Bennett and Siegel, 1987) with ... clay cap will in general be relatively conductive. ... The clay cap is underlain by a zone of pervasive leaching of landfill waste.
11. GEOELECTRICAL CHARACTERIZATION OF COVERED LANDFILL SITES: A PROCESS-ORIENTED MODEL AND INVESTIGATIVE APPROACH
Maxwell Meju
11.1. Introduction Landfill sites commonly use the space available in disused quarries or special purpose-built structures but not all past landfill operations were adequately controlled or documented such that the site boundaries, and the type and volume of fill are unknown in some old covered landfill sites. Even in controlled sites, the final form and depth extent of the landfill may not conform to those indicated in the original plan submitted to the regulatory authorities during the application for a site license. Thus, a significant amount of work is required in order to accurately define the relevant parameters of a covered landfill site. Our hydrogeophysical interest in landfill sites lies in assessing the pollution threat they pose since they may contain hazardous substances. In conventional geophysical investigation of landfill sites, the usual goals are to determine the geometrical characteristics (size and shape) of the repository and the physiochemical properties of the infill. Of the several non-invasive geophysical methods used in landfill studies, the electrical and electromagnetic (EM) methods are the most popular owing to their inherent ability to detect changes related to variations in fluid content, chemical composition and temperature in the subsurface, and the minimum capital and labor outlay required to use them in small-scale surveys (Whiteley and Jewell, 1992; Meju 2000). Since the presence of saline fluids in the ground enhances its ability to conduct electrical current, it is possible to locate a leachate plume by measuring the resistivity distribution in the subsurface. The main ground resistivity measurement techniques employed in landfill studies are the direct current (dc) resistivity and/or induced polarization (IP) methods (e.g. Barker, 1990; Ross et al.,1990; Carpenter et al., 1990, 1991; Meju, 2000; Weller et al., 2000) and transient electromagnetic (TEM) methods (e.g. Buselli et al., 1990, 1992); but the radiofrequency magnetotelluric (RMT) method (e.g. Tezkan et al., 1996) and self potential (SP) method (e.g. Naudet et al., 2003, 2004) are rapidly emerging as powerful tools for landfill investigations (see also Chapter 9).
319 H. Vereecken et al. (eds.), Applied Hydrogeophysics, 319–339. C 2006 Springer. Printed in the Netherlands.
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The focus of this chapter is on the development and application of a consistent process-oriented geoelectrical framework for investigating old nonengineered landfill sites where the necessary record of operations is no longer available or never existed, as in the case of unauthorized dumping grounds. The adopted approach draws from concepts in geotechnics and contaminant biogeochemistry, and stresses the complex geometry of landfill sites, the heterogeneous material compositions, and the attendant biogeomorphiccum-biogeochemical processes in landfill environments. The analogy between landfill waste decomposition processes and weathering of geological materials permits the development of a conceptual geoelectrical model for old covered landfill sites (Meju, 2000). The geochemical observations that leachate properties vary consistently with age (e.g. Farquhar, 1989; DoE, 1996) and the observations that some geoelectrical and hydrochemical parameters of leachate are interrelated (e.g. Meju, 2000; Naudet et al., 2003), suggest the possibility of predicting fill-age and hydrochemical properties using geoelectrical information (e.g. Meju, 2000, 2002). Practical examples will be used to illustrate the various aspects of the proposed conceptual geoelectrical model.
11.2. Landfill Processes and Geoelectrical Signature To understand how the attendant processes in landfill environments can influence our geoelectrical measurements, it is instructive to examine the consistent features of models derived from geological, geotechnical, biological and biogeochemical observations on landfills and rock weathering that can be adopted as the basic building blocks for any geoelectrical model for landfill sites. Three main features of landfill sites (complex geometry, heterogeneous material composition, and complex biogeomorphic processes in harsh environmental conditions) are adapted into a simple geoelectrical model in this section. 11.2.1. CHARACTERISTICS OF LANDFILL SITES
11.2.1.1. Structure of Landfill Sites The old non-contained landfill sites come in various shapes, sizes and depths, many being located in disused quarries, opencast coal mines or other convenient holes in the ground. They may be situated above, below or astride the regional water table. In some landfill sites, a lining of relatively impermeable material may be present, or the waste may be in direct contact with granular or crystalline geological materials. In landfill capping, a soil cover layer is required when returning the site to agricultural or amenity use and steep sided cover systems are often incorporated in the cover design (see Figure 1) to maximize the landfill capacity (Hall and Gilchrist, 1995). The cover system may
GEOELECTRICAL CHARACTERIZATION OF COVERED LANDFILL 321
Figure 1. Geometry of an old covered landfill site. The landfill cover may be a domed cap or have a steeply-sloping side. A basal liner may be absent or breached allowing a 3-D contaminant plume to migrate downgradient from the site
be multi-layered or single-layered in some old landfills. There may be basal floor slopes to promote leachate drainage to sumps but it can be expected that many pre-regulation landfill sites may have inadequate basal containment and leachate collection systems; there will also be cases where the landfill bottom is neither graded nor lined. Landfills generally range in thickness from about 3 to 40 m.
11.2.1.2. Nature and Characteristics of Landfill Wastes Landfill deposits are characterized by complex material composition, nonuniform compaction within each layer, non-uniform decomposition process, non-uniform settlement and varying pore fluid composition (Fang, 1995). The deposits may be intermixtures of domestic and industrial wastes, soils and exhumed geological materials (see Table 1). The composition of urban waste will vary from community to community, from country to country (Table 1), and from season to season (Fang, 1995). The wastes in old landfill sites may not be as well compacted as in modern regulated landfill practice and will thus have substantial internal permeability. They will in general consist of degradable and non-degradable materials (food and garden wastes, ashes, paper, textiles, plastics, metals, building waste, mill tailings, organic liquids etc) but it is their chemical composition that is important when assessing their potential for groundwater pollution (Meju, 2000). Since landfills are a complex mixture of anthropogenic deposits, their physical properties would show a wide range of variation. The published electrical resistivity of solid waste and contaminated substrate range from 1.5 m to ca. 20 m (e.g. Knight et al., 1978; Laine et al., 1982; Everett et al., 1984; Carpenter et al., 1991; Meju, 2000) with the associated leachate being highly conductive (Whiteley and Jewell, 1992; Meju, 2000).
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TABLE 1. Composition (percentage by weight) of typical municipal solid wastes (Meju, 2000) Types Food and garden/ Organic wastes∗ Paper, paper board Metal Glass Wood Textiles, rugs Rubber, leather, (bonesb , wood∗ ) Plastic Earth, ash, cinder Construction rubble Others
New York City, USA
Osaka, Japan
Zagreb, Croatia
Meruelo, Spain
Ankara, Turkey
Beijing, China
19.3
17.7
23.3∗
52.0∗
50.8∗
45
58.8 7.6 8.6 2.5 0.8 0.8
37.1 5.5 12.3 2.5 4.0 0.3
20.5 2.4 7.3 1.1 3.8 2.6b
21.9 3.2 4.1 2.4
8.9 1.2 1.4 — 1.4
5 1 1
0.8
15.2
10.8
8.3 1.4
1∗
28.2 6.7
2.1 32.4 1.8
1
46
Data compiled from Sowers (1968); Yamamura (1983); Kovacic et al. (1995); Sanchez-Alciturri et al. (1995) and Wasti (1995). The superscripts refer to original classifications at data sources.
11.2.1.3. Formation and Migration of Landfill Leachate and Biogas Degradable domestic waste will decompose into organic and/or inorganic soils plus other byproducts depending on its chemical composition. Initially, it undergoes a short-term process of mechanical decomposition and settlement causing its physical properties to change (Fang, 1995). Owing to the heterogeneous nature of the usually organic-rich domestic waste, the distribution of settlement will be non-uniform (Fang, 1995) and often leads to severe fracturing of the top seal of the landfill cover. The top seal is then highly vulnerable to erosion and infiltration of rainwater and snow-melts. Long-term physical, chemical (notably hydrolysis, hydration, carbonation, oxidation and solution) and microbial degradation processes lead to the dissolution or deterioration of landfill materials, gas generation and production of leachate. Initially, the microbial degradation of landfill waste occurs under aerobic conditions. As the oxygen becomes depleted by the microbial activity, anaerobic conditions rapidly set in and the biodegradation of organic materials becomes anaerobic. Methane gas is generated bacterially from the abundant organic materials under the prevailing anaerobic conditions. Infiltrating rainwater, groundwater, or other liquids disposed of within the wastes will dissolve some soluble mineral constituents of the landfill once the absorbent or field capacity of the fill is exceeded and free drainage of water can occur. This leaching process may remove the common mineral elements or the bonding materials causing changes in matrix cement or the
GEOELECTRICAL CHARACTERIZATION OF COVERED LANDFILL 323 TABLE 2. Typical changes in leachate concentrations with age of landfill waste Age of waste Leachate parameter
0–5 years
5–10 years
10–20 years
>20 years
TDS pH BOD COD Ammoniacal N Total P Chloride Sulphate Calcium Sodium + potassium Magnesium + iron Zinc + aluminium Alkalinity (CaCO3 ) σw
10000–25000 5–6 10000–25000 15000–40000 500–1500 100–300 1000–3000 500–2000 2000–4000 2000–4000 500–1500 100–200 10000–15000 1500–4000
5000–10000 6–7 1000–4000 10000–20000 300–500 10–100 500–2000 200–1000 500–2000 500–1500 500–1000 50–100 1000–6000 750–1500
2000–5000 7–7.5 50–100 1000–5000 50–200
