Abstract

 The canals of the Venice, Italy are subjected to a progressive silting, and periodic dredging operations, which have been performed since ancient time to ensure navigation and acceptable sanitary conditions. In the period from the early 1960s to the early 1990s, however, the dredging was substantially interrupted, and a layer of sediment, with a thickness of up to 1 metre, formed in the whole network. This reflects the pollutant inputs of that thirty year period. In 1994, an extensive program of analysis of total pollutant concentrations (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, hydrocarbons, PAHs, PCBs and chlorinated organic pesticides) in the sediment began to assess the contamination prior to dredging operations and to select appropriate disposal sites. The entire canal network, which is approximately 38 km-long, was sampled by taking sediment cores at 50 m-intervals, resulting in 775 sites. The investigation was concluded in 1999 and a large geo-referenced database was compiled that included information on the organic matter content, grain-size distribution and geo-technical characteristics of the samples. The sediment is heavily contaminated from a variety of pollutant sources. According to the Italian Ministry of the Environment, Copper, Pb and Zn are the main contributors to the harmful levels of the sediment contamination. The spatial distribution of concentrations in the network reflects both the presence of point and non-point sources of pollutants, and furnishes information on the mechanisms that govern pollutant transport and accumulation in the system. The comparison between concentration data obtained in the upper and lower layers of the sediment column shows a general diminution of pollutant inputs, in particular Zn, PAHs and pesticides to the system in recent years.