Permafrost and terrain conditions at northern drilling-mud sumps: Impacts of vegetation and climate change and the management implications

Permafrost can provide a containment medium for drilling wastes deposited to in-ground sumps, but tall shrubs may proliferate on covers causing snow to accumulate, active layers to deepen and the ground to thaw. We evaluate these effects using a 2-dimensional heat transfer model to simulate the thermal evolution of sumps in warm (− 3.0 °C mean annual ground temperatures (MAGT)) and cold (− 6.0 °C MAGT) permafrost under varying snow and climate conditions characteristic of the Mackenzie Delta region. Application of climate and snow normals for Inuvik, Northwest Territories, south of treeline, and Tuktoyaktuk, on coastal tundra, maintained wastes within frozen ground at temperatures below − 1.5 °C in warm permafrost and − 3.0 °C in cold permafrost, respectively. A gradual increase in snow depth from 0.17 m to 1.5 m simulating the effect of shrub growth on snow accumulation, caused thawing by the third decade. In the absence of shrub growth and increasing snow, moderate climate warming (0.09 °C/year) also caused sump thawing after 35 years for the warm scenario, but for the cold scenario wastes remained below − 2 °C through to year 40. Climate warming and increasing snow depths hasten thermal degradation. Modeling results indicating sump degradation due to deepening snow were corroborated by snow and ground temperature measurements, observations of collapsed shrub covered sumps in the Mackenzie Delta region and the local absence of permafrost where deep snow accumulates over mineral soils. Although thawing increases the mobility of sump contents, the associated subsidence of the sump and adjacent areas may inhibit lateral movement of the wastes. Several factors combine to influence the integrity of sumps in permafrost indicating the need for a long-term management strategy.