Deconvolving geochemical micro-spatial variability of an unconsolidated aquifer through chemometric and geostatistical techniques

Substrate properties are pivotal in shaping porewater chemistry and groundwater quality, serving as the primary driving factors. While spatial analysis of geochemical distribution has been extensively explored in hydrochemistry, the application of geostatistical techniques in tropical island soil investigation remains lacking. In this study, we aim to characterise the geochemical and micro-spatial variability of soil along the flow path of groundwater via integration of chemometric and geostatistical techniques. The study was conducted across a ~ 350 m² area of unconsolidated soil profile (2.75 m depth × 125 m length) on a sedimentary island’s aquifer. The results revealed that multivariate statistical analysis coupling iso-factor scores maps can provide key insights to constraint the subsurface geochemical behaviour. Marine and lithogeneous (natural-derived) materials were found to significantly influence the metals distribution across the soil profile. Clay and sesquioxides (Al₂O₃ and Fe₂O₃) from the high-relief area appear to influence the micro-variability of the inland soil, while seawater mixing impacts the subsoil geochemistry. This study concludes that natural sources, pedogenic processes, and seawater intrusion are the major drivers of metals variability in the tropical island aquifer. These findings provided a scientific overview of metals distribution in a seawater–freshwater mixing zone and significant references for the vertical and horizontal distribution of metals in a sandy soil profile. In particular, the chemometric and geostatistical analysis can be potentially used as robust tools for investigating biogeochemical cycling and the source and sink of groundwater pollutants. It is envisaged that future studies could employ a similar approach to explore other islands worldwide.