Wastewater dispersion measurements in the Fraser River estuary, Canada, for the Annacis Island Wastewater treatment plant outfall

Abstract

Detailed measurements of effluent dilution and dispersion were completed in 1995 in the Fraser River estuary, Canada, as part of a larger study to design an environmental effects monitoring program. Wastewater is discharged at rates of 4-9 m3/s through an 18-port, 3-barrel diffuser in about 10 m of water in a tidal estuary 24 km upstream of the mouth. Rhodamine WT dye was used as a tracer and coordinated in situ measurements were carried out using a towed fluorometer system and a profiling system, each operated from separate vessels. Dispersion measurements were obtained for high and low river discharge conditions and for the complete tidal cycle. The surveying methods proved to be effective for both weak and strong current conditions. It was found that vertical mixing was rapid in the unstratified portion of the estuary and usually complete within 200-500 m of the diffuser. Lateral mixing was relatively slow and cross-channel mixing was not observed even at distances exceeding 12 km. Maximum intertidal zone exposure to effluent occurred on ebb tides but at low concentrations (dilution > 300:1) and confined to the north bank of the river. Minimum dilutions, of the order of 5:1 to 10:1, were found over the outfall at slack tide as the current reversed.