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The chemical association of metal species in sediments provides indications about the release of trace metals by processes with toxic effects under certain environmental conditions. The knowledge of the way those metals are bond to sediments, serves to recognize specific sources of pollution in systems. Speciation plays an important role as far as bioavailability and toxicity are concerned. The accumulation of metal particles in sediments occur by the following mechanisms: a) Adsorption to the material fine particles (clay particles surface); b) precipitation of the element as carbonates, sulfates or oxides; c) co-precipitation of the element with iron and manganese oxides; d) complexation with the organic matter; e) incorporation into the crystal lattice of silicates. Currently, five phases are considered when studying the bioavailability of trace elements in sediments: a) exchangeable phase, MgCl2 (causes change in salinity); b) leachable phase, acetic acid (causes pH change); c) reducible phase, hydroxylamine hydrochloride (allowing the release of metals in anoxic conditions); d) oxidizable phase, hydrogen peroxide (causes the degradation of organic matter); e) pseudo-residual phase, aqua-regia (allows the release of metals associated with minerals). In the last two fractions, metals are strongly bound to the sediment constituents and are not available for animals and plants. The organic phase is relatively stable in nature and metals present therein are removed by strong oxidizing conditions. The metals contained in the pseudo-residual phase measure the degree of environmental pollution, since a large amount of metals present in this phase indicates a lower degree of pollution.

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