Metaux lourds dans les sediments de l'estuaire de la loire

Abstract

Estuaries, being interfaces between fluviatile and marine environments, are the scene of considerable physico-chemical and biological gradients which can have an effect on the distribution of metallic elements between the solid and dissolved phases. The purpose of the present study was to determine the relative influence of each parameter in this distribution. The study included two series of samples (ICOLO 26 and ICOLO 32) taken by means of the Schipeck grab at 19 selected points of the estuary as indicated in Fig. 1. Within the framework of this study we first investigated and were able to determine the origin and evolution of the metallic elements in the Loire estuary. Pollution level Table 1 and Fig. 2 summarize the results: firstly, the metallic element contents are relatively low, indicating that pollution is not very strong. Secondly, the contents measured in the different samples vary considerably due to the great heterogeneity of sample composition and particle-size distribution. Upstream-downstream evolution Figure 2 shows that the longitudinal profiles for metals are very similar to those obtained for the different characteristics of the sediments. Such an observation has already been made by various authors. Owing to the existence of these relationships, we believe, as do Boust et al. (1981), that it is preferable to interpret the results relative to metals obtained on a given site in terms of the various characteristics of the sediment. In Fig. 3, the metallic element contents (ICOLO 26 samples) is thus plotted against organic carbon. As shown in the figure, there is a very good relationship between the two parameters. Figures 2 and 3 indicate that the contents are uniform throughout the estuary with no evidence of further significant sedimentation this uniformity being due to tidal action. Owing to this uniformity, the impact of significant discharges is highly localized except for the wastewater from the city of Nantes as a whole, whose impact which is felt along the entire estuary. The relationship between copper and volatile matter for the ICOLO 32 samples, as represented in Fig. 4, shows that the points fall into two groups, which are the same whatever the explanatory parameter involved. Though the results are more uniform for the ICOLO 26 than for the ICOLO 32 samples, Fig. 6 clearly shows that the “marine” points stand out in both series of samples. This is indicative of a reduction in contents in the downstream direction, especially noticeable beyond point B (Zn, Cr, Ni) or even M (Cd, Cu, Pb). The seasonal evolution is clearly shown on the “metal-characteristic of sediment” graphs by the comparison of the slopes of the lines linking the same point sampled during the two sampling campaigns. The example of copper is shown in Fig. 7. On the whole, analysis of the results derived from the study of one characteristics of the sediment, indicates that only limited enrichment or depletion occurs at a point over a period of time. This probably reflects a good constancy over time of the metallic flux in the estuary. There is little doubt that changes in the composition of the sediment, will cause changes in the trapping of the metallic flux. In this case, variations in contents do not reflect alterations in the metallic flux over the sediment, but the varying capacity of the sediment under investigation to trap the same polluting flux according to is own composition. This study was also productive in the investigation of the various phenomena responsible for the reduction in metal contents during estuarial transit, which phenomenon can be expected to occur in a large number of estuaries. The impact depends on the chemical properties of the element under investigation. Solubilization of metallic elements due to to increased salinity occurs but plays only a secondary role in the upstream-downstream reductions. This is presumably because of the low percentages of metal absorbed in relation to the total amount of metals associated with the sediments. The reduction in metal contents related to the degradation of organic matter also occurs but it is of little significance compared with the overall phenomenon. The disappearance of metals associated with organic matter could well be related to salinity. The considerable reduction of contents in the lower estuary markedly occurs downstream of the salinity front and precisely where the first sediments characteristic of the marine environment are encountered. Consequently, the most obvious parameter responsible for the reduction in the metallic element contents is the mixing of relatively laden fluviatile sediment with “clean” marine sediments.