Engineering Geology

E n g i n e e r i n g G e o l o g y
Table 8.5. Classification of landfill sites based on their hydrogeology (after Barber, 1982).
With kind permission of the Water Research Council
Designation Description Hydrogeology
Fissured site, or site with Material with well- Rapid movement of leachate via
rapid subsurface liquid developed secondary fissures, joints or through coarse
flow permeability features sediments. Possibility of little
dispersion in the groundwater or
attenuation of pollutants
Natural dilution,
Permeable materials with Slow movement of leachate into
dispersion and little or no significant the ground through an unsaturated
attenuation of secondary permeability zone. Dispersion of leachate in the
leachate
groundwater; attenuation of
pollutants (sorption,
biodegradation, etc.) probable
Containment of leachate Impermeable deposits Little vertical movement of
such as clays or shales, leachate. Saturated conditions
or sites lined with exist within the base of the landfill
impermeable materials
or membranes
However, there is no guarantee that clay, soil cement, asphalt or plastic lining will
permanently remain impermeable. Thus, the migration of materials from the landfill site into
the substrata will occur eventually, although the length of time before this happens is subject
to uncertainty. In some instances, the delay will be sufficiently long for the polluting potential
of the leachate to be greatly diminished. One of the methods of tackling the problem of
pollution associated with landfills is by dilution and dispersal of the leachate. Otherwise,
leachate can be collected by internal drains within a landfill and conveyed away for treatment.
Selection of a landfill site for a particular waste or a mixture of wastes involves a consideration
of economic and social factors as well as the hydrogeological conditions. As far as the
latter are concerned, most argillaceous sedimentary, massive igneous and metamorphic rock
formations have low intrinsic permeability and therefore afford the most protection to water
supply. In contrast, the least protection is provided by rocks intersected by open fissures or
in which solution features are developed. Coarse-grained soils may act as filters leading to
dilution and decontamination. Hence, sites for disposal of domestic refuse can be chosen
where decontamination has the maximum chance of reaching completion and where groundwater
sources are located far enough away to enable dilution to be effective. The position of
the water table is important as it determines whether wet or dry tipping takes place, as is
the thickness of unsaturated material underlying a potential site. Unless waste is inert, wet
tipping should be avoided. The hydraulic gradient determines the direction and velocity of the
flow of leachates when they reach the water table and is also related to both the dilution
that the leachates undergo, and to the points at which flow is discharged. Gray et al. (1974)
426