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

Abstract

Sediment and heavy metal accumulation in a small canal, 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 ∼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 210 Pb and 137 Cs 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 137 Cs 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 137 Cs at depth, probably related to the 1963 global fallout input. The 137 Cs profiles are consistent with accumulation rates of ∼2 to 3 cm y −1 . Excess 210 Pb activities show little variability in the upper 25 to 45 cm of the margin cores, and decrease by a factor of ∼2 over the next ∼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.

Substantial Decrease in Contaminant Concentrations in the Sediments of the Venice (Italy) Canal Network in the Last Two Decades—Implications for Sediment Management

“…The limit between L1 and L2, based on change in metal concentrations, cannot be precisely determined and the date of reopening of the canals after dredging is not precisely known (sometime between 2003 and 2004). Consequently, the estimated sedimentation rate of recent sediment in L1 is approximate but consistent with previous estimates for the Venice canals of about 2-3 cm year −1 [30,31]. As the dredging in the studied canals was finalized at latest in 2004, followed by the overflow from adjacent canal sections, we assumed that normal sedimentation resumed in 2005.…”

“…More sediment accumulates at the edges of the canals than in the central part. Cores were collected at a distance of 1-1.5 m from the canal walls, corresponding to about a quarter of the canal width, which should ideally reflect the average sediment accumulation rate at each site [31]. In addition, the surface of the canal bottom was uneven after the excavation, and boat traffic and the location of domestic sewage discharge points can produce small undulations, resulting in a variable accumulation rate in the longitudinal direction of the canal.…”

The effect of groundwater advection on salinity in pore waters of permeable sediments

“…The surrounding land has little, if any, relief. The lagoon's drainage basin covers 1850 km 2 and has an annual average rainfall of 950 mm (Zonta et al 2005). Previous measurements of SGD in this lagoon average 30 cm d 21 in the northern lagoon and 6 cm d 21 in the central lagoon (Rapaglia 2005;Garcia-Solsona et al 2008).…”