Analyses of Sediment Bacteria Monitoring Data from Two Deep Ocean Raw Wastewater Outfalls, Victoria, BC

The sediment bacteria densities in the vicinity of two deep ocean municipal sewage outfalls, Clover Point and Macaulay Point, have been monitored annually since 1985. Each outfall measures 0.9 m diameter and is located in water depths in excess of 60 m. Sediment bacterial densities were used to define the spatial extent of the effluent plume. Thirteen years of sediment fecal coliform density data were analysed to determine whether temporal or spatial trends existed. In general, triplicate field samples were collected at various distances (0–1600 m) from each outfall in numerous compass directions. In addition to the field triplicate data, laboratory triplicate, blank analysis and reference location data were available for assessing the accuracy and the precision of the collected data. Sediment fecal coliform densities were found to be statistically greater in magnitude and above reference densities within 280 m of the outfalls; however, the spatial patterns were not consistent for each year. This region of elevated fecal coliform densities was confined to a distance of twice the length of the diffuser from the outfall. The annual means in the east and west directions at both locations were in general different each year. At Clover Point, the annual fecal coliform densities at the sampling locations did not change in the periods before 1994 and after 1994. Macaulay Point data were similar except that there were two anomalies, namely an increase in the annual means in the period after 1994 at 400 m east, and a decrease in the period before 1994 at 1600 m east. Because of the complex dynamics of fecal coliform bacterial cells in the receiving water, the sediments and the wastewaters, as well as the imprecision of the measurements of sediment fecal coliform densities, sediment fecal coliform densities are a qualitative measurement of the effect of the discharge on the local sediment quality. Very few statistically significant spatial or temporal patterns were found in the historical monitoring data, reflecting the qualitative nature of these measurements even with an extensive quality control and quality assurance program and large data sets.