Assessment of groundwater salinization in coastal aquifers under climate-driven sea-level rise using field surveys and isotope tracers

Thesis Abstract

Coastal groundwater resources are increasingly threatened by salinization driven by climate-induced sea-level rise, which facilitates seawater intrusion and elevates groundwater chloride concentrations, compromising freshwater supplies and ecosystem health. This study addresses the knowledge gap on spatial and temporal salinity dynamics in vulnerable coastal aquifers and the relative roles of sea-level rise, salinity transport pathways, and isotope signatures in delineating recharge and intrusion processes. The aim is to quantify salinization trends, identify dominant controlling mechanisms, and develop a robust framework for predicting future salinity scenarios under climate change. Specific objectives are (1) to map spatial patterns of groundwater salinity and stable isotope compositions (?18O, ?2H) across representative coastal aquifers; (2) to determine seawater intrusion extents using geophysical surveys and hydraulic head measurements; (3) to assess temporal trends in salinity using time-series data from continuous groundwater monitoring wells over a five-year period; (4) to quantify groundwater mixing and recharge sources through tritium and 3H-3He dating complemented by Cl/B enrichment indices; (5) to develop a predictive model integrating sea-level rise projections, aquifer hydraulic parameters, and isotope tracers to forecast salinity under multiple climate scenarios; and (6) to formulate management recommendations for sustainable abstraction and mitigation strategies. The study adopts a mixed-methods, longitudinal field design integrating hydrogeochemical analysis, isotope geochemistry, and numerical modeling. The population comprises coastal aquifers in three geographically distinct settings with documented intrusion risk. A stratified random sampling approach yields 60 groundwater wells for spatial coverage, 15 monitoring stations for continuous hydrological data, and 20 piezometers for vertical profiling. Data collection employs standardized groundwater sampling for major ions, stable isotopes (?18O, ?2H), tritium (3H), carbon-14 where feasible, and chloride concentrations, complemented by direct measurements of electrical conductivity, pH, temperature, and groundwater level. Isotope analyses are conducted using isotope ratio mass spectrometry (IRMS) for ?18O and ?2H, and accelerator mass spectrometry (AMS) for tritium. Geophysical surveys (Electrical Resistivity Tomography and seismic refraction) delineate subsurface salinity interfaces. Groundwater age-dating employs 3H-3He dating and lumped-parameter models to estimate residence time distributions. Data quality is ensured through calibration against global standards, duplicate samples, blanks, and cross-laboratory validation. Analytical methods include descriptive statistics to characterize salinity distributions, regression analyses to relate salinity to distance from the coastline, hydraulic gradient, and sea-level metrics, and time-series analyses to detect trends. Isotopic data are interpreted using Craig-Gordon evaporation models and mixing lines to identify freshwater–seawater end-member contributions. A multi-criteria appraisal combines chloride mass balance, ionic ratios (Na/Cl, Ca/Mg), and isotope signatures to partition intrusion versus increased evaporative concentration. A numerical groundwater flow model, calibrated with observed heads and salinity profiles, incorporates sea-level rise scenarios from region-specific climate models and stratified aquifer properties. Model validation uses observed salinity fronts and observed vs. simulated time series. The study also evaluates adaptive management options, running scenario analyses for pumping reductions, recharge enhancement, and barriers, with cost-benefit perspectives. Anticipated findings include (i) pronounced salinization in near-shore zones correlated with rising sea levels and reduced aquifer storage; (ii) distinct isotopic signatures separating seawater intrusion from direct evaporation or concentrated recharge; (iii) lag times between sea-level rise and groundwater response indicating vulnerability of deeper aquifers; (iv) robust predictive relationships enabling scenario-based forecasting of salinity fronts up to 30 years ahead under Representative Concentration Pathway-like scenarios. The research is expected to contribute to knowledge by advancing integrated hydrogeochemical and isotope-based approaches for intrusion assessment, improving understanding of time-dependent salinity responses to climate drivers, and delivering a transferable framework for coastal water-resource management. The study concludes that climate-driven sea-level rise intensifies coastal groundwater salinization through a combination of seawater intrusion and altered recharge, with isotope tracers providing critical discrimination of processes. Recommendations include prioritizing monitoring networks with isotope-capable laboratories, implementing managed aquifer recharge where feasible, adopting pumping rules to minimize drawdown-induced intrusion, and developing decision-support tools that couple climate projections with aquifer-specific vulnerabilities.