Heavy metal contents in surface soils along the Upper Scheldt river (Belgium) affected by historical upland disposal of dredged materials
Introduction
For several decades, periodical dredging of river sediments has been necessary to allow for shipping traffic on the river Scheldt. Since the 1990s dredged sediments have been disposed in confined disposal facilities. Previously, sediments were disposed along the shores in the alluvial plain without concern for the potential presence of contaminants. An important change in the Scheldt river system was affected by the broadening and deepening of the river to allow for ship traffic up to 2000 tons. These works were executed between 1960 and 1980. The river profile had to be broadened and deepened, leading to a changed sedimentation pattern. The resulting dredged pure sandy soil materials were used for levelling the wet parts of the alluvial plain.
With respect to the river Scheldt, much research has primarily focused on the river estuary downstream Rupelmonde. Studies about the upstream part in contrast are scarce, although most of the heavy metal input in the estuary is caused by upstream pollution (Regnier and Wollast, 1993). In suspended matter and bottom sediments of the Scheldt Estuary, Verlaan et al. (1998) and Verlaan (2000) could distinguish between a fraction of fluvial origin and a fraction of marine origin because of the elevated concentrations of Cr, Pb, Zn, Cu and Cd in the former. Both water quality, suspended matter quality and sediment quality is improving in the Scheldt estuary since two decades (Zwolsman et al., 1996, Baeyens, 1998, Zwolsman, 1999). Zwolsman (1999) states that improvement of the water quality eventually will lead to an increased mobilisation of heavy metals in the estuary because of an increased oxygen supply to the sediment.
Important sources of heavy metal pollution in the catchment of the river Scheldt are the transboundary industrial fluxes from northern France. Transboundary influx of polluted water is screened yearly at the Flemish border (VMM, 1997, VMM 2000a). Approximately 90% of Cd and Cr in the Flemish surface water are estimated to originate from transboundary pollution (VMM, 2000b). Seuntjens et al. (1996) developed an integrated sediment quality assessment tool for the Flemish region. The results of this so-called triad approach revealed that the quality of sediments in the upstream part of the Scheldt in Flanders was very bad based on biological, ecotoxicological and physicochemical criteria (Ministerie van de Vlaamse Gemeenschap, 1995, De Cooman et al., 1998, De Deckere et al., 2000).
Research concerning the geochemistry of heavy metals in soils derived from landfilled dredged sediments is a recent topic and shows that adverse effects of heavy metals depend upon the field conditions, of which redox potential and pH are of prime importance. Tack et al. (1996) showed that the solubility of Cd, Cu, Pb and Zn as a function of pH increased strongly in an oxidising environment compared to the reduced environment. Metal mobility and availability in selected upland disposal sites in Flanders has been studied in considerable detail (Singh et al., 1996, Singh et al., 1998, Tack et al., 1998, Tack et al., 1999). Sequential extraction revealed low residual fractions compared to total contents. This may reflect an important anthropogenic input of metals in these dredged sediment-derived soils (Zhang et al., 1990, Vicente-Beckett, 1992). Based on DTPA extraction, Zn, Cd and Cu were estimated to be highly plant-available (Singh et al., 1998). Elevated heavy metal concentrations in the pore water equally raised concerns for enhanced metal availability for plant uptake (Tack et al., 1998). In contrast, long-term leaching and migration of metals to ground water predicted from leaching tests was estimated to be of no environmental concern (Tack et al., 1999). Field trials where polluted dredged sediment was applied as a topsoil revealed no migration of metals or organic pollutants 16 months after application (Ruban et al., 1998). However, depending on the geometry of the area, metal transport from the sediment disposal site to surrounding areas by surface runoff may be of concern (Singh et al., 2000).
Several approaches were used to evaluate sediment quality as a function of time. Geochronology was determined by comparison of sediment analyses from different periods (Zwolsman et al., 1996), by the use of radionuclide time tracers (Zwolsman et al., 1993, Winkels et al., 1998) or by calculation of sediment deposition rates on sampled points based on time series of monthly surveys (Wiese et al., 1997). An advantage of the use of deposited sediments is that they integrate the fluctuating input of heavy metals over rather long time scales (Regnier and Wollast, 1993).
To estimate the geographical extent and the environmental consequences of historical sediment disposal on land along inland water rivers in Flanders, a detailed survey was initiated in 1997 on behalf of the Flemish Authorities. The primary goal of our study was to survey the alluvial plains of the Upper Scheldt for the presence of old dredged sediment landfills, and to measure and appraise the heavy metal contamination at these sites. Besides, we made an attempt to study the evolution of heavy metal concentrations in these dredged sediment-derived soils as a function of the period of landfilling and the location of the landfill from the data.
Section snippets
Study area
The study area for this research was the Flemish part of the Upper Scheldt (Fig. 1). The catchment area of the river Scheldt covers approximately 21 600 km2 in the north-west of France, the west of Belgium and the south-western part of the Netherlands. The river can be divided in three parts, the Upper Scheldt, the Scheldt river and the Scheldt estuary, which in turn is subdivided in the fluvial, the upper and the lower estuary. The Upper Scheldt is defined as the upstream part of the river
Results
Research in the archives of the Flemish community revealed that in the most upstream part of the study area small quantities of sediment were dredged on specific spots with high sedimentation rates such as sluices, and at the Kortrijk-Bossuit canal outfall. Only once, in 1966, larger quantities section were dredged from a larger section (Table 3). In the downstream part of the river, the situation was similar until 1968, but since 1980, when the calibration works for ship traffic up to 2000
Discussion
This survey provides important information about the pollution of the terrestrial compartment of the Upper Scheldt river system which is connected to pollution of the water body through dredging activities. Landfilling of dredged sediments caused a pollution with a more permanent character than the sediment pollution itself. When sediments settle and accumulate on locations with reduced current, reduction processes effectively immobilise metals in the sediments (Gambrell, 1994), thus decreasing
Conclusions
Dredged and landfilled Upper Scheldt sediments were strongly polluted with Cr, Zn, Cd and Pb a long time before 1965 and no clear quality improvement is observed since. Landfilling of heavy metal contaminated dredged sediments caused an important contamination in the terrestrial part of the alluvial system. The ecological restoration of these dredged sediment-derived soils will be a time-consuming process. However, the presence of dredged sediment landfills in the alluvial plain is an indirect
Acknowledgments
This project was carried out with financial support from the Waterways and Marine Affairs Administration (AWZ) of the Ministry of the Flemish Community. We are grateful to Carine Buysse, Els Mencke, Anya Derop and Athanaska Verhelst for the accurate soil analyses and to Rik Delameillieure for the help during the field survey. Thanks also to Raf Lauriks and Jürgen Samyn for the critical review and to Paul Quataert for the statistical support.
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