Grain Size Effect on Trace Metals in Contaminated Sediments Along the Iranian Coast of the Caspian Sea

Sediments are integral and inseparable parts of the aquatic environments, so any environmental program concerning water quality would be incomplete without the proper study of its sediments. The elemental concentration of sediments not only depends on anthropogenic and lithogenic sources, but also upon the textural characteristic, organic matter content, mineralogical composition and depositional environment of sediments. The generally accepted opinion is that the smaller the size of the sediment fraction, the larger the amount of trace metals bound to this fraction, and that trace elements are mainly present in the clay/silt particles with grain size less than 0.063 mm. In general, this is due to the increase in specific surface area of the smaller fractions and to the surface properties of clay minerals. Forty two separated size fractions of the sediments collected from fourteen sampling stations along the southern Caspian Sea were analyzed employing aquaregia digestion followed by Inductively Coupled Plasma–Optical Emission Spectroscopy to evaluate the pollution level and the trends in samples collected along the Iranian coast of the Caspian sea in August 2005. Selective partitioning of the studied contaminants in sediment fractions was observed, with a minimum content in the very fine sand fractions of grain size 0.075-0.125 mm. Anomalously high concentrations of trace metal content in the medium and coarse sediment fractions on the seashore direction of the Sea was explained by rapid sea level rise and its impact in eroding beaches, formation of large agglomerates formed from smaller sediment fraction particles, and the presence of heavy minerals or coarse fractions of terrigenous origin.