Mapping the 3D subsurface structure of a legacy landfill using a multi-geophysical approach

Legacy landfill sites often contain buried trenches that are not well delineated, not documented to modern standards, with subsurface voids that can act as preferential pathways for water, sediment and contaminants. To evaluate whether geophysical methods can help identify these features in a landfill, we conducted integrated electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) survey along three collocated transects; at a closed 1950’s landfill site in Ontario, Canada. The survey used a multi-electrode ERT array and a 250 MHz GPR to image shallow structures associated with historical waste trenches and potential soil-pipe development.

The survey results reveal a consistent pattern: high-resistivity anomalies in ERT between 1-2 m depth align with wide, crossing hyperbolas in the respective GPR profiles. The high-resistivity anomalies are interpreted as waste trenches. Located between the identified trenches, elongated resistive zones corresponding to GPR troughs and dipping reflectors, are tentatively interpreted as sedimentary layers with possible soil-pipe-like connections. These results, overlaid with site monitoring data, will improve the overall clarity and allow a better understanding of these subsurface structures. Three trench-like structures with two connecting anomalies are imaged along each transect, demonstrating a repeating subsurface pattern. Deeper ERT anomalies (4-9 m) lack GPR counterparts due to the limited penetration of the 250 MHz antenna Further surveys will use different antenna types to achieve deeper resolutions.

The study results show that combining ERT resistivity contrasts with GPR hyperbola geometry, provides a reliable means of mapping buried trenches and potential erosion pathways at legacy landfill sites. Next steps include expanding the survey grid, forward modeling, integrating other geophysical methods that complements ERT and GPR, and developing 3D interpretations to support long-term environmental monitoring and risk assessment.