I don't know how you would clean that up.
Well, that's a can of worms issue that I'll not mention here. And anyone who respects their secret clearances shouldn't say much either. But one has to remember things were done differently back them. And that's that.
To answer you’re question more specifically about chlorinated hydrocarbons, the answer is usually DNAPLS (pronounced dean-apples; Dense Non-aqueous phase liquids). It’s simple really, the chlorinated hydrocarbons are more dense (heavier) than water and so they will permeate into the cracks of the aquifer and are thus impossible to clean. Therefore, it’s sometimes best to just monitor them (via monitoring wells) and make sure the plume (contaminated water) stays within a certain contain area.