in order to improve the understanding and management of this fragile resource. Through the application of the Freshwater Lens Assessment Protocol (FLAP), saltwater intrusion and its influence on the freshwater lens were evaluated. The research incorporated geologic assessment and field mapping, hydrogeologic data analysis and hydrochemical analysis in parallel with social research (i.e. surveys of well owners and semi-structured interviews with key stakeholders distributed through three tiers of government) to yield a group of data sets that have contributed towards the development of an island-wide observational model. The spatial and multi-temporal analysis of typical chemical and physical data has established the spatial vulnerability of the aquifer. The multi-parameter field data collection provides a thorough FLAP methodology that can be used for rapid groundwater quality classification for each monitoring location. The approach allows evaluation of the current abstraction management schemes and the ability to improve design by instituting more realistic and science-based targets. A management instrument in the form of a vulnerability indicator assessment designed in relation to/specifically for aquifer zoning is proposed. A compelling message has been derived from the combination of geologic characterisation, hydrogeologic analysis and hydrochemical data sets resulting in an island-wide observational model that identifies areas of saltwater intrusion. Social data sets reveal themes and attitudes taken by national, provincial and local government respondents on issues involving weak regulatory oversight and limited enforcement of current water resource management policy that endanger the FWL. Additional insight is given into how unfettered land development is creating a negative impact on FWL sustainability. The integration of findings advances new knowledge and supports the efficacy of the FLAP as a non-invasive instrument for freshwater lens assessment in a low-income country context. The FLAP addresses a current gap in the current knowledge base through the implementation of a locally derived, nimble and affordable assessment protocol; the output of this will inform stakeholders on technical and social issues surrounding the sustainability of the freshwater lens.