This is a kind of a can of worms type discussion, so here are cliff notes. I'll be happy to answer any other questions surrounding it, but there's just so much to discuss.
I tend to be against it, simply because it's not implemented safely. There are ways to do it 'safely', well so that the negative risks to human health are significantly minimized, but that's usually not the way its done. Main issues are:
1) The injection fluid is proprietary, and the composition of it does not need to be shared. But generally, there are harmful compounds in these fluids.
2) The formations that house the natural gas reservoirs are very deep in PA - thousands of feet in some instances. Typically they install casing into the borehole, which is steel pipe all the way down to the bottom of the hole. However, they do not always put casing the whole way down. When this is the case, there is no way to determine where the injection fluid is going to go. This isn't often the case, but does happen.
3) They attempt to pump out the injection fluid, but they are never able to remove all of the fluid. There is residual injection fluid left in the formation. Normally, this is very deep and the aquifer is usually not used, most drinking water wells are no where near that deep.
4) The disposal of the fracking fluid is either through treatment, or just injecting it back into the ground and leave it there. You run into the same issues as mentioned above.
5) I've been on many of these sites. The drillers and workers don't really give a shit about the environment. They don't follow the health and safety plans, or the environmental protection plans. They move from Site to Site. For example, the last time I was up there (this was 2015-2016 timeframe), many of the drillers just came from North (or South - I can't remember exactly) Dakota because that's where the work was. Then, that work dried up so they packed their family in their trailer and moved them to PA for 6 months. These guys don't give a shit, they just move forward.