Coastal Groundwater Sustainability in Keralam, India: A Critical Review of GIS-and Remote Sensing-based Seawater Intrusion Vulnerability Assessment

Abstract

Keralam's 590-kilometre coastline hosts nine of the state's fourteen districts, an exceptionally dense coastal population, and a hydrogeologically distinctive combination of narrow lateritic and alluvial coastal aquifers, an extensive 1,500-kilometre backwater and estuarine network centred on the Vembanad, Ashtamudi and Kallada systems, and intense monsoon-driven recharge that alternates seasonally with acute groundwater stress. This critical review synthesises evidence from studies published predominantly between 2018 and 2026 in peer-reviewed and indexed journals to assess how Geographic Information Systems (GIS), satellite remote sensing, and parametric vulnerability indices, principally GALDIT and DRASTIC together with locally modified variants such as GALDIT-U, have been applied to characterise seawater intrusion (SWI) risk across Keralam's coastal aquifers. The review finds that district-level research coverage is markedly uneven: Ernakulam, Kozhikode and, more recently, Thiruvananthapuram have been investigated using different combinations of vulnerability indexing, hydrochemical analysis, electrical resistivity and numerical modelling, while Kollam, Thrissur, Kannur and Kasaragod remain comparatively underexamined despite documented salinity problems. Keralam's laterite-dominated hydrogeology and strong monsoonal recharge generally suppress GALDIT-index vulnerability relative to deltaic coasts elsewhere in India, but urbanisation, backwater proximity, groundwater over-extraction and coastal erosion produce sharply localised high-vulnerability pockets, particularly around Kochi, Kozhikode and Thiruvananthapuram's urban and harbour fringes. Sea-level-rise projections under RCP4.5 and RCP8.5 scenarios have also been incorporated into groundwater and seawater-intrusion modelling, while explainable machine learning and Monte Carlo uncertainty frameworks are only beginning to be applied within the state. The review concludes by identifying priority gaps: uneven district-level and seasonal monitoring coverage, limited hydrochemical or geophysical validation of GIS-based vulnerability maps, minimal integration of dynamic land-use changes into index parameterisation, and an absence of state-wide, standardised comparative assessment across Keralam's full coastal gradient.