Factors affecting metal mobility and bioavailability in the
superficial intertidal sediment layer of the Scheldt estuary
G. Du Laing,∗ D. Vanthuyne, F. M. G. Tack, and M. G. Verloo
Laboratory of Analytical Chemistry and Applied Ecochemistry, Ghent University (UGent), Coupure Links 653,
B-9000 Gent, Belgium
∗
Corresponding author: E-mail: [email protected]
Many factors and variables govern trace metal behaviour in soils and sediments in complex ways. Understanding metal behaviour in intertidal wetland systems is further complicated because it concerns highly dynamic
systems that are continuously subjected to quickly changing environmental conditions governed by alternating
low and high tides. We studied the effects of various influencing factors, such as hydraulic regime, organic matter
and salinity on metal mobility and bioavailability in the superficial calcareous intertidal sediment layer of the
Scheldt estuary. These sediments contain elevated levels of particularly Cd, Cr and Zn. Flooding regime and the
supply of organic matter significantly affect pore water metal concentrations and hence potential mobility and
bioavailability. Fe, Mn and Ni pore water contents in the upper intertidal sediment layer increased as a result of
frequent flooding, whereas Cd, Cu and Zn contents decreased. Organic matter can act as a sink for metals, but it
can also induce dissolution of metals which were previously bound to solid sediment compartments, especially
Fe, Mn and Ni. Salinity particularly favoured Cd mobility and bioavailability in oxidised sediments, which was
confirmed by field monitoring data, but it also affected Zn, Fe and Mn mobility. More detailed speciation analysis
of metals in the pore water is needed to improve our understanding about the contribution and importance of
various processes in determining the observed metal behaviour.
Keywords: wetlands, river, organic matter, salinity, redox, flooding
Introduction
The Scheldt river is also highly polluted, receiving
industrial and domestic wastewaters. There is a high
degree of organic and inorganic contamination (Paucot and Wollast, 1997). Several former and current
wetlands along the river Scheldt have been contaminated by metals because of either flooding and overbank
sedimentation or land disposal of dredged sediments
(Vandecasteele et al., 2002). This contamination constitutes an important obstacle when managing, restoring or creating intertidal wetlands. Wetlands can only
be created and sustainably managed if processes affecting metal mobility and availability are thoroughly
understood and metal fate can be predicted.
We studied the effects of various influencing factors,
such as hydraulic regime, organic matter and salinity.
The Scheldt river sources in the north of France,
and continues its flow in Belgium through the Walloon
Region and the Flemish Region towards the North Sea
outlet in the Netherlands. Its river catchment covers
a surface of 20331 km2 in one of the most populated
and industrialised areas of Europe. The downstream
Scheldt basin is a typical coastal estuary characterised
by a small river discharge, but subjected to a large tidal
influence with amplitude of 4.5 m at the mouth. It is
a unique estuary in Europe as the salinity intrusion
extends more than 110 km upstream. The variations in
tidal level can still be observed at Ghent, 150 km from
the river mouth.
33
C 2007 AEHMS. ISSN: 1463-4988 print / 1539-4077 online
Aquatic Ecosystem Health & Management, 10(1):33–40, 2007. Copyright DOI: 10.1080/14634980701212969
34
Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
Many factors and variables govern trace metal behaviour in soils and sediments in complex ways. Understanding metal behaviour in intertidal wetland systems is further complicated because it concerns highly
dynamic systems that are continuously subjected to
quickly changing environmental conditions governed
by alternating low and high tides.
Properties and pollution status of
superficial intertidal sediments in
the scheldt estuary
Scheldt river intertidal sediments are typically rich
in carbonates. Carbonate contents between 2 and 7%
(w/w) explain the neutral to slightly alkaline pH (7.2–
8.6) of these sediments. This also causes the sediments
to exhibit a strong buffering capacity against acidification. High levels of carbonates and of organic matter (between 2 and 21% w/w) point towards the expectation that these sediments will tend to accumulate
metals. At the same time, these properties will tend to
reduce availability of metals. Clay content is another
highly important factor in controlling metal retention
and availability. Out of 26 locations sampled along the
Scheldt river downstream of Gent, 10 samples were
classified as silty clay, and 6 as silty clay loam according to the USDA texture triangle. Other samples were
classified as sandy loam (4), sandy clay loam (2), sand
(1) and loamy sand (1).
Mean, minimum and maximum metal contents of
the superficial intertidal sediment layer at the 26 sampled locations along the Scheldt river downstream of
Gent can be found in Table 1. Some percentiles of the
distribution are also presented there. Cd, Cr and Zn
contents were particularly high. These contents are elevated compared to normal baseline concentrations in
soils in Flanders (Tack et al., 1997) and even regularly exceed regulatory trigger values for remediation
in the Flemish Region (VLAREBO, 1996). The ranges
of baseline concentrations and trigger values for remediation are also presented in Table 1. Consistent with
the notion that clay and organic matter have a great
affinity to bind trace metals, their contents were significantly correlated with metal concentrations in the
sediments (data not shown).
Metal availability and mobility as
affected by the hydrological regime
The supply of oxygen in superficial intertidal sediment layers primarily depends on the flooding regime.
When soils are flooded, biological and microbiological
activity combined with limited oxygen diffusion under
these conditions causes oxygen depletion and thus establishes reducing conditions. In shortage of oxygen,
soil micro-organisms start to use other electron acceptors such as nitrate, manganese and iron. In highly reducing conditions, sulfate reduction and methanogenesis will occur (Hadas et al., 2001). The dominating
mineralization process mainly depends on the availability of all products and micro-organisms involved.
Although different electron acceptors theoretically will
be oxidised in a sequence as redox potential decreases,
overlapping processes have been observed (Peters and
Conrad, 1996).
Typical for intertidal sediments, they will be reaerated during emerged periods. These alternating
aerobic and anaerobic conditions will have specific
Table 1. Mean, minimum, maximum metal contents, 25, 50, 75 and 90 percentiles of the distribution of the metal contents and the ranges of
the baseline concentrations and trigger values for remediation (mg kg−1 DW) of the superficial (0–20 cm) intertidal sediments on 26 sites
along the river Scheldt downstream of Ghent.
Mean
Minimum
Maximum
Percentiles
25
50
75
90
Range
Baseline contents
Remediation values
Cd
Cr
Cu
Ni
Pb
Zn
7.5
1.1
16.7
133
31
224
88
10
180
35.8
7.7
65.2
140
15
281
583
40
982
5.4
6.7
9.3
14.3
97
139
170
200
58
84
120
166
29.1
34.8
45.8
54.2
108
135
176
252
364
631
817
902
0.6–2.7
1.6–6.8
34–60
119–210
15–28
181–333
8–21
88–235
38–92
189–461
56–142
538–1369
Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
influences on most of the processes regulating the speciation of metals in soils (Calmano et al., 1993 in
Charlatchka and Cambier, 2000) such as:
r sorption/desorption onto different solid components;
r adsorption/coprecipitation onto hydrous oxides of Fe
and Mn;
r formation/decomposition of soluble and insoluble
metal inorganic complex compounds;
r dissolution of carbonates, metal oxides or hydroxides;
r precipitation as insoluble sulfides under highly reducing conditions and their dissolution of sulfates
under oxic conditions.
Calmano (1996) emphasized that future research
should focus on kinetics of metal species transformations and metal release as affected by changing redox
conditions.
As most of the studies previously focussed on shifts
in metal speciation as a result of exposure of sulphidic
sediments to permanently oxic conditions (e.g. after
disposal of dredged sediments on land: Tack et al.,1996;
Stephens et al., 2001; Caille et al., 2003), we now primarily focussed on metal fate upon flooding and exposure to frequently alternating hydrological conditions.
An oxidised dredged-sediment derived soil was selected and subjected to different flooding regimes. To
that aim, the recipient was periodically submerged into
or lifted from deionised water, acidified to pH 4 to
simulate the most significant effects of acidic rainfall. The recipients were subjected to several hydrological regimes during 98 days: (R 1) permanently
flooded, (R 2) alternately two weeks flooded and one
week emerged, (R 3) alternately two days flooded and
eight days emerged, (R 4) alternately two days flooded
and two days emerged, (R 5) continuously on field
capacity (Figure 1). Metal concentrations were measured in pore water extracted from the soil using Rhizon soil moisture samplers (Eijkelkamp, Giesbeek, The
Netherlands), and in collected percolates.
Figure 1 depicts changing metal pore water concentrations in an initially oxidised sediment that was
subjected to different flooding regimes. Pore water Fe
and Mn concentrations that markedly and steadily increased during the entire duration of the experiment
reflected reducing conditions that are being established
within hours after flooding. Also several trace metals
such as Ni and Cr increased (data not shown). Concentrations of Cd, Cu (Figure 1) and Zn, in contrast, decreased with time in the same treatment. The sediment
35
kept at field capacity exhibited low pore water concentrations of Fe, Mn and Ni, but on the other hand Cd, Cu,
Cr and Zn concentrations were relatively high in this
treatment. Alternating hydrological conditions resulted
in fluctuating metal concentrations in the pore water.
The shorter the dry period and the longer the wet period, the more the profile of metal concentrations with
time corresponded to that of the flooded sediments. The
faster the alterations between emerged and flooded periods, the higher was the total metal export by the percolates, and hence the actual metal mobilisation, even
at lower concentrations in the pore waters.
The quantification of the contribution of different
processes to the fluctuations in pore water metal concentrations in the current experimental setup is limited by the difficulty in conducting reliable speciation
studies. The sampling technique involves a slow withdrawal of pore water by applying vacuum. Limited volumes can be recovered, and because of the time needed,
the solution collected in the vacuum tubes is likely to
change during sampling. For example, it was observed
that precipitates of Fe/Mn-oxides were being formed in
the tubes.
Metal mobility as affected by
organic matter supply
In intertidal sediments, phytoplankton, phytobenthos and macrophytes are sources of different types of
organic matter. In the intertidal zones of the Scheldt
estuary common reed (Phragmites australis) is a
widespread, dominant plant species and a major source
of organic matter for the upper sediment layer. It
forms dense stands that are among the most productive
ecosystems in temperate areas. Moreover, these reed
plants are only lightly grazed in the living state and
the greatest part of the primary production ultimately
enters detrital systems (Polunin, 1982).
The fate of metals after decomposition of organic
matter originating from plants is unsure. Several authors claim that upon mineralization, the metals previously bound to the organic matter will be remobilised
into the environment (Alloway, 1995). Others claim
that metals will be transferred from the more available
fractions to e.g. highly insoluble organic complexes
with strongly humified litter and eventually become
buried as long-term sinks (Paré et al., 1999). We monitored heavy metal contents of decomposing leaf blades,
stems and sheaths in litter bags anchored on the top sediment layer in an intertidal zone of the Scheldt estuary
during 16 months, starting from October 2001. Most
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Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
Figure 1. Part A + B: Temporal fluctuation of Fe and Cu concentrations (mg l−1 ) in the pore water of initially oxidised sediments subjected
to different flooding regimes (R 1: y, R 2: , R 3: , R 4: and R 5: ◦). Part C: Flooding regime denoted with white for dry periods and
black for submerged periods.
metal contents in reed litter increased considerably
during decomposition, although samples were thoroughly washed with deionised water preceding analysis. Trapping of sediment particles and associated metals seemed to be very important, which is explained
by the top layer of the marsh sediments being periodically resuspended by tidal wave action. This increases
the chance that mud particles enter into the litter bags.
There are, however, also strong indications that passive
metal sorption and fungal activity are important in determining metal accumulation in decomposing litter of
reed plants. Fungal dynamics proved to be highly cor-
related with metal contents in stem tissue except for Cr
and Zn, both elements of which the contents remained
relatively stable during the experimental period. This
correlation suggests an involvement of fungal activity
in metal accumulation in stem tissue by a) direct incorporation in fungal mass; b) enhanced binding of metals to the decomposing litter due to complexation between extracellular fungal products and metals (Gadd,
1993); c) by induced changes in litter quality by mineralization, e.g. increasing availability of phenolic units
as lignin breaks down, offering many potential metalbinding sites (e.g. Senesi et al., 1987). To clear out
Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
which mechanisms could drive metal accumulation by
decaying organic matter in intertidal sediments under
the influence of microbial decomposers, more specific
research is needed.
The seasonal supply of organic matter can not only
act as a sink, but also as a source. As mentioned above,
changing redox potentials as a result of ongoing aerobic and anaerobic decomposition processes can induce
shifts between dissolved and solid element species in
the sediments. Organic matter can be a limiting factor for these processes. Supply of easily biodegradable
organic matter can thus result in a faster consumption
of oxygen in aerobic sediments and nitrate reduction,
reductive dissolution of oxic manganese and iron minerals and sulphate reduction in anaerobic sediments,
affecting metal mobility (Tanji et al., 2003). Some organic fractions can also directly act as soluble or insoluble complexing agents, increasing or decreasing
metal mobility. The interaction with metals and solubility of these complexes in pore water depends on several
factors, e.g. pH, Ca contents and cation exchange capacity of the sediments (Kalbitz and Wennrich, 1998).
Moreover, decomposition of organic matter can affect
metal speciation by release of CO2 and lowering of the
pH (Charlatchka and Cambier, 2000). The dominating
type of interaction between organic matter and metals
is expected to depend upon the amount and type of
the organic matter (Kashem and Singh, 2001), which
is affected by the decomposition status.
We studied shifts between dissolved and solid element species resulting from the supply of organic matter to oxic metal-polluted calcareous intertidal Scheldt
sediments in a greenhouse experiment similar to the
one described above. The sediments used were mixtures of a dredged sediment derived soil (B) with organic matter originating from reed plants and willows.
These plant and tree species were selected as they are
the most abundant and productive ones in the intertidal zones of the Scheldt estuary. Mixtures were prepared with concentrations (w/w) of 3.5% (RL1) and
7% (RL2) reed leaves, 10% (RS1) and 20% (RS2) reed
stems and 7% (W) willow leaves. These mixtures were
subjected to both a permanently flooded and an alternating hydrological regime of 1 week emerged and 2
weeks flooded.
Figure 2 illustrates some of the results. Temporal
trends of the Fe and Zn concentrations in pore water
could be observed when initially oxidised sediments
were permanently flooded after addition of organic
matter originating from reed plants and willows. Fe
was mobilised to a much larger extent when organic
matter was added to the sediments (RL1, RL2, W1,
37
RS1, RS2), compared to the blank sediments (B). This
was observed both for permanently flooded (Figure 2)
and alternately flooded sediments (data not shown).
The same was observed for Mn and Ni. Supply of
easily biodegradable organic matter results in a faster
consumption of oxygen, nitrates and Fe/Mn-oxides as
electron acceptors by micro-organisms, concurrently
releasing Fe and Mn and probably associated Ni. After 50 to 70 days, the Fe contents started to decrease
again, probably due to the initiation of sulphide precipitation or decreasing biodegradability of the organic
matter. Figure 2 also shows that Zn pore water concentrations increased substantially to 3.5 mg l−1 when
organic matter originating from reed stems was added
to the sediments RS1 and RS2. The effect of the supply
of organic matter from reed leaves was much smaller.
However, Zn contents in the reed stems and leaves were
similar and stems are expected to decompose somewhat slower. Thus, attributing the differing Zn concentrations in the pore water to leaching of Zn from
the stems during decomposition would not be consistent. The differences should probably be attributed to
different interactions of stems and leaves with soluble Zn during decomposition. Cu contents decreased
and fluctuated around the detection limit when organic
matter was added to sediments that were permanently
flooded (data not shown). This is probably due to the
fact that Cu has a very strong affinity for organic matter
(Yin et al., 2002), which only slowly decomposes into
smaller, more mobile fractions in flooded sediments
(Seybold et al., 2002). The contents fluctuated at higher
levels when the sediments were alternately subjected
to flooded and emerged conditions. These fluctuating
conditions probably resulted in an increasing organic
matter decomposition rate (Seybold et al., 2002), resulting in the release of smaller, more mobile organic
matter fractions and associated Cu. Cd contents were
found to be continuously low in all treatments after an
initial transition phase of about 20 days.
Metal mobility as affected by
salinity
The Scheldt estuary is unique in Europe as the
salinity intrusion extends more than 110 km upstream.
Salinity can affect metal mobility and bioavailability.
Particularly Cd in solution is readily complexed by
chlorides (Hahne and Kroontje, 1973). An increase
of the salinity is also associated with an increase in
the concentrations of major elements (Na, K, Ca, Mg)
which can compete with heavy metals for the sorption
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Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
Figure 2. Temporal trends of the Fe and Zn concentrations in pore water (mg l−1 ) when initially oxidised sediments were permanently
flooded after supply of organic matter originating from reed plants and willows (B: no organic matter; RL1: 3.5% reed leaves; RL2: 7% reed
leaves; W: 7% willow leaves; RS1: 10% reed stems; RS2: 20% reed stems; contents on w/w basis).
sites (Tam and Wong, 1999). Both factors could result in increasing metal availabilities with increasing
salinity.
To study potential effects of salinity, sediments were
continuously flooded by water of different salinities,
prepared by adding NaCl to deionised water (0.5; 2.5
and 5 g l−1 ), using the same type of experimental setup
as described above. Sediments involved in the experiment were (1) an initially oxidised low-salinity sediment, (2) an initially oxidised high-salinity sediment
and (3) an initially reduced sediment high in sulphide
content. The mean acid-volatile sulphide (AVS) content
of the reduced sediments was 995 mg kg−1 , whereas it
was less than 50 mg kg−1 in the oxidised sediments.
Cd availability was indeed affected by the salinity.
Increasing availability was observed with increasing
salinity, but the magnitude of the effect depended on
the sediment. The effect was small in the high original salinity, but very large and important in the oxidised sediment with low initial salinity (Figure 3).
Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
39
Figure 3. Cd concentrations in the pore water (μg l−1 ) as a function of time for initially oxidised, low-salinity sediments, initially oxidised,
high-salinity sediments and initially reduced, sulphide-rich sediments during continuous flooding by water of different salinities (0.5 g l−1 ;
2.5 g l−1 ; 5 g l−1 NaCl); DL = detection limit.
In agreement with the experiments involving different
hydraulic regimes, Cd concentrations decreased with
time. In the reduced, sulphide-rich sediments, Cd concentrations in the pore water remained very low, irrespective of the salinity, suggesting a continued control of the solubility by sulphides. Field observations
confirmed the increasing Cd availability with increasing salinity in superficial, oxidised sediments along the
river Scheldt. Cadmium contents in ground-dwelling
spiders e.g. increased with salinity, despite markedly
lower sediment total metal contents in the high-salinity
sites (Du Laing et al., 2002). Increasing availabilities
with increasing salinities were also observed for Zn, Fe
and Mn, but the effect was found to be smaller.
hence potential mobility and bioavailability. Fe, Mn and
Ni pore water contents in the upper intertidal sediment
layer increased as a result of frequent flooding, whereas
Cd, Cu and Zn contents decreased. Organic matter can
act as a sink for metals, but it can also induce dissolution
of metals which were previously bound to solid sediment compartments, especially Fe, Mn and Ni. Salinity
particularly favoured Cd mobility and bioavailability in
oxidised sediments, which was confirmed by field monitoring data, but it also affected Zn, Fe and Mn mobility.
More detailed speciation analysis of metals in the pore
water is needed to improve our understanding about
the contribution and importance of various processes
in determining the observed metal behaviour.
Conclusions
Acknowledgements
Superficial intertidal sediments in the Scheldt estuary contain elevated levels of particularly Cd, Cr and
Zn. Flooding regime and the supply of organic matter
significantly affect pore water metal concentrations and
This research was funded by Ghent University and
by the Belgian Federal Science Policy Office and was
part of the WETMAT-project (EV/02/32A) that fits in
the Second Scientific support plan for a sustainable
40
Du Laing et al. / Aquatic Ecosystem Health and Management 10 (2007) 33–40
development policy (SPSD II). Part 2: Global change,
ecosystems and biodiversity.
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