First, some of them do,clearly, get into the water. See the documentary Gasland. Second, the companies that pump these chemicals into the ground have made it very difficult to do actual science on the situation. Also, see Gasland for lots of examples of that.

EDIT: also, since many of the chemicals are liquids and they are injected into an aquifer that feeds wells, the only thing we really need to know is: are any of the chemicals lighter than water. Even if they are not, once the water is pumped out, the remaining liquids will certainly be next.