This article was downloaded by: [Northeastern University]On: 23 November 2014, At: 03:00Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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Mercury Volatilization from ThreeFloodplain Soils at the Central ElbeRiver, GermanyAnja During a b d , Jörg Rinklebe a d , Frank Böhme a e , RainerWennrich c , Hans-Joachim Stärk c , Sibylle Mothes c , Gijs Du Laing f

, Elke Schulz b & Heinz-Ulrich Neue aa Helmholtz Centre for Environmental Research—UFZ, Department ofSoil Chemistry , Halle/Saale, Germanyb Helmholtz Centre for Environmental Research—UFZ, Department ofSoil Ecology , Halle/Saale, Germanyc Helmholtz Centre for Environmental Research—UFZ, Department ofAnalytical Chemistry , Leipzig, Germanyd University of Wuppertal, Department D , Wuppertal, Germanye KataLeuna GmbH Catalysts , Business Support , Leuna, Germanyf Faculty of Bioscience Engineering , Ghent University , Gent,BelgiumPublished online: 22 Jun 2009.

To cite this article: Anja During , Jörg Rinklebe , Frank Böhme , Rainer Wennrich , Hans-JoachimStärk , Sibylle Mothes , Gijs Du Laing , Elke Schulz & Heinz-Ulrich Neue (2009) Mercury Volatilizationfrom Three Floodplain Soils at the Central Elbe River, Germany, Soil and Sediment Contamination: AnInternational Journal, 18:4, 429-444, DOI: 10.1080/15320380902962395

To link to this article: http://dx.doi.org/10.1080/15320380902962395

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Soil and Sediment Contamination, 18:429–444, 2009Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320380902962395

Mercury Volatilization from Three Floodplain Soilsat the Central Elbe River, Germany

ANJA DURING,1,2,4 JORG RINKLEBE,1,4 FRANK BOHME,1,5

RAINER WENNRICH,3 HANS-JOACHIM STARK,3

SIBYLLE MOTHES,3 GIJS DU LAING,6 ELKE SCHULZ,2

AND HEINZ-ULRICH NEUE1

1Helmholtz Centre for Environmental Research—UFZ, Department of SoilChemistry, Halle/Saale, Germany2Helmholtz Centre for Environmental Research—UFZ, Department of SoilEcology, Halle/Saale, Germany3Helmholtz Centre for Environmental Research—UFZ, Departmentof Analytical Chemistry, Leipzig, Germany4University of Wuppertal, Department D, Wuppertal, Germany5KataLeuna GmbH Catalysts, Business Support, Leuna, Germany6Faculty of Bioscience Engineering, Ghent University, Gent, Belgium

Wetlands at the riverside of the UNESCO Biosphere Reserve “Central Elbe” are highlycontaminated by heavy metals, especially mercury (Hg). The Hg-polluted Elbe flood-plain soils turn out to be a source of gaseous mercury via Hg volatilization from soil intothe atmosphere. A modified field sampling method was used to measure total gaseousmercury (TGM) volatilization from three different sites at the Elbe River. The modifiedsetup had a reduced chamber size and contained an internal gas circulation system.An in-ground stainless steel cylinder minimizes Hg volatilization from adjacent soilair. Cold vapor atomic absorption spectrometry (CV-AAS) was used to determine TGMamalgamated on gold traps. Sampled TGM amounts ranged between 0.02 and 0.63 ng(absolute), whereas the calculated Hg fluxes varied from 2.0 to 63.3 ng m−2 h−1. Themodified system should allow measurements of Hg volatilization at various sites with ahigh spatial resolution, which should enable the study of interrelations between TGMemission and several key factors influencing Hg emission from floodplain soils at theElbe River and other riverine ecosystems in the near future.

Keywords Hg fluxes, River Elbe, total gaseous mercury (TGM), wetland soils

Introduction

Mercury (Hg) is poisonous for organisms (Von Burg and Greenwood, 1991). It is oneof the most highly bioconcentrated trace metals in the human food chain (Lindberg andStratton, 1998). Consequently, environmental pollution by Hg remains a major concern

Address correspondence to Anja During, Helmholtz Centre for Environmental Research—UFZ,Department of Soil Chemistry, Theodor Lieser Str. 4, 06120, Halle/Saale, Germany. E-mail:anja.during@ufz.de or anja.during@uni-wuppertal.de

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430 A. During et al.

in many regions (Munthe et al., 2001). The environmental chemistry of Hg was reviewedextensively (e.g. Ebinghaus et al., 1999; Lin and Pehkonen, 1999; Zhang et al., 2003). Animportant aspect is the reduction of Hg(II) to Hg(I) or Hg(0), which is promoted by very lowredox potentials occurring in permanently or frequently flooded soils and sediments. Underthese conditions Hg may also exist in organometallic forms, of which monomethylmercury(CH3Hg+) and dimethylmercury [(CH3)2Hg] are the most important. Hg(0) and (CH3)2Hgare volatile species. Volatilization from soil to atmosphere is considered to be an importantpathway (Du Laing et al., 2009), and worldwide atmospheric transport has been reported(Poissant and Casimir, 1998). A diversity of natural and anthropogenic sources (e.g. Bashand Miller, 2007; Pacyna et al., 2005; Yan et al., 2003) contributes to the release of Hg tothe atmosphere. According to Pacyna et al. (2006), the estimated Hg emission rate fromanthropogenic sources in the year 2000 was comparable to estimates of Hg emissions fromnatural sources.

During the last century, large amounts of heavy metals from anthropogenic and ge-ogenic origin were accumulated in the floodplains of the Elbe river system (Kowalik et al.,2003; Rinklebe, 2004; Devai et al., 2005; Overesch et al., 2007; Rinklebe et al., 2007).Until 1996, 1500 ± 500 tonnes of Hg were deposited in parts of the Elbe floodplains withina period of only 60 years (Wallschlager, 1996). Consequently, floodplains at the Elbe par-tially reveal highly elevated Hg contents that frequently exceed the threshold values ofthe German Soil Conservation Law (BBodSchG, 1998; BBodSchV, 1999). Although Hgemissions from European soils were reported extensively (e.g. Xiao et al., 1991a; Xiaoet al., 1991b; Lindberg et al., 1995; Wallschlager, 1996; Wallschlager et al., 2000), fieldmeasurements and reliable estimates of Hg fluxes from these highly polluted floodplainsoils at the Elbe River and its tributaries are scarce. However, Wallschlager et al. (2002)found average Hg fluxes of 43 ± 5 ng m−2 h−1 at a selected contaminated wetland site nearthe Lower Elbe River (Northern Germany).

During the past 20 years soil-air exchange rates of Hg were determined at different sitesin various studies to assess related risks (e.g. Schroeder et al., 1989; Schroeder and Munthe,1998; Schluter, 2000; Lindberg et al., 2002; Wallschlager et al., 2002; Poissant et al., 2004;Ravichandran, 2004; Bohme et al., 2005). Mercury fluxes between soil and atmosphere weremostly detected by dynamic flux chambers (Wallschlager, 1996; Wallschlager and Wilken,1996) and some employed micrometeorological methods (Lindberg et al., 1995; Kimet al., 1995). These established systems are complex, stationary, and expensive, hinderingmeasurements of Hg fluxes at several field sites. Our objectives were: i) to quantify Hgemissions from three selected floodplain soils at the Central Elbe River; and ii) to improvethe sampling method to determine Hg volatilization from soils developed by Bohme et al.(2005).

1. Material and Methods

1.1. Study Sites

The study sites are located in floodplains at the Central Elbe River in Germany (Figure 1).The site Worlitz is situated at stream kilometer 242 (51◦51′27′′N, 12◦23′06′′E), the siteBreitenhagen at stream kilometer 290 (51◦57′15′′N, 11◦54′57′′E), and the site Sandauat stream kilometer 417 (52◦48′00′′N, 12◦02′15′′E). The long-term annual precipitationranges from 470 to 570 mm and the mean annual air temperature is approximately 8.0◦C(Rinklebe, 2004). The sites were selected after large-scale conventional soil mapping and

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Mercury Volatilization from Three Floodplain Soils 431

Figure 1. Location of the study areas: Worlitz, Breitenhagen, and Sandau at the Elbe River, Germany.

nine years of comprehensive field pedological research in floodplains of the Elbe River(e.g. Langer and Rinklebe, 2009; Rinklebe, 2004; Rinklebe and Langer, 2006; Rinklebeand Langer, 2008; Rinklebe et al., 1999; Rinklebe et al., 2005a; Rinklebe et al., 2007;Walder et al., 2008). Soils of the low-lying terraces revealed largest concentrations ofpollutants due to the high contents of soil organic matter and fine mineral particles retainingthe pollutants as a result of sedimentation through frequent or extended flooding periodswith low flow-rates (Rinklebe et al., 2007). The soils are classified as “Tschernitzen”(originally Russian: Tschernosem = black earth; Tschernitza = black-earth-like floodplainsoil; compare Altermann et al., 2005; Rinklebe et al., 2007), or as Mollic Fluvisolsaccording to FAO/ISRIC/ISSS (1998). Such soil type is common on lower-lying terracesat the Elbe River (Rinklebe, 2004; Rinklebe et al., 2007). Former studies revealed highmetal contaminations, especially of Hg, in these soils (Devai et al., 2005; Overesch et al.,2007; Rinklebe, 2004; Swaton et al., 2003) (Table 1).

The dominant plant species at the sites Worlitz and Sandau are Urtica dioica L.and Calystegia sepium (L.) R. BR. (grassland) and those at the site Breitenhagen areQuercus robur L., Salix alba L., Fraxinus pennsylvanica MARSHALL, and Urtica dioicaL. (woodland). The sites are periodically flooded by the Elbe River with amplitudes of upto 8 m mainly depending on snowmelt during winter and spring and heavy rainfalls during

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Tabl

e1

Prop

ertie

sof

the

stud

ied

soils

[Mol

licFl

uvis

ols

(FL

m)]

(eac

hva

lue

isca

lcul

ated

with

dupl

icat

esa

mpl

esex

cept

bulk

dens

ityw

here

10sa

mpl

espe

rho

rizo

nw

ere

take

n)

Sand

Silt

Cla

yH

gD

epth

pH[2

000–

63µ

m]

[63–

2µ

m]

[<2

µm

]C

org

Nto

tB

ulk

dens

ity[a

qua

regi

a]Si

te[c

m]

[CaC

l 2]

[%]

[%]

[%]

[%]

[%]

Cor

g/N

tot

[gcm

−3]

[mg

kg−1

]

Wor

litz

0–9

5.8

4439

175.

010.

3514

.30.

803.

49–

326.

237

4617

4.77

0.34

14.0

0.98

4.0

32–8

55.

512

5929

2.37

0.19

12.5

1.03

3.3

Bre

itenh

agen

0–10

5.7

1550

356.

260.

4813

.00.

7812

.910

–40

5.7

1650

345.

260.

3515

.00.

8715

.740

–50

6.5

2345

323.

830.

2813

.70.

908.

0Sa

ndau

0–18

6.7

3143

268.

080.

5913

.70.

819.

518

–49

6.4

3840

228.

690.

5017

.40.

8410

.049

–65

6.8

4336

2111

.88

0.33

36.0

0.88

10.8

432

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Mercury Volatilization from Three Floodplain Soils 433

spring and summer (Rinklebe, 2004). The site Breitenhagen is additionally influenced byits tributary Saale River.

1.2. Sampling, Pre-treatment, and Analysis of Bulk Soil

Soil samples were collected in soil profiles according to genetic horizons. Sampling wasperformed in four replicates of about 1 kg, which were pooled to one sample per horizon(Rinklebe, 2004). Soil material was air dried and sieved to <2 mm. Subsamples wereground in an agate disc mill. Soil properties were determined according to standard methods(Schlichting et al., 1995) as follows: Total C (Ct) and total N (Nt) were measured using aC/N/S-Analyzer (Vario EL Heraeus, Fa. Analytik Jena). Inorganic C was quantified aftertreatment with phosphoric acid (15%) and IR-Detection of the evolved CO2 with a C-MAT550 (Fa. Stroehlein). Soil organic C (Corg) was calculated as the difference between Ct andinorganic C. Particle-size distribution was determined by wet sieving and sedimentationusing the pipette sampling technique. Soil pH was measured in 2.5:1 0.01 M CaCl2: soilsuspension. Total Hg concentrations of soil samples were quantified after digestion usingaqua regia (37% HCl: 65% HNO3, 3:1) ignoring that certain parts may remain in theresiduum. Mercury in soil extracts was measured by cold vapor AAS (CV-AAS). Blanks,triplicate measurements of Hg in extracts, and analysis of diluted mercury standards (Merck)were routinely included for quality control. The given soil chemical and physical resultsrepresent arithmetic means of duplicate samples. Maximum allowable relative standarddeviation between replicates was set to 10%. All concentrations were calculated on dryweight basis (105◦C, 24 h). Bulk densities represent arithmetic means of ten replicates perhorizon using 250 cm3 soil cores. Mercury stocks in different soil layers were calculatedfrom Hg concentrations and bulk densities of each soil horizon.

1.3. Sampling Conditions

Field measurements of Hg fluxes were conducted between the 22nd and 25th of Augustin 2005. Based on the temperature pattern, highest Hg volatilization rates were expectedaround noon, although incident solar radiation on the soil surface, which especially occursaround noon, might have a significant impact on the results. Gabriel et al. (2006) indeedreported greater diurnal variations of TGM fluxes compared to seasonal flux variations whenstudying different urban soil surfaces. Accordingly, flux measurements were performedbetween 9:00 and 10:00 am. Soil temperature at 10 cm depth was directly measured witha cut-in thermometer. Water content of soil samples at 10 cm soil depth was quantified inthe laboratory using an electronic moisture analyzer (MA 30, Sartorius). The study sitesBreitenhagen and Worlitz were additionally equipped with soil-hydrological monitoringstations, which allowed monitoring of soil water content, water tension, and soil temperatureat 20 cm depth every two hours. Moreover, water level, precipitation, and air temperature(at 2 m above the soil surface) were monitored at these sites. These data are presented inTable 2.

1.4. Total Gaseous Mercury (TGM) Sampling System

When measuring total TGM in ambient air, elemental gaseous Hg (Hg◦), dimethylatedgaseous Hg, and particulate phase Hg can be included (Landis et al., 2005). Lindberg et al.(2002) and Wallschlager et al. (2002) reviewed the numerous well-established techniquesto collect and analyze TGM. Munthe et al. (2001) compared methods for sampling and

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Tabl

e2

Sam

plin

gco

nditi

ons

(mea

n±

stan

dard

devi

atio

n)at

the

stud

ysi

tes

(n.d

.:no

tdet

erm

ined

)(v

alue

sof

wat

erco

nten

t,so

ilte

mpe

ratu

reat

10cm

dept

han

dai

rte

mpe

ratu

rean

dpr

essu

reat

10–2

0cm

wer

eca

lcul

ated

each

with

3sa

mpl

es,o

ther

valu

esw

ith6

sam

ples

)

Soil

para

met

erA

irpa

ram

eter

(ove

rsu

rfac

e)

Wat

erco

nten

t[V

ol.%

]Te

mpe

ratu

re[◦ C

]Te

mpe

ratu

re[◦ C

]Pr

essu

re[h

Pa]

Site

Dat

e10

cmde

pth

20cm

dept

h10

cmde

pth

20cm

dept

h10

–20

cm2

m10

–20

cm

Wor

litz

22.0

8.05

25.1

8±

1.30

49.4

8±

1.18

16.8

0±

0.00

15.8

7±

0.23

20.3

3±

0.32

16.4

3±

0.46

1005

.50

±4.

95B

reite

nhag

en25

.08.

0535

.87

±1.

7038

.51

±2.

7716

.40

±.0.

0016

.28

±0.

8317

.76

±0.

1217

.18

±0.

1599

7.50

±2.

12Sa

ndau

23.0

8.05

22.3

8±

3.01

n.d.

16.9

5±

0.07

n.d.

18.8

0±

0.26

n.d.

1003

.50

±2.

12

434

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Mercury Volatilization from Three Floodplain Soils 435

analysis of different atmospheric Hg species. In most cases, dynamic flux chamber systemshave been employed. With the aim of measuring actual fluxes of Hg from a defined soilarea with high precision we optimized the system of Bohme et al. (2005), which allowsrapid estimates of potential Hg emissions of a site. The optimized sampling setup usesan air circulation system, resulting in a continuous gas movement over the soil surface.Thus, a reduced air pressure can be avoided, which is definitely a major advantage ofthe adapted method. In addition, the size of the glass socket has decreased, which facili-tates the handling during sampling. Due to this reduced size, the equipment also enablessampling at higher spatial resolution. This allows an improved coverage of site condi-tions and soil structure, as well as a better assessment of the spatial heterogeneity ofHg emissions.

The developed TGM measuring system (Figure 2) is composed of a glass chamber (1)with a TeflonTM frame (2) and four inputs (3) for air circulation silicone tubes (4), a piece toconnect the chamber to the gold traps (5), two gold traps to trap emitted Hg (6), and a batteryoperated pump (7) with an integrated gas flow meter (8). Three of these glass chamberswith a volume of 0.65 L each were randomly distributed on a plane soil surface of 1 m2 afterremoving above ground parts of vegetation (Figure 3). The TeflonTM frame was installedgas-proof at the bottom of the glass chamber and fixed to an in-ground stainless-steel tubecylinder. The steel cylinder with 10.0 cm height and a mean diameter of 11.35 cm definesthe TGM measurement area to 0.01 m2. Two gold traps arranged in series to prevent Hg-losses were connected to the chamber and at the same time with the battery-powered pump.Silicone tubes with an inner diameter of 4 mm establish the connections between both goldtraps, the pump and the glass chamber. The achieved continuous gas circulation within thesystem and the in-ground steel cylinder avoid a vacuum and minimize Hg volatilizationfrom adjacent soil air. A comparison between different parameters of the method developed

Figure 2. Sampling setup to determine Hg volatilization from soils (modified from Bohme et al.2005): glass chamber (1), with TeflonTM frame (2), four inputs (3), silicone tubes (4), connectingpiece (5), two gold traps (6), battery operated pump (7), with integrated gas flow meter (8).

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436 A. During et al.

Figure 3. Left side: Experimental installation of the system developed by Bohme et al. (2005). Rightside up: Glass chamber of the modified TGM sampling system with integrated air circulation in thefield. Right side down: Measurement using the modified TGM sampling system including replicationsand battery operated pumps in the field.

by Bohme et al. (2005) and our modified sampling setup is given in Table 3 and Figure 3.Gas sampling was performed for one hour with at flow rate of 2 L min−1. To determinethe background TGM concentration of the surrounding air, a blank sample was taken byopening a gold trap tube directly over the soil surface for some minutes. Total gaseous

Table 3Comparison of the method developed by Bohme et al. (2005) with the modified TGM

sampling system

ParameterMethod developed byBohme et al. (2005)

Modified TGM samplingsystem

Covered soil surface 0.145 m2 0.01 m2

Chamber volume 65 L 0.65 LFlow rate 6 L min−1 2 L min−1

Measurement duration(pumped gas volume)

1 hour (360 L) 2 hours (240 L)

Specific features • small teflon frame betweenchamber and soil surface

• wide Teflon frame toconnect chamber within-ground steel cylindergas-tight

• no stainless-steel cylinder • in-ground stainless-steeltube cylinder of 10 cmheight

• no replications • 3 replications at 1 m2

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Mercury Volatilization from Three Floodplain Soils 437

mercury trapped in the gold traps was determined by CV-AAS (Perkin-Elmer/type FIMScoupled with a prototype for amalgam technique from Analytik Jena AG) in the laboratory.

1.5. Calculation of Mercury Fluxes

The mercury fluxes were calculated according to the equation given below:

F = Cabs

A∗ Q,

where F is the total flux of Hg in ng m−2 h−1, Cabs is the mean Hg concentration in ng m−3

calculated from the measured mass of Hg (ng, absolute) (blank corrected) multiplied with1000/v (v = volume in L h−1), A is the covered surface area of the chamber in m2, and Qis the flushing flow rate of the air which is flushed through the chamber in m3 h−1.

The average hourly Hg fluxes of each sampling site are given as arithmetic means,medians, and standard deviations of the three replicates per sampling site (Table 4).

2. Results

2.1. Correlation Between TGM and Pumped Gas Volume

In preliminary experiments we studied different measurement durations at constant gas flowrate, coinciding with different total pumped gas volumes. Consecutive measurements wereconducted in triplicate on an area of 1 m2 when flushing with air at a flow rate of 2 L min−2.The measurement duration was 1 h, 2 h, and 4 h and therefore the pumped volume totalledabout 120 L, 240 L, and 480 L. Correlations between TGM amounts and pumped gas volumeare presented in Figure 4. TGM amounts amalgamated on the gold traps were doubled withduplication of pumped volume and redoubled with quadruplication of the gas volume.

Figure 4. Correlation between the mean values of TGM amounts and pumped gas volumes (flowrate: 2 L min−1) at the sites of Worlitz, Breitenhagen, and Sandau in August 2005 (r2: correlationcoefficient).

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Tabl

e4

Tota

lga

seou

sm

ercu

ry(T

GM

)co

llect

edpu

mpe

dga

svo

lum

e,ca

lcul

ated

Hg

fluxe

spe

rm

2 ,and

Hg

stoc

ksin

the

stud

ied

soils

per

m2

dow

nto

50cm

dept

h(T

GM

and

pum

ped

gas

volu

me

calc

ulat

edea

chw

ithth

ree

sam

ples

)

Hg

flux

[ng

m−2

h−1]

Hg

stoc

ksin

the

soils

[mg

m−2

]T

GM

Pum

ped

gas

Site

[ng

abs.

]vo

lum

e[L

]M

ean

Med

ian

SDn

0to

−10

cm0

to−3

0cm

0to

−50

cm

Wor

litz

0.09

±0.

0712

2.2

±4.

08.

5711

.86

5.71

328

410

6817

58B

reite

nhag

en0.

31±

0.33

119.

4±

1.6

30.9

727

.67

30.7

73

320

1399

2388

Sand

au0.

51±

0.03

120.

8±

4.2

50.0

851

.40

2.28

377

023

9340

84

438

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Mercury Volatilization from Three Floodplain Soils 439

2.2. Soil Properties and Sampling Conditions

Soil properties and concentrations of Hg extracted with aqua regia are listed in Table 1.Soil organic carbon (Corg) and Hg concentrations were relatively high down to considerabledepth in the Mollic Fluvisols. Soil texture is dominated by silt. Soil acidity ranged from pH5.5 (Worlitz) to pH 6.8 (Sandau). Mercury concentrations in the three soil profiles exceededthe precautionary value of the German Federal Soil Conservation and Contaminated SitesOrdinance (BBodSchV, 1999) and even its action values for grassland (BBodSchV, 1999),which are set at 2 mg kg−1 (0–10 cm) and 3 mg kg−1 (10–30 cm). Mercury concentrationsin bulk soils were highest in Breitenhagen and lowest in Worlitz.

Soil temperature was about 17◦C and soil water content ranged between 22.4 and35.9 Vol.% at 10 cm depth during the measurement period (Table 2). At the woodland siteBreitenhagen soil water content was higher at 10 cm depth in comparison to the other siteswhile air temperature over soil surface was lower (Table 2).

2.3. Volatilized Hg Amounts and Calculated Hg Fluxes

Different Hg emissions were detected at the three study sites (Table 4). Mean TGM amountswere highest in Sandau and lowest in Worlitz. The Hg emissions collected on gold trapsrange from 0.02 to 0.63 ng absolute (individual data not shown in Table 4). In Breitenhagenone replicate always showed higher TGM than the other two replicates.

The calculated mean Hg fluxes ranged between 8.6 and 50.1 ng m−2 h−1. Spatialheterogeneity was high at the site Breitenhagen (Table 4).

2.4. Mercury Stocks in Soils

Table 4 presents Hg stocks within different soil layers down to 50 cm depth at the studysites. The study site Sandau reveals the highest Hg stocks at different soil depths. LowestHg concentrations and stocks in bulk soil were found at the site Worlitz. They increasedupstream in the order Worlitz < Breitenhagen < Sandau. No significant correlations havebeen found between any Hg stocks (neither in 0 to −10 cm, 0 to −30 cm, 0 to −50 cm nor0 to −100 cm soil depths) and Hg fluxes.

3. Discussion

The sampling system developed by Bohme et al. (2005) was modified and optimizedin function and size to be able to measure TGM emission at higher spatial resolution.During preliminary experiments with the modified system, we found significant correlationsbetween TGM amounts and pumped gas volumes (Figure 4) (r2 ≥ 0.8). These indicate aquite homogenous Hg concentration in the soil air during sampling times up to 4 hours,which also demonstrates that the increase of sampling time in comparison to the initialsystem of Bohme et al. (2005) is not expected to affect the measured mercury emission.

The floodplain soils at the Elbe River belong to a type of Hg-contaminated soils thathave accumulated large Hg amounts from Hg-polluted water bodies due to depositionof polluted suspended particles and organic matter during periodical flooding (Ebinghauset al., 1999; Rinklebe, 2004). Mercury is still enriched in sediments and suspended particlesof the Elbe River and its catchment area, which is reflected in elevated Hg concentrationsin bulk soils. Highest values of Hg concentrations in bulk soils were found at the siteBreitenhagen (Table 1), where the River Saale, which is polluted by large Hg amounts

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from former abandoned industrial areas, joins the Elbe River. However, the soil in Sandaurevealed the highest Hg stocks at the different depths (Table 4).

The measured TGM amounts and calculated Hg fluxes (Table 4) confirmed that Hg isbeing emitted from soil to atmosphere in considerable amounts at all sites monitored. Thiscould be related to the high Hg concentrations and stocks in the bulk soils, although nosignificant correlations have been found between Hg stocks and Hg fluxes. Mercury fluxesat the three study sites exceeded the mean global Hg fluxes if natural emissions, whichvary between 0.7 and 1.1 ng m−2 h−1 (Lindqvist et al., 1991; Mason et al., 1994; Schroederand Munthe, 1998). Simulated daily averaged natural Hg emissions of the eastern part ofNorth America, as determined by a new model with data gathered from May to September2000, varied between 1.8 and 3.7 ng m−2 h−1 (Gbor et al., 2006). Ericksen et al. (2006)reported mean background Hg emissions from soils of the United States ranging from −0.1to 2.7 ng m−2 h−1. These values of non-enriched sites across the United States are clearlybelow the Hg emissions measured at the Hg-contaminated floodplains in our study. Thefluxes we measured are similar to those reported by Wallschlager and Wilken (1996), whofound Hg fluxes between 20 and 500 ng m−2 h−1. However, the Hg emission fluxes wemeasured are obviously lower than the fluxes of up to 100 ng m3 in the air directly abovethe surface of northern Elbe floodplain soils. This corresponds to a 30-fold increase of thelocal background concentration. In turn, the Hg emission fluxes we measured are obviouslylower than the fluxes of up to 11544 ng m−2 h−1, which have been reported for miningareas in China (Wang et al., 2005). Long-term seasonal observations done by Gabriel andWilliamson (2008) have shown that the main part of TGM emitted from bare soil in anurban sampling area (Tuscaloosa, Alabama, USA). Nevertheless these TGM fluxes (5.69 ±5.79 ng m−2 h−1) fall clearly below the TGM values of our study. This might be attributedto the lower total Hg concentrations in the bulk soil (cp. Table 1 and 0.025–0.047 mg kg−1

by Gabriel and Williamson, 2008) and the influence of the water table in the floodplainsoils of this study.

The mercury fluxes in our study were highly variable. This was clearly observedwithin the site Breitenhagen (Table 2). Magarelli and Fostier (2005) also found such highvariations when measuring Hg fluxes simultaneously at small distance from each other usingtwo dynamic flux chambers. High soil heterogeneity, e.g. variations in pore size distributionwithin the upper soil layer that can occur even on a small scale and spatial variation ofenvironmental conditions in floodplains, probably have caused these variations. Mercuryemission is indeed considered to be affected by soil characteristics, such as e.g. substratetype, surface type, Hg concentration, and speciation (Zhang et al., 2003). The soils westudied reveal high and variable contents of soil organic matter and Cd (Rinklebe, 2004;Rinklebe and Langer, 2006; Rinklebe and Langer, 2008; Rinklebe et al., 2005b). Thiscertainly could have affected the Hg emission, as Wallschlager et al. (1998) deduced thathumic substances and the presence of Cd2+ enhance the abiotic reduction of Hg2+ to Hg◦.Environmental conditions, such as solar radiation, temperature, precipitation, wind, andatmospheric chemistry, have also been reported to affect Hg emission (Zhang et al., 2003).Carpi and Lindberg (1998) and Wang et al. (2006) reported higher Hg fluxes from openfield soils as compared to shaded forested soils. Shadow on the soil surface caused byvegetation decreases the temperatures at the soil surface and thus the Hg volatilizationrate. Consequently, solar radiation was classified as a crucial factor influencing the Hg flux(Wang et al., 2006). However, the cover of vegetation does not seem to influence the Hgemissions in our study to an important extent: Breitenhagen (woodland) revealed higher Hgfluxes than Worlitz and partially Sandau (both grasslands). However, ground vegetation wasremoved before measuring carefully. Next to the effects of solar radiation and temperature

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being previously described (e.g. Carpi and Lindberg, 1998; Feng et al., 2003; Frescholtz andGustin, 2004; Magarelli and Fostier, 2005; Wang et al., 2005), lower Hg emissions at night(e.g. Poissant et al., 2004; Gbor et al., 2006) and higher Hg emissions in summer (Gabrielet al., 2006) have been reported. However, in our current study, sampling was performedat each site at the same time and at similar temperatures. Consequently, variations in solarradiation and temperature did probably not contribute to the variation of Hg emission.Diurnal and seasonal variations of TGM volatilization rates in floodplains of the Elbe Riverwill be studied in the near future.

4. Conclusions

High Hg fluxes were quantified from three Hg-contaminated floodplain soils (Mollic Flu-visols) at different sites at the Elbe River (Germany), using a mobile and easy-to-handlesampling system. The used sampling technique was optimized from a previous setup by in-ducing an air circulation within the system that creates a continuous gas movement over thesoil surface. The modified sampling system allows measurements of Hg volatilization withhigh spatial and temporal resolution. The latter should be enabled to study interrelationsbetween TGM emission and several key factors influencing Hg emission from floodplainsoils at the Elbe River and other riverine ecosystems in the near future.

Acknowledgements

This study is supported by a research grant of the European Fund for Regional Development(EFRE), of the Ministry of Agriculture and Environment, and advice of the Departmentof Environmental Protection (LAU) of the Federal German state Saxony-Anhalt (FKZ:76213/01/05). We would like to thank Analytik Jena AG for supplying the device used toestablish the connection of the gold traps with CV-AAS. We are also grateful to Mr. H.Dittrich for his technical assistance and Mr. J. Steffen for analyzing total gaseous mercury.

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