Critical shear stress of sunken, No. 6 heavy fuel oil in fresh water.

A series of flume- and laboratory-based experiments defined and quantified the thresholds of sunken oil transport using No.6 heavy fuel oil mixed with kaolinite clay. When the sunken oil became mobile, the current-induced bed shear stress exceeded a threshold value specific to the oil, known as critical shear stress (CSS). The oil's CSS was evaluated as a function of water velocity, water temperature, oil condition, and sediment size. Based on experimental results, the stages of oil transport were defined and empirical relationships using the oil's kinematic viscosity (vo) and sediment size were developed to predict oil CSS at each transport stage. For vo<2 × 104 cSt, multiple thresholds of movement were observed: (1) gravity dispersion, (2) rope formation, (3) ripple formation, and (4) break-apart/resuspension. When vo> 6 × 104 cSt, transport was more likely to occur as a single event with the oil remaining intact, saltating over the bed in the direction of flow.

Parsing the toxicity paradox: Composition and duration of exposure alter predicted oil spill effects by orders of magnitude.

Oil spilled into an aquatic environment produces oil droplet and dissolved component concentrations and compositions that are highly variable in space and time. Toxic effects on aquatic biota vary with sensitivity of the organism, concentration, composition, environmental conditions, and frequency and duration of exposure to the mixture of oil-derived dissolved compounds. For a range of spill (surface, subsea, blowout) and oil types under different environmental conditions, modeling of oil transport, fate, and organism behavior was used to quantify expected exposures over time for planktonic, motile, and stationary organisms. Different toxicity models were applied to these exposure time histories to characterize the influential roles of composition, concentration, and duration of exposure on aquatic toxicity. Misrepresenting these roles and exposures can affect results by orders of magnitude. Well-characterized laboratory studies for <24-hour exposures are needed to improve toxicity predictions of the typically short-term exposures that characterize spills.

Validation of oil fate and mass balance for the Deepwater Horizon oil spill: Evaluation of water column partitioning.

Model predictions of oil transport and fate for the 2010 Deepwater Horizon oil spill (Gulf of Mexico) were compared to field observations and absolute and relative concentrations of oil compounds in samples from 900 to 1400 m depth <11 km from the well. Chemical partitioning analyses using quantitative indices support a bimodal droplet size distribution model for oil released during subsea dispersant applications in June with 74% of the mass in >1 mm droplets that surfaced near the spill site within a few hours, and 1-8% as <0.13 mm microdroplets that remained below 900 m. Analyses focused on 900-1400 m depth <11 km from the well indicate there was substantial biodegradation of dissolved components, some biodegradation in microdroplets, recirculation of weathered microdroplets into the wellhead area, and marine oil snow settling from above 900 m carrying more-weathered particulate oil into the deep plume.