Comparative Analysis of Groundwater Depletion in Coastal Aquifers Worldwide

Thesis Abstract

Groundwater resources in coastal aquifers are increasingly stressed by over-extraction, saltwater intrusion, and climate-driven recharge variability, compromising freshwater security for millions of people and agrarian systems. This study addresses the essential problem of understanding comparative patterns and drivers of groundwater depletion across coastal regions, with the aim of informing integrated water-resource management and policy interventions. The specific objectives are to (i) quantify spatiotemporal trends in groundwater storage across a globally representative set of coastal aquifers, (ii) identify the primary anthropogenic and hydrogeological drivers of depletion, including pumping intensity, irrigation practices, aquifer properties, and sea-level rise, (iii) compare resilience and depletion thresholds under varying governance and governance capacity, and (iv) develop a cross-regional framework of best practices and policy levers to mitigate depletion while protecting coastal aquifer salinity integrity. The methodological approach combines a multi-method research design anchored in comparative cross-sectional analysis and grounded in the theory of coupled human-natural systems and the socio-hydrology framework. The population comprises coastal aquifers from five world regions (Americas, Europe, Africa, Asia, and Oceania) selected to capture diverse hydrogeological regimes and institutional contexts. A stratified sampling strategy identifies 40 aquifers with reliable long-term monitoring data (minimum 20 years) and accessible pumping, recharge, and salinity datasets. Data collection integrates remotely sensed groundwater storage proxies from GRACE, borehole water-level records, groundwater withdrawal statistics, land-use and irrigation data derived from national databases, and sea-level rise measurements from satellite altimetry. Instrument validity is ensured through cross-validation between independent data sources and temporal alignment with calibration periods. Data analysis proceeds in three integrated streams. First, time-series analysis employs augmented Dickey-Fuller tests for stationarity, Mann-Kendall trend tests, and multiple-imputation techniques to handle missing data, complemented by seasonal decomposition to isolate recharge and pumping effects. Second, regression-based causal inference uses panel data models (fixed effects and difference-in-differences) to quantify relationships between depletion rates and drivers such as groundwater abstraction intensity, irrigation efficiency, aquifer hydraulic conductivity, recharge variability, and sea-level rise metrics. Third, a comparative policy analysis applies a framework drawing on institutional capacity theory and governance effectiveness to assess how regulatory stringency, data transparency, and stakeholder participation modulate depletion outcomes. Model specification includes robustness checks with alternative lag structures and instrumental variables to address endogeneity concerns. In addition, salinity intrusion risk is examined through salinity-accumulation indices and groundwater age dating where available. Expected findings indicate substantial heterogeneity in depletion trajectories linked to governance quality and hydrogeological resilience. Regions with robust data systems and enforceable pumping quotas exhibit slower depletion and reduced salinization compared with areas suffering data scarcity and weak enforcement. The analysis anticipates a threshold-driven response where depletion accelerates beyond certain pumping intensities and recharge deficits, particularly under rising seawater intrusion pressures. The study also anticipates identifying best-practice configurations, such as integrated aquifer management with transparent metering, managed aquifer recharge where feasible, and adaptation of irrigation practices to minimize non-beneficial withdrawals. The study contributes to knowledge by offering a globally comparative, methodologically rigorous assessment of coastal aquifer depletion that links hydrogeochemical signals with governance mechanisms, advancing the empirical basis for trans-regional policy transfer. It provides a diagnostic framework for policymakers to tailor interventions to local hydrogeology and institutional context, with implications for climate adaptation, urban water security, and sustainable agriculture. The main conclusion is that coastal aquifer sustainability hinges on synchronized improvements in data infrastructure, transparent governance, and adaptive management of pumping and recharge to mitigate both depletion and salinity risks; the recommendations emphasize (1) establishing standardized groundwater monitoring networks, (2) implementing regionally calibrated pumping quotas tied to aquifer storage trends, (3) expanding managed aquifer recharge where hydrogeologically feasible, and (4) strengthening transboundary collaboration and knowledge sharing to scale effective governance solutions.