A Fine Line Between Dispersion and Retention: Oil-Sediment Interactions in Nearshore Marine Environments

Abstract

When crude oil that is spilled in a marine environment approaches the shoreline, it can interact with suspended sediment to form either microscopic oil-mineral aggregates (OMAs), or macroscopic surface residual balls (SRBs) or submerged oil mats (SOMs). The formation of OMAs benefits oil spill remediation by naturally dispersing oil, while the formation of SOMs and SRBs can trap oil near the shore and lead to the re-oiling of beaches for years following a spill. OMAs, SRBs and SOMs all form in nearshore environments when oil interacts with sediment; yet, no relationships between them have been developed in current literature. This work strives to connect research between microscopic (OMAs) and macroscopic (SRBs and SOMs) aggregates of oil and sediment by systematically reviewing and analyzing literature regarding either size-class of aggregate with a specific focus on their formation, fate, and impacts in nearshore environments. Following this review, the results of a laboratory study to determine the effect of crude oil weathering on oil-mineral aggregation are presented. It is well accepted that oil needs to be highly weathered to form SRBs and SOMs, but there is currently a lack of understanding of how the weathering of oil will affect OMA formation. Based on literature, we identified two conflicting hypotheses: OMA formation 1) increases with weathering as a result of increased asphaltene and polar compound content; or 2) decreases with weathering as a result of increased viscosity. Controlled laboratory experiments were carried out to systematically test these two conflicting hypotheses. Experimental results using light, intermediate, and heavy oils, each at five weathering stages, show a decrease in OMA formation as oil weathers.