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 210Pb and 137Cs 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 137Cs 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 137Cs at depth, probably related to the 1963 global fallout input. The 137Cs profiles are consistent with accumulation rates of ∼ 2 to 3 cm y− 1. Excess 210Pb 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.

Introduction

The need to safeguard shallow-water marine environments from anthropogenic impacts has increased the attention in the last decades to the processes of heavy metal contamination in sediments. In particular, the management of highly polluted systems (i.e., dredging, ecosystem remediation) requires a precise understanding of the mechanisms of transfer and accumulation of particles and associated contaminants. This knowledge is even more important in systems such as the canal network of the city of Venice, which receives a large amount of urban effluents and contaminated materials from a resident population of about 76,000 and an estimated daily presence of 66,000, including tourists and commuters. Periodic dredging is therefore required to permit navigation and to maintain acceptable quality standards in the water column. During the period 1965–1995, dredging operations were substantially interrupted, causing silting up of the canals. With the start of a new phase of dredging, an extensive program of sampling and analysis was performed from 1994 to 2000, to establish disposal criteria for the sediments. Lower pollutant concentrations were generally found in the upper part of the sediment column for all the analyzed chemical species (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, hydrocarbons, polyaromatic hydrocarbons, polychlorinated biphenyls and chlorinated organic pesticides (Zonta et al., 2002)). This decrease can be related to the reduction of both point and diffuse inputs (through transfer of commercial and industrial activities to the mainland), local improvements of sewage treatment by the increased use of septic tanks and, beginning in the early 1960s, the replacement of fossil fuels with natural gas in heating systems. Simultaneous with the new phase of dredging, a series of restoration works was started in the city. These included the general improvement of the sewage system, the consolidation of canal embankments and the restoration of building foundations, resulting in a reduction of particulate, organic matter and contaminant inputs into the network. In this context, the evaluation of the dynamics of sediment accumulation in a test-canal is useful to obtain a baseline for the assessment of the effects of interventions and for comparisons with past and future conditions.

The Venice canal network

The city of Venice is located in the central part of a 550 km2 coastal lagoon (mean depth < 1 m), which is connected to the Adriatic Sea by three inlets. Water exchanges in the canal network of the city are mainly driven by first and second order tidal channels originating from the Lido inlet; these channels include the Canal Grande, which crosses the historical center with a meandering pattern. The typical tidal excursions are 30 to 60 and 80 to 100 cm respectively during neap and spring conditions. Salinity in the system generally ranges between 27 and 33 psu and is affected by tidal exchanges and freshwater inputs from urban effluents. The network is characterized by a complex morphology, with a large number of interlinked canals with different width (from a few to tens of meters) and depth (about 1–5 m). The overall length of the system is approximately 40 km and its surface, which roughly corresponds to 10% of the total urban area, is the site of an intense boat traffic for either commercial and transport purposes.

The pattern of water circulation in the canals is not clearly defined. The movement of water masses is closely related to the size of an individual canal and its proximity to the principal channels. Generally average current velocities during slack water are of the order of 20 cm s− 1, but in the more confined part of the network they are as low as a few cm s− 1. The sluggish circulation prevents materials discharged within the system from being transported outside and progressive silting of the canal bottom results. Under these conditions, resuspension of sediment by boat traffic is the main controlling factor of the cross-section morphology. In fact, as the canal shoals, turbulence induces scouring of the mid-channel portion and increases the deposition along the margins, giving rise to a typical V-shaped profile.

The accumulation of highly contaminated anoxic sludge within the canals (Zaggia and Zonta, 1997) represents a potential risk for the system. The release of toxic compounds from reduced sedimentary phases is in fact possible as a result of short-term events, such as resuspension and aeration of bottom sediments (Collavini et al., 2000), or as the result of remediation activities such as wet dredging.

Methods

In 1997, sediment cores were collected in the Rio di S. Angelo (Figure 1) to determine concentrations of heavy metals and the dynamics of sediment accumulation using the depth distributions of 137Cs and excess 210Pb. The Rio di S.Angelo, last dredged in the 1950s, has typical characteristics of medium-to-small canals (about 5 m-wide) with respect to dimensions, boat traffic and proximity to the Canal Grande, the principal means by which the sampled canal is tidally flushed. Water depth in the centre of the section, referred to the mean sea level, is about 1 m (Figure 2).

Figure 1.

Map of the Rio di S.Angelo showing the locations of the sampling sites of the sediment cores. The location of the study canal within the Venice canal network is indicated in the inset.

Figure 1.

Map of the Rio di S.Angelo showing the locations of the sampling sites of the sediment cores. The location of the study canal within the Venice canal network is indicated in the inset.

Figure 2.

Cross section of the Rio di S.Angelo—corresponding to the Y3 site shown in Figure 1—where the X-cores were collected. The position of X-cores and the depth of the sub-samples (numbered) obtained after sectioning the cores are indicated.

Figure 2.

Cross section of the Rio di S.Angelo—corresponding to the Y3 site shown in Figure 1—where the X-cores were collected. The position of X-cores and the depth of the sub-samples (numbered) obtained after sectioning the cores are indicated.

Four 50 cm-long sediment cores (Y1-Y5) were collected from the longitudinal axis of the canal (Figure 1) at a spacing of 12.5 m by means of a syringe-type piston corer. For the determination of heavy metal concentrations, the upper 50 cm were homogenized to obtain a single sample, following the methodology specifically established by the Italian Ministry of the Environment for the assessment of contaminant levels in the Venice canal sediments (Zonta et al., 2002). Samples were digested in hot 8N HNO3, and the leachate was analyzed by atomic absorption spectrophotometry (AAS). For mercury, a refluxing hot digestion with a H2SO4/HNO3 mixture was performed, and the analytical determination was made by cold vapor AAS technique.

Four additional cores (X1-X4, Figures 1 and 2) were collected across a section at the Y3 sampling site, at distances of 0.4, 1.8, 3.2 and 4.6 m from the western side, respectively. These cores, which had different lengths depending from the penetration of the piston corer, were sectioned in 10 cm intervals. The surface slice from the core X1 resulted in a sample of only 6 cm.

137Cs was measured by nondestructive gamma spectrometry on dried samples. 210Pb was measured by leaching the samples in HNO3/HCl in the presence of 209Po tracer. Samples were evaporated and dissolved in 1.5N HCl and 210Po, the granddaughter of 210Pb, was plated onto silver disks. Measurement of the Pb isotopes was made by alpha spectrometry (Cochran et al., 1998a, 1998b). Supported levels of 210Pb were estimated from 210Pb activities in old (> 100 y) sediments from the nearby Canal Grande.

Results

The Venice canals network is characterized by a large variability of environmental features; a detailed study of the accumulation of contaminants must be therefore conducted at a test site that is representative of the average conditions of the system. With this aim, the representativeness of our study site was assessed by using the cores taken along the longitudinal axis of the canal. Concentrations determined in the Y-cores are given in Table 1; three sets of reference values (a, b, c) are also reported for comparison (Zonta et al., in press; Zonta et al., 1994, Donazzolo et al., 1984).

Concentrations (mg kg− 1 dry weight) of heavy metals determined in the Y-cores.

Table 1.
Concentrations (mg kg− 1 dry weight) of heavy metals determined in the Y-cores.
Site Fe (mg/kg d.w) Mn (mg/kg d.w) As (mg/kg d.w) Cd (mg/kg d.w) Cr (mg/kg d.w) Cu (mg/kg d.w) Hg (mg/kg d.w) Ni (mg/kg d.w) Pb (mg/kg d.w) Zn (mg/kg d.w) 
Y1 18297 307 18.8 4.9 20.1 174 3.5 37.1 209 1007 
Y2 18783 292 15.4 5.1 16.3 190 3.7 39.2 178 940 
Y3 20058 301 18.0 6.0 24.7 199 4.0 37.5 177 1026 
Y4 19269 284 17.8 6.4 22.6 195 4.1 37.9 169 911 
Y5 19332 291 14.8 5.3 20.6 187 4.0 40.1 177 940 
Mean 19148 295 17.0 5.5 20.9 189 3.9 38.4 182 965 
Std dev 658 1.8 0.6 3.1 10 0.3 1.3 16 49 
Reference values 
    (a) — — 15.9 6.2 30.9 249 3.6 36.3 166 797 
    (b) 10411 235 — — 20.2 18 — 22.5 37 122 
    (c) — — — 7.1 16.2 90 1.8 28.9 89 865 
Three sets of reference values are reported for comparison: (a) the entire City canal network (mean values from 740 sites); (b) the southern area of the Cona Marsh; (c) shallow-water areas close to the industrial district of Porto Marghera. 
Site Fe (mg/kg d.w) Mn (mg/kg d.w) As (mg/kg d.w) Cd (mg/kg d.w) Cr (mg/kg d.w) Cu (mg/kg d.w) Hg (mg/kg d.w) Ni (mg/kg d.w) Pb (mg/kg d.w) Zn (mg/kg d.w) 
Y1 18297 307 18.8 4.9 20.1 174 3.5 37.1 209 1007 
Y2 18783 292 15.4 5.1 16.3 190 3.7 39.2 178 940 
Y3 20058 301 18.0 6.0 24.7 199 4.0 37.5 177 1026 
Y4 19269 284 17.8 6.4 22.6 195 4.1 37.9 169 911 
Y5 19332 291 14.8 5.3 20.6 187 4.0 40.1 177 940 
Mean 19148 295 17.0 5.5 20.9 189 3.9 38.4 182 965 
Std dev 658 1.8 0.6 3.1 10 0.3 1.3 16 49 
Reference values 
    (a) — — 15.9 6.2 30.9 249 3.6 36.3 166 797 
    (b) 10411 235 — — 20.2 18 — 22.5 37 122 
    (c) — — — 7.1 16.2 90 1.8 28.9 89 865 
Three sets of reference values are reported for comparison: (a) the entire City canal network (mean values from 740 sites); (b) the southern area of the Cona Marsh; (c) shallow-water areas close to the industrial district of Porto Marghera. 

Heavy metal concentrations show little variation along the canal, suggesting that no point sources are present, or alternatively that the contamination is uniform on a 50 m spatial scale. Moreover, the levels are quite close to the average determined for the entire City canal network (a). Therefore, the sediment contamination in the studied canal can be considered as representative of the prevailing conditions of canals in the historical center of Venice.

Heavy metal concentrations determined in the Cona Marsh (b), a relatively unpolluted area of the northern lagoon, highlight a marked enrichment of some species (up to about 6 and 14 for Zn and Cu, respectively) in the sediment of the studied canal with respect to the lagoon background. Data from the Rio di S.Angelo are in fact comparable or even greater than those found in the sectors close to the industrial area of Porto Marghera (c), which were commonly considered the most polluted site in the lagoon. This feature underlines the fundamental role of the canal network as a sink for contaminants delivered by the urban system.

The results of the four cores (X1–X4) collected in the cross-section, at the location of the Y3 sampling site, show that excess 210Pb (Figure 3) and especially 137Cs (Figure 4) activities are greater toward the canal margins than in the center (core X3). Lower radionuclide activities and metal concentrations in the first section of core X1 are due to a very high content of plaster debris derived from the wall of the overhanging building. A similar pattern of lower values in the canal center (core X3) exists for the radionuclide inventories, although only minimum values are possible in each case because the cores did not reach supported levels of 210Pb or sediment uncontaminated by 137Cs. These patterns support the hypothesis, based on bottom morphology, that sediment is displaced by boat traffic from the center of the canal to the margins.

Figure 3.

Depth distribution of excess 210Pb activity in X-cores. The different core lengths depend on the penetration of the piston corer in the sediment of the canal bottom. Error bars are based on 1 sigma counting uncertainties.

Figure 3.

Depth distribution of excess 210Pb activity in X-cores. The different core lengths depend on the penetration of the piston corer in the sediment of the canal bottom. Error bars are based on 1 sigma counting uncertainties.

Figure 4.

Depth distribution of 137Cs activity in X-cores. The different core lengths depend on the penetration of the piston corer in the sediment of the canal bottom. Error bars are based on 1 sigma counting uncertainties.

Figure 4.

Depth distribution of 137Cs activity in X-cores. The different core lengths depend on the penetration of the piston corer in the sediment of the canal bottom. Error bars are based on 1 sigma counting uncertainties.

It is difficult to use 210Pb and 137Cs to reconstruct chronologies in a system such as Rio di S. Angelo, where physical disturbance is likely. Nevertheless, we observe that cores X1, X2 and X4 show a 137Cs peak at 30 to 45 cm depth. A major pulse input of 137Cs to Venice occurred in 1986 following the Chernobyl accident and we attribute the prominent 137Cs peak in these cores to input from Chernobyl. Although the latter occurred over a short time, the broadening of the peak in these cores can be explained by resuspension and redeposition of sediments accompanied by input of fresh materials that allowed the peaks to be eventually buried.

Core X2 shows the clearest 137Cs pattern, with a pronounced maximum at 35 cm, decreases to 55 cm and significant increases below 55 cm. The latter could be due to inputs from global fallout of 137Cs, which reached a maximum in 1963 in association with the Nuclear Test Ban Treaty. Sediment accumulation rates of ∼ 2 to 3 cm y− 1 are suggested by the 137Cs profile in this core. Excess 210Pb profiles in cores X1, X2 and X4 show relatively little variability in the upper ∼ 25 to 35 cm of the cores (except for the anomalously low values in the upper section of core X1, noted above) but decrease by factors of about 2 to 4 below that depth. If interpreted as due to radioactive decay of 210Pb, these decreases are also consistent with accumulation rates of ∼ 2 cm y− 1. Another limit on sediment accumulation in the Rio di S. Angelo may be placed by the fact that the canal was last dredged in the 1950s. All the sediment sections sampled had detectable 137Cs, indicating that they were deposited after the last dredging. Thus at least 60 to 80 cm (the lengths of the cores) of sediment were deposited in approximately 40 y, yielding minimum rates of accumulation of 1.5 to 2 cm y− 1.

The concentrations of heavy metals in the X-cores are reported in Table 2. They generally correspond to the values of Y-cores (Table 1). In particular, the mean values calculated for the whole set of X-cores closely match the concentrations measured in the Y3-core, which was taken in the same section. These average concentrations also show that the investigated site is characterized by a uniform sediment contamination across the section.

Concentrations of heavy metals (mg kg− 1 dry weight) and percent water content determined in the X-cores. Samples are arranged by increasing depth, according to the scheme of Figure 2.

Table 2.
Concentrations of heavy metals (mg kg− 1 dry weight) and percent water content determined in the X-cores. Samples are arranged by increasing depth, according to the scheme of Figure 2.
Sample id. Fe Mn As Cd Cr Cu Hg Ni Pb Zn H2O content 
Core X1            
    7 15825 296.0 11.2 2.3 25.0 107 2.0 38.4 79 413 37.6 
    6 17880 288.0 13.6 3.7 25.9 179 3.7 38.5 122 649 43.9 
    5 19475 285.0 14.1 4.6 29.9 202 4.2 40.5 142 789 45.9 
    4 18915 282.0 14.8 4.8 30.7 206 3.9 37.0 143 764 46.2 
    3 19690 296.0 14.4 4.0 29.5 196 3.9 36.5 136 708 46.6 
    2 20275 317.0 15.1 6.7 26.4 193 4.1 39.1 156 1000 46.3 
    1 22040 329.0 23.8 13.1 33.5 253 4.2 41.6 178 1430 46.8 
Core X2           
    15 18740 286.0 16.2 4.4 28.1 200 4.3 35.9 154 781 48.9 
    14 19460 287.0 15.8 4.3 30.5 199 4.8 36.4 163 765 47.5 
    13 19215 285.0 13.8 4.5 28.3 202 4.0 37.0 163 741 45.7 
    12 18730 297.0 12.9 4.2 23.6 173 4.1 35.1 141 711 45.1 
    11 19245 289.0 16.0 4.6 22.9 184 4.1 36.6 157 776 45.1 
    10 20270 310.0 21.6 8.7 25.3 209 4.3 43.3 174 1135 43.5 
    9 26350 318.0 33.4 13.7 33.1 257 4.0 48.2 186 1465 46.7 
    8 19225 310.0 28.6 14.7 34.5 285 4.3 44.8 203 2096 46.2 
Core X3           
    21 18560 285.0 10.3 4.7 24.8 203 4.1 36.1 141 751 46.8 
    20 18115 289.0 15.7 4.1 21.6 198 3.6 36.3 147 757 43.5 
    19 19030 291.0 14.4 4.6 24.9 172 3.6 36.1 154 765 46.5 
    18 23815 333.0 27.6 11.9 24.0 246 4.1 45.8 217 1779 46.2 
    17 20375 287.0 22.2 7.8 21.2 207 3.7 39.0 218 1170 43.6 
    16 20200 293.0 19.5 7.4 33.4 225 3.9 39.7 191 1137 45.6 
Core X4           
    28 18260 298.0 15.7 3.8 22.3 193 3.7 34.8 133 678 48.9 
    27 18270 291.0 14.7 4.0 26.4 189 3.7 36.0 140 710 45.2 
    26 18970 282.0 15.4 4.3 23.7 200 4.1 34.2 152 702 48.1 
    25 18145 288.0 13.7 3.4 31.3 153 3.1 39.2 134 539 43.4 
    24 20575 306.0 16.8 4.2 30.1 189 3.4 37.0 150 723 45.5 
    23 21950 317.0 21.5 7.0 29.3 191 3.5 43.7 181 923 44.0 
    22 22340 313.0 21.5 6.0 31.6 186 2.9 38.7 164 745 41.1 
Sample id. Fe Mn As Cd Cr Cu Hg Ni Pb Zn H2O content 
Core X1            
    7 15825 296.0 11.2 2.3 25.0 107 2.0 38.4 79 413 37.6 
    6 17880 288.0 13.6 3.7 25.9 179 3.7 38.5 122 649 43.9 
    5 19475 285.0 14.1 4.6 29.9 202 4.2 40.5 142 789 45.9 
    4 18915 282.0 14.8 4.8 30.7 206 3.9 37.0 143 764 46.2 
    3 19690 296.0 14.4 4.0 29.5 196 3.9 36.5 136 708 46.6 
    2 20275 317.0 15.1 6.7 26.4 193 4.1 39.1 156 1000 46.3 
    1 22040 329.0 23.8 13.1 33.5 253 4.2 41.6 178 1430 46.8 
Core X2           
    15 18740 286.0 16.2 4.4 28.1 200 4.3 35.9 154 781 48.9 
    14 19460 287.0 15.8 4.3 30.5 199 4.8 36.4 163 765 47.5 
    13 19215 285.0 13.8 4.5 28.3 202 4.0 37.0 163 741 45.7 
    12 18730 297.0 12.9 4.2 23.6 173 4.1 35.1 141 711 45.1 
    11 19245 289.0 16.0 4.6 22.9 184 4.1 36.6 157 776 45.1 
    10 20270 310.0 21.6 8.7 25.3 209 4.3 43.3 174 1135 43.5 
    9 26350 318.0 33.4 13.7 33.1 257 4.0 48.2 186 1465 46.7 
    8 19225 310.0 28.6 14.7 34.5 285 4.3 44.8 203 2096 46.2 
Core X3           
    21 18560 285.0 10.3 4.7 24.8 203 4.1 36.1 141 751 46.8 
    20 18115 289.0 15.7 4.1 21.6 198 3.6 36.3 147 757 43.5 
    19 19030 291.0 14.4 4.6 24.9 172 3.6 36.1 154 765 46.5 
    18 23815 333.0 27.6 11.9 24.0 246 4.1 45.8 217 1779 46.2 
    17 20375 287.0 22.2 7.8 21.2 207 3.7 39.0 218 1170 43.6 
    16 20200 293.0 19.5 7.4 33.4 225 3.9 39.7 191 1137 45.6 
Core X4           
    28 18260 298.0 15.7 3.8 22.3 193 3.7 34.8 133 678 48.9 
    27 18270 291.0 14.7 4.0 26.4 189 3.7 36.0 140 710 45.2 
    26 18970 282.0 15.4 4.3 23.7 200 4.1 34.2 152 702 48.1 
    25 18145 288.0 13.7 3.4 31.3 153 3.1 39.2 134 539 43.4 
    24 20575 306.0 16.8 4.2 30.1 189 3.4 37.0 150 723 45.5 
    23 21950 317.0 21.5 7.0 29.3 191 3.5 43.7 181 923 44.0 
    22 22340 313.0 21.5 6.0 31.6 186 2.9 38.7 164 745 41.1 

The depth profiles of As, Cd, Cu, Pb and Zn show general increases of concentrations with depth (shown in Figure 5A–E by means of a gray scale) Although the profiles show significant differences across the canal, a clear increase of concentration with depth is observed in all the cores, despite the presence of some minor deviations. The observed trends emphasize the consistent reduction of the contaminant inputs in the canal network during the more recent period. An assessment of the chronology of this important change would be particularly informative to understand how the contaminant loads are accumulated in the system and to define quality standards for management purposes in the present and future scenarios. Despite the limitations posed by sediment redeposition on the radionuclide chronologies, the fact that maxima in metal concentrations are found in sediment layers deeper than the 137Cs maximum, likely produced by inputs from the Chernobyl accident, suggests that reduction in contaminant inputs started in the 1970s.

Figure 5.

Vertical profiles in X-cores. Concentration ranges are shown by means of a gray scale. The positions of the 137Cs maximum from the Chernobyl accident (dots) are connected by a dashed line. (A) As, (B) Cd, (C) Cu, (D) Pb, (E) Zn.

Figure 5.

Vertical profiles in X-cores. Concentration ranges are shown by means of a gray scale. The positions of the 137Cs maximum from the Chernobyl accident (dots) are connected by a dashed line. (A) As, (B) Cd, (C) Cu, (D) Pb, (E) Zn.

Conclusions

Profiles of excess 210Pb and 137Cs in the sediments of the Rio di S. Angelo show that sediment accumulation is more rapid near the canal margins than in the center. The combined action of boat traffic and tidal hydrodynamics determines the resuspension of bed materials in the center and an increased deposition at the margins, leading to the characteristic V-shape of the canal cross section. Rates of sediment accumulation are estimated to be 1.5–2 cm y− 1 (minimum values) from the thickness of sediment deposited since the last dredging and ∼ 2 to 3 cm y− 1 from the 137Cs and excess 210Pb profiles.

The profiles of heavy metal concentrations (As, Cd, Cu, Pb and Zn) in the sediment cores show clear decreases toward the sediment-water interface. This decrease, approximately since the 1970s based on the 137Cs profiles, is a common feature of the whole canal network and can be related to the reduction of both point and diffuse inputs.

Acknowledgements

We thank Mauro Frignani and Luca Bellucci (CNR – ISMAR, Bologna) for making their detectors available for the 210Pb and 137Cs work and for discussion of the results. We also wish to thank Gianfranco Magris, Ruggero Ruggeri and Francesco Simionato (CNR – ISMAR, Venice) for assistance in sampling.

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