Transport of Sewage-Contaminated Sediment in Northeastern Hamilton Harbour

Abstract

Dispersal patterns associated with sewage discharges from an STP outfall in northeastern Hamilton Harbour were obtained in 1992 and 1996 using coprostanol as a tracer. Both surveys revealed a distinctive shore-parallel pattern, characterized by a south-eastward trend and a second transport direction toward the northwest and southwest. As physical support for the tracer patterns, vertical profiles of currents near the outfall were monitored during four time periods from 1991 to 2000. Current data indicated the flow at the outfall was complicated; there was flow in all directions at all depths. However, the cumulative surface flow followed the prevailing wind (toward the east and northeast), while the bottom flow was approximately in opposition to the surface flow (to the west-northwest). Combining this circulation structure with the seasonal variation in buoyancy of the effluent discharge, the bi-directional transport pattern interpreted from the tracer distribution could be explained using a model in which the STP effluent plume was advected by local current at a position in the water column where the plume was stable. During January through August when the effluent was denser than ambient harbour water, it tended to be advected by flows lower in the water column moving the plume toward the north to west and into the central basin. During the rest of the year, it was lighter and more influenced by surface currents, which advected the plume onshore (toward the east and northeast) where it was deflected by shoreline geometry in a shore-parallel direction toward the southeast.