The Use of Chemical Dispersants to Control Oil Spills in Shallow Nearshore Waters

Abstract

Oil slicks should be dispersed in shallow nearshore waters to prevent oil from stranding. Field and laboratory studies show that chemically dispersed oil causes fewer adverse biological effects, and those are transitory. Spilled oil attains an average thickness of 0.1 mm or less in an hour or two on the water surface. Thus maximum concentrations in water are 100 ppm at 1 m and 10 ppm at 10 m. Most volatile/soluble hydrocarbons that cause toxicity evaporate from the slick in a few hours. Soluble hydrocarbons under field slicks and in chemically dispersed oil plumes are generally a factor of 150 to 1 million lower than those found to cause mortality of a wide range of organisms in laboratory studies. Chemical dispersants appear to protect some organisms, apparently by reducing oil droplet “stickiness.” A relatively few mechanically dispersed crude oil droplets, compared to many chemically dispersed droplets, caused (1) increased larval abnormalities in herring egg exposures and (2) increased petroleum content in adult coho salmon tissue. Chemical dispersion of crude oil prevented (1) mortality of mature mangrove trees in Panama and (2) adverse effects on intertidal organisms in Maine and immediate subtidal organisms in the Arctic. An oil spill is large or small relative to the receiving water. Scenarios of large and small spills show that there are no environmental reasons for not using chemical dispersants. A review of control capabilities shows that spills exceeding 160 m3 (1000 bbl)/day can only be accomplished by multiengine aircraft spraying dispersant, with mechanical methods assisting in critical areas. This assumes that the oil is dispersible. If not, most oil from spills over 160 m3/day may strand on shores. To be most effective, dispersant spraying must be initiated early, meaning that preapproval is necessary. Reduced adverse environmental effects along with greater control capabilities indicate that chemical dispersants should be used even in shallow nearshore waters.