Decadal and seasonal water quality trends downstream of urban and rural areas in southern Alberta rivers

Seasonal and long-term water quality changes downstream of urban and agricultural land uses were compared using chloride, fecal coliform (FC), Escherichia coli and discharge from five long-term river monitoring sites in the Bow and Oldman watersheds in southern Alberta. Water quality data from up- and downstream locations of two major urban centers (Calgary and Lethbridge) and single sampling locations downstream of three agricultural sites were evaluated. Significant monotonic, decadal increases in chloride mass fluxes observed downstream of both urban areas were consistent with increasing chloride fluxes in wastewater effluent from increasing populations. Significant step function decreases in FC concentrations downstream of the two urban centers (89% at Calgary, 70% at Lethbridge) observed after UV disinfection were introduced at upstream wastewater treatment plants, suggesting wastewater disinfection improved river water quality. Significant monotonic decreases in pathogen indicators were found at only one of the three agricultural sampling locations. Seasonal variations in indicator bacteria were consistent with a constant source at the urban downstream sites, while variable seasonal loading patterns at the agricultural sites were attributed to seasonally changing land use. This suggests that the urban centers are more significantly mitigating pathogens in rivers than rural areas despite their significant growth.