River Mixing and Microorganism Survival, Slave River, N.W.T.

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The study reported herein involved investigations of the transverse mixing and microorganism survival within the Slave River downstream of Fort Smith, N.W.T. The Fort Smith wastewater treatment and disposal system consists of a three cell lagoon system, followed by a continuous discharge to the Slave River through a submerged near-shore outfall. The study included field bacteriological sampling and tracer tests under open water and ice covered conditions. The purpose of the tracer tests was to provide an accurate measurement of the wastewater mixing (the dilution and distribution) within the river. The tracer data serves as a reference against which the time-dependent survival of microorganisms may be assessed. The mixing studies were conducted using continuous injection of Rhodamine WT fluorescent dye and sampling at several selected cross sections over a 40 km reach of the river. The tracer tests indicated the mixing in the upper portion of the study reach was considerably lower than expected from test results on other natural streams and modelling using reach-averaged geometry, velocity and transverse mixing coefficient would prove unsatisfactory. However, a numerical modelling scheme using local parameters was found satisfactory and used to find local values of the transverse mixing coefficient. Bacteriological sampling and analysis for total and fecal coliforms were conducted for each sampling station. After accounting for the physical mixing the limited data indicated a barely discernable microorganism die-off rate within the study reach under ice cover conditions. During the summer the high efficiency of treatment within the lagoon system reduced indicator concentrations in the river to levels which prevented any evaluation of the die-off rate. The numerical model for the mixing was adapted to allow prediction of the concentration of microorganisms within the wastewater plume for various decay rates.

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