Gasoline is a combination of fuels. They are all different size molecules with different evaporation rates. It also has additives in it. What you are seeing is normal. The lighter portions of the gasoline evaporate first, (aka. butane) the heaviest last, and the additives might not evaporate on ground for an extended period of time. At the same time being a good solvent some of what you see as residual might have been pulled from whatever the gasoline leaked onto. On page 5 of this word article there is a partial list of what is in gasoline. https://www.atsdr.cdc.gov/toxprofiles/tp72-c3.pdf
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