My first thought at "Cookie Dough Tree" was this (NSFW-ish): http://www.thefrisky.com/2010-04-12/a-tree-that-smells-like-semen/ http://www.thefrisky.com/2010-0

My first thought at "Cookie Dough Tree" was this (NSFW-ish): http://www.thefrisky.com/2010-04-12/a-tree-that-smells-like-semen/ http://www.thefrisky.com/2010-04-12/a-tree-that-smells-like-... OP's contaminants are a class of chemicals called dense nonaqueous phase liquids (DNAPLs) and they have a nasty habit of collecting at the bottom of aquifers (they're denser than water), making them particularly hard to clean up. They eventually degrade into vinyl chloride, which is a carcinogenic and toxic gas with a (yup) sweet smell. (This is probably glossing over a lot, but it's the summary I got from my boyfriend who does site investigation for soil and groundwater contamination—It could be summed up as "never live downgradient from a dry cleaning establishment").