Impact of dredging in a shallow costal lagoon: a sediment quality triad approach. I. trace metals and Corphium bioassays.

The general aim of this work is to detect spatial and temporal differences in the sediments of a shallow coastal lagoon (Pialassa Baiona, Italy) before and after dredging operations. The area – which covers a surface of about 1100 ha - is located in the Emilia-Romagna Region 10 km north of the town of Ravenna, and bordered by the Northern Adriatic Sea. Natural and man-made changes over the time lead to existing physical features resulting in a number of different basins of less than 1 m in depth broad an irregular in shape, separated by levees and crossed by a network of artificial channels. From 1958 to 1976 the lagoon had been heavily impacted by industrial pollution, and mercury was among the most important pollutants, which nowadays contaminates the sedimentary compartment. Four replicates sediment samples were collected with a box-corer at each of twelve stations in channel and wetland areas selected as control and impact areas according to the dredging plan, and analysed for a set of heavy metals (Cd, Cu, Cr, Hg, Ni, Pb, and Zn). Mercury content in sediments was determined by Cold Vapour Atomic Absorption Spectrometry (CVAAS) after hot digestion with H2SO4/ HNO3 at 80° C for 3 h. Heavy metals underwent microwave digestion prior to analysis by Graphite Furnace Atomic Absorption (GFAA) using a Perkin-Elmer HGA-800 Instrument. Parallel sediment toxicity testing was conducted with a 10-day mortality test using the amphipod Corophium insidiosum. Preliminary results of the pre-dredging phase are here presented with reference to heavy metals content and toxicity responses in sediment samples.