Chloride as an Indicator of Non-point Source Contaminant Migration in a Shallow Alluvial Aquifer

Non-point source (NPS) contamination in the context of surface and ground water interaction was investigated in the Elbow River, Alberta, Canada. Groundwater flow direction, water table elevation, baseflow recession, chloride, nitrate and microbiological measurements were made to determine the interaction of several small tributaries and a small hamlet with Elbow River-associated groundwater. Groundwater flow in the alluvial aquifer adjacent to the Elbow River is predominantly subparallel to the river, but can vary seasonally by as much as 30°. Bedrock constricts the alluvial aquifer downgradient of the tributaries and the hamlet, causing groundwater to emerge into the Elbow River at this point. Chloride mass flux estimates suggest that septic effluent from the hamlet enters the groundwater immediately downgradient of the hamlet and discharges to the river from 5 to 12 km downgradient. A tributary creek which joins the Elbow River just above the bedrock constriction also contributes significant chloride to the river, and had significantly higher bacterial loads than the Elbow River, suggesting that land uses on this creek are also a significant source of contamination. Geologic sources, cattle grazing on the alluvial aquifer, road salting, golf course fertilizer use and wildlife are also potential contaminant sources.