Speaking as an environmental scientist with a really quite good working understanding of the hydrogeology of bedrock and surficial aquifers, petroleum contaminiation, as well as environmental fate and transport of toxins I can say this.
Fracking -can- be done responsibly. And when the price of the product retrieved justifies the amount of research necessary to prove that there is a negligible chance of expose to humans - at EACH WELLHEAD INDIVIDUALLY - backed up with some serious on-site data collection and environmental monitoring... and I mean of a sort that grad students dream of being able to do, nothing like what 'monitoring' is today. Then it -might- be safe. But I am certanly speaking of a research cost in the 100-400K per wellhead range.
So, basically, after all the other oil is gone, oil would be valuable enough to barely pay for fracking to be done safely. That's my opinion, and I find and clean up (yeah, right, I manage it) contaminated groundwater.
It's more valuable than oil, clean water... You can go a day without oil.