Quantifying baseflow and water-quality impacts from a gravel-dominated alluvial aquifer in an urban reach of a large Canadian river

Groundwater discharge and non-point source (NPS) loading were evaluated along an urban reach of an eastern-slopes Rocky Mountains river (Bow River, Canada) to understand sources of water-quality impacts and baseflow. The discharge did not increase measurably over a 16-km reach. Groundwater in the river-connected alluvial aquifer was a mixture of river and prairie groundwater, with elevated chloride concentrations (average 379 mg L^–1) from road salt. Alluvial groundwater was the major NPS of chloride discharging to the river. Although the mass-flux based estimates of groundwater discharge were small (mean 0.02 m³ s^–1 km^–1, SD = 0.04 m³ s^–1 km^–1, n?=?30), the associated chloride mass flux over 16 km was significant (equivalent to that discharged from the city’s largest wastewater-treatment-plant effluent). Although local groundwater baseflow was previously thought to contribute significantly to overwinter baseflow in this reach, little contribution was measured in this study. Low baseflow generation is consistent with long-term river discharge data that show almost all of the baseflow generation occurs in the Rocky Mountain reach. Thus, local watershed areas are important for water-quality protection, but climate change in the headwaters is most salient to long-term flow.