|MadSci Network: Chemistry|
There are many factors affecting the spreading of a thin layer of gasoline. Please check these webpages for more information:
Oil spill prevention
Although there’s no clear-cut calculation for the determination of spill coverage, here’s a site with some insight:
Movement of pollutants
Your question seeks a clear-cut determination of the coverage of gasoline over water. However, this is just not one of those clear-cut calculations. The factors in the API and EPA websites lean toward chaotic mathematics. I’m sad to say that the sources for answers such as yours are through mishaps. Spills that occur accidentally are also remediated as soon as possible. Gasoline has many compounds detrimental to living organisms. Materials like benzene, MtBE, xylene, plus many more that are either carcinogenic or poisonous. While reading articles, I did notice a disturbing figure, “...one gallon of gasoline can contaminate 250,000 gallons of water...” If an automobile carries between 20 and 30 gallons of gasoline, that could cause a great deal of damage to a body of water.
Try the links in the MadSci Library for more information on Chemistry.