Heavy metal mobility and bioavailability in sediment fractions of major tourist beaches on the south-west coast of India: Implications for coastal ecological health

The occurrence, distribution, and mobility of heavy metals in environmental media are critical to ecosystem services and sustainability. This study examined concentrations of Copper (Cu), Nickel (Ni), Cadmium (Cd), Iron (Fe), Manganese (Mn), Zinc (Zn), and Chromium (Cr) in five fractions of bulk sediments from four tourist beaches in Kovalam—Pozhikkara, Hawa, Lighthouse, and Chowara—on the southwest coast of India, using sequential extraction. Results showed distinct metal fractionation patterns across beaches. Pozhikkara beach exhibited metals in the order Cu > Fe> Zn > Cr> Ni > Cd> Mn in exchangeable fractions, Cu > Mn> Cd > Cr> Zn > Fe> Ni in carbonate-bound fractions, Cu > Fe> Cd > Cr> Zn > Ni> Mn in reducible fractions, Cd > Fe> Cu > Cr> Ni > Zn> Mn in oxidisable fractions, and Mn > Ni> Cr > Zn> Cd > Fe> Cu in residual fractions. Cu dominated exchangeable fractions at Hawa (F1: 32.96%, F2: 14.35%) and Pozhikkara (F1: 39.52%, F2: 15.19%), while Ni was mainly in residual fractions (F5: 70.36–85.08%) across all beaches, indicating geogenic origin. Pollution and enrichment factors revealed Cu and Cd as high risk, with Pb, Cr, and Cu posing risks to children. Health risk assessments indicated adults faced minimal non-carcinogenic risks (Hazard Index, HI < 1), whereas children were exposed to significant risks with HI values surpassing safety limits for Pb (1.13–3.39), Cr (1.02–1.64), Zn (5.25), and Cu (1.06) at all the beaches. The carcinogenic risk (CR) from As was also above acceptable levels (> 10⁻⁶) for children at Pozhikkara and Lighthouse beaches. Positive Matrix Factorisation analysis suggested anthropogenic and geogenic sources, with anthropogenic sources mainly affecting exchangeable and carbonate-bound fractions. These findings emphasize the need for enhanced monitoring and management to reduce potential health risks.