Natural and anthropogenic inputs of hydrocarbons to the Strait of Georgia

Sediment cores from the Fraser estuary, Vancouver Harbour and Strait of Georgia, suspended sediment samples from the Fraser River and sediment grabs from Vancouver Harbour have been analyzed for alkanes and parent and alkyl PAHs (polycyclic aromatic hydrocarbons). Principal components analysis (PCA) clearly distinguishes mixed sources by separating parent PAHs according to molecular size, and separating alkyl substituted PAHs from higher plant PAHs. We find the Fraser River to be the predominant source for natural and anthropogenic hydrocarbons to the Strait of Georgia. The natural hydrocarbon burden from the Fraser River is augmented principally by petroleum hydrocarbons and combustion PAHs from Vancouver. Contaminated sediments from Vancouver Harbour very likely have also been transported to a major ocean disposal site off the Fraser estuary. Petroleum alkanes, which dominate Fraser River suspended sediment samples, are lost by processes such as dissolution or microbial degradation during transport and sedimentation, while PAHs from the river are delivered essentially unchanged to sediments in the strait. Hydrocarbon composition undergoes little change with depth at a reference location in the Strait of Georgia, indicating that PAH inputs have changed very little since the early part of this century. In Vancouver Harbour the low rate of sediment accumulation coupled with surface mixing has led to the retention of contaminant PAHs within the surface mixed layer, while the rapid delivery of sediments from the Fraser River has buried contaminant PAHs from historical ocean disposal in the Strait of Georgia.