Vertical radar profiling for the assessment of landfill capping effectiveness

In this paper we present the results of the characterization of a large landfill cap by means of ground-penetrating radar (GPR) measurements. The GPR data were collected in boreholes, using a vertical radar profile (VRP) configuration, where one antenna was kept at the ground surface while the other was progressively lowered into the borehole. This yields a vertical profile of GPR velocity from which a moisture content profile can be obtained. VRPs were conducted in 15 boreholes available on-site, all having been drilled through the protective cap and the waste mass into the underlying native soil. The separation between the boreholes (many tens of metres) makes it infeasible to characterize the site via other forms of hole-to-hole GPR measurements, with the exception of a few pairs of holes drilled at the periphery of the waste mass and therefore of limited usefulness. The VRP data allowed for the characterization of the moisture content profile across the waste body down to the water table, very close to the natural land surface, providing evidence that the waste is generally fairly dry (moisture content less than 10% on average). In order to assess the effectiveness of the cap, two surveys where conducted in March and April 2005 using all the boreholes with the aim of identifying moisture content changes due to natural rainfall and especially artificial irrigation over a limited area surrounding one of the boreholes. The time-lapse VRP results show that in the irrigated area a measurable amount of water seeps through the cap and changes the waste moisture content. Elsewhere, we basically observed no changes in moisture content due to natural infiltration during the same period: this fact does not confirm or exclude that some degree of leakage can occur. Direct in-situ permeability measurements, albeit more localized than VRP data, confirm that the landfill cap is not totally impermeable, thus corroborating the results derived from geophysical measurements.

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