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Estuarine, Coastal and Shelf Science 93 (2011) 7e13

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Estuarine, Coastal and Shelf Science

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Relocation effects of dredged marine sediments on mercury geochemistry: Venicelagoon, Italy

Seunghee Han a,b,*, Joris Gieskes a, Anna Obraztsova c, Dimitri D. Deheyn a, Bradley M. Tebo a,d

a Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USAb School of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Koreac Synthetic Genomics, La Jolla, CA 92037, USAdDivision of Environmental & Biomolecular Systems, Oregon Health & Science University, Beaverton, OR 97006, USA

a r t i c l e i n f o

Article history:Received 28 January 2010Accepted 7 March 2011Available online 17 March 2011

Keywords:mercurypore watersediment pollutionvertical profilesVenice Lagoon

* Corresponding author. School of EnvironmentaGwangju Institute of Science and Technology, Gwang

E-mail address: shan@gist.ac.kr (S. Han).

0272-7714/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.ecss.2011.03.004

a b s t r a c t

Understanding the biogeochemical process of Hg is critical in the overall evaluation of the ecologicalimpacts resulting from the reuse of Hg-contaminated dredged sediment. Sediment banks (V1 and V2)were constructed with freshly dredged sediments from a navigational channel in Venice Lagoon, Italy,with the goal of clarifying potential differences in the biogeochemistry of Hg between the reuseddredged sediments and those from surrounding sites (SS1 and S2). Toward this purpose, Hg and mon-omethylmercury (MMHg) concentrations, and Hg methylation rates (MMRs) in the surface 2.5 cmsediments were monitored, along with ammonium, iron, sulfate and sulfide concentrations in the porewaters of banks and surrounding sites from November 2005 to February 2007. Pore water analysesindicate that the bank sediments are characterized by lower levels of sulfate and iron, and by higherlevels of ammonium and sulfide compared to the surrounding sediments. With respect to Hg speciation,the fractions of MMHg in total Hg (%MMHg/Hg) and the MMRs were significantly lower in the bank V1compared to those in the reference site SS1, whereas the %MMHg/Hg and the MMRs were similarbetween V2 and S2. A negative correlation is found between the logarithm of the particle-water partitioncoefficient of Hg and the MMR, indicating that the reduced MMRs in V1 are caused by the limitedconcentrations of dissolved Hg. Organic matter appears to play a key role in the control of MMR via thecontrol of Hg solubility.

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1. Introduction

Dredging and the disposal of dredged materials are importantissues concerning coastal area management. Surface sediment isdredged from estuaries and coastal areas to maintain navigationchannels and often to remove contaminated materials (Alden andYoung, 1982; Levinton et al., 2006). Major concerns have arisenover where to dispose of this dredged material and the ecologicalimpacts of such disposals (Levinton et al., 2006; Burchell et al.,2007). In recent years, dredged materials have been relocated forenvironmentally beneficial purposes, such as the rejuvenationof intertidal habitats (Burchell et al., 2007). However, this type ofrelocation has been practiced only on a small scale due to a lack ofunderstanding regarding the ecological impacts that follow thereuse of dredged sediment. Understanding the biogeochemical

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processes involving these contaminants is critical in the overallevaluation of the ecological impacts resulting from the reuse ofdredged sediment.

To understand the biogeochemical processes of contaminantsinvolved in the reuse of dredged channel sediment in the VeniceLagoon, Italy, the SIOSED (Scripps Institution of OceanographySEDiment research) program was conducted from March 2005 toNovember 2007 (Deheyn and Shaffer, 2007). Sediments weredredged from a navigation channel and transplanted onto twoshallow sites. At the relocated and surrounding (reference) sites,a multidisciplinary study was carried out, including studies on thegeochemistry of trace metals, microbial community, fauna and floracontent, and sedimentary ecotoxicology. This type of monitoringprogramwas essential because the sediment in the Venice Lagoon iscontaminated with various metals and organic pollutants, andconsequently,most of the sediment in the lagoon has been evaluatedas potentially hazardous (Frignani et al., 1997; MAV-CVN, 2004).

Mercury is one of the most serious pollutants in the VeniceLagoon (Bloom et al., 2004; Han et al., 2007a), posing a potential

Fig. 1. Locations of sediment banks (V1 bank in SS1 and V2 bank in S2) and thedredged channel site (SS0) in the Venice lagoon, Italy.

S. Han et al. / Estuarine, Coastal and Shelf Science 93 (2011) 7e138

threat to public health and marine ecosystems. Most Hg contami-nation in the Venice Lagoon originates from past occurrencesbetween the 1950s and 1980s, especially from the chlor-alkalidischarge located in the petrochemical zone of Porto Marghera(Bloom et al., 2004). Thus, Hg concentrations in the subsurfacesediment are often higher than those in the surface sediment(Bloom et al., 2004; Han et al., 2007a). Considering the massbalance calculations using various physical volumes and flows, ithas been demonstrated that Hg flux to Venice Lagoon water isdominated mainly by the resuspension of contaminated sediment(Bloom et al., 2004). The current pollution input from rivers,industry, and precipitation has been estimated to be less than 25%of the total Hg flux, highlighting the importance of the resus-pension flux (Bloom et al., 2004).

We speculated that the reuse of dredged sediment from thenavigation channel may increase the concentration of the moretoxic form of Hg, monomethylmercury (MMHg), compared to thosefound in the reference sediment by several reasons. First, thedredged sediment from the navigation channel may contain higherconcentrations of MMHg than in the reference sediment because ofcommon characteristics of channel sediments, such as enhancedorganic concentrations and microbial activities. Secondly, thesolubility of Hg in the dredge sediment would be higher than thatin the reference sediment, perhaps due to the oxidation of ironsulfide and consequent release of dissolved Hg (Hammerschmidtand Fitzgerald, 2004; Rothenberg et al., 2008) when Hg-contami-nated subsurface sediment is exposed to the surface and air duringthe dredging activity. This process would lead to higher Hg meth-ylation rates (MMRs), if the dissolved Hg is a limiting factor for thenet MMR, as evidenced in several estuarine and coastal sediments(Hammerschmidt and Fitzgerald, 2004; Wolfenden et al., 2005;Hammerschmidt et al., 2008). Finally, a reduction in the dissolvedsulfide concentration in pore waters of the dredge sediments, dueto the potential destruction of anoxic conditions during thedredging activities, may produce favorable conditions for Hg-methylating organisms to uptake inorganic Hg via increasingneutral Hg-sulfide species (Benoit et al., 1999; Drott et al., 2007;Han et al., 2007b, 2008). Based on these assumptions, experi-mental banks were built with freshly dredged sediments froma navigational channel in the Venice Lagoon, and Hg speciation andpore water geochemistry were monitored over a period of 18months. In the present study, we report the Hg and MMHgconcentrations, and Hg methylation rates in bank and referencesediments along with dissolved Hg, Fe, ammonium, sulfide, andsulfate concentrations in pore waters.

2. Material and methods

2.1. Study area

The study area consisted of three sites (Fig. 1): SS0, SS1, and S2.Freshly dredged sediment from approximately the top 1 m layer ofsite SS0, a previously dredged channel, was transplanted into SS1and S2 (1.4 m water depth) to create subtidal dredge sedimentbanks, bank V1 in site SS1 and bank V2 in site S2, between October25, 2005, and November 16, 2005. The sediment was dredged fromSS0 using an excavating crane equipped with a clamshell grab ona barge; the sediment (w220 m3) was re-excavated from the bargeand redeposited into the delimited areas that had been designatedfor making the banks. Initially, the relocated sediment was con-tained by wood pilings to protect against immediate erosion. Thewood pilings were removed in June 2006 after the bank sedimenthad compacted and stabilized. The heights of both banks (V1 andV2) were reduced from 1 m to 70 cm following the naturalcompaction and stabilization of the sediment, which was below sea

level even at low tide. Thewidth and length of the banks were 30mand 10 m, respectively (Deheyn and Shaffer, 2007). The surfacesediment in SS1 was more sandy and contained less organic carbonthan that in S2, and the dredged sediment from SS0 containedmoreorganic carbon than the reference sediments (Table 1).

2.2. Sample collection

Sampling of the sediment commenced shortly after the bankconstruction. Surface (2.5 cm) sediment was collected using shortpush cores in November 2005, and February, June, and July 2006,and February 2007. Long piston cores or vibracores (1.5 m or 30 cm)were collected in December 2005, and in May, September, andNovember 2006. The collected cores were extruded and sectioned(0e2.5, 2.5e5, 5e7.5, 7.5e10, 10e15, and 15e20-cm intervals)within 24 h in a N2-filled glove box in the laboratory of Thetis SpA(Venice, Italy), thus preventing the oxidation of sulfide and dis-solved ferrous iron.

After the extrusion and sectioning of cores in the glove box, porewaters were extracted by centrifuging under nitrogen conditions atapproximately 5000 rpm. After filtering the pore water samplesusing 0.45-mm pore size polyethersulfone syringe filters underanaerobic conditions, approximately 10e20 cm3 of the filtered porewater sample was acidified for the analysis of dissolved Hg and Fe,and the remainder was used for measurements of dissolved sulfate,sulfide, and ammonium. The remaining sediment slices werestored frozen for analyses of sedimentary Hg and MMHg. Sepa-rately, approximately 20 g of the sediment slices were sealed inamber glass vials under N2 saturated conditions and transported tothe SIO laboratory in a portable electric cooler (4 �C) for Hgmethylation experiments. Hg methylation experiments werecarried out within 1 week after sampling.

Table 1Total Hg, MMHg, fraction of MMHg in total Hg, MMR, pore water Hg (dHg), log Kd for Hg, and TOC determined in the top 2.5 cm sediment of banks (V1 and V2) and referencesites of banks (SS1 and S2). Sampling seasons are November and December, 2005, February, May, June, July, September and November 2006, and February 2007. Data aremean � SD of n samples.

Hg (ng g�1 dw) MMHg (ng g�1 dw) MMHg/Hg (%) MMR (% hr�1) dHg (pM) Log Kda of Hg TOC (%)

V1 748 � 246 0.37 � 0.19 0.052 � 0.026 0.080 � 0.037 19 � 7.3 5.3 � 0.19 1.1 � 0.15SS1 329 � 83 0.66 � 0.55 0.19 � 0.15 0.17 � 0.085 63 � 69 4.7 � 0.42 0.46 � 0.10V2 785 � 124 0.74 � 0.93 0.094 � 0.11 0.13 � 0.10 33 � 19 5.2 � 0.28 1.3 � 0.13S2 491 � 56 0.86 � 0.64 0.17 � 0.12 0.18 � 0.12 30 � 26 5.0 � 0.29 0.78 � 0.20n 10 10 10 7 9 9 3

a Kd ¼ Cs (mole/Kg)/Cw (mole/dm3), Cs ¼ concentration in particles, Cw ¼ concentration in water.

S. Han et al. / Estuarine, Coastal and Shelf Science 93 (2011) 7e13 9

2.3. Total mercury and monomethylmercury

For the analysis of total Hg in the sediments, approximately 1 gof sediment was digested overnight in a Teflon� bottle at roomtemperature with 8 cm3 of 12 N HCl and 2 cm3 of 14 N HNO3, fol-lowed by dilution with 500 cm3 of Milli-Q water on the followingday (Method 1631). Aliquots of sediment digests (0.1e0.3 cm3)were used to quantify the total Hg by aqueous phase reductionwithstannous chloride solution, trapping onto a gold-coated quartzcolumn, thermal desorption, and detection by cold vapor atomicfluorescence spectrometry (CVAFS; Method 1631). Acidified (0.06 NHCl) pore water samples were treated with ultraviolet (UV) irra-diation prior to aqueous phase reduction and detection by CVAFS(Gill and Bruland, 1990; Choe et al., 2004). Sediment water contentwas determined separately to transform the concentrations of Hg ing-wet sediment to g-dry sediment. The analytical precision of thetotal Hg measurements calculated from the recovery of certifiedreference material (PACS-2, National Research Council of Canada)averaged 98 � 13% (mean � SD, n ¼ 26) and that from the recoveryof the matrix spike (100% of the sample Hg added) averaged100 � 18% (mean � SD, n ¼ 13).

MMHg in sediments (0.5e1 g) was extracted into the organicphase following a reactionwith 5 cm3 of acidic KBr solution,1 cm3 of1 mol dm�3 CuSO4 solution, and 10 cm3 of CH2Cl2. An aliquot(2e5 cm3) of CH2Cl2 was back-extracted to the aqueous phase bypurging out CH2Cl2 with highepurity nitrogen gas as described byChoe et al. (2004). The extracts were analyzed for MMHg throughthe use of aqueous phase ethylation, trapping on a Tenax column,gas chromatography separation, thermal decomposition, anddetection by CVAFS (EPA Method 1630; Liang et al., 1994). Themethod detection limit, estimated as three times the standarddeviation of the method blank, was 0.01 ng g�1. The analyticalprecision calculated from the recovery of certified referencematerial(DORM-2, National Research Council of Canada) averaged 95 � 10%(mean � SD, n ¼ 30) and that from the recovery of matrix spike(100% of sample MMHg added) was 109 � 7% (mean � SD, n ¼ 8).

2.4. Mercury methylation rate

We employed the tracer method using stable Hg isotopes for themeasurement of net MMRs (Hammerschmidt and Fitzgerald,2004). A cold vapor generation system was interfaced to aninductively coupled plasma-mass spectrometer to allow greatersensitivity. In our method, a 200Hg working solution diluted from200Hg(NO3)2 with filtered overlying water was added to sedimentat concentrations of 2 ng 200Hg g�1 of wet sediment undera N2-saturated atmosphere. The sediments were incubated underanoxic conditions for 4 h at appropriate temperatures (fieldtemperature� 2 �C), after which the sediment samples were frozenat �80 �C (Hammerschmidt and Fitzgerald, 2004). The concentra-tions of MM200Hg in incubated sediment samples were detectedusing an inductively coupled plasma-mass spectrometer, after the

separation of MMHg from the sediment samples using an organicextraction method: MMHg in lyophilized sediments (w2 g) wasextracted into the organic phase following a reaction with 5 cm3 ofacidic KBr solution,1 cm3 of 1mol dm�3 CuSO4 solution, and 10 cm3

of CH2Cl2. After organic extraction, MMHg in CH2Cl2 was back-extracted into the aqueous phase using the purging methoddescribed by Choe et al. (2004). Because the current analyticalmethod for MMR does not further separate Hg species after theextraction, unlike the GC method previously described, so thecorrection is necessary. Freeze-dried sediments spiked just prior toextraction were used to determine the amount of inorganic 200HgextractedwithMM200Hg during the organic extraction process. Themethylation of added 200Hg was evaluated as the excess concen-tration of 200Hg versus 198Hg in the sample extracts, as described inEqs. (1) and (2) (Hintelmann et al., 1995):

MM200Hgtracer ¼�X

MM198Hg�X

MM200Hg� Rnatural

�.

½ðRtracer � RnaturalÞ � A200� ð1Þ

MMRð%=hÞ ¼ MM200Hgtracer � 100=200Hgadded� hours of incubation (2)

where MM200Hgtracer is the concentration of MMHg produced fromtracer (200Hg) injection, SMM198Hg is the total concentration ofMM198Hg (the sum of MM198Hg originally present and MM198Hgproduced from tracer injection), SMM200Hg is the total concentra-tion of MM200Hg (the sum of MM200Hg originally present andMM200Hg produced from tracer injection), and A200 is the abun-dance of 200Hg in the tracer solution. The Rtracer and Rnatural are theratio of 198Hg/200Hg in tracer solution and natural ratio of198Hg/200Hg, respectively. The detection limit of MM200Hg producedis a function of the ambient MMHg concentration (w0.5 ngMMHg g�1 dry sediment), the natural abundance of 200Hg, and theprecision of the measurement of 198Hg/200Hg. In this study, thedetection limit averaged 1.3 pg g�1 on a dry weight basis with 1.1%CV (coefficient of variation) in the measurement of 198Hg/200Hg.

2.5. Pore water and sediment geochemistry

Dissolved sulfate was measured using a nephelometric methodbased on the precipitation of barium sulfate by the addition ofexcess BaCl2 crystals to a sample aliquot (Gieskes et al., 1991).Dissolved sulfide was determined as soon as possible using themethod published by Strickland and Parsons (1972), or it waspreserved as zinc sulfide through the precipitation of sulfide byaddition of zinc acetate to 3e5 cm3 sub-samples. Such analysisinvolves the formation of a dye (Lauth’s violet) from p-phenyl-enediamine. The determination of ammonium was based on themethod used in Solorzano (1969), originally developed to detectvery low NH4

þ concentrations in seawater. This method is based on

S. Han et al. / Estuarine, Coastal and Shelf Science 93 (2011) 7e1310

the diazotization of phenol and the subsequent oxidation of thediazo compound by sodium hypochlorite to yield a blue color. Ironwas also measured using a colorimetric method that makes use ofan orange complex with orthophenanthroline after any ferric ironhas been reduced by the hydroxylamine-HCl and the solution hasbeen buffered by sodium citrate (Gieskes et al., 1991).

For the measurement of total organic carbon (TOC) in sediment,approximately 2 g of sediment was lyophilized overnight in acid-cleaned glass bottles. Approximately 0.2 g of dry sediment wasweighed into silver capsules placed in a microtiter plate, and theaccurate weight of each sample was recorded. A small amount ofdilute HCl solution was added repeatedly until the release of CO2was complete. Next, the sediment samples were dried at 60 �C, andcarefully crimped silver capsules were shipped to the UC DavisStable Isotope Facility (Davis, CA). At the facility, the carbon mass indried sediments was measured using an elemental analyzer (PDZEuropa ANCA-GSL).

0 5 10 15 20

)mc(

htped

0

5

10

15

20

Nov 05Dec 05Jul 06Nov 06

0 1 2 3 4 5

ammo

0 10 20 30 40 50

)mc(

htped

0

5

10

15

20

Nov 05Dec 05Jul 06Nov 06

0 10 20 30 40 50

V1 SS1

0 3 6 9

)mc(

htped

0

5

10

15

20

Nov 05 Dec 05Jul 06Nov 06

sulfid

iro

0 1 2 3

0 10 20 30

)mc(

htped

0

5

10

15

20

Nov 05 Dec 05Jul 06Nov 06

0 10 20 3sulfa

Fig. 2. Sediment pore water profiles of ammonium, iron, sulfide, and sulfate in the banks (V1and y-axis labels on V1 apply to all sites.

3. Results

3.1. Pore water geochemistry

The pore water geochemistry for ammonium, iron, sulfide, andsulfate in the upper 20 cm of the sediments is presented in Fig. 2.Sediments of Site SSO served as the source of sediment depositedon top the sediments of Sites SS1 and S2 with an average thicknessof 70 � 10 cm. Cores from Site SS0 indicated that the upper70e100 cm of the sediments at this site are, on average, charac-terized by high concentrations of ammonium and sulfide, as well aslow concentrations of sulfate (Fig. 3) as compared to those foundfrom SS1 and S2. These signals are distinct, because Site SSO islocated in a dredged channel just north of Site SS1.

The cores taken from the banks V1 and V2 are typically char-acterized by steep increases in ammonium between 0 cm and20 cm (Fig. 2). In both V1 and V2, the distributions of dissolved

nium (mM)0 5 10 15 20 0 1 2 3 4 5

0 1 2 3

V2 S2

0 10 20 30 40 50 0 10 20 30 40 50

0 3 6 9e (mM)

n ( M)

0 10 20 30 0 10 20 300te (mM)

µ

and V2) and reference sites (SS1 and S2). Site labels on ammonium apply to all profiles

0 2 4 6 80 10 20 300 5 10 15 20 25

)mc(

htped

0

30

60

90

120

sulfate (mM) sulfide (mM)ammonium (mM)

Fig. 3. Sediment pore water profiles of ammonium, sulfate, and sulfide in Site SS0collected in August 2005, before the dredging activity.

S. Han et al. / Estuarine, Coastal and Shelf Science 93 (2011) 7e13 11

ammonium, shortly after completion of the banks, showed highconcentrations at the surface (4.6 mM in V1 and 2.6 mM in V2 inNovember 2005). This is the result of the very recent deposition ofthe sediment mixture of SSO, which was not entirely homogenousin composition. Below 70 cm, ammonium levels returned towardthe expected low values (<0.2 mM) for the upper parts of Sites SS1and S2 (Supporting Information, S1), upon which the banks wereconstructed.

Sulfate concentration depth profiles generally reflected those ofammonium (Fig. 2). Data for dissolved sulfide in V1 and V2 indicaterelatively shallow maxima between 10 and 15 cm, part of which isinherited from the deposition of SSO sediments, but with a poten-tial production through in situ sulfate reduction processes. Sulfidesare very reactive components in the pore fluids with possibleremoval as metal sulfides, the main cause of the iron disappearancebelow 5 cm of V1 and V2 Cores.

3.2. Mercury speciation in surface sediment

The mean total Hg concentrations in the surface 2.5-cm sedi-ments collected from November 2005 to February 2007 are shownin Table 1. Considering the Hg concentrations found in the surfacesediment from the Adriatic side of the Lido Inlet (120 ng g�1; Bloomet al., 2004), the range of total Hg found in the bank and referencesites is indicative of moderate Hg contamination. The total Hgconcentrations in the bank and reference sites are in agreementwith literature data determined from the surface sediment of theVenice Lagoon (Bloom et al., 2004; Han et al., 2007a) and the rangeof total Hg found commonly in urbanized estuaries (e.g., Ches-apeake Bay, USA, Mason and Lawrence, 1999; Seine River estuary,France, Mikac et al., 1999). An enhanced total Hg concentrationwasnoted in the surface of the banks as compared to the surface of thesurrounding reference sites (t-test, p < 0.05), which agreed withthe large amount of Hg in source (SS0) sediments (450e990 ng g�1

at the depth of 0e100 cm).The MMHg concentration range in the surface 2.5 cm is in

agreement with previous literature values determined from thesurface sediment of the Venice Lagoon (Bloom et al., 2004; Hanet al., 2007a) and typical MMHg concentrations found in urban-ized estuarine sediments (e.g., Patuxent River estuary, USA, Benoitet al., 1998; San Francisco Bay, USA, Choe et al., 2004; Bay of Biscay,France, Stoichev et al., 2004). When comparing the bank andreference sediments, there were no statistically significant differ-ences between V1 and SS1, and between V2 and S2 (t-test, p > 0.05for each), which was notably different from the total Hg results.

The values of %MMHg/Hg in the bank and reference sedimentscorresponded to the lower range of those in other estuarine andcoastal sediments (0.1e0.75%, Fitzgerald et al., 2007). Whencomparing the bank and reference sediments, the %MMHg/Hg wassignificantly lower in V1 than SS1 (t-test, p < 0.05) and similarbetween V2 and S2 (t-test, p > 0.05). The close correlation between

%MMHg/Hg and MMR has been shown in surface lake and estua-rine sediment when MMRs were measured by short-term incuba-tion (4e48 h) of Hg isotopes, suggesting the dominance of themethylation process over demethylation or net transport processes(Hammerschmidt and Fitzgerald, 2006; Drott et al., 2008;Hammerschmidt et al., 2008).

The net MMR range in the upper 2.5 cm of sediment arecomparable to those values found in coastal sediments of LongIsland Sound, USA (1.4e6.3% day�1, Hammerschmidt andFitzgerald, 2004), and lower than those of freshwater reservoirsediments (2e17% day�1; Gray and Hines, 2009) and continentalshelf sediment (2e15% day�1; Hammerschmidt and Fitzgerald,2006). Data for the net MMR revealed a pattern similar to thatobserved for the fraction of MMHg in total Hg, lower in V1 than SS1but similar between V2 and S2.

4. Discussion

4.1. In situ ammonium production and sulfate reduction in the banksediment

From the data presented in Fig. 2, it appears that the nearsurface sulfate and ammonium anomalies did not persist for muchmore than a few months after the construction of the banks.Thereafter, a diffusive exchange process with the overlying watershad been well established already in a short time. However, thecontinued existence of the concentration maxima in ammoniumand minima in sulfate at about 25 cm (Supporting Information, S1)implies that the diffusive exchange with the overlying water is notthe only process affecting the concentration profiles.

Dissolved chloride concentrations (Supporting Information, S2)showed considerable variability in the near surface sediments inthe depth range of 0e20 cm, especially in the reference sites, SS1and S2 and to a lesser extent in the bank sites, V1 and V2. Partic-ularly in the near surface 10 cm, the chloride concentration changesare largest, presumably as a result of enhanced exchange resultingfrom tidal pumping and/or bioturbation processes. Though clearlythe gradients are not always of a diffusive nature, one canapproximate the exchange between overlying waters in terms ofa simple mixing model using a turbulent diffusion analogy.Whereas the sediment molecular diffusion coefficient for chlorideis probably about 1 �10�5 cm2 s�1 (Li and Gregory, 1974; Krom andBerner, 1980), we can estimate the magnitude of the “effective”mixing coefficient by a calculation of the diffusive path lengthZ ¼ (2Deff t)0.5 over a time period (t) of 1 month. With Deff of5 � 10�5, one calculates a depth (z) of 16 cm and with Deff of(10 � 2) � 10�5, one obtains a depth range (z) of 23 � 3 cm. In V1and V2, 16 cm is appropriate, whereas for SS1 a more appropriatedepth estimate >30 cm and for S2 roughly 25 cm. This greaterdepth at the reference sites is probably related to larger porositiesin these sediments than in V1 and V2, which represents the greaterconsolidation of the SSO sediments (and hence lowers porosities inV1 and V2).

Dissolved ammonium has a similar molecular diffusion coeffi-cient as dissolved chloride (Li and Gregory, 1974) and accordinglywe estimate the “effective” exchange coefficient (Deff) of NH4

þ to be5�10�5 cm2 s�1 in V1 and V2. If the average concentration gradientof ammonium is roughly 4 mM per 10 cm, it is possible to calculatean exit flux from the sediment by means of the formula: J ¼ eDeffdC/dz, which yields a flux of 15 mmol m�2 d�1. This flux is probablya maximum flux, but the magnitude of this flux is similar to thatreported by Scholten et al. (2000) in the vicinity of Porto Margherain the Venice Lagoon. With a standing stock of roughly1500 mmol m�2 in the upper 50 cm of the pore water column, thisflux would diminish the pore fluid’s ammonium concentrations in

TOC (%)0.0 0.5 1.0 1.5 2.0

goLKd

gHfo

3

4

5

6

7r2 = 0.42, p < 0.0001

Fig. 4. The particle-water partition coefficient (Kd) of Hg as a function of organiccarbon content at the depths of 0e2.5, 2.5e5, 5e7.5, 7.5e10, 10e15, and 15e20 cm ofall sampling sites collected in June 2005, August 2005, and May 2006. Top 2.5 cmsediments are colored black.

S. Han et al. / Estuarine, Coastal and Shelf Science 93 (2011) 7e1312

about three months. Clearly, this is not the case, and we suggestthat substantial production of ammonium resulting from organicmatter decomposition via sulfate reduction must continue in theupper 50e70 cm of bank Sites V1 and V2. Probably, as a result of thedeposition of the SSO sediments, containing higher levels oforganic carbon, continued consumption of organic carbon throughsulfate reduction must be important. Thus both sulfide andammonium production, combined with diffusion into the overlyingwaters and the underling reference sediments (SS1 and S2) hasreached a potential steady state.

4.2. Importance of mercury solubility in the production ofmonomethylmercury

The results shown in Table 1 suggest that the lower MMR foundin the bank V1 compared to the reference site SS1 is associated withthe solubility of Hg (log Kd). Indeed, a weak to moderate correlationhas been reported between the log Kd of Hg and the net MMR ofinorganic Hg in several estuarine and coastal sediments(Hammerschmidt and Fitzgerald, 2004, 2006; Hammerschmidtet al., 2008). The existence of a correlation between the log Kd ofHg and MMR suggests that the partitioning of total Hg between thedissolved and particulate phases exerts a significant control overMMHg production over the long term. A direct comparisonbetween the dissolved Hg and the bulk sediment MMHg furthersupports this idea: the lower levels of sediment MMHg in V1 thanin SS1 are related to the lower levels of dissolved Hg in V1 than inSS1 (Table 1).

Organic matter appears to play a key role in the control of Hgsolubility: a positive linear relationship was found between the TOCand log Kd, particularly for the surface sediment (Fig. 4). In additionto organic matter, solid FeS may play a part in the control of Hgsolubility. The diffusive flux calculations described in section 4.1predict an abundance of solid FeS, both inherited from SS0 andnewly produced in situ, in bank sediments, which would promotea decrease in Hg solubility. The apparent steady state of ammoniumconcentrations in V1 and V2 supports this suggestion: the diffusiveefflux of ammoniumwas well balanced by the in situ production ofammonium, which is mainly processed via microbial sulfatereduction. Sulfide produced by sulfate reduction would cause FeSprecipitation, with consequent removal of dissolved Fe from thebank sediment pore waters.

5. Summary

In summary, the MMR and the %MMHg/Hg in the top 2.5 cmlayers of sediments were lower in the dredged sediment bank V1when compared to the undisturbed reference site SS1. This was

mainly due to the higher content of organic matter in V1 than inSS1, which caused a decrease in the dissolved Hg concentration ofV1. The organic matter contents were more similar between V2 andS2, resulting in comparable levels of MMR and %MMHg/Hg. Thepresent study suggests that relocation of the dredged sedimentfrom the navigation channel causes decreases in MMHg concen-trations, as compared to MMHg levels found in the referencesediments, when the TOC levels are significantly higher in thedredged sediment than in the surrounding sediment.

Acknowledgments

This research could not have been conducted without thegenerous help of the research members at Thetis SpA (Venice,Italy): Andrea Berton, Matteo Conchetto, Emiliano Molin, ChiaraCastellani, and Fabrizio Perin. We give special thanks to themembers of the SIOSED program: Lisa Levin, Douglas Bartlett,Farooq Azam, Osmund Holm-Hansen, Hany Elwany, and TonyRathburn. This material was produced in the framework of theSIOSED project (to DDD), and supported byMagistrato alle Acque diVenezia, Italy (Venice Water Authority) through the ConsorzioVenezia Nuova and Thetis S.p.A. Any opinions, findings, andconclusions or recommendations expressed in this paper are thoseof the authors and do not necessarily reflect the views of theMagistrato alle Acque di Venezia (Venice Water Authority), Con-sorzio Venezia Nuova, or Thetis S.p.A.

Appendix. Supplementary material

Supplementary data associated with this article can be found inthe online version, at doi:10.1016/j.ecss.2011.03.004.

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