HEAVY-METAL POLLUTION STUDIES BY RESUSPENSION EXPERIMENTS IN VENICE LAGOON

A mechanical device for sediment resuspension has been used in Venice Lagoon for sampling extremely thin surface sediment layers, allowing the characterization of the top few tenths or hundredths of a millimetre. The age of the resuspended layer was estimated at two sampling stations from the sedimentation rate to be about 2 weeks. Zinc and iron were determined (nitric acid extraction) in particles of the resuspended layers and in the corresponding underlying sediments. Lower metal concentrations were detected in sediments (43-323-mu-g/g for Zn and 9200-17400-mu-g/g for Fe) with respect to resuspended particles (63-424-mu-g/g for Zn and 12,200-25,400-mu-g/g for Fe). The remarkable concentration decreases of zinc (both in resuspended particles and in sediments) from the inner areas of the lagoon (Porto Marghera) seaward, can be attributed to the decreasing effects of industrial pollution. The relatively high concentrations of iron observed in areas far from Porto Marghera, can be explained by the contribution of runoff waters or by contamination from nearby polluted sediments. Higher metal concentrations in resuspended particles were observed in April than in February (1987). Concentration increases for zinc (from 7 to 91%) were more substantial than for iron (from not significant to 42%). Zinc uptake by plankton, followed by deposition of residual organic materials onto the sediment surface is suggested as an explanation of the higher metal concentrations detected in April. These results reflect the particular features of the proposed technique, which, at least for cohesive and fine grained sediments, seems to be promising for future applications in studies both on the tracing of metal pollution sources and on the migration processes of these contaminants between sediment and overlying water.