Faculteit Bio-ingenieurswetenschappen
Academiejaar 2012 – 2013
A sediment record of lead contamination in the Zenne River, Belgium
Daan Renders Promotor: Prof. dr. ir. Filip Tack Copromotor: dr. Olivier Evrard
Masterproef voorgedragen tot het behalen van de graad van Master na Master in de Milieusanering en het Milieubeheer !
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Acknowledgements
This work is an important step in my studies. I would never have been able to write this
dissertation without the help and support of several people.
Firstly, I want to thank my promoters Prof. dr. ir Filip Tack and dr. Olivier Evrard to give me
the opportunity to continue my research about sediment contamination in the Zenne River and
for their help, suggestions, support and their continuous interest in my work.
Secondly, I want to thank dr. Sophie Ayrault for the additional isotopic analyses and the
suggestions she gave. I also want to thank my cousin Micky for reading some drafts of this
work and for giving me some suggestions about the use of statistics.
Finally, I want to thank my friends, parents and my sister Riet for their continuous support,
motivation and help.
Aalst, June 2013.
Daan Renders
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Table of contents
Acknowledgements ................................................................................................................... iii!
Table of contents ....................................................................................................................... iv!
Abstract ..................................................................................................................................... vi!
Samenvatting ............................................................................................................................ vii!
List of figures .......................................................................................................................... viii!
List of tables .............................................................................................................................. ix!
List of abbreviations ................................................................................................................... x!
1! Introduction .......................................................................................................................... 1!
2! Literature review .................................................................................................................. 3!
2.1! The Zenne river and its basin ........................................................................................ 3!
2.2! Evolution of Pb contamination in the Zenne ................................................................. 5!
2.2.1! Immediate past of Pb contamination in the Zenne River ....................................... 5!
2.2.2! Long-term evolution of Pb contamination in the Zenne River .............................. 8!
2.3! Pb usage in Belgium .................................................................................................... 12!
3! Problem statement .............................................................................................................. 14!
4! Materials and Methods ....................................................................................................... 16!
4.1! Preliminary work ......................................................................................................... 16!
4.2! Determination of the total Pb-concentration ............................................................... 19!
4.3! Determination of the isotopic composition of Pb ........................................................ 20!
5! Results ................................................................................................................................ 21!
6! Discussion .......................................................................................................................... 23!
6.1! Pb contamination in the DRO1 and EPP1 sediment cores .......................................... 23!
6.2! A local background concentration for Pb? .................................................................. 27!
6.3! Lead contamination sources ........................................................................................ 29!
6.3.1! Anthropogenic Pb sources in the Zenne basin ..................................................... 29!
6.3.2! Determination of the fraction of contributing Pb sources .................................... 36!
7! Conclusions ........................................................................................................................ 39!
7.1! General conclusions .................................................................................................... 39!
7.2! Scope for future research ............................................................................................. 40!
8! References .......................................................................................................................... 41!
Attachment 1 ............................................................................................................................ 49!
v
Attachment 2 ............................................................................................................................ 50!
Attachment 3 ............................................................................................................................ 51!
Profiles analysed by Verstraelen (1998) .............................................................................. 51!
Profiles analysed by Callebaut (2001) .................................................................................. 53!
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vi
Abstract Two sediment cores collected along the Zenne river in Belgium were studied in order to
assess the sources and history of lead (Pb) contamination. The Zenne basin (1168km2) is the
densest inhabited sub-catchment (1200 inhabitants/km2) within the Scheldt basin and has a
long history of industrialisation. The Zenne flows through the capital region of Brussels, the
largest urban area of Belgium. Both the industry and large population created a large pressure
on the river.
A sediment core collected in Eppegem, downstream of Brussels, was analysed for Pb
concentrations with ICP-QMS. The core was dated from 1974 to 2011. Lead concentrations
up to 775 mg/kg in this core were significantly above the mean Pb concentration of the upper
crust (20 mg/kg) but decreased significantly in more recently deposited sediment layers. This
decrease is probably related to the de-industrialisation of Brussels since the 1970s and the
stricter environmental regulations. A second, non-dateable, sediment core collected in
Drogenbos, just upstream of Brussels, was also analysed. The average Pb concentration in this
core was at 90 mg/kg (!2 = 14,9) significantly lower than the concentrations measured in
Eppegem, but still above the Pb concentration of the upper crust. The difference in Pb
concentration between Drogenbos and Eppegem outlines the large contribution of Brussels to
Pb contamination of the Zenne.
The isotopic composition of Pb in the two sediment cores was also analysed. The average
isotopic signature of Pb in the core of Drogenbos was significantly different from the isotopic
signatures measured in the sediment core of Eppegem. It appeared that the contribution of
leaded gasoline to Pb contamination in Eppegem was higher than in Drogenbos. In the
Eppegem sediment core a significant shift towards the isotopic signature of the Earth’s crust
was observed in the more recently deposited sediment layers. This shift reflects the decrease
in contribution of leaded gasoline to Pb contamination in the Zenne and explains partly the
decrease in total Pb concentration in the same core.
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Samenvatting Twee sedimentkernen die verzameld werden langs de rivier Zenne in België werden
onderzocht om de bronnen en de geschiedenis van loodvervuiling in het Zennebekken na te
gaan. Het Zennebekken (1168 km2) is het dichtst bevolkte bekken (1200 inwoners/km2)
binnen het stroomgebied van de Schelde en kent een lange periode van industrialisatie. De
Zenne stroomt door de hoofdstad van België, Brussel, het grootste stedelijke gebied van het
land. Zowel de industrie en de grote bevolking in het Zennebekken veroorzaakten een grote
antropogene druk op de rivier.
Een gedateerde sedimentkern die verzameld werd in Eppegem, stroomafwaarts van Brussel,
werd met ICP-QMS geanalyseerd naar loodconcentraties. De kern behelst de periode tussen
ca.1974 en 2011. Concentraties lood (Pb) tot 775 mg/kg zijn gemeten in deze kern en zijn
significant hoger dan de gemiddelde Pb concentratie van de aardkorst (20 mg/kg). De
gemeten concentraties dalen significant in de meer recente sedimentlagen. Deze daling is
waarschijnlijk veroorzaakt door de de-industrialisatie van Brussel sinds de jaren 1970 en de
strenger wordende milieuwetgeving. Een niet-dateerbare sedimentkern uit Drogenbos, net
bovenstrooms van Brussel, werd ook geanalyseerd. De gemiddelde Pb concentratie in deze
kern was 90 mg/kg (!2 = 14,9) en is significant lager dan de gemeten concentraties in
Eppegem, maar is ook hoger dan de gemiddelde Pb concentratie van de aardkorst. Het
verschil in Pb concentratie tussen Drogenbos en Eppegem toont aan dat Brussel een grote
invloed heeft op de Pb vervuiling van de Zenne.
De isotopische samenstelling van Pb in de twee sedimentkernen werd ook geanalyseerd. De
gemiddelde isotopische signatuur in de kern uit Drogenbos was significant verschillend van
de isotopische samenstelling die gemeten werden in de sedimentkern uit Eppegem. Lood
afkomstig van gelode benzine bleek een groter aandeel te hebben in de sedimentkern van
Eppegem dan in die van Drogenbos. In de Eppegem kern werd ook een significante
verschuiving in isotopische samenstelling doorheen de tijd gezien. Deze verschuiving
weerspiegelt de vermindering in het gebruik van gelode benzine in het Zennebekken en
verklaart deels de significante daling van de totale Pb concentratie doorheen de tijd.
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viii
List of figures Figure 2.1: Geographical and hydrological presentation of the Zenne basin ............................. 3!
Figure 2.2: Total Pb concentration in the riverwater as a function of time in Eppegem (data
Instituut voor hygiene en epidemilogie (up to 1989) and VMM(1989-now). Note that the
outliers in the data are not shown. ...................................................................................... 6!
Figure 2.3: Total Pb concentration in the river water as a function of time in Anderlecht (data
VMM). ................................................................................................................................ 6!
Figure 2.4: Location of the different sediment cores and profiles in overbank sediments along
the Zenne ............................................................................................................................ 9!
Figure 2.5: Vertical distribution of Pb in the profile of Weerde (Swennen and Van der Sluys,
1998). The depths are given in cm. .................................................................................. 10!
Figure 2.6: Production and consumption of refined lead in Belgium (data International Zinc
and Lead Study Group, personal communications) ......................................................... 13!
Figure 4.1: Localisation of the sampling sites Eppegem and Drogenbos in the Zenne basin. . 17!
Figure 5.1: The total Pb concentration, the calculated EFs and the isotopic ratios 206Pb/207Pb
and 208Pb/206Pb measured in the EPP1 sediment core as a function of depth and time. The
EF was calculated with respect to the mean concentration in the upper continental crust,
determined by Taylor and Mclennan (1995). ................................................................... 22!
Figure 6.1: Atmospheric Pb emissions in Belgium (data Pacyna and Pacyna, 2000) .............. 27!
Figure 6.2: Enrichment Factors of the EPP1 samples with respect to a constructed local
geochemical background (EF local) and with respect to the upper continental crust (EF
crust). ................................................................................................................................ 29!
Figure 6.3: Three isotopes plot of sediment cores EPP1 and DRO1 and different ores that
were potentially imported in the Zenne basin. The mixing line between the isotopic
composition of the crust and the Broken Hill ore is also indicated. ................................. 33!
Figure 6.4: Three isotopes plot of the sediment samples EPP1 and DRO1 and different
anthropogenic activities in the Zenne basin. The mixing line between the isotopic
composition of the crust and leaded gasoline is also indicated. ....................................... 34!
Figure 6.5: Three isotopes plot of the EPP1 samples. The depth and determined age of each
sample is given. ................................................................................................................ 36!
!
ix
List of tables Table 2.1: Pb concentrations measured in riverbed sediments of the Zenne (data VMM,
available on vmm.be/geoview). The sapling locations are ranked from downstream to
upstream. ............................................................................................................................ 7!
Table 2.2: Pb concentrations in active stream sediments (data from Swennen et al. (2000) and
Callebaut (2001)). The Locations are ranked from downstream to upstream sites. ........... 8!
Table 2.3: Minimum, maximum and mean Pb concentrations and EFs in the different profiles
along the Zenne (adapted from data Verstraelen, 1998 and Callebaut, 2001). The
concentrations are reported in mg/kg and the EFs are calculated with respect to the upper
crustal background determined by Taylor and McLennan (1995). The locations are
ranked from upstream to downstream locations. .............................................................. 12!
Table 5.1: The total Pb concentration, the calculated EFs and the isotopic ratios 206Pb/207Pb
and 208Pb/206Pb measured in the DRO1 sediment samples. The EF was calculated with
respect to the mean concentration in the upper continental crust, determined by Taylor
and Mclennan (1995). ....................................................................................................... 21!
Table 6.1: Comparison between Pb concentration in sediment samples of the Zenne (EPP1)
and the Seine (M1, data Ayrault et al., 2012) ................................................................... 26!
Table 6.2: Isotopic signatures of different Pb ores potentially imported in the Zenne basin. If
multiple samples were analysed in the studies, we give the range of the obtained results.
.......................................................................................................................................... 30!
Table 6.3: Mean isotopic ratios of Pb in different environmental samples in the Zenne basin or
other relevant areas. If multiple samples were analysed in the studies, we give the range
of the obtained results. ...................................................................................................... 31!
Table 6.4: The fraction of leaded gasoline that contributed to the different samples, according
to different models ............................................................................................................ 38!
!
x
List of abbreviations EF: Enrichment Factor
ICP-QMS: Inductively Coupled Plasma Quadrupole Mass Spectrometry
VMM: Vlaamse Milieu Maatschappij; Flemisch Environmental Agency
Pb: Lead
Al: Aluminium
Cs: Caesium
WWTP: Waste Water Treatment Plant
HIC: Hydrologisch Informatie Centrum; Hydrological Information Centre of Flanders
1
1 Introduction The metal lead (Pb) has since long been used extensively by humans. Elevated lead
concentrations in an ice core record collected in Greenland revealed that anthropogenic lead
production started already 6000 years ago (Hong et al., 1994). The use of lead and lead
emissions into the atmosphere have increased since then to reach a first maximum during
Antiquity. Together with the decline of the Roman Empire lead emissions into the
environment decreased significantly. However, since the Middle Ages and the Renaissance
lead production has been increasing till now (Hong et al., 1994; Alfonso et al., 2001; De
Vleeschouwer et al., 2007). Lead emissions have shown the strongest increase since the
Industrial Revolution. This strong increase was mainly caused by coal combustion, the use of
leaded gasoline, metallurgic activities and waste incineration (Caplun et al., 1984; Monna et
al., 1997; Komárek et al., 2008). Lead additives for gasoline (e.g. tetraethyl lead) were
introduced in the 1920s as an anti-knocking agent for combustion engines and quickly
became one of the major sources of lead contamination in the environment (Nriagu, 1990;
Komárek et al., 2008).
Together with the increasing use of lead throughout history, the toxicity of the element to
humans was revealed. Prolonged exposure to lead, even in low concentrations, causes adverse
effects to the human health, e.g. changes in the neurological development of children and
cardiovascular diseases (Järup, 2003; Von Storch et al., 2003; Farmer et al., 2011). The
increased awareness about the environment since the 1970s and the knowledge about the
toxic effects of lead, first led to the prohibition of lead-based paints, lead water pipes and
food-cans in the developed countries in the 1970s. In the next decades, leaded gasoline was
gradually phased out and finally completely banned with the Aarhus Treaty. In this treaty,
signed in 1998, most European countries agreed to use only unleaded gasoline by the year
2005 (Von Storch et al., 2003). Also in most other countries in the world regulations about the
usage of lead additives in gasoline are made (Nriagu, 1990). All these measures and
regulations on the use of lead, caused a sharp decrease in anthropogenic lead emissions
(Nriagu, 1990; von Storch et al., 2003; De Vleeschouwer et al., 2007).
Despite the recent decrease in the emissions of lead into the environment, lead contamination
is still problematic. In rivers for instance, the major part of the total concentration of lead in
2
water is usually associated with suspended sediment (Lesven et al., 2009; Magnier, 2012).
Deposition of contaminated sediments creates a temporary sink for lead and other metals in
the river system. However, hydrological and geomorphological changes in the river system
can convert a sink into a source for lead contamination and the stored metals can be
reintroduced into the environment (Walling et al., 2003). Also due to changes in pH or other
physico-chemical parameters metals can be mobilised in deposited sediment, creating a
secondary source for metal contamination in rivers (Argese et al., 1997, Petersen et al., 1997,
Cappuyns and Swennen, 2004). Both mechanisms can cause a recontamination of a water
body long after the initial contamination. Therefore, it is important to know how much
contaminated sediment is stored in the alluvium of a river catchment and how severe lead
pollution was in the past.
In this study, we consider the case of the Zenne River in Belgium, a tributary of the Scheldt
River. The Zenne River flows through the densest inhabited part of Belgium (i.e. the capital
region of Brussels; Garnier et al., 2012) and is one of the most polluted rivers in Belgium and
Europe (Billen et al., 1999; Swennen and Van der Sluys, 1998). First, we give an overview
about the contemporary knowledge about Pb contamination in the Zenne basin. After this
overview, we formulate the research questions and hypotheses.
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3
2 Literature review
2.1 The Zenne river and its basin
The Zenne (also called Senne) is a river in Belgium within the Scheldt basin (Figure 2.1). It
rises in the vicinity of Naast and Soignies at a height of 120m TAW and flows into the river
Dijle near Heffen, to the north of Mechelen. The length of the river is 103 km. The upper
reaches of the river (upstream of Halle) are deeply incised into the Palaeozoic calcareous
rocks and slates. Downstream of Halle the river incised in the Tertiary sands (Brusselian
formation) and created a wide floodplain during the Quaternary (De Béthune, 1961). The
Zenne catchment drains an area of 1168 km2 and receives most water from rain and
wastewater discharges (Cappuyns and Swennen, 2007). The Zenne drains through the Dijle
and Rupel into the Scheldt estuary providing around 12% of the fresh water input of the
estuary (Baeyens et al., 1998). The river flows through the three Belgian regions and through
the provinces of Hainaut, Walloon Brabant, Flemish Brabant and Antwerp (Renders, 2012).
Figure 2.1: Geographical and hydrological presentation of the Zenne basin
4
The Zenne basin has a long economical and industrial history. In the Middle Ages the Zenne
was used for transport between Brussels and other cities in the Scheldt basin. However, the
shallow and meandering river was not suitable for fast transport. To create a faster route
between Brussels and the Scheldt, a canal between Brussels and Willebroek (also called the
Zeekanaal) was dug between 1477 and 1561. A second canal, between Charleroi and
Brussels, was dug in the first half of the 19th century (Deligne, 2003). Several interaction
points exist between the Zenne and both canals, making the hydrological system of the basin
complex. A detailed description of this hydrological system is beyond the scope of this work,
but can be found in IMDC et al., (2005).
The construction of the canal between Brussels and Charleroi was a triggering factor for the
industrial development in the Zenne basin in the 19th century as most coal from the mines in
Hainaut was transported via this canal. Also raw materials needed in the production processes
of the different factories were mainly transported via the canals. The developing industry was
active in textile, chemical, metal, paper and beer brewing industries. All these industries used
the water of the canals and the Zenne in their production process and disposed their waste
water in these water bodies (Deligne, 2001). Until the 1970s, the industry in the region around
Brussels was one of the biggest employers in Belgium. From Clabeq to Vilvoorde a lot of
heavy industry was present along the Zenne. However, since the 1960s the region began to
de-industrialise. Today, almost no industry is present anymore in the region, only a limited
number of factories are still active (Vandermotten et al., 2009).
The industrial development in the Zenne basin also caused a major population increase and a
subsequent tendency to urbanisation in the 19th and 20th century. Today, the Zenne basin is
the densest inhabited catchment (540 inhabitants/km2 and 25% of urban area) within the
Scheldt basin (Garnier et al., 2012).
Both the industrial development and the consequent population increase, created a large
anthropogenic pressure on the environment in the Zenne basin. The organic and chemical
waste loads in the river increased enormously, creating several negative effects like odour,
decrease in biological activity and spread of diseases (Billen et al., 1999; Deligne, 2001;
Garnier et al., 2012).
5
Starting in 1860, measures were taken by the authorities to decrease the negative effects
created by the contamination. However, these measures were largely ineffective or counter-
productive because the technological knowledge to deal with contaminated rivers was not yet
available. The best-known measure from this period is the covering of the Zenne in the city
centre of Brussels. This measure made the Zenne an important part of the sewing system of
Brussels, making the contamination even worse (Deligne, 2001, 2003).
Due to the covering of the river in Brussels, the water quality of the Zenne was not a priority
for the authorities for several decades. Despite the improved scientific and technological
knowledge and environmental regulations, it took until 2000 before the first wastewater
treatment plant (WWTP) was implemented in the region of Brussels. It even took until 2007,
when a second WWTP was constructed, before all domestic wastewater of the region of
Brussels was treated before it was discharged into the Zenne (Garnier et al., 2012). !
2.2 Evolution of Pb contamination in the Zenne
In this part we will review the existing studies and measurement campaigns that consider Pb
in the Zenne basin. We make a distinction between the immediate past (1970s-today) and the
long-term evolution of Pb contamination.
2.2.1 Immediate past of Pb contamination in the Zenne River
The Environmental Agency of Flanders (VMM) regularly reports on the physical and
chemical water quality of non-navigable rivers in Flanders. In this framework the total Pb
concentration is also regularly reported. All the measurements of the VMM are publically
available on www.vmm.be/geoview. Along the Zenne different monitoring stations exist, but
for most of them, no long or continuous data series are available on Pb. However, in
Eppegem, downstream of Brussels, a long series of Pb-concentration measurements exists
with only hiatus between 1989 and 1996, 1999 and 2003. At the same location, the Institute of
Hygiene and Epidemiology, the predecessor of the VMM, measured Pb concentrations in
1978, 1979 and 1982. In Figure 2.2 the measured Pb concentrations are shown. However, to
improve readability, three outliers (larger than mean concentration + 3 standard deviations)
and concentrations below the detection limit are removed from the data. We observe a
significant decrease in Pb concentration in time (spearman rank (") of -0,239, p<0,01). The
outliers were 311 µg/l in June 2006, 780 µg/l in February 1979 and 246 µg/l in December
1978.
6
Figure 2.2: Total Pb concentration in the riverwater as a function of time in Eppegem (data Instituut voor hygiene en epidemilogie (up to 1989) and VMM(1989-now). Note that the outliers in the data are not shown.
Also in Anderlecht, just upstream of Brussels a continuous series of measurements exists
(Figure 2.3). This series is not as long as the one of Eppegem, but some interesting
characteristics can be noted. The spearman rank (" = -0,252, p<0,01) is of the same order of
magnitude as the one observed in Eppegem, but the mean Pb concentration in Eppegem is
significantly higher (Mann-Whitney test, p<0,01) than the Pb concentration in Anderlecht.
This indicates that the region of Brussels contributes for a large part to the Pb contamination
in the Zenne.
Figure 2.3: Total Pb concentration in the river water as a function of time in Anderlecht (data VMM).
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The large temporal variation in Pb concentrations (and overall, in several metals) in the water
is explained by Magnier (2012). She observed a high correlation between metal
concentrations and the discharge of the Zenne. During high discharges more sediment is
transported, and because most Pb (>95%, measured at different time intervals) in the Zenne
appears to be associated with the particulate fraction, the concentration in the water will
increase (Magnier, 2012).
The VMM also sampled the sediments of the riverbed of the Zenne at eleven locations
between 1992 and 2011. In Table 2.1 these results are given (the data are freely available on
vmm.be/geoview). Concentrations range between 31 mg/kg (in Lembeek, 2008) and
370mg/kg (in Vilvoorde, 1992). The VMM uses a reference concentration of 14 mg Pb/kg to
assess the severity of contamination in rivers in Flanders. All measured concentrations are
well above this reference value (between 2 and 26 times higher). Unfortunately, there are too
little samples taken at the same locations or in the same year to outline spatial or temporal
trends.
Table 2.1: Pb concentrations measured in riverbed sediments of the Zenne (data VMM, available on vmm.be/geoview). The sapling locations are ranked from downstream to upstream.
Location Year Pb (mg/kg)
Eppegem 2010 143
Vilvoorde, Havendoklaan 2006 364
2002 300
Vilvoorde, Houtkaai 2007 61
2003 102
Vilvoorde, Sluisstraat 2005 257
Vilvoorde, Budasteenweg 1992 370
Anderlecht, Verwelkomingsstraat 2006 127
2002 121
Ruisbroek, Broekweg 1992 72
Beersel, Zennebeembeden 2006 134
Lot, Zennestraat 2005 91
Lembeek, Heldenstraat 2008 31
2004 186
2000 100
Lembeek, Perregatstraat 2011 272
8
Swennen et al. (2000) and Callebaut (2001) collected recently deposited sediment samples on
the surface of the riverbanks along the Zenne. The Pb concentrations in these samples are
given in Table 2.2. It can be seen that the Pb content in the sediments downstream of Brussels
is higher than the Pb concentrations in Tubize. Except for the Eppegem sample, these
locations are too far away from the sampling locations of the VMM. Therefore we cannot
compare them. The sediment samples of Eppegem analysed by the VMM (see Table 2.1) and
Callebaut (2001) were collected with a time interval of 10 years. Despite this time difference,
the Pb content in both samples is of the same order of magnitude.
Table 2.2: Pb concentrations in active stream sediments (data from Swennen et al. (2000) and Callebaut (2001)). The Locations are ranked from downstream to upstream sites.
Location year Pb content (mg/kg) Source
Hofstade 2000 126 Callebaut (2001)
Weerde 1992 511 Swennen et al. (2000)
Eppegem 2000 286 Callebaut (2001)
Tubize 1992 22 Swennen et al. (2000)
2.2.2 Long-term evolution of Pb contamination in the Zenne River
Overbank sediments are deposited on the riverbanks and in the floodplain when the discharge
of a river exceeds the bank-full discharge. These sediments will accumulate in a sequence of
thin layers, with each layer corresponding to a flood event. Each layer is characterised by the
chemical and mineralogical composition of the suspended sediment present in the river at the
time of deposition. Therefore, overbank sediments can be used to reconstruct a chronology of
metallic contamination (Ottesen et al., 1989).
Along the Zenne, several sediment cores and profiles were collected in overbank sediments
and floodplains during the last decades (see Figure 2.4). All these profiles were subjected to
geochemical analyses to determine the vertical distribution of the total metal content.
However, no dating or isotopic analyses were done.
9
Figure 2.4: Location of the different sediment cores and profiles in overbank sediments along the Zenne
The first profile along the Zenne river that was examined in detail, was reported by Swennen
and Van der Sluys (1998). This profile is situated in Weerde, downstream of the industrial
areas of Brussels and Vilvoorde (see Figure 2.4). The vertical distribution of Pb is given in
Figure 2.5. In this figure, it can be seen that Pb concentrations reach very high levels.
Concentrations above 500 ppm are recorded at a depth of > 2m. The Pb concentrations reach
a maximum (> 1100 ppm) in the middle of the profile. From there, Pb concentrations decrease
towards the top. Swennen and Van der Sluys (1998) statistically analysed 66 profiles along
several rivers in Belgium and concluded that all analysed samples in the profile of Weerde
contained outlying Pb concentrations.
10
Figure 2.5: Vertical distribution of Pb in the profile of Weerde (Swennen and Van der Sluys, 1998). The depths are given in cm.
Based on the results of Swennen and Van der Sluys (1998), two master thesis studies
(Verstraelen, 1998 and Callebaut, 2001) were conducted to examine the metal contamination
more into depth. In Figure 2.4 it can be seen that Verstraelen (1998) sampled along the whole
course of the Zenne. Callebaut (2001) sampled only in the most downstream part of the
Zenne.
The comparison of the Pb content in different sediment samples is difficult, because this
concentration varies depending on grain size and organic carbon content, which are
determining factors (Cundy and Croudace, 1995). Therefore, we calculate enrichment factors
(EFs). EFs allow us to make a comparison between the different sediment samples and to
make an assessment about the severity of the Pb contamination in the sediments (Loska et al.,
1997). EFs are a comparison between the measured Pb concentrations and a background
concentration. They are calculated using equation 1.
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Note that in equation 1 Pb concentrations are normalised by a reference element Ref. In
literature Scandium (Sc) is mostly used as a reference element, but also Aluminium (Al),
Titanium (Ti) and Thorium (Th) are used (Shotyk, 1996; Loska et al., 1997; Audry et al.,
2004; Le Cloarec et al., 2011). All these reference elements are related to the clay content or
11
grain size, because trace metals preferentially sorb onto fine sediment fractions (Cundy and
Croudace, 1995). It is assumed that this reference element varies only naturally and is not
influenced by human activities. EFs are ratios which are straightforward to interpret and are
frequently used in geochemical studies (see for example: Hong et al., 1994; Shotyk, 1996;
Audry et al., 2004; De Vleeschouwer et al., 2007; or Le Cloarec et al., 2011). In our study, Al
(80400 mg/kg in the upper continental crust Taylor and McLennan, 1995) will be used as a
reference element because it is the only reference element measured in all available data and
studies about the Zenne. The use of Al as a reference element has a major drawback because
Al is also used in anthropogenic activities. Therefore, the Al concentration in a sediment
sample might not reflect the pre-industrial Al level. As a background concentration we use the
mean concentration of Pb in the upper continental crust (i.e. 20mg/kg), determined by Taylor
and McLennan (1995).
In Attachment 3 the measured Pb concentrations and the according EFs are given. In Table
2.3 the maximum, minimum and mean EFs and Pb concentrations are summarised. In general,
the highest concentrations and EFs are reached at the top of the profiles. Concentrations and
EFs decrease with increasing depth, indicating an increasing Pb contamination through time.
It can be seen that in several profiles (Quenast, A, B, BI, C, D, E and F) low EFs and Pb
concentrations are measured at the base. This can indicate that pre-industrial sediment is
sampled at these locations. The highest concentrations and EFs are measured in Lembeek and
Buizingen, downstream of the industrial areas of Clabecq, Tubize and Halle. It is striking that
in the profiles of Callebaut (2001) lower concentrations and EFs are recorded than in the
profiles of Verstraelen (1998). This might be explained by slightly different sampling
strategies. Verstraelen (1998) sampled in the immediate vicinity of the river. In contrast,
Callebaut (2001) sampled further away from the river, in the floodplain. Thus, the locations of
Callebaut (2001) are less regularly flooded than the locations of Verstraelen (1998). Callebaut
(2001) remarked that the concentrations measured in her profiles are much lower than those
measured in the profile collected in Weerde. She explained this difference by the relative
position of the profiles to the river. The samples of Weerde were collected in the immediate
vicinity of the river.
12
Table 2.3: Minimum, maximum and mean Pb concentrations and EFs in the different profiles along the Zenne (adapted from data Verstraelen, 1998 and Callebaut, 2001). The concentrations are reported in mg/kg and the EFs are calculated with respect to the upper crustal background determined by Taylor and McLennan (1995). The locations are ranked from upstream to downstream locations.
Location Min Max Mean Std dev
Pb EF Pb EF Pb EF Pb EF
Quenast 8 3,5 42 19,1 25 12,8 10,0 5,44
Lembeek 42 21,9 152 118,6 100 63,5 46,1 33,62
Buizingen 48 27,2 326 124,9 199 87,7 111,1 36,78
Lot 62 36,6 120 79,1 100 63,1 26,9 18,46
D 7 0,7 172 21,7 3,8 4,8 0,38 8,33
E 7 0,6 57 5,1 24,2 2,1 19,99 1,77
F 5 0,7 81 10,4 23,1 3,0 29,20 3,74
A 13 1,7 134 21,3 55,2 7,5 34,49 5,48
C 4 1,0 59 9,3 22,3 3,1 17,53 2,81
BI 6 0,5 42 12,0 14,3 2,3 11,94 3,41
B 4 0,9 77 12,2 18,5 2,4 19,89 2,91
Hofstade 38 16,6 258 87,9 93 36,4 74,6 23,87
2.3 Pb usage in Belgium
No statistics or data exist about the historical use of Pb in the Zenne basin. However, every
year, the International Zinc and Lead Study Group (IZLSG) publishes an estimate of the
amount of refined Pb produced and consumed in several countries. In Figure 2.6 the available
data for Belgium are given. It can be seen that the production of refined Pb increased slightly
since the 1960s. The consumption of refined Pb however, remained relatively stable till the
mid 1990s. Since then the consumption started to decline. The production of refined Pb in
Belgium has always been larger than the consumption, which indicates that a major part of the
produced refined Pb in Belgium is exported.
13
Figure 2.6: Production and consumption of refined lead in Belgium (data International Zinc and Lead Study Group, personal communications)
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14
3 Problem statement From the literature review it became clear that there exists a high level of Pb contamination in
the Zenne basin since long. However, only indications of the evolution of Pb contamination
through time are available. Also little information is available about the sources of Pb
contamination.
As discussed before (see section 2.2.2), undisturbed alluvial sediments can be used as a proxy
for the long-term evolution of metal contamination (Ottesen et al., 1989). However, only the
total concentration of lead in sediments is not sufficient to make a detailed assessment about
the sources of lead contamination. For this, the isotopic signature of lead can be used. The
isotopic composition of lead in contaminated sediments can be interpreted as a mixture of
different lead sources with an unique isotopic signature (Monna et al., 1997; Komárek et al.,
2008; Vanhaecke and Kyser, 2012).
In this work, a reconstruction of Pb contamination in the Zenne basin through time is made.
Therefore, we make use of a dated sediment core that was collected along the Zenne River in
the framework of a previous research about sedimentation and metal contamination in the
Zenne (see Renders, 2012). The Pb concentration and the isotopic composition of Pb in the
sediments are used to evaluate the contribution of different sources of lead contamination
through time.
The following research questions were posed:
1. Can we see an evolution in the concentrations and isotopic composition of Pb through
time?
2. Is it possible to distinguish different sources of Pb contamination in the Zenne basin
based on the isotopic composition of Pb?
3. Is it possible to explain trends or peak events of the Pb contamination through time
based on the isotopic compositions of Pb?
Based on these research questions we formulated the following research hypotheses:
1. A decreasing trend is visible in the total lead concentration. The isotopic composition
of lead in the sample converges towards the isotopic composition of lead in natural
samples; i.e. the anthropogenic influence becomes smaller through time.
15
2. Based on the isotopic composition of lead, we can distinguish two or three different
sources of lead contamination in the Zenne basin.
3. The isotopic composition of lead enables us to explain the trend or peak events in the
lead contamination.
16
4 Materials and Methods
4.1 Preliminary work
On 16th August 2011 two sediment cores, which will be referred to as EPP1 and DRO1, were
collected in overbank sediments of the Zenne river, in the framework of a previous
geochemical research (Renders, 2012). The locations of both sampling sites are indicated on
Figure 4.1. Both cores were collected with a percussion corer. A PVC-tube with an outer
diameter of 5cm and a length of 1m was inserted into a steel tube. This steel tube was pushed
down into the sediments with a jackhammer. A hydraulic pump was used to retrieve the steel
tube from the sediments. After the retrieval of the PVC-tube with the sediment core inside,
this tube was sealed at the top and at the bottom.
In the laboratory, the PVC-tubes were sawn in half lengthwise. The sediment cores were
described visually and afterwards divided into samples of ca. 1cm thickness using a plastic
knife. These samples were weighed immediately after their removal of the PVC-tube. They
were dried in an oven at ca. 50°C for at least 5 days. The samples were weighted again after
drying and subsequently stored in airtight plastic bags.
EPP1 was collected on the left bank in Eppegem, about 6km downstream of Brussels, just
upstream of the confluence of the Tangebeek with the Zenne and just downstream of a
connection between the canal between Brussels and Willebroek and the Zenne (the so-called
Hevels van Vilvoorde). This core of about 72 cm long consisted out of silt (median grain size
of about 50 µm over the entire length of the core), except at 44 cm and 64 cm depth, where
two light coloured layers of coarse sand (median grain size of 165 µm) were present. The
organic carbon content in EPP1 varied between 1 and 2% over the entire length of the core,
except at 44 and 64cm depth (the coarse sand layers) where it was comprised between 0,2 and
0,7%.
DRO1 was collected on a small terrace located on the right bank of the Zenne in Drogenbos,
just upstream (ca. 1,5 km) of the Capital Region of Brussels. This coring location is the last
site where the river has a natural-looking, meandering course before it pursues its flow in
underground galleries through the city of Brussels. DRO1 was ca. 78,5cm long and consisted
out of sandy material in the top 17cm. The rest of the core was characterised by finer material
17
with a lot of organic debris. The lowest 15cm contained plastic and brick fragments. No grain
size analyses were done on this core.
Figure 4.1: Localisation of the sampling sites Eppegem and Drogenbos in the Zenne basin.
Several samples of both cores were subjected to gamma spectrometry analysis in the
Laboratoire des Sciences du Climat et de l’Environnement in Gif-sur-Yvette, France. This
allows dating a sediment core by measuring the activity of gamma-emitting radionuclides in a
sample. Two widely-used proxies: Caesium-137 (137Cs) and excess-Lead-210 (210Pb-xs) were
used to date the sediment cores (see for example Audry et al., 2004; Meybeck et al., 2007; or
Le Cloarec et al., 2011).
18
137Cs is an artificial radioisotope with a half-life time of 30,17 years. It is produced by nuclear
fission. It has been released in the environment only at distinct moments, i.e. the nuclear
weapon tests in the 1960’s, the Chernobyl accident in 1986 (Walling and He, 1997) and the
Fukushima accident in 2011 (Evrard et al., 2012). Sediment exposed at the surface is enriched
in 137Cs during such fall-out events. Subsequent deposition of sediment will bury the enriched
surface, creating strata with high 137Cs content. The layers or strata within an undisturbed
sediment sequence with high 137Cs activities can be linked to one of the distinct fall-out
events. By interpolating the age between two levels of high 137Cs activity, an age-depth
distribution can be obtained (Walling and He, 1997).
210Pb is a natural radioisotope with a half-life time of 22,3 years. It is like 226Ra a decay
product of Uranium-238 (238U). 222Rn is produced in soils by the decay of 226Ra and because 222Rn is a gas in normal atmospheric conditions, it can escape to the atmosphere. The amount
of 222Rn that escapes depends on the soil characteristics such as permeability and moisture
content. 222Rn further decays into 210Pb. Thus, 210Pb is produced both in the atmosphere and in
soils. There is a constant atmospheric fallout of 210Pb (called the excess or unsupported 210Pb,
or 210Pb-xs). Therefore, the total 210Pb activity in a soil is the result of two origins: the
supported 210Pb activity (i.e. the 210Pb activity produced in the soil or sediment) and the 210Pb-
xs activity. When fresh sediment deposits cover a sediment surface, the excess activity will
start to decay by its half-life time. A constant sedimentation rate can be estimated from the
decrease in 210Pb-xs activity with depth. Only the total 210Pb activity can be measured in a
sample. The supported activity, however, can be estimated by measuring the activities of one
or more parent elements of 210Pb in the soil. This estimated supported activity can then be
subtracted from the total activity to calculate the excess activity in a sample (Du and Walling,
2012).
The results of the gamma spectrometry and dating attempts (not shown here, see Renders,
2012) revealed that DRO1 could not be dated. The sediment in this core experienced probably
post-depositional mixing, but the presence of 137Cs in the samples indicates that the sediment
was deposited after the first occurrence of this isotope in the atmosphere in the 1950’s. EPP1,
in contrast to DRO1, could be dated (see Attachment 1 for a summary of the results). From
both proxies (137Cs and 210Pb-xs) a sedimentation rate of ca. 2 cm/year was derived. An age-
depth relationship was established based on this sedimentation rate. The 72 cm long EPP1
core thus covers the timespan 1973-2011. The age-depth relationship of EPP1 was confirmed
19
by historical documents about works carried out on this riverbank and thus has a high
credibility.
After gamma spectrometry and ICP-QMS measurements (see section 4.2), the samples of
EPP1 were analysed for grain size and organic carbon content. The grain size was measured
with laser diffraction (Beckman Coulter LS 13320 Particle Size Analyser) in the laboratory of
geomorphology at the KULeuven. Organic carbon content was measured by means of the
method of Walkley and Black (1934). A detailed description of these results can be found in
Renders (2012).
4.2 Determination of the total Pb-concentration
The total Pb-concentration in the EPP1 and DRO1 samples was determined with Inductively
Coupled Plasma Quadrupole Mass Spectrometry, ICP-QMS (X Series, ThermoElectron,
France). ICP-QMS requires a total solubilisation of the sediments. Therefore, a subsample of
80 to 100 mg of the dried samples was taken. This subsample underwent a pre-treatment
before ICP-MS analyses could be carried out. This pre-treatment consisted of three successive
steps.
First, 15ml HNO3 65%: HCl 30% (3:1) was added to the subsample. This attack takes three
days at room temperature and dissolves Ca and Mg from the sediment. The NO2 which is
formed by the reaction is evaporated at 90° for two hours. The remaining liquids in the
sample were removed by pipetting and the sediment was rinsed three times with 10 ml 0.5M
HNO3 to ensure the removal of Ca and Mg and to avoid that these elements will influence the
reactions in the next step. 10 ml of HF 48,9%: HNO3 65% (1:1) was added to the sample in
the second step. This reaction takes place at room temperature in closed vessels and attacks
the siliceous minerals. After 24h the sample was evaporated for 3-5 days at 100°C to remove
the hexafluorosilic acid, which was formed in the reaction. The third step oxidized the organic
matter in the sample. Therefore, after the second evaporation, 12ml of HNO3 65%: HClO4 69-
72% (1:1) was added to the residue and heated at 120°C over five days in closed vessels.
Again, the final solutions were evaporated to remove the perchloric acid, formed by the
reaction. The pipetted liquids from the first step were again added to the sample and the
sample was then evaporated to near dryness. After the evaporation, 1ml of 65% HNO3 was
added to the sample. This solution was then again evaporated to near dryness. This step was
repeated three times in order to minimise the residue of chloride ions in the sample. For ICP-
20
MS analyses the final samples were dissolved in a 0,5N HNO3 solution. The pre-treatment of
the samples and the ICP-MS analysis were done in the same laboratory as the gamma
spectrometry.
4.3 Determination of the isotopic composition of Pb
There exist four stable isotopes of lead, 204Pb, 206Pb, 207Pb and 208Pb. The last three isotopes
are produced by the radioactive decay of 238U, 235U and 232Th respectively and are the so-
called radiogenic isotopes of lead. The isotopic composition of lead in a sample is defined by
the relative abundances of these four Pb isotopes and is in literature usually reported as 208Pb/206Pb and 206Pb/207Pb isotope ratios. Time and the initial U, Th and Pb concentration
control these relative abundances (Bollhöffer and Rosman, 2002). The Earth’s crust and ores
are characterised by a local variability in elemental and isotopic composition. This variability
makes it possible to use the isotopic composition of Pb to distinguish natural, local Pb from
Pb originating from other areas or ores. When Pb contamination occurs, the isotopic signature
of contaminated environmental samples will be a mixture of the natural, local isotopic
signature and the signatures of the different Pb contamination sources (Vanhaecke and Kyser,
2012).
The sediment solutions prepared for the determination of the total Pb concentration (see
above) were also used to determine the lead isotope ratios 208Pb/206Pb and 206Pb/207Pb with
ICP-QMS. Five replicates per sample were made with following experimental conditions: 3
channel reading, 30ms dwell time and 100 sweeps. Every three samples a reference material
was analysed (NIST SRM-981). The 2! errors on both isotopic ratios reached 0,23%.
21
5 Results The measured total Pb concentrations and the isotopic ratios 206Pb/207Pb and 208Pb/206Pb in the
EPP1 core are graphically presented in Figure 5.1. The measured data are also tabulated in
Attachment 2. Concentrations ranged between 186 and 775 mg/kg. A local minimum (311
mg/kg) was present at 43cm depth, coinciding with a coarse sand layer. The concentrations
decreased significantly with depth (spearman rank, " =-0,806, p<0,01). We also calculated
EFs according to equation 1. The mean Pb concentration of the upper continental crust
(Taylor and McLennan, 1995) was used as a background concentration and Al was used as
the reference element. The EFs range between 36 and 116 and indicate that the measured Pb
concentrations are well above the mean crustal lead concentration. The spearman rank of -
0,755 (p<0,01) indicates that the EF’s decrease significantly through time. The maximum EF
is reached at 45 cm depth, which corresponded with the year 1987. The 206Pb/207Pb isotopic
ratio varies in a narrow range between 1,1481±0,0009 and 1,1578±0,0032. This isotopic ratio
does not show significant changes with depth or time (spearman rank " = -0,02; p=0,938).
Also the 208Pb/206Pb isotopic ratio does not significantly change with depth or time (spearman
rank " = -0,232; p=0,354). This isotopic ratio varies between 2,1014±0,0092 and
2,1242±0,0087.
The measured data in the five DRO1 samples are tabulated in Table 5.1. No dating was
available for this core, and only five samples were analysed. Therefore, we only discuss the
mean concentrations, EFs and isotopic ratios and no trends. The mean concentration of Pb in
the DRO1 samples was 90 mg/kg with a standard deviation (!2) of 14,9. The mean EF was 16
(!2 = 3,8). The mean 206Pb/207Pb isotopic ratio was 1,167 (!2 = 0,003) and the mean 208Pb/206Pb isotopic ratio 2,099 (!2 = 0,003).
Table 5.1: The total Pb concentration, the calculated EFs and the isotopic ratios 206Pb/207Pb and 208Pb/206Pb measured in the DRO1 sediment samples. The EF was calculated with respect to the mean concentration in the upper continental crust, determined by Taylor and Mclennan (1995).
Depth (cm) Pb (mg/kg) Pb EF 206Pb/207Pb 208Pb/206Pb
1 86 20 1,164±0,003 2,100±0,006
3 72 18 1,165±0,003 2,098±0,007
40 82 11 1,172±0,003 2,100±0,006
60 98 15 1,166±0,003 2,096±0,006
70 111 14 1,169±0,002 2,103±0,009
22
Figure 5.1: The total Pb concentration, the calculated EFs and the isotopic ratios 206Pb/207Pb and 208Pb/206Pb measured in the EPP1 sediment core as a function of depth and time. The EF was calculated with respect to the mean concentration in the upper continental crust, determined by Taylor and Mclennan (1995).
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6 Discussion
6.1 Pb contamination in the DRO1 and EPP1 sediment cores
In Drogenbos (DRO1) the measured Pb concentrations and EFs were elevated. This indicates
that there is Pb contamination upstream of Brussels. This finding is confirmed by several
contaminated Pb profiles of Verstraelen (1998) which were collected upstream of Brussels.
The DRO1 samples can be compared with the samples of Lot (Verstraelen, 1998) because
both sampling locations are situated only a few 100 m from each other. The Pb concentrations
measured at both locations are of the same order of magnitude (72-110 mg/kg for DRO1 and
62-120 mg/kg for Lot). However, the EFs of Lot and DRO1 are significantly different (Mann-
Whitney test, p<0,05). This is explained by a different lithology at both sampling locations.
Metals sorb preferentially to small particles (i.e. clay minerals, Cundy and Croudace, 1995).
The DRO1 samples have a higher Al concentration, indicating a higher clay content. Also in
the other upstream profiles of Verstraelen lower Al concentrations were observed than in
DRO1 (see Table 2.3). These differences in results between our research and the study of
Verstraelen (1998) can be explained by various factors. Firstly, Verstraelen (1998) used
different techniques to determine the elemental concentrations (XRF in stead of ICP-QMS).
No information about the digestion process used by Verstraelen (1998) is avaialable.
Secondly, one can question the representativity of the material sampled in Drogenbos. Indeed,
at the base of the sedimentcore several brick fragments and plastic was found and next to the
sampling location several works were done between 2001 and 2011 (Renders, 2012).
Therefore, it is possible that the material sampled in the DRO1 core is deposited by man (e.g.
during works at the bank) and not by the river. Thirdly, local variations in lithology can
explain the different Al contents. Despite local variations in lithology, it is striking that the Pb
EFs in the DRO1 samples are relatively low compared to the samples of Verstraelen (1998).
Finally, the use of Al as a reference element in the calculation of EFs influences the results as
well. Al is used in antropogenic activities and therefore might not reflect the pre-industrial Al-
concentration of the sediment sample. Unfortunately, no grain size analyses were done on the
DRO1 samples, so the correlation between Al concentration and the clay content is unknown.
However, other elements related to the clay content without antropgenic uses (e.g. Ti and K)
are also elevated in the DRO1 samples (results presented in Renders, 2012). This indicates
that the DRO1 samples have a higher clay content than the other upstream samples.
24
The results showed that in Eppegem, downstream of Brussels, high levels of Pb were
measured. The concentrations and EFs in EPP1 were significantly higher than those measured
upstream of Brussels (Mann-Whitney test, p<0,05), indicating major Pb contamination.
Ajmone-Marsan and Biasioli (2010) showed that, in general, urban soils contain high
concentrations of Pb, due to the high usage of this metal in antropogenic, urban activities.
Therefore, we can attribute part of the Pb contamination in Eppegem to Brussels. The
remaining part of Pb contamination is probably caused by the industry along the Zenne and
Brussels-Willebroek canal. The sampling location of EPP1 is situated just downstream of the
‘Hevels van Vilvoorde’. This is a construction between the canal Brussel-Willebroek and the
Zenne to divert water from the canal to the Zenne in case of high water levels in the canal
(IMDC et al., 2005) When water is diverted from the canal to the Zenne, the water level in the
Zenne will rise, probably causing sedimentation at the sampling location in Eppegem.
Therefore, EPP1 reflects the metal contamination in both the canal and Zenne.
The significant decrease in both the Pb concentrations and EFs towards the surface in the
EPP1 core indicates a decreasing Pb contamination of the Zenne downstream of Brussels
through time. A major cause for this decrease is likely the de-industrialisation of Brussels and
the surrounding areas since the 1970s. Today, virtually no industry is present anymore in the
region (Vandermotten et al., 2009). A second influencing factor is probably the fact that the
environmental regulations in Europe and Belgium have become more strict since the 1970s
(Garnier et al., 2012). Thus, the remaining industry (e.g. Audi-Vorst and Tessenderlo Chemie
in Vilvoorde) needs to meet these regulations about environmental emissions of pollutants
and less Pb is emitted to the environment. Also the implementation of the WWTP Aquiris in
2007 may have had an influence on the Pb content. This WWTP treats the wastewater of 1,4
million inhabitants of the city of Brussels. It is designed to remove organic matter and
nutrients (Garnier et al., 2012), but some sediment is also removed during the treatment.
We observe two local maxima and a local minimum in the calculated EFs. The maxima occur
in 1987 (45 cm depth) and 2005 (12 cm depth). Renders (2012) already suggested that the
observed peak at 12 cm depth might be caused in June 2006. In this month (14/06/2006), the
VMM measured 311 µg Pb/l in the river water, one of the highest concentrations ever
measured in Eppegem. The high Pb concentration in the water was probably caused by high
discharges in the river and thus, high amounts of contaminated sediment transported in the
river (Magnier, 2012). Another explanation might be an industrial discharge of contaminants
25
in the Zenne or canal. However, more research is needed to confirm this hypothesis. The
maximum at 45 cm depth can be caused by the same mechanisms, but no information of the
VMM or its precursor is available for this period. The minimum occurs in 1988, just after the
maximum at 45 cm depth. It coincides with a coarse and sandy layer. This minimum is
probably not caused by a sudden drop in Pb contamination, but by a non-linear relationship
between Al and the grain size. If Al were linearly related with the grain size, the grain size
would not have an effect in the calculation of the EFs.
It is striking that the Pb concentrations and EFs of the EPP1-core are significantly higher
(Mann Whitney test, p<0,01) than those measured in the profiles of Verstraelen (1998) or
Callebaut (2001) (see section 2.2.2). Only in the Pb profile of Weerde, analysed by Swennen
and Van der Sluys (1998), concentrations of the same order of magnitude were measured.
Also the Pb levels in the riverbed samples collected downstream of Brussels (see Table 2.1
and Table 2.2) are of the same order of magnitude as the EPP1 samples of the same age. The
different concentrations and EFs between EPP1 and Weerde and the profiles of Verstraelen
(1998) and Callebaut (2001) are probably explained by the relative position of the river to the
sampling locations. The EPP1 and Weerde samples are probably collected in riverbed
sediments instead of in the less contaminated floodplain sediments.
Metal contamination in the floodplains of the Seine river in France was analysed in depth by
Le Cloarec et al. (2011) and Ayrault et al. (2012). These studies used the same methodology
as was applied in this work and Renders (2012). Therefore a comparison between the Zenne
and the Seine, two highly urbanised and industrialised river catchments, is possible. No Al
concentration is reported by Le Cloarec et al. (2011), so EFs with respect to the upper
continental crust cannot be calculated, therefore we compare the measured Pb concentrations.
In Table 6.1 the Pb concentrations in a dated sediment core (M1) from downstream of Paris
are reported, also the Pb concentrations measured in samples of EPP1 with corresponding
ages are given. Note that only some samples of M1 are reported, because this core covers a
much longer time span than EPP1. The concentrations measured in the EPP1 core are higher
than those measured in the Seine, despite the Seine is heavily polluted with Pb (Le Cloarec et
al., 2011; Ayrault et al., 2012). However, it needs to be noted that grain size and organic
carbon can influence this comparison (Cundy and Croudace, 1995). As noted above, the
relative location of the cores to the river can also influence the comparison.
26
Table 6.1: Comparison between Pb concentration in sediment samples of the Zenne (EPP1) and the Seine (M1, data Ayrault et al., 2012)
Year Pb (mg/kg) in EPP1 Pb (mg/kg) in M1
2011 332 n.a.
2001 (M1)/ 2000 (EPP1) 309 74
1986 596 136
1980 (M1)/ 1979 (EPP1) 775 162
1974 690 n.a.
1938 n.a. 421
It is important to note the limitations about the use of sediment records along a river to
reconstruct the metal contamination through time. Bølviken et al. (2004) state that the vertical
variations of the chemical composition in overbank sediments do not always represent
variations of river characteristics through time.
Firstly, atmospheric deposition of airborne contaminants can occur onto the sediments. Thus,
the sediments do not only represent changes in the characteristics of the river water, but also
changes in airborne contaminants, which are not necessarily produced in the river catchment
(Bølviken et al., 2004). Pacyna and Pacyna (2000) showed that anthropogenic Pb emissions
into the atmosphere in Belgium are substantial. In Figure 6.1 it can be seen that the
atmospheric Pb emissions decreased since the 1970s. Especially the contribution of the
combustion of gasoline decreased strongly between 1975 and 1985. Also the emissions
associated with the other contributing activities decreased. However, the determination of the
deposited Pb from the atmosphere is unknown. In the case when multiple sediment cores are
compared to each other, it is assumed that each core received the same amount of atmospheric
depositions through time. In the case of the EPP1 core, the decrease in atmospheric Pb
concentrations, and thus a decrease in atmospheric Pb deposition, can also partly explain the
decreasing concentrations with time in the sediment core.
27
Figure 6.1: Atmospheric Pb emissions in Belgium (data Pacyna and Pacyna, 2000)
A second major influencing factor is the secondary mobility (leaching) of contaminants after
the deposition of the sediments. Leaching is influenced by pH, organic carbon content, grain
size, time or biological activity (Cappuyns and Swennen, 2004; Du Laing et al., 2009). No
tests were conducted to determine the potential for leaching of Pb in the EPP1 core, but the
mobility of Pb in sediments is in general negligible (Kober et al., 1999; Alfonso et al., 2001;
Sonke et al., 2002).
A last important confounding factor is the potential post-depositional mixing of the sediments
caused by anthropogenic activities, lateral migration of the river or bioturbation (Bølviken et
al., 2004; Wijnhoven et al., 2006; Du Laing et al., 2009). However, the presence of a reliable
dating on the EPP1 core leads us to rule out the possibility of redistribution of sediment after
its deposition (Renders, 2012).
6.2 A local background concentration for Pb?
The discussed EFs were calculated with respect to the mean crustal Pb and Al concentration
of the upper crust as determined by Taylor and McLennan (1995). However, this mean
concentration of the upper crust does not reflect local variations in the Pb concentration of
soils and sediments. In the literature review (see section 2.2.2), we reported several low Pb
concentrations measured at the base of different profiles along the riverbanks of the Zenne
"!
'"""!
#"""!
+"""!
$"""!
,"""!
%"""!
'),,! ')%,! ')*,! ')&,! '))"! ')),!
(%&3/8'*)#
8'*)#
;3<=.!
6=8=53!9./01234/5!
>?7/@45=!2/8A1734/5!
B./5!?50!73==@!9./01234/5!
C/5DE=../17!8=3?@!8?51E?231.45F!
G3?34/5?.H!E1=@!2/8A1734/5!
28
(Verstraelen, 1998 and Callebaut, 2001). These profiles were probably not contaminated at
their base and therefore reflect the natural Pb concentrations in the Zenne basin.
From all available Pb profiles in sediments in the Zenne basin, only one was dated: EPP1.
This core is heavily contaminated and only covers a very recent time span (see section 4.1).
The lack of dating on the other profiles makes it difficult to select pre-industrial samples.
Therefore, we choose a conservative approach to construct a local background concentration
for Pb in the Zenne basin. We only select the deepest (and probably oldest) samples of the
profiles if the Pb levels of the sample above and the deepest samples from the other profiles
are of the same magnitude. To exclude grain size effects in the selection of samples, we
normalise the Pb concentration with the reference element Al. Based on this approach, we
selected the deepest samples of profiles B, C, BI, D and F (Callebaut, 2001) and the deepest
sample of Quenast (Verstraelen, 1998). The profiles analysed by Callebaut (2001) are situated
in the lower reaches of the Zenne. In contrast, the profile of Quenast is situated near the
source of the Zenne. This spatial distribution of the selected samples makes them
representative for the whole catchment.
The mean Pb concentration of the selected samples is 6 mg/kg (!2 = 1,54). This value is much
lower than the mean concentration of the upper continental crust (20 mg/kg) determined by
Taylor and McLennan (1995). This difference can be explained by two factors: (i) local
variability and (ii) the clay content of the sediments. Trace elements will preferentially sorb to
fine particles, i.e. clay particles. As explained above, Al is associated with clay minerals
(Cundy and Croudace, 1995; Shotyk, 1996). The upper continental crust contains more Al
(8,04%) than the sediments collected in the Zenne basin (3,69%). Therefore, the
concentrations of trace elements like Pb will be higher in the upper continental crust than in
the sediments of the Zenne basin. The low Al content in the Zenne basin (compared to the
mean upper continental crust) is probably caused by the geological substrate of the Zenne
basin: Tertiary and Quaternary cover sands (De Béthune, 1961). However, more dated pre-
industrial sediment samples spread over the entire catchment are needed to confirm the
relevance of this background concentration.
The determined background concentration of Pb can be used to re-calculate the EFs. In Figure
6.2 the calculated EFs with respect to the local background are shown for the EPP1 core. The
calculated EFs for the EPP1 core vary between 55 and 176, which is very high. The EFs with
29
respect to the local background are higher than those with respect to the upper continental
crust, but the same pattern is observed.
Figure 6.2: Enrichment Factors of the EPP1 samples with respect to a constructed local geochemical background (EF local) and with respect to the upper continental crust (EF crust).
6.3 Lead contamination sources
The total Pb concentration in environmental samples (e.g. sediments) can consist of a mixture
of different Pb sources. Therefore, the isotopic composition of Pb in a sample will be a
mixture of the isotopic compositions of the Pb-sources that have contributed to the sample.
These contributing Pb-sources are also called end-members (Vanhaecke and Kyser, 2012).
6.3.1 Anthropogenic Pb sources in the Zenne basin
The origin of Pb used in the Zenne basin has probably changed throughout history. In Table
6.2 the isotopic signatures of different Pb ores are given. These ores are known to produce Pb
for different European cities and regions throughout history (Sonke et al., 2002; De
Vleeschouwer et al., 2007; Farmer et al., 2011; Ayrault et al., 2012) and were potentially
imported in the Zenne basin. The mean isotopic composition of the continental crust,
determined by Millot et al. (2004) is also given as a reference.
')*+!
')*&!
')&+!
')&&!
'))+!
'))&!
#""+!
#""&!
"!
'"!
#"!
+"!
$"!
,"!
%"!
*"!
"! ,"! '""! ',"! #""!
8'*)#,=E>1#
2'9(:#,$-1#
!"#?@#
-A!IJ!2.173!
-A!IJ!@/2?@!!
30
Table 6.2: Isotopic signatures of different Pb ores potentially imported in the Zenne basin. If multiple samples were analysed in the studies, we give the range of the obtained results.
206Pb/207Pb 208Pb/206Pb Reference
Belgian ores 1,167-1,188 2,075-2,108 Dejonghe, 1998
Congolese ores 1,076-1,163 2,065-2,235 Sonke et al., 2002
German ores 1,150-1,189 2,056-2,112 Sonke et al., 2002
Spanish ore (Rio Tinto) 1,164 2,101 Marcoux, 1998
Australian ore (Broken Hill) 1,040 2,220 Townsend et al., 1998
Mean continental crust 1,205 2,062 Millot et al., 2004
Belgian pre-industrial
sediment (Kempen region)
1,2045 2,058 Sonke et al., 2002
Pb imported from different ores is processed and mixed in different anthropogenic activities
(e.g. industrial processes or gasoline additives). Therefore, these activities will also have
distinct Pb isotopic signatures (Komárek et al., 2008). In Table 6.3 isotopic signatures of Pb
in different environmental samples collected in the Zenne basin or other relevant areas are
reported. No reliable information about the isotopic composition of gasoline additives in
Belgium exists. Therefore, we present the isotopic composition of gasoline additives in
France and the Netherlands.
As discussed before (see section 2.1) the industry in the Zenne basin started to grow
exponentially since the construction of the canal between Charleroi and Brussels. This canal
was used to transport coal from the mines in Charleroi (Deligne, 2001, 2003). Therefore, we
assume that the majority of the coal combusted in the Zenne basin originated from the mines
in the south of Belgium. The isotopic composition of Pb released by combustion of Belgian
coal (determined by Walraven et al., 1997) is also tabulated in Table 6.3.
Isotopic ratios of Pb can be interpreted in ‘three isotopes plots’. In such a plot the isotopic
ratios 206Pb/207Pb and 208Pb/206Pb are plotted against each other. In this plot, the measured
isotopic compositions in the samples will fall within a triangle formed between the plotted
isotopic compositions of three distinct Pb sources or end-members (Vanhaecke and Kyser,
2012). A three-isotope plot is interpreted the same way as texture triangles used to classify
soils.
31
Table 6.3: Mean isotopic ratios of Pb in different environmental samples in the Zenne basin or other relevant areas. If multiple samples were analysed in the studies, we give the range of the obtained results.
206Pb/207Pb 208Pb/206Pb Reference
Gasoline in France 1,084 2,182 Mona et al., 1997
Gasoline in the Netherlands 1,062 2,28 Hopper et al., 1991
Aerosols in Brussels (1972-75) 1,141-1,146 2,115-2,119 Petit, 1977
Aerosols in Charleroi 1,129 2,1331 (Petit, 1974)
Precipitation in Brussels (1974-75) 1,137-1,146 2,106-2,121 Id
Metallurgic industry in Hoboken
(1974)
1,179-1,184 2,064-2,084 Id
Belgian coals and coal ashes 1,170-1,180 2,091-2,098 Walraven et al., 1997
The three isotopes plots (Figure 6.3 and Figure 6.4) reveal that the isotopic composition of
both sediment cores was very distinct. The isotopic signatures of the DRO1 samples lie closer
to the isotopic composition of the crust (see Table 6.2) than the EPP1 samples. This is
probably caused by the upstream position of Drogenbos to Brussels and the consequent lower
levels of Pb contamination.
We also see that all analysed samples fall on a mixing line between the isotopic composition
of the crust and the isotopic composition of the Broken Hill ore (Figure 6.3). The Australian
Broken Hill ore was mainly used to produce Pb additives for gasoline in Western Europe
(Komárek et al., 2008). This was also the case for Belgium. The relative contribution of Pb
from Broken Hill in Pb additives in Belgian gasoline ranged between 45 and 61% between
1970 and 1974. The remaining fraction originated from Canadian and South African ores
(Petit, 1977). This explains why gasoline additives can be identified as an end-member in
Figure 6.4.
The isotopic composition of gasoline additives changed through time due to changing
mixtures of Pb originating from different ores. In France for example, the contribution of Pb
from Broken Hill to leaded gasoline varied between 50 and 80% between 1980 and 1995.
Consequently, the 206Pb/207Pb isotopic ratio ranged between 1,06 and 1,10 in the same period
(Véron et al., 1999). This temporal variation in isotopic signature of leaded gasoline might
partially explain the different signature of Pb additives in the Netherlands and France. In the
following, we assume the isotopic composition of Pb additives used in France is the same as
32
the one of Belgium. Indeed, the different atmospheric samples (aerosols and precipitation)
collected in Belgium (Petit, 1974 & 1977) have an isotopic composition that lie on the mixing
line between the isotopic signature of leaded gasoline in France and the natural end-member.
The deviations of the determined isotopic compositions around the mixing line between the
natural end-member and leaded gasoline can be caused by (i) the uncertainty of the isotopic
compositions of the two end-members that define the mixing line or (ii) the presence of a
third, but unknown end-member.
In Figure 6.3 it can be seen that the DRO1-samples have a signature close to the one of the
Spanish Rio Tinto ore. According to Fletcher (1991), Spain was the biggest Pb producer in
Europe since 1878 and Spanish Pb was exported to the entire world. Ayrault et al. (2012)
showed that, by means of isotopic analyses, historic urban Pb (i.e. before the introduction of
leaded gasoline) in Paris mainly originated from the Rio Tinto ore. This might be the case for
Brussels and the Zenne basin as well. However, the isotopic composition of Pb in the DRO1
samples possibly also resembles a mixture between Pb originating from Belgian or German
ores and leaded gasoline (see Figure 6.3 and 6.4). The average and most recent (2011) 206Pb/207Pb ratio of the EPP1-samples (1,154 ± 0,003) is equal to the signature of the ‘urban
Pb’ in Paris as reported by Ayrault et al., 2012. Ayrault et al. (2012) defined urban Pb as the
mixture of historical Parisian lead (i.e. Pb originating from Rio Tinto) and Pb originating from
leaded gasoline. This strengths the hypothesis that historic urban Pb in Brussels originated
from the Spanish ores, because downstream of Brussels (i.e. in Eppegem) the isotopic
signature of Pb is a mixture between the natural end-member, Pb originating from leaded
gasoline and Pb introduced in the catchment before the introduction of leaded gasoline.
Unfortunately, to our knowledge, no long time series of isotopic signatures of Pb in urban
areas in the Zenne basin or Belgium exist to proof this hypothesis. Therefore, the isotopic
compositions of dated pre-industrial sediment samples are needed.
33
Figure 6.3: Three isotopes plot of sediment cores EPP1 and DRO1 and different ores that were potentially imported in the Zenne basin. The mixing line between the isotopic composition of the crust and the Broken Hill ore is also indicated.
!"#$%
!"#&%
!"#'%
!"(#%
!"(!%
!"($%
!"(&%
!"('%
!"!#%
!"!!%
!"!$%
("#!% ("#$% ("#&% ("#'% ("(#% ("(!% ("($% ("(&% ("('% ("!#% ("!!%
!"# $%&
!"' $%(
!"'$%&!")$%(
)**(%
+,-(%
./012%3456672%82%96:"%!##$;%
</7=8>%?566%3@7A>18>B%82%96:"%(CC';%
,57%@5>27%349/D70E"%(CC';%
<86F59>%7/81%3+8G7>FH8"%(CC';%
.7>F76818%7/81%3I7>=8%82%96:"%!##!;%
J8/K9>%7/81%3I7>=8%82%96:"%!##!;%
<86F59>%L/8M5>B012/596%18B5K8>2%3I7>=8%82%96:"%!##!;%
34
Figure 6.4: Three isotopes plot of the sediment samples EPP1 and DRO1 and different anthropogenic activities in the Zenne basin. The mixing line between the isotopic composition of the crust and leaded gasoline is also indicated.
!"#N#%
!"(##%
!"(N#%
!"!##%
!"!N#%
!"O##%
("#$#% ("#&#% ("#'#% ("(##% ("(!#% ("($#% ("(&#% ("('#% ("!##% ("!!#%
!"# $%&
!"' $%(
!"'$%&!")$%(
)**(%
+,-(%
./012%3456672%82%96:"%!##$;%
4829660/F5D%5>B012/P%?7Q7=8>%3*8252"%(CRR;%
<86F59>%D796%D7KQ01257>%3S96/98T8>%82%96:"%(CCR;%
</011861%98/71761%(CR#U1%3*8252"%(CRR;%
J91765>8%7V%2H8%W82H8/69>B1%3?7LL8/%82%96:"%(CC(;%
J91765>8%7V%X/9>D8%347>>9%82%96:"%(CCR;%
*/8D5L529257>%5>%</011861%3*8252"%(CRR;%
.H9/68/75%98/71761%3*8252"%(CR$;%
35
To see if the relative contribution of leaded gasoline decreased since the stricter regulations,
we have a more detailed look at the EPP1 samples in a three isotopes plot (Figure 6.5). A
significant change (spearman rank ! = -0,510, p<0,05) is observed in the isotopic signatures
of the EPP1 samples in Figure 6.5. This indicates that the contribution of leaded gasoline
decreased in time, and the isotopic compositions of the sediments shifted towards the natural
end-member. This finding is in agreement with the findings in other studies about the
temporal evolution of Pb contamination in Europe (e.g. Monna et al., 1997; Sonke et al.,
2002; De Vleeschouwer et al., 2007; Ayrault et al., 2010) and the decrease of Pb emissions
from combustion of leaded gasoline (Pacyna and Pacyna, 2000; von Storch et al., 2003).
There are some other interesting features in Figure 6.5 to observe. First of all, the sample at 3
cm depth (dated as 2009) has the lowest 206Pb/207Pb isotopic ratio of all EPP1 samples, despite
the fact that it has one of the lowest EFs. This might indicate that the total Pb concentration in
this sample originates for a higher percentage from e.g. gasoline. Different explanations are
possible for the low ratio. Firstly, an increase in leaded gasoline emissions in the environment
can decrease the 206Pb/207Pb isotopic ratio. However, this hypothesis is unlikely because
leaded gasoline is not available anymore in Belgium since the 1990s (Von Storch et al.,
2003). Secondly, a decrease in the contribution of the industry or a decrease in coal
combustion increases the relative contribution of ‘old’ emissions of Pb originating from
gasoline. A final hypothesis is based on the coincidence of the low 206Pb/207Pb isotopic ratio
with the temporary closure of Aquiris, the biggest WWTP of Brussels in April 2009. If the
sediment that is normally retained in this WWTP (i.e. soil particles from the city centre of
Brussels) is enriched with Pb originating from leaded gasoline, the 206Pb/207Pb isotopic ratio
will decrease.
A second interesting sample is the one at 40 cm depth, dated 1990. In Figure 6.5 it can be
seen that it does not fall on the mixing line between the isotopic composition of gasoline and
the pre-industrial sediments. A third, but unknown, end-member has probably contributed
substantially to this sample.
36
Figure 6.5: Three isotopes plot of the EPP1 samples. The depth and determined age of each sample is given.
6.3.2 Determination of the fraction of contributing Pb sources
Until now, we only qualitatively discussed the Pb isotopic data. However, theoretical models
exist to determine the relative contribution of different end-members to a sample.
In literature a simple, two-end-member model based on the isotopic composition is frequently
used (Monna et al., 1997; Komárek et al., 2008; Ayrault et al., 2012). This model is given by
equation (2):
!"#$%&!''%
("#$%&!!)%
*"#$%&!!+%
,$*"#$%&!!,%
'!"#$%&!!-%
'&"#$%&!!*%'*"#$%&!!(%
',"#$%&!!&%
&'"#$%&!!!%
&."#$%'))+%
(!"#$%'))*%
.!"#$%'))!%
.("#$%')++%.*"#$%')+,%
.,"#$%')+-%
*!"#$%')+.%
-!"#$%'),)%
,!"#$%'),.%
&$'!!%
&$'!*%
&$''!%
&$''*%
&$'&!%
&$'&*%
&$'(!%
'$'.-% '$'.+% '$'*!% '$'*&% '$'*.% '$'*-% '$'*+% '$'-!%
!"# $%&
!"' $%(
!"'$%&!")$%(
37
!! !
!"!"#
!"!"#!"#$%&
! !"!"#
!"!"#!
!"!"#
!"!"#!! !"!"#
!"!"#!
!""
(2)
where (206Pb/207Pb)i, for i = sample, A and B, the isotopic composition of the sample and two
end-members A and B. In the three-isotope plot (Figure 6.4) we saw that most variation
within the determined isotopic compositions was explained by two sources of Pb: leaded
gasoline and the natural isotopic composition of the crust.
Another, more complicated, model was used by Shirahata et al. (1980). This model
incorporates the isotopic composition of Pb and the Pb concentration of the sample and two
end-members A and B. It is given by equation 3:
!" ! !
!"!"#
!"!"#!"#$%&
!" !"#$%& !!"!"#
!"!"#!!" !
! !"!"#
!"!"#!
(3)
The determined EFs can also be used to assess the anthropogenic Pb fraction in a sample
without the use of the isotopic composition. This can be done by eq. (4):
f = 1 - (1/EF) (4)
It is easy to show that equation 4 is related to the model used by Shotyk (1996) and Sonke et
al. (2002) to determine the anthropogenic contribution of Pb within the total Pb concentration
of a sample. This model is given by eq. (5)
!" !"#$%&%'(")* ! !" !"!#$!!"#$%& ! !" !"#$%&!"!" !"#$%
(5)
38
In case of eq. 3 and 5, the anthropogenic fraction can be calculated as the ratio between the
anthropogenic Pb concentration and the total Pb concentration of a sample.
All these models have been applied on the DRO1 and EPP1 samples. We chose leaded
gasoline of France and the upper crust as end-members (see Table 6.3 for the isotopic
signatures and references). The results are summarised in Table 6.4. The average fraction of
Pb originating from leaded gasoline in the DRO1 samples is always significantly lower
(Mann Whitney test, p<0,05) than the fractions calculated in the EPP1 samples. This could be
expected from the relative position of the EPP1 and DRO1 samples on the mixing line in
Figure 6.4. It is striking that the fractions that were obtained with eq. 2 are significantly lower
(Mann Whitney test, p<0,05) and not significantly correlated with the results obtained with
eq. 3 and 4 (Spearman rank test, p>0,05). The results obtained with eq. 3 and eq. 4 are highly
significantly correlated (spearman rank of 0,901; p<0,01), which is in agreement with the
findings of Sonke et al., (2002). The obtained results of eq. 3 and eq. 4 show that the
anthropogenic fraction of Pb decreases significantly through time (Spearman rank of
respectively -0,788 and -0,755, p<0,01). This confirms our finding that the influence of
leaded gasoline decreases through time. The different results between eq. 2 and eqs. 3 and 4
are probably caused by the incorporation of the total Pb concentration in the model. In the
results obtained with eq. 3, we see that anthropogenic fractions up to 104% are obtained. This
is not realistic, but is probably caused by the use of the total Pb concentration in the samples
and thus, differences in organic carbon content and grain size are not incorporated.
Table 6.4: The fraction of leaded gasoline that contributed to the different samples, according to different models
Average DRO1
("2)
Range EPP1
Eq. 2 31% (2,6) 39-47%
Eq. 3 82% (4,2) 94-104%
Eq. 4 93% (1,6) 97-99%
Three-end-member models are also developed in literature (e.g. Li et al., 2012) but are
complicated to apply and need a lot of additional data about the different end-members. In the
case of the EPP1-core it is not relevant to apply these models, because we only identified two
end-members.
39
7 Conclusions
7.1 General conclusions
To reconstruct the lead (Pb) contamination in the Zenne basin, we used a dated sediment-core
(referred to as EPP1) that was collected along the Zenne downstream of Brussels, and a non-
dated sediment core collected in Drogenbos (upstream of Brussels). The following three
research questions were formulated.
1. Can we see an evolution in the concentrations and isotopic composition of lead
through time?
2. Is it possible to determine different sources of lead contamination in the Zenne basin
based on the isotopic composition of lead?
3. Is it possible to explain trends or peak events of the lead contamination through time
based on the isotopic compositions of lead?
The following conclusion forms the answer to these questions.
The Pb concentrations (up to 775 mg/kg) and enrichment factors revealed a high level of Pb
contamination of the Zenne, but they decrease significantly through time. This decrease is
probably caused by the de-industrialisation of Brussels and the stricter environmental
regulations that were introduced since the 1970s. In Drogenbos, the average Pb concentration
was 90 mg/kg ("2 = 14,9). This is significantly lower than the concentrations measured in
Eppegem. This difference in concentrations indicates a large contribution of Brussels to Pb
contamination in the Zenne. Still, the Pb concentrations in Drogenbos are well above the
mean Pb concentration of the upper crust. This indicates that also Pb contamination exists
upstream of Brussels.
The average isotopic composition of Pb in the Drogenbos samples (206Pb/207Pb = 1,167; "2 =
0,003 and 208Pb/206Pb = 2,099; "2 = 0,003) was significantly different from the isotopic
composition of the EPP1-samples (206Pb/207Pb = 1,148-1,158 and 208Pb/206Pb = 2,101-2,124).
We used three-isotopes plots to determine different sources or end-members of Pb
contamination. In these plots it became clear that, in addition to the natural isotopic
composition, leaded gasoline contributed significantly to the Pb contamination in both the
EPP1 and Drogenbos samples. However, leaded gasoline contributed significantly more in the
40
EPP1 samples than in the Drogenbos samples, probably because Drogenbos is situated
upstream of Brussels. We also found indications that the Pb used in the Zenne basin before
the introduction of leaded gasoline probably originated from the Spanish Rio Tinto ore.
In the three-isotopes plots we saw that the isotopic composition in the EPP1-core shifted
significantly from the isotopic composition of leaded gasoline towards the natural end-
member through time. The decrease in time of the contribution of leaded gasoline to Pb
contamination in the Zenne was also observed in the determined fractions of leaded gasoline
within the samples. These results reflect the decreasing use of leaded gasoline due to the
changed regulations since the 1970s and explain partially the decreasing Pb concentration
through time.
7.2 Scope for future research
To make an assessment about the degree of Pb contamination we calculated enrichment
factors. We used the mean Pb concentration of the upper continental crust as a background
value in these calculations. However, by doing this, local variations in background
concentration are not taken into account. Therefore, we made an attempt to construct a local
background concentration for Pb in the Zenne basin based on results published in literature.
To confirm this background concentration, more dateable sediment cores are needed. These
dateable sediment cores should reach sediments of pre-industrial age and must be located in
different parts of the Zenne basin. Ideally, these sediment cores must be collected in the upper
reaches of the catchment and just upstream and downstream of Brussels. In this way, the
spatial and temporal evolution of Pb contamination in the whole catchment can be
characterised.
These dateable sediment cores are also needed to test the hypothesis if the Spanish Rio Tinto
ore was the main source for Pb in the Zenne basin before the introduction of leaded gasoline.
Detailed historical data about the industry, import and ways of production in the Zenne basin,
if existing, could also contribute to determine the different sources of Pb contamination.
Little studies on the isotopic composition of Pb in the Zenne basin, or central Belgium, exist.
More research about this subject can give more insight and understandings in the evolution of
Pb usage in Belgium.
41
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!
49
Attachment 1 In this attachment the results of the gamma spectrometry on the EPP1 sediment core are
given. These results were presented in Renders (2012). Note that the activities of both proxies
are normalised by the Th content of the samples. This was done to remove the grain size
effects. The peak in 137Cs activity at 46-47 cm depth is attributed to the Chernobyl accident in
1986, indicating a sedimentation rate of 1,88 cm/year. The 210Pb-xs activity decreases
exponentially with depth. The logarithmic function fitted on the 210Pb-xs depth distribution
allowed Renders (2012) to determine a sedimentation rate of 2,07 cm/year. More information
about the dating and its reliability can be found in Renders (2012).
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50
Attachment 2 In this attachment, the results of the ICP-QMS analyses on all samples are tabulated.
name
sample
Depth
(cm)
Year Pb
(mg/kg)
206Pb/207Pb 2s 208Pb/206Pb 2s
EPP1 00 0 2011 331,7 1,1547 0,0026 2,1118 0,0100
EPP1 03 3 2009 186,3 1,1481 0,0009 2,1154 0,0016
EPP1 05 5 2008 n.d 1,1561 0,0008 2,1067 0,0017
EPP1 07,5 7,5 2007 245,0 1,1549 0,0006 2,1086 0,0023
EPP1 10 10 2006 283,6 1,1526 0,0039 2,1151 0,0097
EPP1 12 12 2005 285,5 1,1577 0,0008 2,1054 0,0018
EPP1 15 15 2003 224,4 1,1578 0,0007 2,1043 0,0016
EPP1 17 17 2002 250,9 1,1549 0,0006 2,1063 0,0013
EPP1 21 21 2000 308,8 1,1548 0,0010 2,1058 0,0030
EPP1 24 24 1998 341,9 1,1536 0,0006 2,1082 0,0024
EPP1 30 30 1995 596,6 1,1553 0,0054 2,1055 0,0074
EPP1 40 40 1990 606,9 1,1547 0,0046 2,1242 0,0087
EPP1 43 43 1988 311,3 1,1520 0,0049 2,1116 0,0075
EPP1 45 45 1987 600,9 1,1526 0,0039 2,1114 0,0082
EPP1 47 47 1986 595,8 1,1516 0,0045 2,1037 0,0068
EPP1 50 50 1984 559,0 1,1486 0,0042 2,1094 0,0076
EPP1 60 60 1979 775,2 1,1569 0,0029 2,1014 0,0092
EPP1 70 70 1974 690,4 1,1578 0,0032 2,1113 0,0094
!
51
Attachment 3 In this attachment, the existing Pb profiles in sediments along the Zenne are given. These
profiles were analysed by Verstraelen (1998) en Callebaut (2001). The EFs were calculated in
this work.
Profiles analysed by Verstraelen (1998)
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