An integrated geophysical study of a landfill and its host sediments

摘要

Electromagnetic, magnetic and ground-penetrating radar (georadar or GPR) data recorded on dense grids have been used to investigate, a Swiss landfill and its host sediments. Specific objectives were to located clusters of steel drums that may contain dangerous chemicals, delineate the landfill borders, map the depth to the ground water table, determine the nature of the host sediments with emphasis on outlining potential pathways and barriers to fluid flow, and detect any leaking contaminants. Large volumes of ferrous material were identified throughout the landfill, with particular high concentrations of iron and steel situated below two broad areas and beneath a relatively narrow zone along its southern and western boundaries. Magnetic anomalies along these boundaries were consistent with the presence of several discrete centres of ferrous material buried at depths ～6 m, each containing material that was either much greater in volume or much more magnetic that a 50, 000-litre steel storage tank. Landfill borders could be defined to no better than ～10-15 m on the basis of standard apparent electrical conductivity and magnetic maps; broad high-amplitude anomalies due to buried ferrous waste extended well beyond the borders. Definition of the borders to ～2.5 m was provided by there-dimensional georadar data and by a novel coloured map that effectively combined information contained in the apparent electrical conductivity and reduced-to-the-pole magnetic data. Outside of the landfill, apparent electrical conductivities were uniformly <7 mS/m, consistent with the presence of ubiquitous dry gravel and sand at depths shallower than ～6 m. Guided by information supplied by two boreholes, the georadar data allowed both minor and major lithological variations to the traced beneath large undisturbed areas of the study site. Reflections from the upper surfaces of four extensive gravel sheets were mapped beneath the entire region southeast of the landfill. A prominent oval-shaped feature and conspicuous lineations on timeslices were interpreted as an ancient scour pool and the remains of ancient meandering stream channels, respectively. The most prominent georadar reflection, which was recorded everywhere outside of the landfill at travltimes >160 ns, originated from the groundwater table and a nearly coinciendent interface between highly permeable fluvial (above) and weakly permeable lacustrine (below) sediments. Structural relief as small as <1 m could be discerned on many of the geophysicaly mapped units. Important lithological and structural trends were consistently east-west to northeast-southwest, perpendicular or highly oblique to the present groundwater flow direction. Although our new geophysical data yielded no concrete evidence for contaminants leaking from the landfill, the interpreted lithological and structural details provided important constraints relevant to our understanding of the general flow regime beneath the study site.