For a stormwater drain and was wondering if you would be able to separate two liquids (underground) by boiling off the one at the lower temp rate....

Assuming the two (or more) liquids have different boiling points, you could indeed separate them by boiling, but you would probably want to collect the liquids first so that they could be properly separated. This process is called distillation. The liquid with the lowest boiling point will boil off before the others at this temperature. By collecting the vapor using laboratory equipment, this liquid can be separated out. This requires a stillof some...

Assuming the two (or more) liquids have different boiling points, you could indeed separate them by boiling, but you would probably want to collect the liquids first so that they could be properly separated. This process is called distillation. The liquid with the lowest boiling point will boil off before the others at this temperature. By collecting the vapor using laboratory equipment, this liquid can be separated out. This requires a still of some kind. Otherwise, unless there were a sufficient system for allowing the vapor to escape, boiling the liquid underground would probably result in a large amount of vapor in the chamber, which would re-condense into liquid again when it cooled and mix back in with the liquid in the drain. 

Another way of separating out two liquids from a mixture is centrifugation. This is a process of spinning liquids of varying density in a centrifuge so that they separate out into layers by density. 

Both of these would work for separating out components of a mixture; however, it would probably require isolating or collecting the mixture first so that the proper equipment could be used.