Multi-layer stratification of groundwater temperatures in Ordovician and Silurian aquifers at the Victor Diamond Mine, as evidence of groundwater recharge under past cold climate conditions and limited deep groundwater circulation in James Bay Lowlands, near Attawapiskat, Ontario

English Abstract:

The Victor Diamond Mine is located in the James Bay Lowlands, approximately 90 km west of Attawapiskat. The mine site is underlain by up to 270 m of flat lying sedimentary bedrock capped by up to 60 m of marine sediments from the post glacial Tyrell Sea. Groundwater temperatures collected from monitoring locations around the mine reveal three horizons of stable groundwater temperature, in addition to a shallow horizon of seasonal variation. These horizons are: an upper horizon of approximately 150 m of predominantly limestone bedrock with temperatures between 4.0 to 4.5 °C; a middle horizon of predominantly limestone from approximately 150 to 220 m depth with temperatures between 2.5 to 3.3 °C, and a lower horizon from 220 to 270 m depth of mudstone with temperatures greater than 5.0 °C. While the temperature of the upper horizon matches the average annual groundwater temperature in the overburden suggesting the temperatures are advection controlled, and the temperature of the lower horizon can be explained by the geothermal gradient, the temperature of the middle horizon is anomalously low. Based on groundwater chemistry data, previous investigators at the site had hypothesized that there was relic Tyrell Sea Water trapped in some aquifers. The temperature data presented herein supports this hypothesis. The presence of a wide area of low temperature groundwater at the site, approximately 4,400 years after the retreat of the Tyrell Sea and 7,900 years after the retreat of the glaciers suggests that there is limited groundwater circulation in the deeper limestone aquifers of the James Bay Lowlands. Furthermore, given the Tyrell Sea was present in the area for 3,500 years, the data implies either more active groundwater circulation during the Tyrell Sea period, or different hydrogeologic conditions possibly driven by a glacial advance over sea water.