URBAN RIVER SAMPLING FOR NON-POINT SOURCE CONTAMINANTS IN THE CONTEXT OF GROUNDWATER–SURFACE WATER INTERACTION

Many agencies conduct long-term river water sampling programs. Increasing concern for non-point source (NPS) groundwater pollution loading to rivers requires insight into bank storage and groundwater – surface water interaction at a watershed scale. River sampling to understand NPS impacts needs to be conducted when bank storage (i.e. groundwater with a residence time of less than one season) has been discharged, and baseflow consists of groundwater with a sufficiently long residence time to have been impacted by overlying land use.

The Bow River is an eastern slopes river that has extensive groundwater – surface water interaction with its sand and gravel alluvial aquifer. Major bank storage occurs during mountain melt, with peak annual flows typically observed in June. The river flows for more than 45 km through the city of Calgary (population 1.3M).

River chloride concentrations (primarily from road salt) were used to separate bank storage from alluvial aquifer groundwater. Historic water quality sampling data was analyzed in combination with a year-long detailed surface water sampling and stream gauging program conducted at five Bow River sampling sites within city limits. The highest NPS solute concentrations were observed near the end of the baseflow period, when baseflow contained the lowest portion of bank storage. The ratio of bank storage to alluvial aquifer ground water in baseflow and mass flux of road salt chloride to the river were estimated.