Geochemical characterisation and spatial distribution of heavy metals in tropical beach sediments along the Udupi coastline, Karnataka, India

Heavy metal distribution in beach sediments was assessed along the Udupi coastline, Karnataka, India. The study covered eleven sites across three morphological zones (swash, mid-beach, backshore) and two contrasting seasons (pre-monsoon, post-monsoon). Sediment samples were analysed for eight elements (Pb, Cr, Zn, Cu, Mn, Fe, Al, K) using total acid digestion and ICP-OES. A locally derived geochemical background threshold (C_bg = Median + 2×MAD (Median Absolute Deviation)) was established to overcome the limitations of generic crustal reference standards (Average Shale Value and Upper Continental Crust) for quartz dominated sandy beach systems. Three outlier detection methods (Tukey boxplot, Median Absolute Deviation, and Cumulative Distribution Function mirroring) showed consistent agreement, capturing a seasonal decline in Pb and Zn post-monsoon. Copper emerged as the most consistent signal of concern. It exhibited a significant post-monsoon increase in background threshold (up to +136.83% under MAD), a reduction in outlier frequency to zero, and an inversion in correlation with lithogenic tracers (Cr and Mn) from positive to negative. These findings reveal a transition from localized, point-source Cu anomalies in pre-monsoon to a more spatially uniform and elevated distribution in post-monsoon, irrespective of lithogenic influences. Correlation analysis further distinguished lithogenic (Cr-Fe-Mn) and aluminosilicate (Al-K) associations from the Cu-Zn anthropogenic signal. Overall, heavy metal distribution along the Udupi coastline reflects two distinct influences: a stable lithogenic background and a seasonally dynamic input, likely anthropogenic. The study provides a locally validated geochemical baseline and emphasizes the necessity of season-specific background thresholds in geologically distinct sedimentary settings.