Characterization and aquatic impacts of combined sewer overflows in Greater Vancouver. British Columbia

Abstract

The Greater Vancouver Regional District has been assessing the environmental concerns and management problems associated with combined sewer overflows (CSOs) as part of their long range liquid waste management plan. There are currently 52 CSO outfalls in the region with some of the major outfalls having 100-150 discharge events/yr concentrated in the winter months. Physical/chemical/microbiological characterization studies of CSO discharge has been investigated at 7 sites. Statistical analysis of these quality data have demonstrated that it can be segregated into three groups based on land use characteristics, namely residential, light industrial, and heavy industrial. These quality characteristics were used along with land use characterization of the 52 collection areas and discharge data to determine the loading of contaminants to the different waterways in the region. The contaminant loading in the region is dominated by the Clark Drive CSO since it contributes 40% of the annual discharge volume to the inner harbor of Burrard Inlet. Contaminant loading calculations from other sources demonstrates that, the sewage treatment plant predominates in the outer harbor, while the CSO and stormwater predominate in the inner and central harbor respectively. At the large Clark Drive outfall, dye tracer and numerical modelling were used to characterize the dispersion plume. Ecological effects of the contaminants were determined using the sediment quality triad. Silver sediment contamination, toxicity bioassays, and benthic invertebrate community structure analysis were useful indicators of the environmental effects of this discharge. Similar studies at the English Bay CSO, which only has 1/90 of the Clark Drive discharge, found sediment contamination levels that exceeded sediment contaminant criteria, but ecological impacts were less severe.