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.

Introduction

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.

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. 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 re-aerated during emerged periods. These alternating aerobic and anaerobic conditions will have specific influences on most of the processes regulating the speciation of metals in soils (Calmano et al., 1993 in Charlatchka and Cambier, 2000) such as:

  • sorption/desorption onto different solid components;

  • adsorption/coprecipitation onto hydrous oxides of Fe and Mn;

  • formation/decomposition of soluble and insoluble metal inorganic complex compounds;

  • dissolution of carbonates, metal oxides or hydroxides;

  • 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 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 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 correlated 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 metal-binding sites (e.g. Senesi et al., 1987). To clear out 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, 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 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). 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.

Conclusions

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 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.

Acknowledgements

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 development policy (SPSD II). Part 2: Global change, ecosystems and biodiversity.

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