The bioavailability of oil droplets trapped in river gravel by hyporheic flows.

Abstract

Little is known about the fate of oil spills in rivers. Hyporheic flows of water through river sediments exchange surface and groundwater and create upwelling and downwelling zones that are important for fish spawning and embryo development. Risk assessments of oil spills to rivers do not consider the potential for hyporheic flows to carry oil droplets into sediments and the potential for prolonged exposure of fish to trapped oil. This project assessed whether oil droplets in water flowing through gravel will be trapped and whether hydrocarbons partitioning from trapped oil droplets are bioavailable to fish. Columns packed with gravel were injected with oil-in-water dispersions prepared with light crude, medium crude, diluted bitumens, and heavy fuel oil to generate a series of oil droplet loadings. The concentrations of oil trapped in the gravel increased with oil loading and viscosity. When the columns were perfused with clean water, oil concentrations in column effluents decreased to the detection limit within the first week of water flow, with sporadically higher concentrations associated with oil droplet release. Despite the low concentrations of hydrocarbons measured in column effluent, hydrocarbons were bioavailable to juvenile rainbow trout (Oncorhynchus mykiss) for more than three weeks of water flow, as indicated by strong induction of liver ethoxyresorufin-o-deethylase activity. These findings indicate that ecological risk assessments and spill response should identify and protect areas in rivers sensitive to contaminant trapping.