A closer look at bioaccumulation of petroleum hydrocarbon mixtures in aquatic worms

Petroleum hydrocarbons (oils) are ubiquitous in the aquatic environment, and adequate risk assessment is thus essential. Bioaccumulation plays a key role in risk assessment, but the current knowledge on bioaccumulation of oils is limited. Therefore, this process was studied in detail, using the aquatic worm, Lumbriculus variegatus, and 14 field-contaminated sediments. The main focus during the present study was on uptake kinetics, the relationship between oil boiling point fraction and uptake, and effects of sediment characteristics. Uptake kinetics became slower with increasing boiling point fraction, but 70 to 90% of the equilibrium situation was reached within the standard exposure duration of 28 d. Worms accumulated sedimentary petroleum hydrocarbons in the range of C(10) to C(34), a range much wider than expected. Biota-to-sediment accumulation factors (BSAFs) for separate boiling point fractions were constant and around the proposed value of 1 to 2 up to C(22), but gradually decreased beyond this point. The decrease was probably caused by a combination of nonequilibrium conditions and enhanced sorption of higher boiling point fractions to sediments; the latter possibly due to the presence of strongly sorbing separate oil phases or black carbon. A negative relationship was observed between BSAF and oil concentration in sediment, which was explained by the presence of separate oil phases at high oil concentrations. These strongly sorbing phases may limit their own availability, particularly when being highly weathered; worms may also avoid them. The observed phenomena have obvious implications for bioaccumulation assessment of oils and suggest that the current risk assessment procedure for oils in sediments may lead to erroneous results.