Sediment properties and their influence on the geochemical composition in the Lagoon of Venice.

The grain-size distribution, mineralogical composition, heavy mineral assemblages, surface area and concentration of major and trace elements have been determined for bottom sediments from a restricted zone of the Lagoon of Venice, which is of environmental concern. The interpretation of various parameters and their inter-relationships has been done using several statistical procedures. The most common sediments are day silts, typical of a low energy environment. Their fine grain-size favours the uptake of trace elements. Three mineralogical assemblages with specific areal distributions have been identified, both from the provenance of detrital materials and from the depositional conditions. The predominance of semistable and unstable heavy minerals was observed. Some heavy minerals were also recognized as tracers of the rivers reaching the lagoon in the past. The surface area appears to be related to the presence of fine particles and/or clay minerals. The abundance of major elements reflects the mineralogical composition. The trace element distribution shows two zones with different contamination, referable to industrial and municipal discharges. The relationships between heavy metals concentrations and sediment properties are variable. The main role was played in some cases by the fine grain-size and surface area, and in other cases by day minerals.