Abstract
Annual flow volumes and contaminant mass discharges from such sources as stormwater runoff, combined sewer overflows (CSOs) and sewage treatment plant (STP) effluents were computed for 47 urban centres located in 17 Canadian Areas of Concern (AOCs). Urban centres were defined as entities with sewage treatment plants servicing populations greater than 1,000.
The relative annual flow volume contribution from individual sources varies significantly among AOC areas. By effluent type, stormwater runoff contributes 17 to 65%, CSOs 1 to 6%, and STP effluents about 35 to 80%. During wet weather, this distribution changes significantly, and stormwater runoff, CSOs and STP effluents contribute about 80, 7 and 13% of the total volume, respectively. In terms of annual solids loadings in the AOCs, stormwater runoff generates 49 to 96%, CSOs 2 to 20%, and STP effluents 4 to 39%. During wet weather, the solid loads are generated almost entirely by stormwater runoff and CSOs.
For calculation of contaminant loadings, the concentration data from large urban and industrial catchments and a few smaller communities with mostly residential land were pooled together and used to compute loadings for other areas. Therefore, the computed loads are considered order-of-magnitude estimates, which are sufficient for planning level analyses. More accurate estimates would require local contaminant concentrations and flow data. The highest annual loadings of toxic contaminants were found for trace metals, followed by total PCBs, and a few trace organic compounds. In general, stormwater runoff contributed the largest contaminant loading. In most cases, a detailed assessment of the relative contaminant contributions from various sources and for various trace organic compounds was not possible because most of the data were at or below the analytical detection limit.