Sediments were collected from 36 sites along the estuary of the Sibuti River in Sarawak, Malaysia, during the South-west and North-east monsoons, and then subjected to metal fractionation for Pb, Cd, Mn, Ba, Zn, Se, Co, Fe, Cu, Al and Cr. Metal concentrations were analyzed for 5 different fractions using the sequential extraction method. Pb, Mn and Cd possessed the greatest mobility, while major percentage of metals found were associated with residual fraction of sediments. Cd, Se and Cr, Fe were found dominantly associated with the carbonate and reducible fractions respectively, whereas Zn was found highly associated with oxidizable fraction of sediments. The factor analysis showed that the absorption of Mn in the upper part of estuary and most of the transition of metals in both seasons was regulated by the synthesis of Mn-oxyhydroxides catalyzed by microorganisms. On the other hand, association of organic matter with Fe and Al-oxyhydroxides work as major mechanism for absorption and settlement in intermediate and lower part of the estuary. Despite being associated with exchangeable fraction, Mn was found to be from geogenic sources along with all other metals except Cd, which is believed to be sourcing from agricultural lands. Ecological risk indices suggest that estuarine sediment is highly contaminated with Se, Co, Cu, Cd and pose considerable to very high risk at 90 % of the stations. Such enrichment is mainly resulting from the oxidation of metals from pyrite concretions and shale beds found in Sibuti and Lambir formations of the river basin. ERL and ERM values suggest Cu and Cr pose maximum threat towards the sediment dwelling living organisms. RAC reported Cd, Mn, Ba and Se in high risk category due to their potential to mobilize frequently, whereas Al, Fe and Cr present no risk towards benthic community.
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