Exploring permafrost’s role in containing legacy contaminants in the Mackenzie River Delta using electrical resistivity tomography

Oil and gas exploration activities of the past century have left the Mackenzie River delta of the Canadian Northwest Territories dotted with more than 200 drilling mud sumps. These sumps are pits created by excavating the ground, placing drilling fluid wastes and drilled cuttings inside and backfilling with excavated material. The initial assumption was that the permafrost would provide a containment and prevent the escape of the contaminated material to the environment. However, as the Arctic warms due to climate change, it is uncertain whether permafrost will keep performing this containing role in the long-term. In this work, we monitor the apparent electrical resistivity of the shallow subsurface using repeat Electrical Resistivity Tomography (ERT) measurements in order to understand the freezing and thawing of the sumps. This also allows us to observe changes in the temperature and ice content of permafrost beneath the sumps and the surrounding tundra. We look at data collected from August 2023 until August 2025 from 4-day to seasonal intervals, examining changes of the sumps over time and contrasting them with the surrounding tundra. With our results we aim to shed light on the thermal effect these legacy contaminated sites have on the surrounding landscape and describe changes that might affect their containment function.