Baseline Levels of Volatile Hydrocarbons and Petroleum Residues in the Waters and Sediments of the Grand Banks

Baseline levels of low molecular weight volatile hydrocarbons and petroleum residues in the Grand Banks area were measured in April 1981 with a focus on the Hibernia and South Tempest sites where exploration for oil was occurring. Concentrations of volatile hydrocarbons ranged from 0.41 to 1.80 nmol/L (geometric mean = 0.74 nmol/L) in the water column and 0.05–3.20 mmol/L in the surficial bottom sediments. The former, almost exclusively methane, were of recent biological origin, while the latter, which also contained ethane, propane, and butane, were probably related to petroleum. There was no visible evidence of surface slicks at the time, and floating particulate petroleum residues were absent from most locations. Concentrations of dissolved/dispersed petroleum residues in the sea surface microlayer ranged from 14 to 440 μg/L (geometric mean = 28.9 μg/L) and in the water column from 0.05 to 4.1 μg/L. Concentrations of petroleum residues in the surficial bottom sediments ranged from 0 to 7.3 μg/g. While these levels are among the lowest found anywhere in the waters and sediments off eastern Canada and in the eastern Arctic, there was evidence that the oil industry, even at the level of its activity at the time, was having a detectable impact on background levels of petroleum-related substances in the sea surface microlayer and the surficial bottom sediments.