The Physical Fate of Weathered Crude and Emulsified Fuel Oils as a Function of Intertidal Sedimentology

Abstract

The purpose of this series of field experiments was to study the initial behaviour and fate of contrasting oil types stranded on a wide range of sediment shorelines. Measurements were made of sediment grain size, moisture content, firmness and drainage parameters to assess their influence on oil infiltration. Oil was applied at 1 lm^-2 (Nigerian crude) and 2 lm^-2 (emulsified fuel oil) to a series of replicated field plots laid out at ca. mean high water of spring tides. Visual measurements of oil cover and infiltration were backed up by sediment hydrocarbon analysis carried out at intervals over a 1 year period. Residence times of the oils ranged from 3 days to > 1 year. The emulsified oil tended to infiltrate only as far as permitted by antecedent moisture content and permeability and, on muddy shores, only via large pores. Some plots were firmer as a result of oil application and this may increase oil residence time in a way analogous to the formation of “asphalt pavements”. Oil uptake was reduced on low shear strength sediments, but this may have been an indirect effect of waterlogging. Drainage of the surface sediments was a prerequisite to significant oil infiltration, but water table movements did not encourage, by “tidal pumping”, penetration deeper than ca. 2 cm. Finally, sediment % mud content may be used to predict the likely oil residence time, possibly because this sediment parameter is very sensitive to energy level, and also because mud particles retain surface-bound contaminants more strongly than do sands.