HYDROGEOCHEMICAL SETTING OF THE ATHABASCA RIVER: CHARACTERIZING THE GEOCHEMICAL AND ISOTOPIC LABELLING OF NATURAL SOURCES OF SALINITY

The Athabasca Oil Sands Region is an important oil reserve for Canada and the world. Identifying impacts of oil sands development to surface and groundwater resources is complicated the Athabasca River is incised directly into bitumen saturated sands of the McMurray Formation, and other saline Cretaceous and Devonian units. The input of natural saline groundwater from these formations has been attributed as the cause for the large increases in solutes that occurs in the river north of Fort McMurray, and better understanding the background inorganic and organic inputs from the different geological units is necessary to identify potential tracers or indicators of potential oil sands process water impacts on surface or groundwater quality. Here we compile new isotope data collected from various seep sampling campaigns with regional groundwater and river water datasets to better understand the potential sources of dissolved solutes entering the Athabasca River from natural groundwater discharge. Electrical conductivity surveys conducted along the Athabasca River were used to identify areas with elevated terrain conductivity to detect areas where high salinity groundwater could be discharging to the river. Porewater samples from the alluvial sediment in potential discharge areas were obtained using drive point piezometers installed between 1- 3m below the sediment interface. The porewater, groundwater and river water isotope data provide information about the sources of the water (δ¹⁸O and δ²H), and solutes (δ³⁴S-SO4, δ¹⁸O-SO4, ⁸⁷Sr/⁸⁶Sr) and groundwater ages (³H, ¹⁴C). The porewater in the alluvial sediment showed variable degrees of mixing with the overlying Athabasca River water, but the geochemical and isotopic composition of these samples are consistent with discharge of Cretaceous and Devonian Formation waters. The distribution of the seep geochemistry and some of the bulk river chemistry and isotopic labelling are related to changes in geology along this stretch of the Athabasca River. The results of this investigation provide insight into the geochemical evolution of riverine water quality, and help identify the natural background sources of salinity and organics that need to be understood to develop appropriate tracers for identifying anthropogenic inputs and for monitoring network design.