Trace Metal Leachability of Land‐Disposed Dredged Sediments

Abstract

The potential risk of surface and ground water contamination by trace metals leached from contaminated land disposed dredged sediment is a major environmental concern. The objective of this study was to evaluate trace metal leachability, leachate quality and environmental impacts of land-disposed dredged sediments. In addition to chemical characterization and determination of potential and actual leachability, dredged sediments were also subjected to solid phase fractionation using a sequential extraction procedure.Potential leachability, defined as the maximum metal pool that may become available for leaching at a constant pH 4, decreased in the following order: Zn ~ Cd > Mn > Ni > Co > Cu ~ As > Pb > Cr. Potential metal leachability was controlled mainly by solid phase distribution of metals in sediments. The acid extractable fraction of most metals correlated well with the potentially leachable metals extracted with a solution of pH 4 (r 2 > 0.67) except for Pb. The kinetics of metal release were determined using the cascade leaching test with water at pH 4. Except for Ni and Zn in the first fraction, metal concentrations in all consecutive leaching fractions remained below the maximum permissible level in water for human consumption. Actual metal leachability expressed as a percentage of potential leachability varied widely among trace metals and decreased in the order: As > Cu > Cr> Ni > Zn > Cd > Co > Pb. Our results indicate low risk of surface and ground water contamination resulting from land disposal of dredged sediments. 4Large quantities of sediments are dredged each year to clean and maintain the depth of navigational waterways, harbors and estuaries worldwide. In the Flemish region of Belgium alone, an average of 3,940,000 m 3 is disposed on land annually (Demoen, 1989). Land disposal of these dredged sediments may affect the surrounding environment due to the presence of harmful organic compounds and trace metals. Among other disposal alternatives such as sea dumping, landfilling and incineration, confined land disposal (upland disposal) is frequently adopted because of the increasing pressure to ban all sludge dumping at sea. Moreover, it is easy and cost effective. However, upland disposals may cause changes in their redox potential, pH and organic matter content of the land-disposed dredged sediments (LDDS). These physico-chemical changes may enhance the leachability and bioavailability of trace metals in sediment (Moore et al., 1988;Förstner, 1993;Gambrell, 1994), which may restrict the beneficial utilization of LDDS, particularly in landscape restoration, agriculture and construction.The leachability and bioavailability of trace metals in soils and sediments depend on their chemical and physical associations (Bubb and Lester, 1991; Tack and Verloo, 1993;Ma and Rao, 1997). Metals in soils and sediments may form specific mineral phases, be loosely bound on exchangeable sites, co-precipitate, be adsorbed onto mineral phases, be fixed by organic matter and sulfides or be structurally boun...

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