That's an excellent question.
I'll amplify on the comment I made earlier. It's all about the scale of an operation. It takes upwards of 30,000 cubic metres of water to hydraulically fracture a shale gas well. That's 50 to 100 times more water than has traditionally been used in conventional wells. That also implies that the waste stream coming back from that well is going to be equally large.
When the industry says that they've had vast experience, 60 years of experience, with hydraulic fracturing, what they fail to say is that they've had fewer than 10 years of experience on a large scale using these unconventional methods to develop gas from shale.
It comes down to two things--one, the larger volume of fluids being used and fluid waste being produced, and two, the absolute necessity, because of the geological differences between gas distribution and shale and gas trapping in traditional wells, that it take a large number of wells per square kilometre, three wells per square kilometre. That means thousands and thousands, or tens of thousands, of wells for a particular place.