Martin, J.M., Nirel, P. and Thomas, A.J. (1987) Sequential extraction techniques: ... Stafford, E.A., Simmers, J.W., Rhett, R.H. and Brown, C.P.. (1991) Interim ...
Land Contamination & Reclamation, 6 (3), 1998
© 1998 EPP Publications
Land disposal of heavy metal contaminated dredged sediments: a review of environmental aspects Satya P. Singh,1 Filip M. G. Tack2 and Marc G. Verloo2 1. Department of Environmental Technologies, Alberta Research Council, 250 Karl Clark Road, Edmonton, Alberta, T6H 5X2, Canada. 2. Laboratory of Analytical Chemistry and Applied Eco-Chemistry, University of Gent, Coupure-Links 653, B 9000 Gent, Belgium Received December 1997; accepted March 1998
Abstract Land disposal of metal contaminated dredged sediments may adversely impact the surrounding environment. Effects can be immediate, or time delayed and non-linear, and are controlled by a number of geochemical factors and by the physico-chemical characteristics of the dredged material. This review paper focuses on various environmental aspects related to upland disposal of dredged sediments. Physico-chemical changes occurring after land disposals, potential pathways for metal migration from land disposal sites, characterisation of the pollution status of dredged materials, evaluation and classification strategies and various alternatives for the handling of dredged sediments are discussed. The ultimate goal is to decide upon the best alternative for handling dredged sediments.
INTRODUCTION
limited and volumes to be disposed of must therefore be strongly reduced (Van Hoof 1991). The application of dewatered and stabilised dredged materials in landscaping, agriculture, construction, restoration of previous industrial sites and covering of landfills could be a valuable way for its beneficial utilisation (Strubbe et al. 1993). However, one must be able to guarantee that the pollutants present will not affect the surrounding environment in the short, medium and long term (Bradshaw 1993). In this context, the determination of physical and chemical characteristics of metals in sediments is an indispensable preliminary phase in deciding the best option for handling dredged sediments. For the proper assessment of the environmental impact resulting from land disposal or from its alternative uses, the processes controlling the mobilisation, transportation and retention of sediment associated heavy metals need to be understood and quantified. This review paper focuses on various environmental aspects related to upland disposal of heavy metal contaminated dredged sediments. An overview is given of physico-chemical changes occurring after land disposals and potential pathways for metal migration into the ecosystem from upland disposal sites. Various chemical and biological methods are presented that may be applied to gain insight into the pollution status of sediment.
Massive amounts of sediments are dredged in order to maintain the depth of the navigational waterways, harbours and estuaries worldwide. Land disposal of these dredged materials may affect the surrounding environment due to the presence of harmful components such as organic compounds and heavy metals. Most organic pollutants are micro-biologically decomposed by different pathways, and finally these are converted into CO2 and water (De Brabandere 1991). Depending on the constituents, the whole process may take from a few hours to several months (Bouwer 1993; Genouw et al. 1993; SchulzBerendt 1993). The behaviour and fate of heavy metals is governed by various physico-chemical processes, which dictate their availability and mobility in soil and sediment systems (Tack and Verloo 1995). Depending on their binding forms, these sediment-associated metals are more or less available for mobilisation and subsequent uptake by plants and living organisms (Förstner 1989; Lund 1990; Ramos et al. 1994). Because strategies for treatment and beneficial use of contaminated dredged materials are still in the development stage, dredged sediments are disposed of into confined areas (Beyer et al. 1990; De Haan et al. 1993). Other disposal alternatives include dumping in open water and application to intertidal and upland depositions (Van Hoof 1991; Farrah and Pickering 1993). Because land disposal is easy and cost-effective, it is the most widely adopted disposal alternative. In densely populated areas, the available area for confined disposal is
PHYSICO-CHEMICAL CHANGES FOLLOWING LAND DISPOSAL OF DREDGED MATERIALS The behaviour of heavy metals in soils and sediments 149
Land Contamination & Reclamation / Volume 6 / Number 3 / 1998
depends on their physico-chemical properties as well as on environmental conditions prevalent in the immediate surroundings (Gambrell 1994; Van Gestel et al. 1995). Important parameters include pH, redox conditions, organic matter content, texture and the presence of adsorbing materials such as oxides and hydroxides of aluminium, iron and manganese and clay minerals (Bolt and Bruggenwert 1976; Alloway 1990; Yong et al. 1992; Van Riemsdijk and Hiemstra 1993). Brandon et al. (1993) summarised pronounced physico-chemical changes in some properties of contaminated estuarine dredged materials after land disposal (Table 1). Following upland disposals dredged materials are subjected to drying and oxidation. The pH of the unamended sediment dropped from 7.6 in 1983 to 3.2 by the end of 1986. The substantial drop in pH of dredged materials is assumed to result from decomposition of organic matter and oxidation of sulphide minerals, while carbonates are not sufficient to provide buffering against acidification (Gambrell et al. 1991). pH is the main factor dictating the behaviour of heavy metals in soils and sediments, and directly affecting the adsorption and solubility of metals (Smal and Salomons 1995). Small shifts in pH can strongly affect dissolved metal levels. The metal solubilities as a function of decreasing pH increased strongly for Cd, Cu, Pb and Zn in oxidised sediment (Tack et al. 1996) Redox conversion may occur due to periodic changes in the hydrological regime. During dredging, aeration of the sediment suspensions can raise the Eh and subsequent temperature increases (e.g. during draining and drying of land disposed sediments) can promote microbiological and chemical processes (Farrah and Pickering 1993). Oxidation of sulphides in dredged sediment takes place as soon as the sediment comes in contact with oxygen. In a number of cases, this caused strong changes in pH. During the first stage of metal sulphide oxidation, the dissolved metal concentration increases because insoluble metal sulphides are transferred to the adsorbed phase (Figure 1). The continuous oxidation of iron sulphides causes consumption of calcium carbonate’s buffering capacity and once this is completed, a sharp drop in pH causes a further
Concentration in solution Dredged Material on land – no buffering capacity Sulphide phase to adsorption + oxidation of ironsulphides Dredged material on land – oxidised
Sulphides phases to adsorption
Reduced Sediment
Concentration in solid
Figure 1. Increase in heavy metal concentrations in solution when metal sulphides become unstable in an oxygen rich environment (Salomons 1995) increase in solubility of sediment associated metals such as Hg, Zn, Pb, Cu, and Cd (Förstner 1993a; Gambrell 1994). On the other hand, the mobility is lowered for Mn and Fe in upland disposal environments. The lower solubility of iron can be explained by the conversion to insoluble iron oxyhydroxides (Kabata-Pendias and Pendias 1984). The solubility of these oxides decreases 1000-fold for each unit increase in pH (Lindsay 1979). The organic matter content in an upland disposed dredged material tends to decrease with time (Gambrell and Patrick 1978; Gambrell 1994; Tack et al. 1996). Decomposition of soil organic matter is the gradual breakdown and mineralization of nonliving macro-organic matter by soil organisms. This depends directly on the biological activity of the soil and hence is closely related to soil temperature, moisture content, pH, oxygen availability, mineral nutrient status and stress from toxic chemicals (Doelman 1995; Schulin et al. 1995). Due to the
Table 1. Changes in soil characteristics after upland disposal of contaminated estuarine dredged materials (Brandon et al. 1993) Control Plots Parameter
Amended*
Unamended 10/83
11/85
6/86
10/89
10/83
11/85 **
6/86
10/89
7.6
3.2
3.2
3.4
7.6
NS
NS
4.4
Salinity ppt
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