GROUNDWATER EVOLUTION IN POST-GLACIAL ALBERTA: THE FIRST 10,000 YEARS

Continental glaciation brings fresh unweathered bedrock and glacial sediments into the near-surface geologic environment where deglaciation exposes them to renewed hydrogeologic processes. Study of post-glacial environments provides insight into controls on the geochemical evolution of groundwater systems as well as contemporary processes that may evolve where climate change initiates decay and receding of permafrost.

Shallow groundwater systems in Alberta, deglaciated ~10,000 years ago, are hosted by Upper Cretaceous to Paleocene bedrock formations of the Western Canada Sedimentary Basin (WCSB), poorly lithified post-Paleocene gravels and finer clastics deposited on the Tertiary erosion surface, and Pleistocene glacial sediments. A small area of northeastern Alberta is underlain by exposed rocks of the Precambrian Shield and overlying Devonian through Upper Cretaceous rocks of the WCSB.

Shallow groundwater in Alberta is young, with δ²H, δ¹⁸O, and tritium values typically indicating modern to submodern recharge. However, evidence for older, cold climate recharge is also preserved in some regions, and is interpreted to indicate ‘fossil’ waters recharged by glacial melt waters more than ~10,000 years ago.

Groundwater salinities and compositions in Alberta are highly variable and reflect processes active in a young geological landscape. In the near-surface regime, groundwater can be subdivided into hydrochemical facies reflecting relatively recent atmospheric contact versus those that have evolved long since such contact. The overall evolution of salinity, as well as that of pH and major cations, indicates co-evolution of groundwater and soils and may reflect kinetic controls on silicate dissolution. Redox zonation is complex as oxidation of glacial sediments and near-surface bedrock is incomplete. Anthropogenic perturbations also lead to inadvertent release of arsenic and other compounds. We suggest that the diversity of groundwater compositions, the wide ranges in salinity, and natural and anthropogenic occurrences of arsenic and other trace metals in Alberta groundwaters reflects their evolution in an immature and evolving post-glacial landscape.