CAN RIVER WATER DATA BE LEVERAGED TO UNDERSTAND GROUNDWATER CIRCULATION FOR A LARGE AREA?

Developing the liquid-rich shale gas plays in west-central Alberta relies on non-saline water for hydraulic fracturing, which is often sourced from surface water and shallow groundwater. Understanding and managing the cumulative effects of water use for large areas requires scientifically defensible geoscience at a comparable regional scale. As part of evaluating Alberta’s groundwater inventory, the Alberta Geological Survey is completing a hydrogeological characterization project for a 22000 km² region that is forested and relatively unpopulated. For such a large area of interest, we sought to use river water as an accessible integrator of groundwater circulation. Synoptic sampling of environmental tracers along three rivers was completed during low flow conditions in 2015. Analytes included major ions, stable isotopes of water, radon, SF₆, tritium, and noble gases. The sampled lengths of each river varied from 75 to 170 km, with an average sample spacing of about 20 km. All rivers were found to have SF₆ and tritium concentrations with an apparent age of approximately 7 years and stable isotopic signatures that indicate groundwater recharge from snowmelt. The interpretation of river water being sourced from localized groundwater flow systems agrees with the very limited hydraulic head data for the region. Spatial variation of radon concentration in river water does not appear to reflect variation in bedrock geology, but when combined with other analytes, provides a first-order estimate of variation in groundwater discharge. Concurrent hydrostratigraphic mapping and modelling has found gravel-capped plateaus with relatively shallow bedrock, and a highly heterogeneous bedrock formation. A conceptual model of groundwater circulation in the region is being developed by integrating the river water data, limited groundwater data, and results of hydrostratigraphic modeling. These data and conceptualization of the groundwater interaction with surface water will advance the understanding of regional water resources at a scale suitable for managing cumulative effects and better inform the development of energy resources.