tarballs. Even though gasoline has the highest water solubility of all oil types, dissolved concentrations under slicks decrease rapidly by evaporation.
Light refined products, such as diesel, No. 2 fuel oil, jet fuels, and kerosene, are narrow-cut fractions that have low viscosity and spread rapidly into thin sheens. They do not tend to form emulsions except under very cold conditions. They evaporate more slowly (compared to gasoline) and incompletely; therefore, they are ranked as “medium” in terms of their horizontal transport or movement. As low-viscosity, moderately persistent oils, light distillates tend to disperse readily into the water column by even gentle wave action. Thus, they have the highest potential of any oil type for vertical mixing. There is also a greater potential for dissolution to occur, from both surface sheens and droplets dispersed in the water column. The water-soluble fractions are dominated by two- and three-ringed PAH, which are moderately volatile and may affect aquatic biology. Thus, spills of light distillates have the greatest risk of impacting water-column resources. Light distillates are not very adhesive; therefore, they do not adhere strongly to sediments or shoreline habitats. Loading levels on the shoreline are relatively low because of the thinness of sheens on the water surface and the low adhesion of stranded oil. The constituents of these oils are light to intermediate in molecular weight and can be readily degraded by aerobic microbial oxidation. Long-term persistence in sediments is greatest under heavy loading and reducing conditions where biodegradation rates for anaerobic bacteria are low.
Crude oils contain a wide range of compounds, from light to heavy; thus, they are affected by many fate processes. Evaporation can remove about one-third of the volume of a medium crude oil slick within the first day, but there will always be a significant residue. Many crudes will emulsify readily, a process that greatly reduces subsequent weathering rates. As a result, crude oil spills close to shore often strand and persist on shorelines, particularly on permeable substrates such as gravel beaches and sheltered habitats such as marshes. Crude oils tend to adsorb heavily onto intertidal sediments, with the risk of subsequent erosion of oiled sediments from the shoreline and deposition in nearshore habitats. Under high-energy, nearshore conditions, oil and sediments can mix and be transported to the bottom sediments. For spills that are transported offshore, the slicks eventually break up into fields of tarballs that can be transported long distances because they are so persistent. The water-soluble fraction of crude oils include a wide range of PAH. Dissolution from slicks and stranded oil can persist for weeks to years.
These oil types, such as No. 6 fuel oil, bunker C, and heavy slurry oils, lose only up to 10 percent of their volume via evaporation. Some products are so viscous that they cannot form emulsions, but many emulsify shortly after release. They show low natural dispersion because the oil is too viscous to break into droplets. These oils have the lowest water-soluble fraction; thus, loadings to the water column are generally low under slicks. Spills of heavy distillate quickly break up into thick streamers and then fields of tarballs that are highly persistent. The heavy distillate can be transported hundreds of miles, eventually stranding on shorelines and posing significant impacts to birds and other marine animals such as turtles. Because of their high density, these releases are more likely to sink after picking up sediment, either by mixing with sand in the surf zone or after stranding on sandy shorelines. Some heavy distillates are so dense that they are heavier than brackish or sea water and will not float when spilled.
Large volumes of produced water are discharged in offshore oil-producing areas. Produced water is treated to remove most free oil prior to discharge; however, the water still contains a moderate amount of soluble and volatile petroleum hydrocarbons. The concentrations of benzene, toluene, ethylene and xylenes (BTEX) in produced water vary by almost a factor of 10, with an average concentration of about 5 mg/L. The polynuclear aromatic hydrocarbons (PAH) also vary by about an order of magnitude, with an average concentration of about 1 mg/L. Because these compounds are already in solution, their concentrations are reduced rapidly by volatilization and dilution, particularly when released to open, well-mixed waters. Elevated levels of contaminants in sediments typically extend 100-200 m from the discharge point. There are occasional events in the treatment process that result in light sheens, but they are expected to disperse rapidly with a low risk of stranding on shorelines and no risk of emulsification or tarball formation.
Operational discharges from vessels generally occur more than 50 miles from shore and are concentrated along shipping lanes. Discharges are composed of bilge water, cargo tank washings, fuel oil sludge, and other oily wastes, which are all considered moderate in terms of their loss by evaporation and dissolution, formation of tarballs, and potential for long-distance transport. Vessel discharges pose a low risk of vertical mixing because the releases are generally viscous. Because these discharges are released in offshore waters, there is low potential for contact with sediments and sedi