Fracking takes place thousands of feet down, below impermeable rock, well below any water supply. What gets pumped down there is not coming back up except though the well head it went down.
The devil is always in the details. Yes, the fluids go through the wellbore. Which is a steel pipe thousands of feet long sealed with a cement liner. Now, this liner is a complicated thing - it's deep in the ground and hard to see. The well bore / liner also has a number of seals and fail safes. If done correctly, there is very little chance of damage to the rocks from the fracture site to the surface. If done incorrectly, there is quite a bit of chance that liquids will push out and some chance that this liquid will interfere with the aquifer. Or worse (ie, Deepwater Horizon)
Not everyone in the oil patch is competent and compulsive enough to do everything right all of the time. We CAN enforce bore testing and make these sorts of issues very, very unusual. AFAIK, only Texas and Louisiana have strong enough oversight to drastically limit wellbore 'excursions' (obviously, nothing is perfect). The Dakotas, Pennsylvania, NY have very little regulatory control over the wellbore quality.
Thus, you can expect a higher number of failed wellbores and thus pollution. Remember, even in unregulated states this doesn't usually happen, but it can be quite expensive to test and re cement a failed well (ie. Deepwater Horizon). Anyone trying to cut costs is going to let a marginal casing job slide.