HYDROGEOCHEMISTRY OF CRYSTALLINE BEDROCK UNDER DEEP PERMAFROST CONDITIONS

Changing environmental conditions on the surface of the earth can impact deep underground constructions. In order to predict the long-term performance of a geologic repository in Canada or northern Europe, the influence of periglacial and glacial conditions must be considered. This research strives to enhance the scientific basis for safety and performance assessment and to derive constrained permafrost scenarios relevant to the evolution of crystalline groundwater flow systems. Very little has been published regarding cryogenic processes and hydrogeology in crystalline rock under deep permafrost conditions. Thus, an objective of the field study is to make sure information obtained is transferable to a generic nuclear waste repository in a similar geologic environment.

The study was conducted at the Lupin mine in Nunavut Territory, Canada, where permafrost extends to 540 meters below ground surface (mbgs). Geologic and structural data was provided by the mining company. Satellite images, thermal conditions, and seismic surveys were utilized in structural interpretations of the site. Groundwater and gas samples were collected at various levels throughout the mine to a depth of 1130 mbgs. Many samples obtained from frozen levels of the mine were contaminated by Na-Cl brines used for drilling. Boreholes located directly under the permafrost indicate that an unsaturated zone or pockets of “dry permafrost,” may exist. It is currently too early to say whether this unsaturated zone is the result of 20 years of mine activities, limited recharge underneath permafrost, or a by-product of the freezing process.

Samples from deeper in the mine provide insight into chemistry of deep groundwaters below the permafrost. Waters range from brackish to saline, are Na-Cl dominant, and lack indication of contamination (high SO₄, NO₃, and tritium). Carbon isotope analyses of gases indicate an organic source of methane. Matrix fluid boreholes, leaching experiments, and analysis of chlorine isotopes are underway to determine sources and evolution of chloride in the system.