Natural Cleaning of Shorelines Following the Exxon Valdez Spill

Abstract

ABSTRACT Field observations have identified a number of natural processes that contributed to the cleanup of the Exxon Valdez spill. This paper focuses on physical processes that remove oil residue from shoreline sediments. Data from shoreline surveys at 16 monitoring sites in Prince William Sound show significant reductions in both surface oil coverage and sediment oil content during the period from September 1989 to March 1990, while cleanup activities were suspended. Natural cleaning appeared to be dominated by wave action during storms. Changes in oiling conditions on high- and moderate-energy beaches can be explained as the result of wave action, based on beach profile data and time-lapse photography. However, observations from low-energy areas indicate that rates of change there were often as high as those from the more exposed and energetic wave environments. Studies of the oiled sediments have shown that interactions between the residual oil and fine mineral solids, such as clay particles, played an important role in the natural cleaning process. These interactions formed flocculated aggregates of a solids-stabilized emulsion in which small oil droplets are coated with micro-sized mineral fines and surrounded by seawater. In this form, the oil no longer sticks to the sediment surfaces so that it can be removed by low-energy waves or tidal washing. The large specific surface area of the flocculated oil promotes its eventual biodegradation by indigenous microorganisms.