1. INTRODUCTION 
As populations increase worldwide, the multiplying 
numbers of people not only require more space in which 
to live, but also generate increasing amounts of waste. 
In Great Britain one of the oldest and most popular 
methods of disposing of this waste is the practice of 
landfilling; usually use being made of exhausted sand 
and gravel quarrires with the ultimate aim of returning 
the land to some sort of use. Often reclaimed land is 
used for recreational purposes (playing areas, parks, 
etc); however, due to the scarcity of good quality 
agricultural land, the pressure is now to attempt to return 
the sites to agricultural use. 
Only recently have the environmental implications of 
burying putrescible household waste been fully 
appreciated. In the anaerobic conditions that quickly 
become established in the landfill environment, 
microorganisms break down the decomposible elements 
of the refuse resulting in the production of leachate and 
'landfill gas'. Landfill gas is a combination of methane, 
carbon dioxide, oxygen, nitrogen and other trace gases, 
the actual proportions and quantities depending mainly 
on the age of the landfill and the original composition of 
the waste (Figure 1). Unlike solid contaminants which 
generally remain at fixed locations, landfill gas can travel 
some distance through permeable ground following the 
path of least resistance; the presence of landfill gas has 
been recorded at distances of up to 200m from the edge 
of sites (Flower, 1976; Leone et al, 1977; Carpenter, 
1986). Its lateral migration is encouraged by the 
practice of completing stites with a relatively 
impermeable clay cap to reduce infiltration into the site 
and so minimise leachate production. Gas will migrate 
either due to diffusion or as a result of the existance of 
pressure gradients, the type of movement depending on 
the conditions at the site (changes in atmospheric 
pressure and ambient temperature, moisture conditions 
within the cap, changes in the water table, rates of 
generation of gas, etc). 
There are two main environmental consequences of 
landfill gas migration: 
1. Gas may accumulate in confined spaces (such as 
under floors of buildings) until concentrations of 5 to 
15% by volume of methane exist, when there is serious 
risk of explosion. 
2. Gas may accumulate in sufficient quantities in the 
root zone of trees and other vegetation on, or adjacent to, 
sites such that the plants are subjected to stress and 
suffer impaired growth. Although vegetation damage 
related to landfill gas is well documented (Flower, 1976; 
Leone et al, 1977; Hewitt & McRae, 1985; Tankard, 
1987), the precise effects, reasons for the effects, and 
the concentrations and combinations of gases which 
influence plant growth, are complex and poorly 
understood. The influence of gas on plant growth and 
health is probably due to the exclusion of oxygen from 
the root zone, either as a result of gas being emitted at 
such a rate that normal soil gases are physically 
displaced, or as a result of the oxidation of methane to 
carbon dioxide by soil micro organisms (Hoeks, 1972; 
Pankhurst, 1973; Spreull & Cullum, 1987). 
Landfill gas-induced stress may manifest itself as 
chlorosis, reduction in plant height, yield or percent 
cover, early maturation, defoliation, and, in severe 
cases, death of the plant. This may have serious 
consequences for the return of completed sites to 
agricultural use; as well as for the surrounding 
agricultural and forested land. The problems on the site 
are exacerbated by soil movement and restoration 
practices; in many cases the soil is not given time to 
re-establish good structure prior to the planting of crops. 
To date, there is no data available to quantitively 
evaluate the effect on crop yields attributable to landfill 
gas. 
Thus, emission of landfill gas from completed sites 
may pose a serious problem in that it affects the 
subsequent utilisation, reclamation, and redevelopment 
of the sites and their surroundings. Despite this there is 
little information published evaluating the methods and 
methodology of detecting and monitoring the gas. 
Thus, the aim of the research was to relate the 
environmental impact of migrating landfill gas as 
indicated by changes in vegetation health to gas 
concentrations. If a correlation between gas levels and 
vegetation health can be established then remote sensing 
may provide a cost-effective and timely method of 
identifying and monitoring gas migration. 
NON-METHANOGENIC STAGES METHANOGENIC STAGES 
FIGURE 1. Variation of Landfill Gas Production with Time 
(Farquhar & Rovers, 1973). 
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