Joe,

I know of no specific evidence.  I do know that a proper cement and casing design, combined with the fact the hydrocarbons are >1 mile below the water table, is the best defense against the issues of contaminated water.  As with any design or construction project, if one cuts corners or does not have a firm grasp on the principles, contamination can occur.  There is also the issue of surface contamination, as the amounts of product become very large for some wells.  To me, informed oversight and regulation would be the way to go, al a house inspections during building.

The bigger issue with shale gas is the water used to perform the treatments.  Many companies are working and coming out with systems to treat and recover the produced water (either injected or naturally occuring salt water from the formation) as it flows from the well with the intention of reusing for the next set of wells.  Once the cost of these systems are brought down to a lower level, and the inevitable increase in the cost of fresh water (hauling, permits, and the like) occurs, we will reduce the water use for these treatments, but not eliminate it.  I assume some really smart people are looking at using either less water in general, or some other means of propelling the required "sand" into the formation to produce the gas.