benthic fluxes of trace metals in the lagoon of venice

www.elsevier.com/locate/microc

Microchemical Journal

Benthic fluxes of trace metals in the lagoon of VeniceB

C. Turettaa, G. Capodaglioa,b,*, W. Cairnsa, S. Rabarb, P. Cescona,b

aInstitute for the Dynamics of Environmental Processes-CNR, Dorsoduro 2137, 30123 Venezia (I), ItalybDepartment of Environmental Sciences-Univ. Ca’ Foscari Venice, Dorsoduro 2137, 30123 Venezia (I), Italy

Accepted 20 June 2004

Available online 30 September 2004

Abstract

To assess the exchange and the mobility of trace metals between sediments and water and their geochemical behaviour, experiments were

carried out within the sphere of the CORILA project for the safeguarding of the Venice lagoon. Trace element exchanges were examined for

approximately 60 h at two sites in the central part of the Venice lagoon (Italy): the first one is located in front of the industrial area of Porto

Marghera (Tresse) and the second one in front of Campalto, near the causeway (Campalto). The experiments were carried out using a benthic

chamber monitored for pH, dissolved oxygen, salinity, and temperature. The temporal trend of metals inside the benthic chamber was

examined in relation to changes of pH and dissolved oxygen. Diffusive metal fluxes were also assessed by determination of the vertical

distribution of metals in pore water.

Al, As, Cd, Cu, Fe, Mn, Mo, Sb, U, V and Zn were determined by ICP-SFMS. The metal concentrations for the lagoon samples were in

agreement with expected values; the concentration ranges (min–max in ng/ml) were: Al 0.24–0.61, As 1.42–2.27, Cd 0.050–0.182, Cu 0.81–

2.46, Fe 0.25–1.66, Mn 11.59–31.66, Mo 6.50–10.62, Sb 0.139–0.516, U 1.7–3.3, V 0.69–3.21, Zn 5.20–21.51.

Positive fluxes for the Tresse and Campalto experiments were determined for Cd (0.21 and 0.18 pmol/cm2/h), Zn (62 and 67 pmol/cm2/h),

Cu (0.29 and 0.50 pmol/cm2/h) and Mn (19 and 12 pmol/cm2/h), while negative fluxes were determined for iron (�3.5 and �6.3 pmol/cm2/

h). Other elements showed differences in behaviour for the two experiments; the fluxes, for the Tresse and Campalto experiments,

respectively, were 5.1 and �6.9 pmol/cm2/h for molybdenum, 0.25 and �0.18 pmol/cm2/h for arsenic and 1.3 and �8.4 pmol/cm2/h

vanadium. Therefore, the different characteristics of the two areas affect the mobility of trace elements, which can derive from differences in

the environmental characteristics of the two areas or seasonal difference in which the experiments were carried out.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Benthic fluxes; Trace metals; Venice

1. Introduction

The Venice lagoon, located in Northern Italy, is an open

system, and it is important to know the exchanges through

all its boundaries to understand the fate of elements that

arrive into the lagoon from all the surrounding areas. The

lagoon has been subjected to important anthropogenic

inputs: domestic sewage, agricultural drainage and various

wastes from the industrial area. These inputs have pro-

0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.microc.2004.06.003

B Presented at the XI Italian-Hungarian Symposium on Spectrochem-

istry, Venice, Italy, October 19–24, 2003.

* Corresponding author. Institute for the Dynamics of Environmental

Processes-CNR, Dorsoduro 2137, 30123 Venezia (I), Italy.

E-mail address: [email protected] (G. Capodaglio).

gressively deteriorated the quality of the lagoon ecosystem.

Significant amounts of these pollutants are accumulated in

sediments, which may constitute a potential source of

secondary pollution: chemicals may be recycled many times

through the sediment–water interface before being perma-

nently buried or removed through the lagoon inlets.

Diagenetic processes contribute to the remobilisation of

elements, which are temporarily stored in sediments and

may be dissolved in pore water and then diffuse to the

overlying bottom water. In this context, the sediment–water

interface represents an important exchange surface, which

presents the greatest gradient in chemical and physical

properties. Fluxes of elements through this interface, named

bbenthic fluxesQ, affect element concentrations in both pore

water and the overlying bottom water [1]. With the aim of

79 (2005) 149–158

C. Turetta et al. / Microchemical Journal 79 (2005) 149–158150

understanding and quantify the inputs of trace metals in the

waters of the Venice lagoon, the processes of remobilisation

of these elements from sediments to overlying waters,

related to oxygen concentrations, were studied using a

benthic chamber [1–4].

A benthic chamber is a device based on a simple

principle to estimate fluxes at the sediment water interface:

a known seawater volume and known sediment surfaces are

isolated inside the chamber during the experiment period.

Concentration changes in the enclosed water over time are

used to calculate fluxes of elements into or out of the

sediment [5]. Parameters such as pH and oxygen give

important information about the changes inside the chamber.

Water samples were periodically collected and analysed to

follow the temporal trend of the studied trace elements;

benthic fluxes were estimated from changes in concentration

over time.

The use of a benthic chamber to determine the flux of

trace elements at the sediment–water interface is very useful

in understanding and quantifying the changes of trace

element concentrations from an oxygenated to an almost

anoxic environment, and in knowing the fate of trace metals

and nutrients when low oxygen conditions occur in bottom

waters; this phenomenon frequently occurs in the lagoonal

organic rich waters [2].

To simulate the hypoxic conditions that may occur in

lagoon water, in particular due to weather conditions, any

water exchange between the exterior and the interior of the

chamber must be precluded and the natural decrease in the

oxygen content was monitored during the experiments. To

describe the processes that control the exchanges and the

mobility of trace metals between different environmental

compartments with sufficient detail, samples must be

collected at an adequate frequency.

Benthic chamber experiments were carried out in two

polluted sites in the central part of the Venice lagoon, one

close to the industrial area of Marghera and the other close

to Campalto, not far from the international airport of Venice

and close to the past solid waste unloading area of S.

Giuliano. Benthic fluxes were estimated by measurements

of changes of concentration of Al, As, Cd, Cu, Fe, Mn, Mo,

Sb, U, V and Zn as a function of time.

Fig. 1. Venice lagoon map. Locations of benthic chamber experiments.

2. Material and methods

2.1. Chemicals and laboratories

All materials used for sampling, treatment and storage of

samples and solutions were carefully chosen, acid-cleaned

and conditioned to minimize sample contamination [6,7].

Preparation of all materials, i.e., bottles for sample

storage and dilution, standard solutions, and vials for

analyses, was carried out in a clean laboratory equipped

with a class 100 laminar flow bench available at the Institute

for the Dynamics of Environmental Processes, located in the

Department of Environmental Sciences-University Ca’

Foscari Venice. The laboratories and procedures have been

described elsewhere [8,9].

2.2. Sampling and sample handling

Water samples were collected during two benthic

chamber experiments carried out in the central part of the

Venice lagoon (Fig. 1), one close to the industrial area of

Marghera (site A—bTresseQ) and the other close to

Campalto, not far from the international airport of Venice

and close to the solid waste unloading area of S. Giuliano

that was used in the past (site B—bCampaltoQ). The

chambers were constituted of a box of 90-l volume

(60�60�25 cm), obscured on the top, to minimize light

effects, and closed by flexible polyethylene walls to

compensate for differences of pressure produced by

sampling and tide changes. The chambers were monitored

for pH, dissolved oxygen, salinity, and temperature, by a

multi-parametric probe (mod. 556, YSI, Ohio, USA), and

samples for trace elements were collected every 3–4 h, for

approximately 60 h. The sampling was carried out using a

pump, the water was filtered using a 0.20-Am filter cartridge

(Sartorius Sartobran, Gottingen, Germany); the sampling

system and filtration apparatus were previously repeatedly

rinsed with an acid solution (ultrapure water with 0.2%

ultrapure HCl). After filtration the samples were stored in a

freezer at �20 8C until analysis.

Two sediment cores were collected in June 2003, one in

the Marghera area and the other in Campalto, to extract the

pore water to determine benthic fluxes by an independent

method; about 30-cm cores were collected by one piston

corer, they were immediately closed and placed in a glove

box conditioned by nitrogen to eliminate oxygen. Con-

Table 1

Certified and measured values in CRM-CASS-4 (ng/ml)

Element Certified valuea Measured valuea

As 1.11F0.16 1.19F0.10

Cd 0.026F0.003 0.028F0.003

Crb 0.144F0.029 0.169F0.007

Cub 0.592F0.055 0.601F0.030

Feb 0.713F0.058 0.706F0.044

Mnb 2.78F0.19 2.82F0.17

Mo 8.78F0.86 8.59F0.49

Pb 0.0098F0.0036 0.0098F0.0024

U 3.0c 2.7F0.2

V 1.18F0.16 1.21F0.15

Znb 0.381F0.057 0.363F0.020

Al – 0.468F0.059

Sb – 0.240F0.018

a Uncertainties are expressed as 95% confidence limit for the mean.b A-Flow nebulizer coupled with Teflon spray chamber.c Information value only.

C. Turetta et al. / Microchemical Journal 79 (2005) 149–158 151

temporary overlying water was collected at two different

depths (subsurface and bottom water). The cores were

sliced at intervals of 0.5–1 cm, with greater detail at the

top of the sediment core. The sediment was centrifuged to

extract the pore water, which was quickly filtered in an

inert atmosphere and was frozen until analysis. Pore water

was analysed to determine the concentration gradient

through the sediment towards bottom water and visa versa

to calculate the diffusive flux (flux controlled only by

diffusion).

2.3. Instrumentation and analytical parameters

Trace metal measurements were performed by ICP-

SFMS (Element2, Finnigan-MAT, Bremen, Germany) fol-

lowing the methodology previously reported [9]. The

instrument was installed in a dedicated laboratory with the

sample introduction area protected by a laminar flow

cabinet. Intensity optimisation was carried out daily, using

a tuning solution of ultrapure water containing 1 ng/ml of

In. Before beginning the analyses, an accurate mass

calibration was performed in low, medium and high

resolution mode using a solution containing elements with

m/z values covering the whole mass range of interest. Al,

Cd, Mo, Sb, U and V were determined in low-resolution

mode (m/Dm=400), while Cu, Fe, Mn and Zn were

determined in medium resolution mode (m/Dm=4000) and

As was determined in high resolution mode (m/Dm=

10000).

Two sample introduction systems were utilised: the first

was direct introduction by a A-flow nebulizer (Elemental

Scientific—100 Al/min) with a Teflon spray chamber (to

determine Cu, Fe, Mn and Zn); the second was direct

introduction by a A-flow nebulizer coupled with a desolva-

tion unit (Aridus, Cetac Technologies, Omaha, NE, USA),

in which the sample gas is heated to 95 8C to prevent droplet

accumulation in the spray chamber, which is then swept by

an Ar flow into a semi-permeable membrane to significantly

reduce the formation of oxides (used to determine all the

other elements). The two introduction systems were coupled

to the ASX-100 autosampler (Cetac Technologies). The

sample introduction systems, the desolvation unit and the

autosampler were maintained and handled under the flow

hood installed above the instrument.

Each sample was 10-fold diluted using ultrapure water

obtained by coupling a Milli-RO system (Millipore, Bed-

ford, MA, USA) with a Purelab-Ultra system (ELGA-

Vivendi Water Systems, Bucks, UK), and acidified with

UPA grade HNO3 (1:10 v/v) and an internal standard

solution of In, Sc, Y and W (1 ng/ml) was added. The use of

appropriate internal standards allows us to correct measure-

ments for changes in sensitivity of the instrument [10]. The

accuracy of the measurements was determined using a

certified reference material (CRM-CASS-4) in which the

trace element concentrations were determined. The obtained

and certified values are reported in Table 1. The measured

values are in good agreement with certified values for the

available data: the difference between the measured and

certified values is less than 1% of the certified value. For Al

and Sb, which are not certified, we observed mean values of

0.468 and 0.240 ng/ml, respectively, which agree with

expected values as reported in the literature for coastal

seawater [10–14].

The quantification of trace elements was carried out by a

matched calibration method. Five aliquots of CASS-4,

filtered (0.45 Am) coastal seawater certified reference

material, handled as just described for the samples, were

spiked with a multi-element standard solution (5, 10, 50,

100, 200 pg/ml).

2.4. Benthic chamber experiment

The two experiments were carried out from 28 to 31

of October 2002 (Tresse) and from 25 to 27 of May

2003 (Campalto) at two different polluted areas in the

central part of the Venice lagoon (Fig. 1). The chamber

was carefully deposited on the bottom using all possible

precautions not to disturb the superficial level of the

sediments. Despite these precautions, a re-suspension of

sediment occurred, which was highlighted by higher

concentrations of some elements in comparison with the

values obtained for samples collected just before the

positioning of benthic chamber.

Fifteen water samples were taken for a period of about

60 h approximately every 3–4 h (2 l) from a 90-l benthic

chamber; one sample was collected outside before settling

the chamber. The samples were collected using a pump

connected to the chamber through an FEP sampling tube.

Samples were collected after pumping approximately 100

ml of water (dead volume of sampling tube). The volume of

water collected was compensate by the flexible walls of the

chamber. Samples were directly filtered on board using the

filter cartridge Sartorius equipped with 0.2-Am cellulose

membrane and stored in a portable refrigerator. Measure-


ments of Eh and pH were performed to monitor the

evolution of the experiments.

To preserve sample integrity with regard to the total

metal concentration [8], they were stored frozen at �20 8Cwithout any treatment until the analyses.

3. Result and discussion

3.1. Physico-chemical characteristics

Salinity, pH, percentage of saturation of dissolved oxygen

and temperature changes during the two experiments are

compared in Fig. 2 and show differences related to characte-

ristics of the different areas and the season. Autumn 2002 was

extremely wet, with temperatures within the average for the

period, while Spring 2003 had rainfall below average for the

period and had exceptionally high temperatures.

In agreement with the hydrological characteristics of

the lagoon, the Campalto area presented a lower salinity

with respect to the Tresse Area. In both the experiments,

the salinity showed an increasing trend, which we can

consider to be of low significance. The salinity variation

was not correlated to the tide level and was very low

with respect to that expected in the two areas. Therefore,

we can consider the chambers well sealed, and the

salinity variation could derive from intrusion of water

through the sediment during the sample collection. At the

beginning, the Tresse experiment showed a low salinity

value that rapidly increased during the first 20 h (from

29.2x to 29.5x), then it was almost constant throughout

Fig. 2. Salinity, pH, oxygen and temperature

the next 25 h and increased again rising to a value of

about 29.6x during the last 15 h. The Campalto

experiment showed an almost constant value of about

28.3x during the first 30 h and then an increase rising

to a medium value of about 28.8x during the last 30 h.

The pH inside the chamber during the two experiments

ranged between 7.95 and 7.60 in Tresse and between 7.70

and 7.27 in Campalto, with a constant decrease during the

first experiment; the second was characterized by a more

rapid decrease during the first 30 h to reach a stable value

over the 30 last hours.

During the two experiments, sub-oxic conditions were

reached in both the benthic chambers, but the dissolved

oxygen concentration showed a different trend in the two

experiments; as for pH, a constant decrease in oxygen

content in the Tresse experiment was evident (from 53.9xto 12.8%), while in the Campalto experiment, we observed

an initial rapid decrease (from 75.2x to 26.2x, during the

first 25 h) followed by a constant value of about 38.0%. The

latter experiment showed a variability well correlated to the

temperature, therefore, those changes in the percentage of

oxygen saturation were deriving from changes in oxygen

solubility.

Due to the low depth of the lagoon in the studied area,

the temperature inside the chamber shows the day/night

variation, characteristic of the autumn and spring periods.

3.2. Trace elements

The concentrations of dissolved elements Al, As, Cd, Cu,

Fe, Mn, Mo, Sb, U, Vand Zn are reported in Table 2a and b.

changes during the two experiments.


Although the chamber was carefully deposited on the

bottom, the pressure applied during the chamber settling

caused a re-suspension of sediments and/or pore-water

released from sediment inside the chamber. This was high-

lighted, for both the experiments, by the increase in

concentration of some elements, e.g., Cd, Cu and Fe, at the

beginning of the experiments with respect to the values

obtained for a sample collected just before the chamber

positioning (time zero).

Data reported in Table 2 were used to calculate the

benthic fluxes. The concentration and the chamber volume

(changing from the initial 90 l to 60 l at the end) were used

to calculate the amount of metals inside the chambers.

However, the calculation of fluxes on the basis of point-to-

point variations contributes to an apparent high variability

of the results. This calculation might work for data with a

very low experimental error but is unsuitable for trace

components subject to a relatively high uncertainty. There-

fore, to reduce the effect of the uncertainty of the data, the

mean variation of amount of trace elements inside the

chambers as a function of time during the experiment was

Table 2

Trace element concentrations (ng/ml) and physiochemical parameters in Tresse (a

Al As Cd Cu Fe Mn Mo Sb

(a)

LV-02-1F 0.329 1.42 0.063 2.29 1.38 12.49 6.16 0.240

LV-02-2F 0.364 1.70 0.086 1.67 1.41 14.88 7.73 0.357

LV-02-3F 0.384 1.59 0.066 1.38 0.77 12.31 6.28 0.407

LV-02-4F 0.390 2.16 0.088 0.84 0.99 20.17 8.91 0.466

LV-02-5F 0.408 2.20 0.059 0.97 1.18 21.86 8.54 0.383

LV-02-6F 0.238 2.16 0.052 1.20 1.66 23.45 10.22 0.510

LV-02-7F 0.467 1.77 0.051 1.32 0.98 16.28 7.70 0.403

LV-02-8F 0.365 2.35 0.075 1.11 1.46 21.34 9.12 0.426

LV-02-9F 0.441 1.94 0.091 1.38 1.13 21.66 11.00 0.516

LV-02-10F 0.437 1.89 0.097 1.19 1.40 18.84 10.26 0.414

LV-02-11F 0.306 1.71 0.089 1.27 0.25 16.73 8.74 0.356

LV-02-12F 0.223 1.87 0.127 0.90 0.39 17.17 10.01 0.421

LV-02-13F 0.253 2.07 0.173 1.31 1.16 24.83 10.76 0.389

LV-02-14F 0.437 2.27 0.118 1.21 1.12 26.24 10.16 0.455

LV-02-15F 0.598 2.22 0.140 1.55 1.31 28.07 10.90 0.311

(b)

LV-03-0B 1.27 1.15 0.099 1.36 2.49 15.82 4.60 0.139

LV-03-1B 1.12 1.63 0.025 1.10 1.53 8.01 4.62 0.196

LV-03-2B 1.22 1.22 0.043 1.06 1.34 1.47 4.35 0.215

LV-03-3B 1.02 1.01 0.038 0.97 2.35 5.90 4.40 0.140

LV-03-4B 1.10 0.86 0.051 1.13 2.88 8.08 4.76 0.188

LV-03-5B 0.98 0.96 0.060 0.96 3.17 8.97 3.92 0.211

LV-03-6B 1.15 1.47 0.052 1.63 2.49 8.57 4.75 0.273

LV-03-7B 1.06 0.95 0.059 1.26 1.81 10.71 4.54 0.203

LV-03-8B 0.99 1.14 0.065 1.08 1.54 3.59 4.58 0.257

LV-03-9B 1.19 1.08 0.048 1.45 1.77 8.24 4.41 0.199

LV-03-10B 0.92 1.12 0.086 1.17 2.00 10.51 4.99 0.164

LV-03-11B 2.01 0.61 0.091 1.11 2.09 10.17 4.66 0.223

LV-03-12B 0.86 1.03 0.094 1.32 2.00 8.30 4.35 0.161

LV-03-13B 0.95 1.04 0.154 1.44 1.12 7.16 3.82 0.181

LV-03-14B 0.86 1.29 0.120 1.52 0.84 6.14 4.04 0.146

LV-03-15B 4.95 0.81 0.133 1.84 3.14 7.38 4.94 0.231

obtained from the slope of the regression line for the plot

picomole vs. hours. The fluxes were then calculated by the

equation:

Fb ¼R

Sð1Þ

where Fb is the mean benthic flux during the experiment, R

is the slope of the regression line in pmol/h and S is the

sediment surface covered by the chamber. Positive fluxes

result from an increase in concentration in the water with

time; conversely, negative fluxes result when the concen-

tration in water decreases with time.

In Fig. 3a–b are reported the plots of trace element

concentration as a function of time and the fluxes

determined during the Tresse and Campalto experiments.

Comparison of results for elements, for which distribu-

tion is controlled by the same chemical processes, e.g., Fe

and Mn, or controlled by the biological activity, e.g., Cd, Cu

and Zn, showed significant flux differences. The manganese

flux was 19 and 12 pmol/cm2/h for the Tresse and Campalto

experiments, while negative fluxes were determined for iron

) and Campalto (b) experiments

U V Zn Salinity % T

(8C)O2

(%)

pH Eh

1.72 1.39 10.14 29.20 18.2 53.9 7.95 247

2.18 1.84 9.91 29.25 18.3 47.7 7.92 227

1.99 1.49 8.13 29.32 18.2 42.0 7.90 197

3.01 2.39 4.82 29.38 18.1 35.4 7.86 153

3.03 2.73 5.75 29.40 17.4 31.6 7.83 142

3.10 3.21 8.99 29.45 16.3 27.1 7.79 141

2.19 1.82 7.00 29.45 16.8 25.1 7.76 132

3.16 2.93 11.19 29.45 17.6 24.0 7.73 130

3.20 3.12 12.08 29.45 18.2 22.5 7.71 128

3.03 2.21 10.75 29.44 17.4 20.6 7.62 132

2.66 2.22 13.00 29.40 16.7 20.8 7.68 126

3.33 2.48 11.92 29.37 16.7 18.7 7.65 127

3.09 2.55 15.58 29.43 18.4 16.1 7.61 129

3.28 2.63 13.22 29.49 17.7 16.1 7.61 136

3.41 2.40 21.51 29.57 17.8 12.8 7.60 146

3.13 1.46 5.35 28.35 22.35 75.2 7.7 80.7

3.30 1.54 4.59 28.19 23.2 61.7 7.61 86.3

3.26 1.33 5.26 28.26 23.94 52.0 7.50 99.9

3.27 1.58 6.23 28.18 23.1 43.0 7.44 134

3.28 1.32 7.81 28.15 22.22 37.5 7.39 181.9

3.26 1.16 9.16 28.23 21.92 29.8 7.33 188

3.30 1.41 15.81 28.31 22.48 26.2 7.31 44.2

3.20 1.17 12.55 28.32 24.02 30.8 7.28 104.8

3.29 1.03 13.42 28.70 24.57 40.6 7.32 117.1

3.26 0.94 13.95 28.87 23.64 39.2 7.32 156.5

3.56 0.82 15.91 28.94 23.54 38.0 7.31 185

3.39 0.69 17.30 28.99 22.97 37.1 7.27 188.8

3.72 1.03 21.25 28.93 23.21 34.5 7.28 116.7

3.31 0.85 18.88 28.80 24.42 39.2 7.31 124.3

3.42 0.89 18.78 28.73 24.52 44.8 7.37 179.9

3.49 1.04 20.43 28.75 24.21 43.1 7.37 231.5


(�3.5 and �6.3 pmol/cm2/h), despite that the distribution of

both the elements is controlled by redox processes. Differ-

ences were also observed in the behaviour of other elements

for the two experiments. The fluxes, for the Tresse and

Campalto experiments, were 5.1 and �6.9 pmol/cm2/h for

molybdenum, 0.25 and �0.18 pmol/cm2/h for arsenic and

1.3 and �8.4 pmol/cm2/h for vanadium. Therefore, the

different characteristics of the two areas affected the

mobility of trace elements.

To confirm the trend in Mn and Fe fluxes, they were also

estimated by determining their concentration in pore water

Fig. 3. (a) Concentration and fluxes of Mn, Mo, As and V in the Tresse (a) and Cam

the Tresse (a) and Campalto (b) experiments.

of two cores collected in the same two areas and in the

bottom water simultaneously gathered (Fig. 4). Diffusive

fluxes were assessed by measuring the gradient concen-

tration at the water/sediment interface as follows:

Fd ¼ DDC

Dxð2Þ

where Fd is the diffusive flux; D is the diffusion coefficient

(estimated to be 10�6 cm2/s); DC is the difference in

concentration between pore water and the overlying bottom

palto (b) experiments. (b) Concentration and fluxes of Cd, Zn, Cu and Fe in

Fig. 3 (continued).


water (pmol/cm3) and Dx is the distance (cm). This is an

estimation of the flux from sediment towards the overlying

water without taking into account the reverse flux from

water to sediment.

The fluxes calculated for Tresse and Campalto on the

basis of Eq. (2) were 140 and 170 pmol/cm2/h for

manganese and were 14 and 24 pmol/cm2/h for iron,

respectively. These fluxes are overestimated because they

consider only the release from sediment, while they do not

assess the contribution from deposition; however, they

emphasize the significant difference in mobility of man-

ganese and iron, as also observed by the benthic chamber

experiments. Because the benthic chamber experiments and

diffusive fluxes were derived from samples collected in

different time, this could be responsible for the differences

of results; however, the flux values obtained for the two

benthic experiments (the first carried out in October and the

second in May) do not show significant differences.

Therefore, we can be confident that the gap in collecting

time is not relevant, and the high values of diffusive fluxes

with respect to that calculated for benthic chamber experi-

ments should be explained by the residual oxygen content in

the benthic chambers, anoxic conditions were not reached

(only ipo-oxic conditions were obtained) and, in those

chemical conditions, the re-precipitations of Mn(VI) and

Fe(III) oxides in the water column is possible.

Fig. 4. Concentration of Mn and Fe in the Tresse (a) and Campalto (b) pore water.


The zinc, cadmium and copper concentration trend is

characterized by a mean positive flux up to the end of the

experiments. The flux of Zn was higher than that for Cd

(about two orders of magnitude); however, if we examine

the difference in concentration in the dissolved phase, the

mean zinc concentration is about 100 times higher than the

cadmium concentration, and we can thus conclude that

they show a similar behaviour. Zinc and cadmium showed

similar fluxes for the two experiments: 62 and 67 pmol/

cm2/h, respectively, for zinc and 0.21 and 0.18 pmol/cm2/

h, respectively, for cadmium. The flux of copper (0.29 and

0.50 pmol/cm2/h, respectively) was estimated after exclud-

ing the first samples, which were affected by sediment re-

suspension during chamber settling (Fig. 3). Although the

mean benthic flux of copper was always positive and

comparable with those determined for cadmium, it is less

significant if we consider its value in terms of concen-

tration increase. The copper concentration increased by

about 20% from the beginning to the end of the experi-

ments, while the cadmium increased its concentration by

about three times. Therefore, we can conclude that though

the processes controlling cadmium, zinc and copper

distributions are always related to biological activity, the

mobility of the latter seems quite different with respect to

the others.

To evaluate the behaviour of the analysed elements, we

have performed a principal components factor analysis (FA)

to construct a probabilistic model explaining the correla-

tions between the sets of variables and generating new

variables that explain the variance associated to the row

data (named factors) [15]. Eh parameter is not used in this

elaboration because it does not add any information to the

model.

Three significant factors were derived from the FA

applied to the data set, which explained 81.2% of the

variance. Factor 1 (52.6% of explained variance) is

characterized by high scores for As, Mn, Mo, Sb, V,

pH and salinity (positive versus), and Al, Fe and O2

(negative versus); factor 2 (17.2%) presented the highest

positive scores for Zn, Cd and lowest for Cu while O2

and pH have negative scores; factor 3 (11.4%) is

characterized by opposition between Cu (positive) and

U (negative). The plot of factor 1 vs. factor 2 (Fig. 5)

highlights a clear separation between the two experi-

ments, which could derive from differences in the

chemical characteristics of the two areas (salinity, organic

carbon content in the sediment, pH, etc.) or seasonal

differences in which the experiments were carried out

(factor 1); factor 2 emphasizes a separation between the

two phases of the experiments on the basis of variations

of micro element content (Cu, Cd and Zn) related to

O2% levels. The loading values for factor 1 show that

Tresse presents higher values of Mn, Mo, As, Sb and V

than Campalto, while Campalto shows higher values of

Fig. 5. Biplot of factor 1 vs. factor 2.


Al, Fe and U and, quite constantly, low values for Mn,

Mo, As, Sb, V and pH. Factor 2 shows a correlation

between Cd, Zn, and, partially, Cu concentration, in

opposition to pH and oxygen concentrations for both the

experiments.

Factor 3 highlights the initial variation of concentration

of some elements due to the settling of the chambers;

therefore, it is characterized by the high loading values for

some elements (Cu, Fe, Cd).

4. Conclusions

The results obtained in the two areas show that trace

elements can be remobilised from the sediment to the

water from re-suspension and/or sub-oxygenation of

water. Some preliminary conclusions can be drawn from

the data.

The two experiments showed different benthic fluxes of

total dissolved Mo, Sb, As, V and Al that can derive from

differences in the environmental conditions (temperature,

salinity, organic carbon content) or from the evolution of

the physico-chemical parameters (oxygen and pH) during

the experiments; probably more factors contribute to the

behaviour of these elements, such as environmental and

seasonal factors. On the other hand, despite the differences

in the concentrations observed in the two sites studied, the

benthic fluxes of total dissolved Cd, Zn, Mn, Fe and Cu

seem not to be particularly affected by differences between

the two sites, but are affected from chemical and diffusive

processes.

At the beginning of the experiment, the benthic flux of

the trace elements studied was often positive, possibly due

to the remobilisation caused by the settling of the benthic

chambers. Therefore, the sediments may be considered an

important source of trace elements for the water column,

especially when physical factors perturb them. Sediment re-

suspension affects the mobility of a larger part of the metals,

while mobility due to chemical or/and diffusive processes

seems to be quite different even for elements with similar

geochemical characteristics.

Acknowledgement

This work was supported by CORILA under the Project

dRole of aerosol and secondary pollution to the chemical

contamination of the lagoon of VeniceT. The authors thank

M. Frignani for the helpful discussion and sediment

sampling and V. Zampieri for the technical support.

References

[1] K.W. Warnken, G.A. Gill, L.L. Griffin, P.H. Santschi, Marine

Chemistry 73 (2001) 215–231.

[2] C. Zago, G. Capodaglio, S. Ceradini, G. Ciceri, L. Abelmoschi, F.

Soggia, P. Cescon, G. Scarponi, Science of the Total Environment 246

(2000) 121–137.

[3] J.P. Shine, R. Ika, T.E. Ford, Environmental Toxicology and

Chemistry 17 (1998) 2325–2337.

[4] I. Rivera-Duarte, A.R. Flegal, Geochimica et Cosmochimica Acta 58

(1994) 3307–3313.

[5] E. Viollier, C. Rabouille, S.E. Apitz, E. Breuer, G. Chaillou, K.

Dedieu, Y. Furukawa, C. Grenz, P. Hall, F. Janssen, Journal of

Experimental Marine Biology and Ecology 285–286 (2003) 5–31.

[6] G. Capodaglio, G. Toscano, G. Scarponi, P. Cescon, International

Journal of Environmental Analytical Chemistry 55 (1994) 129–148.

[7] G. Scarponi, G. Capodaglio, C. Barbante, P. Cescon, The anodic

stripping voltammetric titration procedure for study of trace metal


complexation in seawater, in: S. Caroli (Ed.), Element Speciation in

Bioinorganic Chemistry, John Wiley and Sons, 1996, pp. 363–418.

[8] G. Capodaglio, G. Toscano, P. Cescon, G. Scarponi, H. Muntau,

Annali di Chimica (Rome) 84 (1994) 329–345.

[9] C. Turetta, G. Cozzi, A. Varga, C. Barbante, G. Capodaglio, P.

Cescon, Journal de Physique. IV 107 (2003) 1321–1324.

[10] I. Rodushkin, T. Ruth, Journal of Analytical Atomic Spectrometry 12

(1997) 1181–1185.

[11] S.N. Willie, H. Tekgul, R.E. Sturgeon, Talanta 47 (1998) 439–445.

[12] M. Filella, N. Belzile, Y.W. Chen, Earth-Science Reviews 57 (2002)

125–176.

[13] M. Filella, N. Belzile, Y.W. Chen, Earth-Science Reviews 59 (2002)

265–285.

[14] L. Yang, R.E. Sturgeon, Journal of Analytical Atomic Spectrometry

17 (2002) 88–93.

[15] UNISTAT Statistical Package (2003).

benthic fluxes of trace metals in the lagoon of venice

Documents