Migration of contaminants in groundwater at a landfill: A case study: 7. DC, VLF, and inductive resistivity surveys

Abstract

We report here the results of direct-current (DC), very-low-frequency (VLF) and inductive resistivity surveys of the abandoned landfill at Canadian Forces Base Borden. These surveys were undertaken initially to assist in mapping the migration of contaminants and continued to assess the relative usefulness of the three techniques for the task. All three techniques were successful in outlining the regions of highest landfill-derived dissolved constituents in the sand aquifer, but the electromagnetic methods have a significant advantage over DC resistivity in both resolution and cost. This conclusion is very site-specific, however; we emphasize the importance of evaluating geophysical methods through preliminary modelling before undertaking field work at a new site. Where the contaminant levels at Borden are less than about twice background, mapping their distribution with surface geophysics became unreliable. As has been noted in previous case histories of this type, surface surveys are ultimately limited in their usefulness by natural scatter in the apparent resistivity, variations which are unrelated to the variations in contaminant level. This lithological and topographical “noise” can easily mask the detail of a contaminant pattern and is generally inseparable from contaminant-related variations. If geophysical monitoring is to be useful, it is essential that resistivity surveys be undertaken as part of the pre-landfill hydrogeological studies in order to separate the natural scatter from post-landfill surveys.