Another big problem with these families of contaminants, is they are heavier than water. Contaminants such as oil, gasoline, and diesel, are lighter than water.

Another big problem with these families of contaminants, is they are heavier than water. Contaminants such as oil, gasoline, and diesel, are lighter than water.(Called LNAPL- light non aqueous phase liquids.) and therefore float on water. So when they sink into the ground, they hit a water table and stop, floating on that water table. Water tables are about 5 to 50 meters underground - moving very very, slowly - usually off sight (towards a neighbor.) It is fairly easy to pump that to contaminated layer of water up, treat it, and pump it back down, a bit downstream. Chlorinated and fluorinated compounds, and other similarly constructed organic compounds are DNAPL - dense non aqueous phase liquids. They sink. They hit the water table, and keep sinking, not only below the water table, but further. Finding cracks in the bedrock and they keep sinking. It is almost impossible to get at them to clean them all up . Some small fraction of the DNAPL pollutants may be transported with the ground water, the rest stays and goes deeper. And you can't get at them. You need to drill: very deep, through bedrock, and many holes, which aren't connected. Read as: very very expensive. Once you remove the source of the contaminants,. You can reduce the main part of the problem. They are also somewhat volatile, so they sometimes migrate up. Sometimes as gases. (But that's another story.) Meanwhile, quite often the treatment technique, can make the contaminants more toxic! Oxidizing a contaminant with chlorine in it can turn it into a simpler, less complex contaminant, which may be more reactive, and/or more toxic. In summary it's the sort of a system that is complicated and one has to understand it, and think about the possible scenarios. Often it is cheaper and most realistic to pave such a site, hence isolating the contamination, underground. (In some cases, depending on the results of a risk analysis, continuous or yearly monitoring is mandated, to see if the contamination is spreading off site. For example dry cleaning solvents, may reduce to PVC (poly vinyl chloride), which may further to reduce to Vinyl Chloride - an even more toxic contaminant.