Sediment and heavy metal accumulation in a small canal, Venice, Italy

Sediment and heavy metal accumulation were studied in a canal of Venice (Rio di S.Angelo) that has typical characteristics of medium-to-small canals with respect to dimensions, boat traffic and pollution sources. The tidal dynamics are particularly weak in this approximately 5-m wide canal. Four cores were collected across a section and five additional cores were collected from the longitudinal axis. The canal was dredged in the late 1950s, and therefore the collected cores represent approximately 40 years of deposition. Excess Pb-210 and Cs-137 are present throughout the sediment cores and inventories of both radionuclides are greater toward the canal margins than in the center, indicating enhanced deposition along the margins. The margin cores show a Cs-137 peak at 30 to 45 cm depth that we attribute to input from the 1986 Chernobyl accident. Two of the cores are sufficiently long to show increases of Cs-137 at depth, probably related to the 1963 global fallout input. The Cs-137 profiles are consistent with accumulation rates of approximately 2 to 3 cm y-1. Excess Pb-210 activities show little variability in the upper 25 to 45 cm of the margin cores, and decrease by a factor of approximately 2 over the next roughly 30 cm of the cores. As the canal shoaled with sediment deposition, resuspension of sediment by boat traffic along the canal center likely produced increased deposition along the margins. Concentrations of heavy metals fall within the average of values determined for the whole canal network. Arsenic, cadmium, copper lead and zinc show clear decreases toward the sediment-water interface, suggesting recent reductions of contaminant inputs.