The Effect of Pulp Mill Effluent on Fine-grained Sediment Morphology and Storage in the Fraser River at Prince George, B.C.

Abstract

A growing body of literature is examining the degree of and effects of the flocculation of fine riverine sediments. It has been shown that flocculation changes the morphology of the fine riverine sediment in the water column. The implications of these changes are as yet incompletely understood, but assumed to cause increased sediment deposition. Some recent studies have investigated the effect of pulp mill effluent as a flocculant, but have not satisfactorily ascertained the relationship between the effluent and flocculation. In this study, measurements of the fine sediment morphology upstream and downstream of a pulp mill were compared to measure the effect of pulp mill effluent as a flocculant. The observations were repeated weekly throughout the annual cycle of temperature, sediment concentration and discharge, allowing the effect of the pulp mill effluent to be put into the context of the natural variation of flocculation of river sediments. To measure the actual fine sediment deposition rates with and without the presence of the pulp mill effluent, a series of sediment traps were deployed during low and medium flow conditions. The size fractions trapped in them provided insight into the natural level of deposition and the amount of enhancement associated with the effluent.

The data show that the natural size of flocs in the Fraser River, as represented by differences in the effective and absolute particle size distribution, is small when compared to results from measurements in other rivers. Floc sizes are larger under low-flow conditions, especially under winter ice. The annual range of floc sizes of the natural sediments (~ 10 µm) was much larger than the increase measured due to the presence of pulp mill effluent (~ 1 µm). No statistically significant increase in the rate of fine sediment accumulation in the sediment traps was noted due to the influence of the pulp mill effluent, indicating that, at least in the near-field plume (300 m), it has no effect on deposition.