Trace elements and polychlorinated biphenyl (PCB) congeners in Liverpool Bay sediments
Some trace elements and various PCB congeners were determined in superficial sediments from Liverpool Bay. Some sediment parameters were also determined for a better understanding of the distribution of these contaminants. Lower amounts of fine particles were found in sediments from the southern area of the sampling grid where the hydrodynamic conditions inhibit the deposition and favour the resuspension of particles with small diameter. The distribution of muddy deposits was patchy throughout the bay although the mud deposits near the Burbo Bight seem to be a permanent feature. The concentrations of trace elements and carbon were determined in two grain size fractions. Carbon in the fine fraction increased in sediment samples with lower proportions of fine sediments probably as a result of an increase in the available surface area since these samples apparently had higher proportions of clays. This relationship was scattered suggesting that the inputs of carbon from different sources, i. e., waste disposal, riverine inputs and autochthonous production, overshadow any clear trends between carbon and grain size in sediments. This observation is consistent with the distribution of S13C values. The concentration of organic matter in coarse sediments seemed to be determined by the amount of surrounding fine particles, particularly in samples with fine fraction concentrations higher than 5%. The concentrations of all of the trace elements, except arsenic, in the coarse fraction were correlated with the organic matter content suggesting that the organic phase is an important carrier of metals in the coarse particles. In the fine fraction, however, there was a large variability in the correlations, suggesting different geochemical behaviour between trace elements. For example a contrasting behaviour was observed between mercury and arsenic since the former seemed to be associated with land derived organic matter with no correlations with the content of iron and manganese, whereas arsenic showed a strong correlation with these two elements reflecting its preference for hydrous oxides of iron and manganese. The geochemical behaviour of each particular trace element seems to play an important role in the observed distributions of the elements in Liverpool Bay sediments. PCBs were determined in total sediments. A high correlation between PCB concentrations and the fine fraction content was observed as a result of the preferential association of these contaminants in fine particles. The distribution of normalized PCB concentrations indicated that inputs from the Mersey may be an important source of PCBs. Different patterns were observed in the PCB congener composition of the samples which reflected the different sources and degree of alteration of the "original" pattern found at the source. The changes in the composition were related to the degree of chlorination and substitution pattern since these factors control the physicochemical properties of PCB congeners. The sediments were classified based on their elemental content and on their PCB contents by means of multivariate statistical methods. The sediments were classified in similar groups from their organic and inorganic contents, which suggested that the sources of organic and inorganic contaminants in Liverpool Bay are similar.