Estimation of PCDD/F distribution and fluxes inthe Venice Lagoon, Italy: combining measurement

and modelling approaches

Matteo Dalla Valle b,a, Antonio Marcomini b, Adriano Sfriso b,Andrew J. Sweetman a, Kevin C. Jones a,*

a Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University,

Lancaster LA1 4YQ, UKb Department of Environmental Sciences, University of Venice, Calle Larga S.ta Marta, 2137, I-30123 Venice, Italy

Received 21 February 2002; received in revised form 4 December 2002; accepted 13 January 2003

Abstract

The available experimental information on the occurrence of PCDD/Fs in the Venice Lagoon, Italy, was compiled

and used to calculate fugacities for the environmental compartments of sediment, suspended particulate matter (SPM),

water and air and then used to estimate fugacity ratios and assess the likely net direction of flux between media. The

bottom sediment: SPM fugacity ratios for different PCDD/Fs indicate conditions close to equilibrium, suggestive of the

close coupling of SPM with re-suspended sediment. Sediment/water and the sediment/air fugacity ratios suggest that net

flux directions vary depending on the congener and the location within the lagoon. Net sediment–water–air movement

(i.e. re-mobilisation/volatilisation) is suggested for the lighter congeners from the industrial canals, where the highest

PCDD/F concentrations in the lagoon occur. The tendency to volatilise increases with decreasing congener molecular

weight. In contrast, net deposition (air–water–sediment) appears to be occurring for the heaviest (hepta- and octa-)

substituted PCDD/Fs. OCDF represents a marker of the industrial district of the lagoon, decreasing in concentration

and as a fraction of total PCDD/Fs with increasing distance. The fugacity-based quantitative water air sediment in-

teraction (QWASI) mass-balance model was applied to the central part of the lagoon. The key parameters for the

determination of the model output, identified by a sensitivity analysis, were: the sediment active depth, the sediment re-

suspension and deposition rates, and the total input of PCDD/Fs to the system. The QWASI model also indicates the

tendency for the lighter PCDD/Fs to be released from surface sediment to the water column.

� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: PCDD/F; Budget; Model; Fluxes

1. Introduction

The contamination of the Venice Lagoon, particu-

larly with respect to the polychlorinated dibenzo-p-

dioxins and -furans (PCDD/Fs) and other persistent

organic pollutants (POPs) has received much attention

over the last decade. The lagoon has a long history of

industrial activity and from the 1950s the inner lagoon

area of Porto Marghera expanded as chemical and oil

refining plants were developed. During the last two de-

cades, the decline of the chemical industry and the

adoption of new technologies for emission control and

abatement significantly reduced the aqueous and gas-

eous emissions from the industries to this area. How-

ever, high concentrations of PCDD/Fs, polychlorinated

Chemosphere xxx (2003) xxx–xxx

www.elsevier.com/locate/chemosphere

*Corresponding author. Tel.: +44-1524-593972; fax: +44-

1524-593985.

E-mail address: k.c.jones@lancaster.ac.uk (K.C. Jones).

0045-6535/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0045-6535(03)00048-1

ARTICLE IN PRESS

biphenyls, polycyclic aromatic hydrocarbons and heavy

metals still affect sediments, especially in the central la-

goon, providing a potential source for the lagoon eco-

system and for human exposure, through the ingestion

of contaminated seafood. Other sources are the atmo-

spheric deposition, the discharge of untreated municipal

effluents from the historical centre and the emissions

associated with heavy boat traffic due to local trans-

portation, fishing activities, cruisers and oil tankers.

High PCDD/F concentrations were recently recorded in

sediments of the industrial canals (Bellucci et al., 2000;

Magistrato alle Acque di Venezia, 2000) that serve the

Porto Marghera district. The lagoon received untreated

municipal and industrial effluents until the end of the

1960s; even today municipal effluents from the historical

centre are largely untreated.

The annual consumption of fish and shellfish by

Venetians (much of it cultivated/caught from the la-

goon) is approximately twice the average consumption

recorded in Italy (Zanotto et al., 1999). As a result,

high contaminant levels and high consumption of local

product have led to public health concerns and a desire

to better understand pollutant transfers to the lagoon

biota. In order to gain information about the extent of

this contamination, extensive sampling campaigns have

been conducted in recent years, especially in the central

part of the lagoon. Surface sediment, water, suspended

particulate matter (SPM) and biota (clams, mussels,

crabs and fishes) have all been analysed and physico-

chemical data and environmental parameters have been

recorded (i.e. temperature, pH, organic carbon fraction

in sediment and particulate matter, particulate matter

concentration and fluxes; Sfriso et al., 2000). The con-

taminant contribution of rivers from the catchment area

and of atmospheric deposition were also investigated

(Bettiol et al., 2001; Rossini et al., 2001). On-going

PCDD/F emissions are now much lower than in the past

(Marcomini et al., 1999a), but contaminated sediment of

the industrial canals and surrounding areas can poten-

tially act as a significant long-term secondary source of

PCDD/F as it becomes transported and re-distributed.

High re-suspension of bottom sediments is caused by

intensive fishing for clams (Tapes philippinarum), ship-

ping activity and tidal currents. These processes favour

high sediment–water exchange, whilst the shallow warm

waters of the lagoon encourage high biological produc-

tivity and air–water exchange.

There are several publications addressing the occur-

rence and distribution of POPs in the Venice Lagoon

(Fattore et al., 1997; Marcomini and Della Sala, 1997;

Marcomini et al., 1997, 1999a,b; Green et al., 1999a;

Bellucci et al., 2000; Wenning et al., 2000; Frignani et al.,

2001a,b). However, there are few studies examining the

data together and applying modelling approaches to

gain insights into the key processes governing the fate of

the chemicals in the lagoon. Multi-media environmental

fate models, based on the fugacity concept, have been

satisfactorily developed and applied elsewhere (Booty

and Wong, 1996; Freitas et al., 1997; Suzuki et al., 1998,

2000; Connolly et al., 2000; Woodfine et al., 2000; Miller

et al., 2001). This approach allows the equilibrium status

and net flux direction to be assessed, given information

on the environmental conditions and physico-chemical

properties of the compounds of interest (Mackay, 2001).

In this study the central lagoon was chosen to eval-

uate equilibrium status and to develop a mass-balance

model to predict the steady state concentration levels

and fluxes of individual PCDD/F congeners for key

environmental compartments. The quantitative water

air sediment interaction (QWASI) model was used, as it

allows––once environmental parameters (e.g. total water

surface, mean depth, organic carbon content in sedi-

ment), physical–chemical characteristics (such as vapour

pressure, logðKow) and the Henry�s law constant), inputs

and advective flows are known––to predict concentra-

tion values in all environmental media (Mackay et al.,

1983). The QWASI model has been successfully applied

to Lake Ontario (Mackay, 1989) and has been used

widely, being applicable to water bodies with a known

water and suspended solids budget.

The objectives of the paper were therefore to quantify

the intermedia fluxes of PCDD/Fs given the current

present inputs to the lagoon and to predict the steady-

state concentrations of these chemicals in the environ-

mental compartments of sediment, water, air, water and

air particles. In order to do this we needed to: collect and

summarise the experimental data on PCDD/Fs available

for the Venice Lagoon; compile environmental budgets

for PCDD/Fs for the area; estimate the contemporary

flux direction for each congener; obtain the informa-

tion necessary to develop a steady state mass-balance

model.

1.1. Study area

The Venice Lagoon, located in Northeast Italy, has a

half-moon shape and total surface of 550 km2. The

water surface open to the tidal exchange is 430 km2 with

an average depth of about 1 m and a mean tidal ex-

cursion of approximately �30 cm. It is subdivided in

three main basins by three entrance channels allowing

water exchange with the Adriatic Sea (111� 109 m3/y,

i.e. 3:1–4:5� 108 m3 per tidal cycle), while the average

freshwater inflow is about 35 m3/s. About 1 400 000

people live in the water basin area, i.e. 1970 km2 wide,

corresponding to 4 000 000 inhabitant equivalents if

industrial and agricultural activities are taken into ac-

count (Regione Veneto, 1998). The prevailing wind di-

rection is NE–SW and water temperature ranges from

0 to 30 �C.Due to low depth, high water exchange and scarce

macrophytic coverage compared with the past (Sfriso

2 M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx

ARTICLE IN PRESS

et al., 2001), the central part of the lagoon is now a

rather homogeneous system, with uniform physical–

chemical characteristics. In addition, the intense (and

currently illegal) fishing activity for clams (Tapes phil-

ippinarum) represents a serious threat for the lagoon

ecosystem causing a significant increase in sediment re-

suspension and favouring the re-circulation of particu-

late matter and contaminants. The central part of the

lagoon, between the Malamocco and Lido channels

linking the lagoon to the Adriatic Sea, covers a water

surface of approximately 132 km2 and is quite well

mixed hydrologically. Four sites in this area were the

focus of investigation, selected to represent contamina-

tion gradients. These are (Fig. 1):

Site A (Alberoni) This site (12�18091000E, 45�21018000N) islocated close to the Malamocco channel, where the tidal

exchange with the Adriatic is high.

Site B (Sacca Sessola) (12�18051200E, 45�24037200N) in themiddle of the central lagoon, close to the island of Sacca

Sessola.

Site C (San Giuliano) This site (12�17053200E,45�27073500N) is close to the trans-lagoon bridge and

the mouth of the Osellino Canal.

Site D (Fusina) (12�16007500E, 45�24093600N) near the in-dustrial area of Porto Marghera, by the western indus-

trial channel. It is also affected by the Naviglio-Brenta

river and the proximity to a large power station.

2. Data analysis

Data on sediments, SPM, water, riverine inputs and

atmospheric deposition were obtained from recent in-

vestigations (1998–2000) undertaken by the Water Au-

thority of Venice (Magistrato alle Acque di Venezia

and its concessionaire Consorzio Venezia Nuova). The

sampling campaigns for SPM, water and riverine inputs

were restricted to the central part of the lagoon, while

surficial sediments were sampled across the whole sys-

tem.

The analytical procedures for the determination of

PCDD/Fs used high resolution gas-chromatography

coupled with high resolution mass spectrometry

(HRGC–HRMS) with validated analytical procedures

(e.g. Green et al., 1999b). All samples were processed

according to the same well established procedures (EPA,

1994).

2.1. Sediment

More than 120 samples of surface sediment (0–15 cm)

were collected from the lagoon during an extensive

campaign commissioned by the Venice Water Authority

(Magistrato alle Acque di Venezia, 2000). Most were

from shallow (0.5–1 m) locations, while the rest were

taken from deeper locations (2–5 m) in navigable canals.

The southern and northern parts of the lagoon mud

flats are the least contaminated with PCDD/Fs. Typi-

cally, total 2,3,7,8-PCDD/F concentrations are approxi-

mately 75–90 ng/kg dry weight (Table 1). Concentrations

in the navigable canals are about two times higher. In the

central part of the lagoon, both shallow and deeper se-

diments have, on average, concentrations three times

higher (i.e. �300 ng/kg). Surficial sediments in the in-

dustrial area are much more contaminated, ranging from

680 to 1 81 000 ng/kg (average �82 500 ng/kg). I-TEQ

concentrations show the same spatial trend, although the

Fig. 1. Map of the Venice Lagoon, with the four sampling sites.

M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx 3

ARTICLE IN PRESS

range in values is reduced because of the dominant con-

tribution of the highly chlorinated congeners (OCDD,

OCDF), which have lower TEF ratings. For exam-

ple, the average concentration in the navigable canals,

around 3.3 ng I-TEQ/kg, is not statistically different

from the average concentration in the southern (2.9)

and northern (2.2) parts of the lagoon. The average

concentration in the central lagoon is 6.2 ng (I-TEQ)/kg

with the industrial canals ranging from 20 to 11 000 ng

I-TEQ/kg.

The average PCDD/F concentrations in the mud flats

(Table 1) are comparable to European background ar-

eas, while concentrations in samples of the industrial

canals are amongst the highest values observed any-

where (Fiedler et al., 1999).

The congener profiles in the northern and southern

parts of the lagoon and in the navigable canals are

similar, being dominated by OCDD and OCDF. The

PCDD:PCDF ratio in these areas is typically between

0.7 and 0.9. In contrast, the industrial canals and the

central lagoon (Fig. 2) both have a pattern dominated

by OCDF, giving a PCDD:PCDF ratio of �0.25.PCDFs, especially OCDF, appear to provide a marker

for sediments in the industrial district of Porto Marg-

hera (Isosaari et al., 1999; Frignani et al., 2001a).

Another study undertaken at four stations in the

central lagoon reported concentrations within the range

of to those presented in this study (Table 2a and b,

unpublished data). The PCDD:PCDF ratios decreased

from 0.6 in Alberoni to 0.2 in Fusina, while in San

Giuliano and Sacca Sessola it was around 0.5.

2.2. Suspended particulate matter

SPM was collected using passive traps during two

sampling campaigns, one in September 1998, one in

May 1999 (Table 2a and b). Therefore only the settleable

fraction of the suspended matter was collected.

The deposition rate of SPM was high (up to 4.4

kgm�2 day�1, dry weight, mean value approximately 1.7

kgm�2 day�1), whereas the concentration of particulate

matter in the water column ranged from 27 to 134 mg/l

with an annual mean value of 69 mg/l. This is not sur-

prising; sediment re-suspension and hence SPM loadings

are high in much of the lagoon. The PCDD/F profiles in

both sampling campaigns were almost identical to that

found in the sediment of all sites, suggesting a condition

of near-chemical equilibrium between the two compart-

ments. Minor differences between the four sites could be

observed. There was a decrease in the importance of

OCDD from sites A to D and a simultaneous increase in

OCDF abundance. The concentrations of PCDD/Fs

obtained from the last sampling campaign (May 1999),

expressed as the sum of all 2,3,7,8-substituted congeners,

was very similar at sites A and B (125 and 135 ng/kg,

d.w.) and about 6 times higher at sites C and D (720 and

795 ng/kg respectively, d.w.). If concentration values are

normalised to the organic carbon content, differences

Table 1

Mean concentrations of PCDD/Fs found in the Venice Lagoon sediments (dry weight basis), taken from southern, central and

northern shallow bottoms and from navigable and industrial canals

Southern lagoon Central lagoon Northern lagoon Navigable canals Industrial canals

n¼ 29 n¼ 31 n¼ 21 n¼ 32 n¼ 36

mean� st. dev. mean� st. dev. mean� st. dev. mean� st. dev. mean� st. dev.

ng/kg (d.w.) ng/kg (d.w.) ng/kg (d.w.) ng/kg (d.w.) ng/kg (d.w.)

2,3,7,8-TetraCDD 0.9� 2.5 0.7� 2 0.56� 1.3 0.26� 0.21 6.0� 17

1,2,3,7,8-PentaCDD 0.5� 0.9 0.9� 2 0.33� 0.2 0.38� 0.23 50� 170

1,2,3,4,7,8-HexaCDD 0.34� 0.34 0.75� 0.80 0.34� 0.2 0.58� 0.63 130� 360

1,2,3,6,7,8-HexaCDD 0.4� 0.4 0.9� 1.0 0.42� 0.26 0.84� 0.96 140� 520

1,2,3,7,8,9-HexaCDD 0.5� 0.6 1.2� 1.7 0.46� 0.26 0.85� 1.02 180� 670

1,2,3,4,6,7,8-HeptaCDD 6.4� 11 12� 17 4.8� 3.8 11� 15 2060� 7060

1,2,3,4,6,7,8,9-OctaCDD 28� 48 44� 52 30� 26 58� 85 6270� 23 120

2,3,7,8-TetraCDF 1.3� 2.3 2.7� 3.0 0.8� 0.6 1.4� 1.2 240� 450

1,2,3,7,8-PentaCDF 1.2� 1.3 4.0� 4.5 1� 0.8 1.8� 1.7 610� 1450

2,3,4,7,8-PentaCDF 0.9� 1 2.0� 2.0 0.64� 0.44 1.1� 1.1 240� 580

1,2,3,4,7,8-HexaCDF 2.7� 3.5 11� 12 2.5� 2.4 5.0� 6.3 2040� 3830

1,2,3,6,7,8-HexaCDF 1.3� 1.5 5.6� 6.1 1.4� 1.4 3.0� 3.8 860� 1860

1,2,3,7,8,9-HexaCDF 0.34� 0.20 0.66� 0.62 0.37� 0.20 0.39� 0.29 155� 240

2,3,4,6,7,8-HexaCDF 1.5� 1.9 3.4� 3.8 1.2� 1.1 3.4� 6.1 470� 980

1,2,3,4,6,7,8-HeptaCDF 11� 16 63� 62 15.5� 14 25� 35 11 970� 33 440

1,2,3,4,7,8,9-HeptaCDF 1.5� 1 6.3� 7 2.1� 1.2 2.9� 3.8 2550� 7700

1,2,3,4,6,7,8,9-OctaCDF 19� 24 138� 173 26� 23 52� 84 54 530� 2 31 770

P2,3,7,8-PCDD/F 74� 96 296� 290 87� 64 167� 220 83 000� 3 07 000

I-TEQ 2.9� 4.8 6.2� 6.6 2.2� 1.8 3.3� 3.1 800� 1900

4 M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx

ARTICLE IN PRESS

between the sites were less significant and a gradient

from sites A to D is observed. The results of the previous

sampling campaign were very similar: although con-

centration values spanned about an order of magnitude,

the same trend was observed.

2.3. Water

Concentrations in water were obtained from two

sampling campaigns conducted at the same four sites

(Table 2a and b). Water samples were not filtered;

concentrations therefore refer to ‘‘bulk’’ water combin-

ing the contribution of freely dissolved and particle

bound fractions. No information was acquired about

tidal conditions during the sampling; concentration

values must therefore be regarded as just broadly indi-

cative for the sites in question. The tidal condition (and

thus the flow direction) could be a very important pa-

rameter in determining the quality of the water, in par-

ticular for sites A and B, closer to the sea. Total 2,3,7,8-

PCDD/F concentrations increased from the sea to the

inner parts of the lagoon showing for the first campaign

values of 3.0, 3.0, 19 and 31 pg/l for sites A–D, respec-

tively. In the second campaign concentrations of 4.5, 9,

14 and 22 pg/l were recorded. The gradient ob-

served matches the trend observed in the sediment and

SPM.

2.4. River inflow

The lagoon receives freshwater from 24 tributaries.

Although 9 rivers account for approximately 90% of the

flow, input data are available only for two of these, the

Naviglio-Brenta and the Osellino Canal (Zuliani et al.,

2001), which both discharge in the central lagoon. They

account for about 20% of the total freshwater input to

Fig. 2. Average congeners profile found in the central lagoon sediment (no. 30 samples).

Table 2P2,3,7,8-PCDD/Fs at four representative sampling sites in the central lagoon

Station Sediment SPM Water (pg/l)

ng/kg d.w. Organic carbon

fraction

ng/kg d.w. Organic carbon

fraction

(a) May 1999

A––Alberoni 41 0.005 125 0.022 4.5

B––Sacca Sessola 104 0.007 135 0.009 9.6

C––San Giuliano 565 0.012 791 0.022 14

D––Fusina 600 0.010 720 0.013 22

(b) September 1998

A––Alberoni 42 0.005 118 0.024 3.0

B––Sacca Sessola 121 0.007 214 0.016 3.0

C––San Giuliano 926 0.012 863 0.020 19

D––Fusina 658 0.010 1257 0.016 31

M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx 5

ARTICLE IN PRESS

this area, draining a surface of approximately 300 km2

out of the total 1850 km2. Although the whole riverine

input to the lagoon is approximately 1–2% of the total

amount of water exchanged with the sea, it is an im-

portant source of contaminants carrying approximately

50–60 g PCDD/F a year and at least 1.2 g (I-TEQ)/y

(Marcomini et al., 1999b; Bettiol et al., 2001). The

congener profile of the annual mean input shows a

dominance of OCDD and an increase in concentration

with molecular weight of the congeners (Table 3). Un-

fortunately, concentrations of several congeners were

below the detection limit in most samples. Thus the re-

ported value must be considered a minimum estimate.

2.5. Atmospheric deposition and fluxes

The only available dataset for PCDD/Fs in the at-

mosphere near/above the lagoon was acquired recently

(Rossini et al., 2001) with a monthly sampling of bulk

atmospheric deposition at four stations over 13 months

(7/98–7/99). The deposition rates measured were com-

parable to those found in central European rural areas

(Benfenati et al., 1994; Schroder et al., 1997; Wallen-

horst et al., 1997; Gevao et al., 1998; Fiedler et al., 1999),

ranging from 10 to 42 ng total PCDD/F m�2 y�1 and

about 1–2 ng I-TEQ m�2 y�1 (Table 4).

The southern and northern parts of the lagoon had

similar atmospheric deposition fluxes. Fluxes in the in-

dustrial area were 3–4 times higher than at other sites.

The congener profile was similar at the different sites,

although OCDD was more abundant near the historical

centre of the city, while OCDF and 1,2,3,4,6,7,8-

HpCDF were higher in the industrial area.

No direct measurements of PCDD/F air concentra-

tions have been made to date. It was therefore neces-

sary to make the following estimations (Kaupp and

McLachlan, 1999; Halsall et al., 2000):

Wet ðdissolvedÞ precipitation ¼ Cg

VrKaw

Wet ðparticulate boundÞ ¼ Cg � ðVr � W � KpgÞ

Dry ðgaseousÞ ¼ Cg � Vg

Dry ðparticulate boundÞ ¼ Cg � Vp � Kpg

Summing the equations gives

Total deposition

¼ Cg �VrKaw

�þ Vr � W � Kpg þ Vg þ Vp � Kpg

�

where Cg ¼ gaseous concentration (mol/m3), Vr ¼ rain

velocity¼ 2.2�10�3 m/s¼ 800 mm/y, Vp ¼ particle depo-

sition velocity, Vg ¼ gaseous deposition velocity, varying

from 0.064 to 0.37 cm/s, for individual congeners (Koester

and Hites, 1992), Kaw¼ air–water partition coefficient¼H=RT , H ¼Henry�s law constant, Kpg ¼ particle-gas

partition coefficient. Experimental values obtained by

Lee et al. (1998) at 15 �C (mean annual temperature in

Venice) were used, W scavenging ratio¼ 10 000.

Table 3

River input of PCDD/Fs in the Venice Lagoon

River Naviglio-Brenta Osellino canal Mean input Total Lagoon input

(estimation)

pg/l % of total pg/l % of total pg/l % of total ng/s g/y

2,3,7,8-TCDD <2 0 <2 0 <2 0 – –

1,2,3,7,8-PCDD <5 0 <5 0 <5 0 – –

1,2,3,4,7,8-HxCDD <5 0 1 1.2 0.5 0.2 16 0.5

1,2,3,6,7,8-HxCDD 0.5 0.9 1.3 1.6 0.6 1.1 21 0.7

1,2,3,7,8,9-HxCDD 1.5 2.8 1.8 2.1 1.5 2.7 49 1.6

1,2,3,4,6,7,8-HpCDD 4.8 9.0 5.6 6.8 4.9 8.6 160 5.0

OCDD 35 66 40 48 36 63 1100 36

2,3,7,8-TCDF 0.5 0.9 1.3 1.5 0.6 1.1 20 0.6

1,2,3,7,8-PCDF 0.5 0.9 3 3.7 0.9 1.4 30 1.0

2,3,4,7,8-PCDF 0.5 0.9 2 2.4 0.8 1.2 25 0.8

1,2,3,4,7,8-HxCDF 1.0 1.9 2.3 2.8 1.2 2.1 39 1.2

1,2,3,6,7,8-HxCDF 0.5 0.9 1.7 2.0 0.7 1.1 23 0.7

2,3,4,6,7,8-HxCDF 0.5 0.9 1.8 2.1 0.7 1.2 23 0.7

1,2,3,7,8,9-HxCDF <5 0 1 1.2 <5 0.2 – –

1,2,3,4,6,7,8-HpCDF 2.3 4.4 7.1 8.6 3.2 5.1 100 3.2

1,2,3,4,7,8,9-HpCDF 1.5 2.8 1.7 2.0 1.5 2.7 49 1.5

OCDF 3.6 6.8 11 13.6 5.0 8.0 160 5.0

P2,3,7,8-PCDD/Fs 53 100 82 100 58 100 1900 60

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ARTICLE IN PRESS

From the total deposition, it is possible to estimate

the gaseous concentration. The particulate bound frac-

tion is simply:

Cp ¼ CgKpg

And the total concentration is

C ¼ Cp þ Cg

Air concentrations reflect the profile of atmospheric

deposition and the differences between the four sites

were small with the exception of a pronounced

abundance of OCDF at the industrial district site

(Table 5).

3. Methods

A convenient way to estimate the net flux direction is

to compare fugacity values of a chemical among the

different compartments (Mackay, 2001). The higher the

fugacity, the higher the tendency of the chemical to

transfer to another phase. Fugacity is related to con-

centration by a linear relationship:

C ¼ Zf

where C¼ concentration (mol/m3), Z¼ fugacity capacity

(mol/m3 Pa), f¼ fugacity (Pa).

Z is a proportionality constant, dependent on tem-

perature, on the nature of the compartment and on the

physical–chemical properties of the compound.

Z values are obtained from the following relation-

ships (Mackay, 2001):

ZaðairÞ ¼ 1=RT

ZwðwaterÞ ¼ 1=H

Zpðparticulate matter in waterÞ ¼ 0:41KowqpZwfoc=H

Zbwðbulk waterÞ ¼ Zwð1� fpÞ þ Zpfp

ZsðsedimentÞ ¼ 0:41KowqsZw

where qs ¼ sediment density (kg/l); Kow ¼ octanol–water

partition coefficient; foc ¼ organic carbon fraction; fp ¼volume fraction of particles; qp ¼ particulate matter

density.

Table 4

Atmospheric bulk deposition (ng/m2 y) of PCDD/Fs in the Venice Lagoon

Historical centre Northern lagoon Southern lagoon Industrial area

2,3,7,8-TetraCDD 0.08 <d.l. 0.08 0.03

1,2,3,7,8-PentaCDD 0.13 0.04 0.11 0.07

1,2,3,4,7,8-HexaCDD 0.11 <d.l. 0.09 0.03

1,2,3,6,7,8-HexaCDD 0.19 0.04 0.16 0.18

1,2,3,7,8,9-HexaCDD 0.13 <d.l. 0.04 0.09

1,2,3,4,6,7,8-HeptaCDD 1.7 1.0 1.3 1.8

1,2,3,4,6,7,8,9-OctaCDD 7.2 4.2 4.4 5.6

2,3,7,8-TetraCDF 0.42 0.45 0.47 0.66

1,2,3,7,8-PentaCDF 0.31 0.36 0.31 0.36

2,3,4,7,8-PentaCDF 0.49 0.44 0.45 0.44

1,2,3,4,7,8-HexaCDF 0.84 0.62 0.76 1.1

1,2,3,6,7,8-HexaCDF 0.31 0.22 0.22 0.46

1,2,3,7,8,9-HexaCDF 0.37 0.36 0.39 0.52

2,3,4,6,7,8-HexaCDF 0.03 <d.l. <d.l. 0.13

1,2,3,4,6,7,8-HeptaCDF 1.5 0.93 1.4 3.0

1,2,3,4,7,8,9-HeptaCDF 0.09 <d.l. <d.l. 0.15

1,2,3,4,6,7,8,9-OctaCDF 2.7 1.3 2.7 28

P2,3,7,8-PCDD/Fs 17 10 13 42

I-TEQ 0.80 0.44 0.51 1.6

Table 5

Estimated concentration (fg/m3) in air of selected PCDD/Fs

Northern lagoon Southern lagoon Central lagoon Industrial district

1,2,3,4,6,7,8-HPCDD 20.9 27.0 34.9 35.1

1,2,3,4,6,7,8,9-OCDD 52.6 53.8 89.6 69.6

2,3,7,8-TCDF 8.7 8.9 9.2 12.4

2,3,4,7,8-PCDF 17.4 17.4 17.4 25.2

1,2,3,4,7,8-HCDF 14.5 19.4 20.1 27.3

1,2,3,4,6,7,8,9-OCDF 16.1 33.4 33.4 334

M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx 7

ARTICLE IN PRESS

In order to assess the intermedia fluxes as well as the

concentrations at the steady-state, the QWASI model

was applied to the central part of the lagoon. The Venice

Lagoon is a complex environment, and several processes

need to be taken into account in order to understand

and predict the chemical behaviour (Fig. 3). This area

was chosen because most experimental data were avail-

able. At steady-state it is assumed that total inputs and

outputs in the model environment are equal and con-

stant over time. Three compartments, air, water and

sediment, and two sub-compartments, aerosols and sus-

pended particles in water, were considered in this work.

Aerosols and suspended particles were assumed to be in

equilibrium with the air gas phase and water solution

phase respectively.

The model has been developed on an Excel�

spreadsheet, allowing the user to easily change input

parameter values. The first step was the calculation of Zvalues for all the sub-compartments according to the

equations shown in the previous section (Karickoff,

1981; Mackay et al., 1983). Then the spreadsheet

calculates intermedia fluxes (D values) in terms of

mol Pa�1 h�1 (for a detailed explanation: Mackay, 2001).

Selected input data on chemical properties (Kow, the

Henry�s law constant, the vapour pressure) and envi-

ronmental characteristics (total surface, rainfall rate,

mean depth and so on) are given in Table 6.

The program solved the following system of mass-

balance equations:

Air

Ea þ Dvfw ¼ faðDv þ Dm þ Dc þ DqÞ

Water

Ew þ fsðDr þ DtÞ þ faðDv þ Dm þ Dc þ DqÞ þ fIðDI þ DxÞ¼ fwðDt þ Dd þ Dw þ Dv þ Dj þ DyÞ

Sediment

ðDt þ DdÞfw ¼ ðDr þ Ds þ Db þ DtÞfs

where fa, fw, fI, fs are the fugacities respectively of air,

water, inflow water and sediment while Ea and Ew are

the emissions in air and water.

Once the model has estimated the f value, concen-

trations were calculated in all environmental compart-

ments, by using C ¼ Zf , and total amounts of a given

chemical in the environment estimated using data on

compartment volumes.

4. Results and discussion

Sediment-SPM fugacity ratio. The sediment-SPM

fugacity ratio (Fig. 4a) was obtained using values cal-

culated for PCDD/Fs at the four sampling stations of

the central lagoon for matched samples of sediment and

SPM. The fugacity ratio was always in the interval 0.5–

1.5, suggesting a close coupling between sediment and

SPM. Slight differences in the ratios were noted between

sites, with site A �0.9, site B �1.1 and sites C and D

�1.3. A likely explanation for this trend is that site A

receives SPM from the inner lagoon (which is more

contaminated), thereby lowering the ratio, whilst sites C

and D receive SPM from the outer lagoon, which would

raise the ratio.

Fig. 3. Processes considered in the QWASI model (Mackay et al., 1992).

8 M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx

ARTICLE IN PRESS

Air–sediment and air–water fugacity ratios. The air–

water fugacity ratio (Fig. 4c) was obtained from the

estimated air fugacity values and the fugacity values of

water calculated for the four stations in the central la-

goon. Unfortunately some of the congeners were below

the limit of detection. Site D was the only location where

estimates of the air/water fugacity ratio was possible for

all the selected congeners. The fugacity ratio is <1 for

TCDF and HxCDF at all the stations where it was

possible to measure it. The heavier congeners, on the

other hand, gave ratios >1.

Air–sediment fugacity ratios (Fig. 4b) were obtained

by averaging data for different regions of the lagoon.

Mean concentrations and organic carbon content data

were used, while the air fugacities were obtained from the

estimated air concentrations. Only congeners routinely

present at concentrations above the detection limit were

considered. The fugacity ratios were very different in the

different parts of the lagoon. They increased with con-

gener molecular weight suggesting the lightest congener

(TCDF) will tend to escape from the sediment whilst the

heavier ones (HpCDD and OCDD) show the opposite

behaviour. PeCDF and HxCDF are shown to be close to

equilibrium (fr ¼ 1) in each area of the lagoon except the

industrial area, where volatilisation is predicted to exceed

deposition. The high sediment concentrations suggest a

fugacity gradient from sediment/SPM to water to air for

TCDF, PeCDF and HxCDF, with the other congeners

appearing close to the equilibrium.

It is stressed that predictions about net air–sediment

exchange should be viewed with caution since these

calculations rely on very few measurements of aqueous

PCDD/Fs and estimates of air concentrations, as de-

scribed above.

QWASI model. Fugacity and concentration values of

selected PCDD/Fs from the application of the QWASI

model are given in Table 7. Calculations were run for

those congeners, where better experimental data were

available (e.g. concentration values routinely well above

the detection limit). These were 1,2,3,4,6,7,8-HpCDD,

OCDD, 2,3,7,8-TCDF, 1,2,3,4,7,8-HxCDF and OCDF,

that constitute a representative selection of all PCDD/Fs

congeners. The input to the system resulted summing the

atmospheric deposition and the riverine input. Other

inputs (not considered) into water are represented by

industrial and municipal effluents of the historical cen-

tre, although their present contribution is probably less

relevant (Marcomini et al., 1999b).

Concentrations predicted by the model (Table 7)

are lower than those found experimentally in each

Table 6

Input parameters used in the model

Input parameter Value Source

Total surface and water

surface

170 km2––78% Estimation obtained with a GIS software (Arcview�)

Rainfall rate 0.85 m/y Annual average value

Mean depth 2.58 m Estimation obtained with a GIS software (Arcview�)

Sediment active depth 5 cm Some authors (Suzuki et al., 1998) consider a 1 cm value. Given the high re-

suspension and deposition rates, favoured by the scarce coverage of

macrophytic population and by the effects of the intense fishing activity of

Tapes philippinarum, a value of 5 cm is thought to be more realistic for the

lagoon

Sediment organic

carbon fraction

0.010 The average value of the mean annual measured in four sampling sites in the

central lagoon (see text)

Water particles organic

fraction

0.018 The average value of the mean annual measured in four sampling sites in the

central lagoon (see text)

Temperature 288 K or 15 �C Mean annual temperature in Venice

Sediment deposition

rate

7.45E)05 m/h This value was estimated from sediment fluxes on the central lagoon. It

corresponds to 1.43 cm/y. The deposition rate is lower than the re-suspension

one

Water particle concen-

tration

65 mg/l It is a mean experimental value obtained at four sampling stations

Water residence time 120 h 5 d It is an approximate estimation of the mean residence time of water in the

central lagoon

Water background ad-

vective concentration

Congener dependent It is assumed to be 20% of the minimum observed value, since it is estimated

that 20% of the outflow re-enters the Lagoon

Air background advec-

tive concentration

Congener dependent The mean concentration measured in the northern lagoon was chosen. It is the

less contaminated area

Henry�s constant,Log (Kow), P s

L,

Congener dependent Mackay et al. (1992)

Reaction half-life Congener dependent Mackay et al. (1992); Sinkkonen and Paasivirta (2000)

M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx 9

ARTICLE IN PRESS

compartment (Tables 1 and 2, Fig. 4). As far as the

sediment is concerned, estimated concentrations are at

least a factor 5–10 lower than the measured ones. This is

not surprising, since the model predicts steady state

concentrations, after prolonged exposure to estimated

present inputs. It is known, from recent studies con-

Fig. 4. (a) Sediment/SPM fugacity ratio in four sites of the central lagoon. (b) air/sediment fugacity ratios in the Venice Lagoon.

(c) Air/water fugacity ratio in the four stations of the central lagoon.

10 M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx

ARTICLE IN PRESS

ducted on dated sediment cores (Marcomini et al.,

1999a), that the contemporary emissions are substan-

tially lower than in past decades, and that concentra-

tions in sediments, will retain a record of past emissions,

because of the very slow degradation of PCDD/Fs.

However, the predicted pattern of congeners is broadly

similar to the measured one, although there are differ-

ences in the relative importance of OCDD and OCDF.

This discrepancy is due to the difference between the

congener profiles of the present inputs to the system,

enriched in OCDD, from that of the sediment, enriched

in OCDF.

Aqueous concentrations predicted by the model are

generally higher than the experimental ones by a factor

2, and in one case (OCDF congener) the predicted

concentration is 10 times higher than the measured one.

Considering the above observations and the high vari-

ability of the experimental water concentrations, the

result can be considered satisfactory.

In contrast, atmospheric values are one or two orders

of magnitude lower than the ones estimated from the

experimental data on deposition. This discrepancy can

be partly explained by the fact that concentrations

within a compartment are influenced by concentration

within other compartments, that are lower than the

measured ones. In addition the model does not take into

account the advective input, which can have some in-

fluence, determining the background level of concen-

tration. Finally, the input in the system could be higher

than estimated.

Given the volumes of the three environmental com-

partments, the total amount of the selected congeners in

the central lagoon (within the first 5 cm of sediment) was

calculated (Table 7). As expected the top layer (5 cm) of

sediment contains more than 99% of PCDD/Fs present

in the model environment, with between 2 g (TCDF)

and 105 g (OCDD) for individual congeners.

Another interesting observation can be made by

comparing fugacity values of each congener in the dif-

ferent media. The highest fugacities are associated with

the sediment, with the lowest in air. This suggests that

a net transfer of PCDD/Fs from sediment to water

and then to the atmosphere may be occurring, in par-

ticular in the innermost part of the central lagoon where

concentrations in the sediment are higher than else-

where.

4.1. Sensitivity analysis

A sensitivity analysis was performed to identify the

parameters having the greatest influence on the model

output. Given the independency of the input parameters

and the linearity of the model response, a simple �10%variation of different individual input parameters was

imposed to the model each time. This test was conducted

for TCDF and OCDF, giving identical results (Table 8).

Only a few input parameters appear to substantially

influence the model output. These are the sediment ac-

tive depth, the sediment re-suspension and deposition

rate and the total emission (input) to the system. The

extremely high sediment re-suspension rate in the central

lagoon makes the SPM the principal vehicle in the dis-

tribution of these chemicals in the lagoon environment.

The remaining parameters, such as burial rate, water

depth, deposition velocity, the Henry�s Law constant,

the sediment–water mass transfer coefficient and the

water residence time, all have negligible influences on the

model output.

Table 7

Fugacity and concentration values for selected PCDD/Fs predicted by the QWASI model

1,2,3,4,6,7,8-

HpCDD

OCDD 2,3,7,8-TCDF 1,2,3,4,7-

HxCDF

OCDF

Fugacities

Water (Pa) 1.2E)15 7.5E)15 2.1E)14 4.3E)15 3.1E)16Sediment (Pa) 1.8E)14 1.2E)13 3.3E-13 6.7E)14 4.8E)15Air (Pa) 1.3E)19 4.7E)17 2.0E)15 6.7E)18 2.1E)19

Concentrations

Water (pg/l) 3.9E)04 1.7E)03 4.4E)03 1.1E)03 7.2E)04Sediment (pg/g) 2.5 17 0.4 0.7 4.6

Air (fg/m3) 2.4E)05 5.5E)04 2.6E)01 1.0E)03 3.8E)05

Air particulate

bound (fg/m3)

0.2 0.7 1.6 0.4 0.5

Water particulate

bound (pg/l)

0.3 2.0 0.0 0.1 0.5

Air total (fg/m3) 0.2 0.7 1.8 0.4 0.5

Water total (pg/l) 0.3 2.0 0.04 0.1 0.5

Amount (g) 15 105 2 4 28

M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx 11

ARTICLE IN PRESS

5. Conclusions

The Venice Lagoon is a complex ecosystem with lo-

calised direct (riverine, industrial area and historical

centre) and secondary (contaminated sediment) sources.

Only limited areas such as the industrial canals have

high PCDD/F contamination.

Although predictions of future PCDD/F trends is

difficult, it is possible, given an emission scenario, to

estimate the concentration values in the different media

with a steady-state mass-balance model, such as the

fugacity based QWASI model. The advantages of

the QWASI model are its simplicity and ability to pre-

dict the distribution of a given chemical in a complex

environment. It enables prediction of concentrations,

environmental burdens and intermedia fluxes. The con-

centrations of 1,2,3,4,6,7,8-HpCDD, OCDD, 2,3,7,8-

TCDF, 1,2,3,4,7,8-HxCDF and OCDF estimated by the

QWASI model in the environmental compartments are

lower than those determined by monitoring studies.

Higher past emission are the likely explanation. Com-

parison of the experimentally measured concentrations

with model estimate enables to quantify the contribution

of past emissions to be made.

Model output was rather insensitive to temperature

variations, but sensitive to some parameters which are

rather poorly known, notably the scavenging ratio, the

active sediment depth and the air–water mass transfer

coefficient, thus increasing the model uncertainty. Fur-

ther investigations are therefore required, to quantify

these parameters more accurately.

The model was applied to relatively well mixed and

shallow waters area of about 130 km2. It could be useful

to subdivide this area to obtain sub-areas, characterised

by a higher homogeneity and with a well defined water

circulation pattern. Further improvements could bemade

by the incorporation of lagoon currents and wind fields.

Volatilisation of PCDD/Fs from the Venice Lagoon

is likely to occur for the lighter (and more toxic) cong-

eners, notably in the inner part of the lagoon, close to

Table 8

Sensitivity analysis for OCDF

Parameter Value Range used Water

(variation, %)

Sediment

(variation, %)

Air

(variation, %)

(Xm)

A

(Xm þ 10%)

B

(Xm � 10%)

A B A B A B

Rainfall rate (m/y) 0.85 0.94 0.765 0.00 0.00 0.00 0.00 )9.56 10.38

Water depth (m) 2.58 2.8 2.3 0.00 0.00 0.00 0.00 0.00 0.00

Sediment active depth

(cm)

5 5.5 4.5 )6.41 7.51 )7.29 8.54 )5.36 6.27

Sediment organic C 0.010 0.011 0.009 0.00 0.00 0.00 0.00 0.00 0.00

Water particles

organic carbon

0.0175 0.0193 0.0158 )9.33 10.76 0.00 0.00 )7.80 9.00

Temperature (K) 288 317 259 0.00 0.00 0.00 0.00 0.00 0.00

Sediment re-suspen-

sion rate

7.61E)05 8.37E)05 6.85E)05 6.88 )7.16 )1.97 2.05 5.75 )5.99

Sediment deposition

rate

7.50E)05 8.25E)05 6.68E)05 )7.21 8.43 2.06 )2.41 )6.03 7.05

Water particle concen-

tration (mg/l)

65 72 59 0.00 0.00 0.00 0.00 0.00 0.00

Water residence time

(h)

120 132 108 0.00 0.00 0.00 0.00 0.00 0.00

Burial rate (m/h) 3.40E)08 3.74E)08 3.06E)08 )0.11 0.11 )0.11 0.11 )0.11 0.11

Scavenging ratio 10 000 11 000 9000 0.00 0.00 0.00 0.00 )9.08 11.10

Deposition velocity

(m/h)

10.8 11.9 9.7 0.00 0.00 0.00 0.00 )0.01 0.01

Sediment–water mass

transfer coefficient

(m/h)

1.00E)04 1.10E)04 9.0E)05 0.00 0.00 0.00 0.00 0.00 0.00

Air–water mass trans-

fer coefficient (m/h)

3 3.3 2.7 0.00 0.00 0.00 0.00 8.36 )8.36

Kow 8.20 8.04 7.95 )8.80 12.20 0.00 0.00 )7.36 10.20

Henry constant 0.684 0.752 0.616 0.00 0.00 0.00 0.00 0.00 0.00

P sL (Pa) 9.53E)07 0.0 8.56E)07 0.00 0.00 0.00 0.00 9.90 )10.00Total input in the sys-

tem (mol/h)

5.40E)06 5.9E)06 4.9E)06 10.00 )10.00 10.00 )0.00 8.36 )8.36

12 M. Dalla Valle et al. / Chemosphere xxx (2003) xxx–xxx

ARTICLE IN PRESS

the industrial district of Porto Marghera. For the rest

of the lagoon near-equilibrium or deposition conditions

appear to prevail. However, since calculations of fu-

gacity ratios are based on estimated air concentration,

further experimental investigations to establish air con-

centrations have high priority. Future investigations

should also be carried out to assess seasonal variability.

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