1. Introduction
Waste disposal by landfill is very popular, and the ever increasing demand for larger space
for domestic and industrial wastes from urban areas makes them a necessary part of the human
cycle of activities. Landfill sites commonly use the space available in disused quarries or spe-
cial-purpose-built structures but unauthorised
Ž waste disposal in moats defence ditches around
. ancient city walls and dry river channels can
also be found near some urban areas. These large waste containment facilities are typically
polluted, hence the need for stringent statutory controls. Unfortunately, not all past landfill op-
erations were adequately controlled or docu- mented such that the site boundaries, and the
type and volume of fill are unknown in some covered landfill sites. Moreover, even in con-
trolled 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 li-
cense. Thus, a significant amount of work is required to accurately define the relevant pa-
rameters of a covered landfill site.
Our interest in landfill sites may lie in assess- ing the pollution threat they pose since they
may contain hazardous substances. We may also be interested in gauging their construction feasi-
bility, since some old landfill sites, previously considered as unattractive or marginal building
land, are now being developed for light indus- trial and domestic infrastructure as a result of
the decline in heavy industrial activity and the ever increasing need for urban regeneration in
many old industrialised regions. In standard landfill site investigations, the usual goals are to
Ž determine the geometrical characteristics size
. and shape , the physical properties and the
chemical composition of the fill. Geophysics has an important albeit difficult role to play in
fulfilling parts of these requirements. For exam- ple, geophysical methods can furnish useful data
for locating the boundaries of a landfill site and fill thickness. Physical property distribution in
the subsurface, which may have geotechnical significance, can be derived from the geophysi-
cal data. Additionally, since some geophysical methods respond to changes in the physico-
chemical conditions in the subsurface, useful chemical information may be gleaned from con-
tinuous geophysical site monitoring investiga- tions. A few confirmatory boreholes may serve
to validate the geophysically derived informa- tion.
Of the several non-invasive geophysical Ž
methods used in landfill studies see Whiteley .
and Jewell, 1992 , the electrical and electromag- Ž
. netic EM methods are the most popular owing
to their inherent ability to detect changes related to variations in fluid content, chemical composi-
tion and temperature in the subsurface, and the minimum capital and labour outlay required to
use them in small-scale surveys. Since the pres- ence of saline fluids in the ground enhances its
ability to conduct electrical current, it is possi- ble to locate a contaminant plume by measuring
the resistivity distribution in the subsurface. The two main ground resistivity measurement tech-
niques employed in landfill or groundwater con-
Ž .
tamination studies are the direct current dc Ž
resistivity methods e.g. Cartwright and McCo- mas,1968; Warner, 1969; Stoller and Roux,
1973; Klefstad et al., 1975; Barker, 1990, 1992; Ross et al.,1990; Carpenter et al., 1990a,b, 1991;
. Meju, 1993, 1995a,b and transient electromag-
Ž .
Ž netic TEM methods e.g. Buselli et al., 1988,
. 1992; Meju, 1993, 1995a , but the radio- or
Ž .
audio-frequency magnetotelluric RMT or AMT method is rapidly emerging as a powerful tool
Ž .
for such investigations e.g. Tezkan et al., 1996 Ž
. and the self-potential SP
method is a vital complement to these other techniques in near-
Ž surface groundwater flow detection work e.g.
. Ogilvy et al., 1969; Stierman, 1984 . For land-
fill construction feasibility investigations, a combined approach involving the seismic re-
Ž .
fraction method e.g. Carpenter et al., 1991 is useful. Note that several other geophysical
methods have been successfully applied in land- fill characterisation. For example, where there
are appreciable density contrasts between the fill and the surrounding geological materials
Ž .
e.g. Whiteley, 1983; Roberts et al., 1990 , gravimetric techniques could be used for defin-
ing the geometry and depth extent of the land- Ž
fill. Heterogeneities within the fill e.g. metal .
drums may be mapped using magnetometric Ž
. Ž
and ground probing radar GPR methods e.g. .
Hinze et al., 1990 . The shallow seismic reflec- tion and GPR methods are undoubtedly the best
Ž geological mapping tools in ideal situations e.g.
Hill and Ali, 1988; Ali and Hill, 1991; Beres .
and Haeni, 1991 but old landfill sites constitute an entirely different target in which the loose
fill is typically associated with poor energy propagation and the clayey horizons limit the
usefulness of radar signals.
In comparison with the traditional field tech- niques used in natural resources exploration,
geoelectrical investigation of landfill sites is on a different scale and requires special attention to
details. To start with, the area under investiga- tion is often geometrically constrained and be-
deviled by cultural noise, yet the sampling rate and data quality requirements for target defini-
tion are more stringent. The consequences of inadequate or unsuccessful site investigation
may be more serious in pollution-related stud- ies, with insurance or legal connotations. Thus,
in the present economic and legal climate, ade- quate desk-study and model development are
essential ingredients for a successful cost-effec- tive geoelectrical investigation of old landfill
sites.
The main thrust of this paper is to develop a consistent exploration model for anthropogenic
deposits in oldrabandoned sites where the nec- essary record of operations is no longer avail-
able or never existed, as in the case of unautho- rised dumping grounds. The model will draw
from current concepts in geotechnics and con- taminant geochemistry, and stress the complex
geometry of landfill sites, the heterogeneous material compositions, and the attendant com-
plex biogeomorphic processes in landfill envi- ronments. The analogy between refuse decom-
position processes and weathering of geological materials will be explored, and used to guide
the development of a conceptual resistivity-vs.- depth model for old covered landfill sites. The
versatility of this particular model will be tested using depth soundings from various geographic
regions. In line with the exciting geochemical observation that some chemical parameters of
Ž landfill leachates vary consistently with age e.g.
. Farquhar, 1989; DoE, 1996 , we investigate the
relationships between diagnostic leachate chem- Ž
ical parameters electrical conductivity, total
Ž .
. dissolved solids
TDS and chloride content
and attempt to propose a plausible practical scheme for fill-age and hydrochemical predic-
tions using surface geoelectrical measurements. Some illustrative field examples will be pre-
sented in support of the proposed predictive scheme.
2. Model development
Kategori