Influence of seasonal variability of lower Mississippi River discharge, temperature, suspended sediments, and salinity on oil-mineral aggregate formation.

Under certain conditions, oil droplets that have separated from the main oil slick may become coated by suspended sediments forming oil-mineral aggregates (OMAs). The formation of these aggregates depends on suspended particulate characteristics, temperature, salinity, mixing energy, droplet size and number, and oil properties. The OMAs do not re-coalesce with the slick and tend not to adhere to surfaces, potentially evading surface cleanup measures, enhancing opportunity for biodegradation and reducing shoreline oiling. Potential OMA formation was quantified during four distinct states of the Lower Mississippi River during a typical year using empirical relationships from laboratory and field studies for three common oils and different combinations of discharge, temperature, suspended sediments, and salinity. The largest potential OMA formation for the two lighter oils, up to 36% of the total release volume, was in the winter and spring, when high sediment availability promotes formation. For the denser, high-viscosity oil, the peak potential OMA formation, 9% of the release volume, occurred in the summer, when the salinity was higher. These results provide some evidence that, depending on environmental and spill characteristics, the formation of OMAs could be an important, but unaccounted for, process in the fate and transport of oils released in the Lower Mississippi River and should be included in oil spill dispersion models and post-spill site assessment and remediation actions.