1. Literature Review
1.6 Redox Environments of a Landfill Leachate Pollution Plume
There are numerous reports detailing groundwater contamination as a consequence of inadequate landfill leachate management systems and as a result of primitive landfills devoid of such practices (e.g., Lyngkilde and Christensen, 1992a; Bjerg et al., 1995; Mikac et al., 1998; Roling, van Breukelen, Braster, Lin and van Verseveld, 2001). Therefore, it is inevitable that environmental pollution, with particular reference to groundwater contamination, has been been associated with landfills (Griffin, Shimp, Steele, Ruch, White and Hughes, 1976; Baun, Jensen, Bjerg, Christensen and Nyholm, 2000; Cozzarelli, Suflita, Ulrich, Harris, Scholl, Schlottmann and Christensen, 2000).
The high inorganic (Bjerg et al., 1995) and organic concentration of landfill leachate provides an ideal substrate for microbial processes (Cozzarelli et al., 2000; Roling et al., 2001) within the subsurface environment (Ludvigsen, Albrechtsen, Ringelberg, Ekelund and Christensen, 1999). This coupled with complex chemical reactions can result in significant changes in aquifer geochemistry and microbiology downstream of a landfill, with these changes being mirrored in the sequential development of redox zones in time and space
(Williams and Higgo, 1994; Bjerg et al., 1995; Roling et al., 2001). Methane production, sulphate reduction, iron reduction, nitrate reduction, manganese reduction, and aerobic zones (Lynkgilde and Christensen, 1992a; Lensing, Vogt and Herrling, 1994; Williams and Higgo, 1994; Bjerg et al., 1995; IWACO, 1997; Lovely, 1997; Mikac et al., 1998; Ludvigsen et al., 1999) have been identified as the characteristic redox zones present in a landfill leachate pollution plume, with an overall distribution downgradient from the landfill (Figure 1.1) (Lovely, 1997). In the light of such a statement, emphasis must be placed on the fact that the redox potentials increase away from the landfill thereby reflecting the overall distribution of the individual redox processes stated previously (Bjerg et al., 1995; IWACO, 1997).
Figure 1.1 Typical distribution of redox conditions prevalent in an aquifer polluted by an organic contaminant (Lovely, 1997).
The entrance of a high organic load into the subsurface causes a rapid depletion of oxygen, and as a consequence of this, the kinetics of microbial metabolism becomes dependant on the interactions between the organic carbon present in the migrating leachate, the availability of soluble and insoluble electron acceptors and donors (Lynkgilde and Christensen, 1992a;
Cozzarelli et al., 2000), and the kinetics of the actual redox processes (Lynkgilde and Christensen, 1992a). This results in areas expressing dominant terminal electron-accepting
processes (TEAPs) (Lovely, 1997; Cozzarelli et al., 2000) which confer dominant redox zones within the framework of a leachate plume (Lovely, 1997). Lynkgilde and Christensen (1992a) stated that the existence of such a sequence of redox environments is a hypothesis, based on the assumption that significant quantity of: free oxygen, nitrate, sulphate, iron and manganese compounds being present in the subsurface environment. They further concluded that the absence of an electron acceptor would render the corresponding redox environment non- existent. Bjerg et al., (1995) presented a comprehensive description of the prevailing conditions within the individual redox environments of a typical landfill leachate pollution plume.
Natural attenuation is a process by which the concentration of landfill leachate constituents is reduced by natural phenomenon. Based on their definitions, Senior (1990) and Bagchi (1994) identified the following as the possible attenuation mechanisms in the subsurface:
i. Adsorption ii. Biological uptake
iii. Cation- and Anion-exchange reactions iv. Filtration, and
v. Dilution reactions
Rugge et al. (1995) stated that biological degradation can only be proposed as a possible attenuation mechanism, when there is a failure to associate the disappearance of waste stream constituents in the plume with any of the remaining attenuation processes. Bjerg et al. (1995) and Mikac et al. (1998) further stated that it is of fundamental importance to associate pollutant attenuation/degradation to the prevailing redox environments in the plume.
Such reactions often aid in the development of a specific redox state thereby facilitating similar reactions in such an environment (Bjerg et al., 1995). Bouwer and Edwards (1983a;
1983b) found that numerous organic compounds produced varied responses to biodegradation under differing redox environments. They concluded that the prevailing redox environments played a key role in the biotransformation of these organic compounds, since an essential factor affecting the biotransformation process is the type of electron acceptor available to the microbial systems. In a series of investigations, detailing the distribution and migration of
organic compounds in the subsurface, Williams and Higgo (1994), concluded that prevailing redox conditions and the related microbial populations will play a defining role in determining the degradation fate of the organic compounds. Cozzarelli et al. (2000) further stated that the rate at which organics are degraded by the microbiology of the iron reducing, sulphate reducing and methanogenic zones, depends largely on the balance between the reaction rates prevalent in these zones and the rate at which leachate is supplied to the subsurface.
The vast chemical reactions, typical of a leachate plume, are often dominated by the heterotrophic activities of numerous bacterial groups (Lensing et al., 1994; Williams and Higgo, 1994). The abundance of both anaerobic and heterotrophic organisms decreases with increasing depth in the subsurface, which is possibly a result of a decrease in the supply of essential nutrients and electron acceptors as well as an increase in anti-microbial contaminants through the build-up of xenobiotics (Williams and Higgo, 1994). Ludvigsen et al. (1999) showed that cell numbers decreased with increasing distance from the landfill by analysing phospholipid fatty acid (PLFA) and adenosine triphosphate (ATP) content in samples traversing the area of a landfill leachate polluted aquifer. Methanogens are restricted to the most polluted and reduced section of the plume corresponding to the section closest to the landfill (Rugge et al., 1995; IWACO, 1997; Ludvigsen et al., 1999), while sulphate reducers were shown to decrease with increasing distance from the landfill (IWACO, 1997). Research has shown that in some cases methanogenesis and sulphate reduction are exclusive of each other (Bjerg et al., 1995), but other studies have provided evidence that suggests the co- existence of methanogens and sulphate reducers (Beeman and Suflita, 1987; 1990; Cozzarelli et al., 2000). Lynkgilde and Christensen (1992) identified the iron reducing zone as the largest zone in the plume. The importance of this zone in the oxidation cycle of organic matter was highlighted by Albrechtsen and Christensen (1994) and Roling et al. (2001). Iron-, manganese- and nitrate reducers were identified in pockets throughout the plume in lower concentrations than within the boundaries of the respective active redox zones in the plume (IWACO, 1997).
Researchers agree that the biologically mediated redox environments of a landfill leachate pollution plume plays a central role in the natural attenuation of leachate contaminants, and remains a key factor in determining the ultimate fate of such contaminants in the plume (Lynkgilde and Christensen, 1992a; 1992b; Bjerg et al., 1995; Winderl, Anneser, Griebler, Meckenstock, Lueders, 2008).