Generally hydraulic systems are closed circuit, meaning that the liquid used (generally oil) is filtered and stored in a reservoir. pneumatic systems are often open, as air from the atmosphere is...
Hydraulic fracturing breaks open the rock that contains oil or gas deposits and holds this fracture open with sand or proppant so that the oil or gas is able to flow into the wellbore for surface...
One safe alternative method I know of (working for the oil and gas industry) is called "Terra Slicing" it does not involve shape charged perforation but a environmentally and ecologically safe...