I left a comment in the last post about cleaning up PCE, and I figured that it was past time to have a post with some scientific content.
Some of our biggest headaches in the environmental business are chlorinated solvents. The two biggies are related: tetrachloroethene (PCE, or perc) and trichloroethene (TCE). The former is a dry-cleaning fluid and the latter is a metal degreaser, and they're found practically everywhere. Also, TCE is a daughter product of PCE - if a bug (soil microbe) breaks down PCE, you end up with TCE, which can be broken down further to other, less chlorinated compounds.
PCE and TCE are problems for two reasons. The first is that they're super toxic. The maximum contaminant level (MCL) for drinking water is 5 µg/L (parts per billion). The second is that they are more dense than water.
PCE and TCE aren't terribly soluble. But they don't need to be very soluble to cause problems, with such a low MCL. So when they're spilled on the ground, they tend to sink down through the subsurface as a dense non-aqueous phase liquid (DNAPL), slide down any included surfaces, and pool on top of whatever little bit of resistance they encounter, often a tiny fracture in the bedrock. And from there, they will slowly dissolve over time, contaminating groundwater for decades.
Once in the bedrock, especially in super hard and resistant bedrock that doesn't fracture in predictable patterns (say, granite) and that is, say 50 feet below the ground surface, it's nigh-impossible to find the little pools of DNAPL that cause all the problems. You can drill holes all over the place, you can dig down and blast the bedrock, and you may only succeed in jostling the stuff further downward.
After three decades, we're getting better at finding and treating PCE, TCE, and other chlorinated solvents. But it's still time-consuming, expensive, and occasionally a matter of luck.