In this study, the spatial distribution of the total heavy metals content in bottom sediments, suspended particulate matter (SPM) and seawater samples collected from six stations in the Belgian coastal zone was assessed. Furthermore, the speciation of heavy metals in the various binding phases in the sediment and the SPM samples was assessed by the sequential extraction procedure. In addition to that, the bioavailable fraction in the sediment samples was also evaluated using the EDTA extraction procedure. For all methods, standard procedures recommended by the BCR (Bureau Comimittee de Reference, European Community) were used. A comparison between the EDTA fraction and the first two fractions (F 1 and F2) obtained via the sequential extraction procedure was performed so as to evaluate the importance of the EDTA metals fraction. The factor analysis statistical technique was firstly performed on the various data matrices of the combined chemical and environmental variables to extract the most important chemical and environmental parameters that explain the variability among the variable matrices, then on the data matrices of the chemical parameters alone. The average factor scores of the extracted factors in each round were then combined with the average values of the environmental parameters and used in the redundancy correspondence analysis (RDA) so as to evaluate the spatial importance of the extracted factors and to correlate them with the environmental parameters. It bas been found by the use of statistical analysis that the total metals concentration in sediment and SPM samples of one station (S8) out of six was significantly high. For the sediment samples the total metals concentration exceeded the lower ecological assessment criteria (EAC) which were established by OSPARCOM (1996). We suspect that sludge dumping activities in that particular station resulted in increasing metals concentration. Most metals were preferentially bound to the fine clay and organic fractions. Thus, metals levels in SPM samples were much more important than in sediment and water samples. SPM metals fraction from total concentration ( i.e. sedimentary, particulate and dissolved) ranged from 84% for Cd to 97% for Cu. As far as dissolved metals concentration is concerned, Co was the only metal which varied significantly among the six stations. The dissolved trace metal concentrations observed in this study were within the range of the natural background levels which were established by OSPARCOM (1997) except for Pb, which showed slightly higher concentrations (mean dissolved Pb varied between 0.036 and 0.042µg/l). Regarding metals speciation on the various binding phases in sediment samples, it has been observed on the one hand, that in all sandy sediments, the metals distribution exhibited more or less the same trend with as the largest fraction the residual one (F4). On the other hand, in muddy sediment samples (e.g. station 8), an important fraction was bound to the mobilizable fractions (F1 and F2). In SPM samples, the distribution was comparable to that in sediments, except that a larger importance of non-residual bound phases for all metals and particularly a larger importance of the exchangeable and carbonate fraction for Cd (F1 ranged from 25% to 66% of the total particulate Cd concentration) were observed. Regarding the EDTA metal fraction, our results showed that the correlation between EDTA fraction and F1 fraction improved for higher EDTA fractions. The EDTA fraction was generally smaller than the sum of F1 and F2 except in some cases for Ni and Cd where the F2 fraction was almost negligible. EDTA fraction was correlated well with the organic matter and clay fractions and was found to improve in the sense of increasing organic matter and clay contents in the sediment samples. |
