Heavy metals «versus» clay minerals in the quaternary sediments of the Cortegaça beach formation (Ovar, Portugal)

Abstract

The site under study (Cortegaça beach) belongs to the Ovar municipality and is part of an extensiveregion characterized by flat terrains, consisting mainly of old beach and dunar deposits. Coastal erosionhad exposed the so called Cortegaça beach formation showing the following three sedimentary sequences,from the basis to the top: 1) greenish sandy clay; 2) sand layers with marked cross-bedded showing ontop a bioturbated podzol; 3) bioturbated beach sand and dune sand. The main goal of the present paper isthe establishment of relationships between heavy metals nature and content with the mineral composition,particularly of the clay component, of those sediments. The study of the fine fraction (less than 38 mm)of forty samples collected along three profiles has been carried out. X-ray diffraction was used for mineralogicaldeterminations, whereas atomic absorption spectrometry was used for the determination of theheavy metals: Cu, Ni, Co, Zn, Pb, Cd, Mn and Cr. Mineralogical studies made evident a gradual verticalevolution marked by the following main features: 1) dominance of quartz and phyllosilicates in the finefraction associated to discrete amounts of feldspar and other accessory minerals; evaporite mineralsand iron hydroxides are particularly important in some thin layers; 2) general dominance of illite overkaolinite in the clay fraction with oscillations of the illite/kaolinite content. As regards with heavy metalscontent the following facts deserve to be emphasized: 1) Pb and Cu are more concentrated in thesandy layers showing cross-bedded, whenever iron hydroxides are enriched; 2) Zn, Cr and less clearly,Ni, Co and Cd behave differently of Pb and Cu; 3) Mn is clearly enriched at the top of the 3rd sequence.For the typification and hierarchization of the relationships between the mineralogical and the chemicalvariables geostatistical analysis has been carried out using, in particular, both Principal ComponentsAnalysis and Cluster Analysis.