Various articles about fracking are available here.
Anyway, the articles frequently mention that, in addition to the water pumped at high pressure into the ground, the companies also inject chemicals. I have been wondering why, until I read this article about petroleum. It's nice written and well worth a read if you're interested in this at all.
The new gas deposits are contained in porous rock, kind of like a sponge. Fracturing the rock allows the gas to escape. The rock containing the gas is porous, but it also must be permeable, i.e., the pockets of gas must be somewhat connected so that all (or most, or at least much) of the gas will flow out once it's fractured. Here's the money quote from the article:
Another involves fracturing the reservoir rock by pumping fluids and sand into it under high pressure. The fluids open cracks, and the sand keeps them open to let out the petroleum. This can overcome low permeability, and in the eastern U.S. states large new reserves of natural gas have been found through such techniques. Treating the wellbore with various acids or solvents can also raise permeability.So, I guess the acids and solvents dissolve some rock and create or enlarge the connections among the gas pockets so that the gas will flow out faster.
Well. That's odd. Knowing the reasons for injecting acids and solvents into the ground doesn't really make me feel any better about the risks to the ground water.