After this there are no volatiles left so no more evaporation takes place. The surface oil is thus dispersed by a series of chemical and physical processes , some components are lost by photochemical oxidation. Most of the remaining compounds are broken down by microrganisms on the sea or river bed. The rate of bio-degradation is affected by oxygen availability, temperature, the type of oil, amount of emulsification, and availability of nutrients. As both Phosphorous and Nitrogen may be limiting if these nutrients are added the process can be speeded up.
Contentious subject is that of the disperants used in clean up operations.
The problem with emulsifiers and dispersants is that their surface active agents make the cell membranes of the organisms more permeable and therefore this increases the penetration of toxic compounds. These obviously have to happen pretty quickly after the incidents and they are difficult in rough seas although easy in river. Booms are used to stop the slick spreading further, it may be sucked up or skimmed up by a pump, or in the case or rivers it is sometimes possible to flush the river out, if there are reservoirs or locks upstream. In places such as chemical works or garage forecourts surface drainage can be diverted through interceptor sewers and taken for treatment elsewhere. Another method for cleaning up polluted beaches is to use some absorbent material to soak up the tarry residues.