Fontino cites an ancient law that discusses the pollution of water in fountains, and basically states that no one is allowed to dirty the water of any public fountain. If someone did dirty the water, they could be fined ten thousand sesterces.
In Latin, the word dirty, in this old law, used the word oletato. For the sake of clarity, Fontino explains that the phrase, " Oletato videtur esse olidam facito", which was in the old law, translates to mean, "Oletare [= dirty] meaning "to make dirty".
The fountain in Trastevere was using polluted water. The ancient writers dubbed the fountain “fons olei”, which by this they mean the fountain fed by polluted water”, and by which the later Christians misinterpreted the meaning of the word “oil” because “olei “ is very close to “oletus” which mean oil, hence the idea of the oil fountains, which were in fact just polluted water fountains.
Unfortunately, this means no hidden petroleum!