Evaluation of Hydro-Geochemical Processes Affecting Groundwater Quality in Bozcaada Island Using Water Quality Index, Multivariate Statistical Analyses, and Spatial Distribution Mapping

Freshwater security in Mediterranean islands is under threat due to the decoupling of water demand from natural recharge rates, which is being pushed by climate instability and tourism-induced urbanization. This study assesses the hydro-geochemical mechanisms that influence groundwater quality on Bozcaada Island, Türkiye, to determine its appropriateness for drinking and irrigation. A total of 21 groundwater samples were collected during the dry season (June 2025), coinciding with a demographic surge from approximately 1,200 to over 40000 inhabitants to evaluate the groundwater salinity and water quality parameters (EC, TDS, pH, HCO3⁻, NO3⁻, SO4²⁻, Cl⁻, Na⁺, Ca2⁺, K⁺, Mg²⁺, As, Fe, Mn). The methodological framework combined multivariate statistical analyses, notably Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA), with geochemical modeling, water quality index (WQI) analysis, and GIS-based spatial distribution mapping. Hydro-geochemical analysis shows that the groundwater chemistry in the permeable Fıçıtepe and Kirazlı formations is primarily influenced by lateral seawater intrusion and water-rock interactions. The Principal Component Analysis (PCA) successfully distinguished between geogenic weathering processes and anthropogenic salinity inputs, indicating that seasonal over-abstraction reverses hydraulic gradients in sensitive coastal zones. As a result, hydrochemical facies change from Na-HCO₃ in interior recharge areas to Na-Cl in coastal over-abstraction areas. Increased electrical conductivity (EC) and chloride concentrations were spatially linked with saltwater intrusion and high-populated tourism areas, depicting an important fraction of coastal groundwater unsuitable for drinking usage and irrigation due to elevated sodium adsorption ratio (SAR), residual sodium carbonate (RSC), and magnesium hazard exceeding the limits set by the World Health Organization (WHO) and Turkish drinking water standards. The groundwater is of type Na-HCO₃, Na-CL, and Ca-Mg-HCO₃. The Water Quality Index (WQI) assessment shows that groundwater quality for domestic use is frequently contaminated due to excessive EC, Cl^-, Na⁺, SO₄^2-, F^-, As, Mn, and Fe content, requiring treatment before consumption. As of current, the island depends on submarine pipelines water supply to address its water deficit, the research highlights how important this dependence is during moments of peak demand. The findings' conclusion summarizes that sustainable water security needs the integration of Nature-Based Solutions, especially Managed Aquifer Recharge, to restore hydrodynamic equilibrium and mitigate the continuity of salinization fronts in the coastal aquifer.