Geochemical variation and ecological risk assessment of heavy metals and their bioaccumulation in fish species from two estuaries in Southern India

• Heavy metal enrichment in the Vaippar is Zn > Cu > Cr > Pb > Fe > Mn. • Heavy metal enrichment in the Vembar estuaries is Zn > Cr > Cu > Pb > Mn > Fe. • Enrichment factor, the Vembar estuary is more polluted than the Vaippar estuary. • Estuarine water plays a key role in metal transport between sediments and biota. • Bioaccumulation (BAF, BSAF) confirms metal transfer from sediments to fish and food chain. This study assesses the distribution, contamination status, and ecological risk of heavy metals in sediments, water, and fish tissues from the Vaippar and Vembar estuaries, southern India. Mean sediment concentrations in Vaippar were Zn (327 µg/g), Cu (94 µg/g), Cr (36 µg/g), Pb (33 µg/g), extractable Fe (18 µg/g), and extractable Mn (11 µg/g), while in Vembar they were Zn (181 µg/g), Cr (141 µg/g), Cu (92 µg/g), Pb (90 µg/g), extractable Fe (8.85 µg/g) and extractable Mn (43 µg/g). Metals were significantly associated with fine-grained, organic-rich sediments, highlighting their role as major sinks. The Geo-accumulation Index (Igeo) indicated low to moderate contamination, with values ranging from 0.4 to 1.98 for Zn and Cu in Vaippar Enrichment Factor (EF) values ranging from 1.5 to 14.3 indicate anthropogenic enrichment, particularly for Zn, Cu, Pb and Cr. Pollution Load Index (PLI) values (<1) suggest overall low to moderate contamination in both estuaries. Estuarine waters acted as vectors for metal redistribution between sediments and biota. BSAF values indicate limited sediment-driven bioaccumulation, whereas BAF values suggest moderate waterborne bioavailability, particularly for Cu and Zn. This bioaccumulation indicates trophic transfer and potential risks to ecological integrity and human health given the dependence of local communities on estuarine fisheries. The results provide the first integrated baseline dataset combining sediment geochemistry, water quality, ecological risk indices and bioaccumulation metrics for the Vaippar and Vembar estuaries, where such multi-compartment assessments are currently lacking, emphasizing that pesticides, fertilizers, aquaculture inputs, and industrial effluents are the dominant sources of enrichment rather than natural processes. Overall, the results highlight the need for regular monitoring and management strategies to mitigate heavy metal pollution and safeguard estuarine ecosystems and human livelihoods.