Recent evolution of sedimentary heavy metals in a coastal lagoon contaminated by industrial wastewaters (Pialassa Baiona, Ravenna, Italy).

From 200 to 2002, sediment contamination by Cd, Cu,Hg, Pb and Zn was analysed in the Pialassa Baiona salt marsh, which receives petrochemical waste waters front the industrial district of Ravenna (Italy). The recent contaimination levels were compared with data of previous studies carried out in 1982, to assess whether environmental policies and remedial measures reduced sediment pollution. Sedimentary profiles of Cu and Pb were homogeneous along the uppermost 0-10 cm horizon, which corresponded to the sedimentation in the last 30 years. Concnetrations of Zn atttained a peak up to 800mg kg-1 dry weight) in the 0-4 cm sediment horizon, which was assumed to correspond to the last 10-15 years. A wide-spread contamination by Hg was detected in the alst marsh as well as in the main channel with peaks up to 20-40 mg kg-1 dry weith. Nonetheless, recent sediments resulted less contaminated, since Hg discharge from industrial plants ceased about 20 years ago. Contamination levels by Hg values were two orders of magnitude higher than the international sediment quality standards. Cadmium, which was analysed for the first time in 200-2002, attained a peak in the surface layers (1-2.5 mg kg-1 d.w.), with a progressive decline along the sediment column. Through comparison with pre-industrial values detected in the deeper sediment horizons (before1920). Hg showed the highest enrichment factor, up to 300 times. Cd and Zn concentrations in recent sediments were from 2 to 20 times higher than background values. In terms of possible adverse effects, Hg posed the highest risk, and Cd and Zn were frequently above the recommended thresholds.