Geophysical Techniques in Contaminated Lands Assessment — Do they Deliver?

Abstract

A review of physical property contrasts at landfills indicates that they are highly variable due to different disposal methods, materials and active chemical/biochemical processes. Substantial density, seismic velocity, electrical conductivity and dielectric permittivity contrasts can be expected. Old landfills normally have a low density and seismic velocity. High electrical conductivities may be present where contaminants and operative biological processes increase the free ion concentration of leachate. Organic contaminants may dramatically reduce dielectric constant. Seismic refraction field data from the Lucas Heights Landfill in south-western Sydney has been re-interpreted using the latest computer method. This re-interpretation defined an irregular sedimentary bedrock beneath the fill but did not locate the actual base of the fill due to the lack of seismic velocity contrast between saturated fill and weathered sandstone. Re-interpretation required the incorporation of a hidden layer representing the waste zone and underlying sand/weathered sandstone aquifer. At the same site interpretation of resistivity sounding data in terms of a 3-layered model defined the base of the cover material and the bedrock but could not distinguish fill from the underlying sand and weathered shale. The intermediate layer was highly conductive apparently due to leachate which flowed from the waste into the permeable sand within bedrock depressions defined in the seismic interpretation. The Lucas Heights field example raised problems of the minimum level of contamination which could be detected and the effect of thin contaminated layers. Analysis of these problems using synthetic examples demonstrated that conventional resistivity sounding alone may not provide sufficient vertical resolution if contamination is at a low level or, alternatively, if thin, highly conductive contaminated layers are present. Studies on synthetic electromagnetic (EM) examples with thin conductive layers also indicated that transient electromagnetics (TEM) may have an ability to markedly improve the resolution of highly conductive layers through the process of joint inversion with resistivity sounding data. However, TEM accuracy at landfill sites may be limited as their minimum depth of investigation could be too large for shallow targets. Ground penetrating radar has the potential to resolve fine detail in the subsurface. However, the rapid attenuation of radar signals in conductive materials poses a major limitation to its application at landfill sites. At Greenacre, in western Sydney, ground penetrating radar failed to achieve significant subsurface penetration in fills and saturated clays. Also, erroneous interpretations of contrasts between these materials and buried drums demonstrate the limitations of this technique. It is concluded that although no single geophysical technique can deliver at all contaminated landfill sites, a combination of techniques, underpinned by resistivity should provide the best possible resolution of contaminated zones.I1017