Assessing the Spatial Variability of Groundwater Recharge in Coastal Aquifers

Thesis Abstract

Groundwater recharge in coastal aquifers plays a critical role in sustaining freshwater availability, yet its spatial variability remains insufficiently characterized across different hydrogeological settings, thereby impeding the development of sustainable groundwater management strategies. This study aims to assess the spatial variability of groundwater recharge in the Coastal Belt Aquifer System of the Southeast Region, focusing on how hydroclimatic, geophysical, and land-use factors influence recharge rates. The specific objectives include quantifying recharge across multiple sites, identifying the key factors contributing to variability, and developing a predictive spatial model to inform sustainable management practices. Employing a mixed-methods research design, the study integrates quantitative field measurements with geospatial analysis. The population comprises 50 representative sampling locations selected through stratified random sampling, ensuring coverage across distinct hydrogeological zones and land-use types. Data collection involved the use of portable seepage meters, lysimeters, and isotopic analysis (specifically ?2H and ?18O) to directly estimate recharge at each site over a 12-month period, capturing seasonal variations. Additionally, ancillary data on rainfall, temperature, soil type, land use, and geophysical surveys (such as electrical resistivity imaging) were collected to support multivariate analysis. The validity and reliability of field instruments were ensured through calibration against standard protocols and repeated measurements at selected sites. Spatial data were processed using Geographic Information Systems (GIS), while the relationships between recharge and explanatory variables were analyzed via multiple linear regression, analysis of variance (ANOVA), and geographically weighted regression (GWR). To develop the predictive spatial model, techniques such as kriging were employed to interpolate recharge estimates across unmeasured locations, enabling the creation of high-resolution recharge maps. The expected findings reveal significant heterogeneity in recharge rates influenced predominantly by soil permeability, land-use practices, and hydroclimatic variability. It is anticipated that the study will identify distinct recharge zones, with estimates varying from 10 mm/year in heavily urbanized areas to 250 mm/year in vegetated, undisturbed zones. The multivariate analysis is projected to show that land use and soil properties collectively account for over 60% of the observed variability in recharge. These results will be integrated into a GIS-based predictive model that captures spatial fluctuations, providing a valuable tool for resource managers. This research contributes to the body of knowledge by advancing understanding of the spatial dynamics of recharge processes in coastal aquifers, especially in semi-arid regions subject to climate variability and anthropogenic pressures. It offers a methodological framework combining field data, isotopic techniques, and geostatistical modeling tailored for similar hydrogeological contexts. The findings will inform policymaking aimed at optimizing groundwater extraction, improving recharge augmentation strategies, and designing sustainable land-use planning. The study concludes that targeted protection of recharge zones and the integration of geospatial models into water resource management can significantly enhance aquifer sustainability. Key recommendations include implementing land management practices that preserve natural recharge pathways, establishing recharge monitoring networks, and adopting integrated water resource management policies that consider spatial recharge variability. Future research should explore the impact of climate change projections on recharge dynamics and expand the temporal scope of data collection to refine spatial predictions further. This comprehensive assessment provides a vital foundation for evidence-based groundwater management in coastal settings confronting increasing environmental and developmental challenges.