Using regional datasets of isotope geochemistry to resolve complex groundwater flow and formation connectivity in northeastern Alberta, Canada

Abstract

In the southern Athabasca Oil Sands (SAOS) Region, located in Alberta Canada, individual in-situ oil sands operators have conducted local geochemical and isotope groundwater studies to establish baseline characteristics and to improve understanding of the connectivity of water supply and disposal formations within individual leases. Canada's Oil Sands Innovation Alliance (COSIA) initiated this study to combine the data from these individual studies along with public information into a comprehensive unified dataset to evaluate conceptual models of regional groundwater flow and the degree of interaction and separation between important water supply and disposal aquifers. The unified dataset comprised over 293 groundwater samples from the main water source and disposal hydrostratigraphic units and included geochemical and isotopic tracers that can be used to identify sources (δ18O and δ2H) and age of water (3H) and associated solutes (14CDIC, 36Cl), as well as the sources of salinity and diagenetic processes (δ13CDIC, δ34SSO4, 87Sr/86Sr, δ11B, δ37Cl and δ81Br) and dissolved organics (δ13CDOC). The TDS and stable isotope composition (δ18O and δ2H) of groundwater from several Cretaceous formations were used to identify three main mixing controls. i) Areas where locally recharged groundwater mixes with glaciogenic water were identified by lower δ18O and δ18O values, high TDS, Cl:Br and δ34SSO4, 87Sr/86Sr, and δ37Cl values consistent with dissolution of evaporites. These areas were situated where vertical pathways through otherwise competent formations are likely to occur. ii) Mixing with paleo-waters characterized by high TDS and higher δ18O and δ2H signatures indicating groundwater recharged under warmer climate periods and/or evaporative enrichment were identified in the south, and southwestern portions of the study area. The presence of these paleo-waters indicate slow-moving groundwater zones with limited lateral and vertical connectivity. iii) Areas with potential connectivity to shallow aquifers were identified by their lower TDS values, modern meteoric δ18O and δ2H signatures, HCO3 type waters, and δ34SSO4 and 87Sr/86Sr signatures, consistent with weathering of shallow aquifer material. Geochemical and isotopic indicators suggest that in areas where the Colorado Group aquitard is thin or absent, there is greater connectivity between the Grand Rapids Formation and the shallower aquifers. Interpretation of the new unified dataset has improved understanding of the extent that glacial recharge had on Devonian aquifers, and overlying Cretaceous formations, and identified zones with relatively stagnant groundwater flow, both of which have implications for regional groundwater resource development, monitoring and potential wastewater disposal.