Concerns
Among the environmental concerns are that enormous amounts of water are used and the water is contaminated by the layers of earth it is pumped through as well as the fracking chemicals.
The cement to protect the ground water can get leaky with time, especially if acid water is used.
Radon and mercury are gases which can leak out from the shale formation. Other subsurface materials like lead or arsenic may be mobilized.
It cannot be excluded that fractures in rock formations grow in higher rock/soil formations or up to the surface.
Small earthquakes, so called induced seismicity, might result from changing the balance of forces in the rock formations.
There also are the concerns regarding energy politics, fearing that the hype can change the understanding that shale gas is a transition and not a replacement fuel.
Also methane, which is the main component of natural gas, is a potent greenhouse gas. It is poorly understood what impact increasing shale gas industry has on the climate. [1, 2]