Dispersants are primarily used in conjunction with an oil release into an aquatic environment and predominantly into a marine environment. Dispersants are usually applied to surface oil via spray (airplane, helicopter or boat). Based upon conditions and contact accuracy their use may result in either oil, oil and dispersant mixture, or dispersant only in the water. Under correct application, low concentrations of dispersant alone may be observed in the environment but these will only persist for a few minutes in the open environment. Measured low level concentrations and the transient nature of higher concentrations should be taken into consideration when comparing dispersant application with untreated oil in a net environmental benefit analysis.
Dispersants are a blend of several surfactants that reduce the oil-water interfacial tension and work by enhancing the natural dispersal of oil (which can occur naturally via wave action) into the water column as smaller particles with greater surface area. The increased surface area enables more rapid biodegradation by micro-organisms present in the water column.
Figure 4.2: Environmental exposure route for oil dispersants
For the purposes of this example, dispersants will be considered as a chemical application (dispersant that did not interact with oil when applied). Dispersants are primarily utilised at low levels in offshore water with minimal depth criteria (i.e. 300 meters), but may be used in near shore applications with appropriate approval. Dispersants rapidly dilute in the open ocean (<10 ppm in minutes) and like dispersed oil, may cause temporary impacts to sensitive marine species. These are limited to the immediate spill vicinity (upper layer of water column i.e. top 10 meters) and for a short period after dispersants are applied. These impacts are generally limited to non-motile organisms that have reproductive schemes that can readily recover from large losses.
This example will attempt to identify potential ecosystem services during dispersant use in a variety of water environments including off shore (open water), near shore (coastal) and the transition zones (inlets and rivers). This example does not explicitly condone nor dismiss the use of dispersants in shallow marine or freshwater, however for the purposes of identifying ecosystem services in these zones consistent with the other scenarios, an attempt will be made to capture potential ecosystem services that might be considered in an assessment.