Saltwater Intrusion in Fractured Crystalline Bedrock Aquifer. - A Case Study on Koster Islands, SW Sweden

Abstract: Saltwater intrusion (SWI) is one of the significant threats in many coastal areas, mainly driven by anthropogenic hazards (over-pumping) and climate change (sea level rise). So far, there have been extensive research and techniques to understand and minimize SWI. However, quantification and mechanism of SWI remain unclear and incomplete, especially in fractured crystalline bedrock aquifer. This study presents an extensive literature review about SWI with a focus on fractured crystalline-rock aquifer and an aquifer vulnerability assessment of Koster Islands specific to SWI. Using two modified multicriterion geographical information systems (GIS) based models (WELD-Ld and WELD-Mc), the vulnerability of Koster bedrock aquifers to SWI was assessed. Several parameters were used to capture the geomorphological and physical factors (e.g., fractures) which influence the flow of SWI in bedrock aquifers of Koster Islands. These spatially and semi-quantitatively methods are based on the well-known approach ‘DRASTIC,’ a systematic evaluation technique which incorporates major influencing factors. The WELDLd model uses five parameters (thematic layers) including Well density (W), Elevation (E), Lineament length (L), Distance to the sea (D) and Lineaments density (Ld) and WELD-Mc model uses map-counting (Mc) instead of Lineament density (Ld) calculation. The Mc analysis examines the inhomogeneity of Koster lineament distribution patterns. The impact (rate value) and significance (weight value) of each parameter in the vulnerability assessment was assigned based on previous studies and communication with experts. Lineament density (Ld) is the major influential factor in the vulnerability of the study area, which is related to the highly sheared and densely distributed Koster dyke swarm. The overall vulnerability assessment maps show that the north and northeast part of the islands have a higher risk of SWI. The obtained maps have been compared with observation data from the study area and used to validate the models and identified areas and wells that may be at higher risk of SWI. In conclusion, the acquired results (vulnerability maps) incorporated with field studies can be a useful tool for decision-making and groundwater management.