Entrapment of pollutants in Mediterranean sediments and biogeochemical indicators of their impact

摘要

The transit and fate of certain pollutants (Cd, Pb, Zn and P) transported to the western Mediterranean sea-floor and a method to assess their impact are described. The spatial distribution of pollutant concentrations in the Rhone prodelta shows that their decline with distance from their source is due both to mixing with unpolluted sediments, release from contaminated particles during transit and release from sediments after deposition. Beyond the continental shelf, metals of anthropogenic origin, mainly incorporated in faecal pellets, sink and become entrapped in deep sea sediments. Because subsaturating concentrations of trace metals are often found in surface pore waters and storage, therefore, still occurs, co-precipitation with other metal ions existing in slightly supersaturated states has also to be considered. Although phosphate tends to precipitate as apatite after entering sea water, its 'definitive' storage in sediments is hindered by certain forms of pollution. Low pH and reducing conditions in sediments enhance phosphate release to the overlying water. Eutrophication may then occur in localized areas. The effects of urban waste water contamination on biogeochemical processes in sediments are examined, in particular processes responsible for the transformation of organic nitrogen. Sediment quality appears to the better defined by the effectiveness of diagenetic processes than by pollutant concentration per se. In general, polluted sediments possess weak capacities to transform organic nitrogen relative to the quantities of organic matter that are available. Such sediment characteristics are indicated by the preferential growth of Caulerpa taxifolia over that of Posidonia oceanica.