Lab tests on the biodegradation of chemically dispersed oil should consider the rapid dilution that occurs at sea

Abstract

Most crude oils spread on open water to an average thickness as low as 0.1mm. The application of dispersants enhances the transport of oil as small droplets into the water column, and when combined with the turbulence of 1m waves will quickly entrain oil into the top 1m of the water column, where it rapidly dilutes to concentrations less than 100ppm. In less than 24h, the dispersed oil is expected to mix into the top 10m of the water column and be diluted to concentrations well below 10ppm, with dilution continuing as time proceeds. Over the multiple weeks that biodegradation takes place, dispersed oil concentrations are expected to be below 1ppm. Measurements from spills and wave basin studies support these calculations. Published laboratory studies focused on the quantification of contaminant biodegradation rates have used concentrations orders of magnitude greater than this, as it was necessary to ensure the concentrations of hydrocarbons and other chemicals were higher than the detection limits of chemical analysis. However, current analytical methods can quantify individual alkanes and PAHs (and their alkyl homologues) at ppb and ppm levels. To simulate marine biodegradation of dispersed oil at dilute concentrations commonly encountered in the field, laboratory studies should be conducted at similarly low hydrocarbon concentrations.