evaporative cooling towers to release excess heat or discharge water to nearby rivers. Natural gas combined-cycle power (NGCC), which captures the exhaust heat generated by combusting natural gas to power a steam generator, are considered the most efficient large-scale thermal power plants. One study found that the life-cycle demand for water from coal power in Texas could be more than halved by switching the fleet to NGCC. All told, shale gas development in the United States represents less than half a percent of total domestic freshwater consumption, although this portion can reach as high as 25 percent in particularly arid regions. Hazards Drilling depths of 1,000 to 3,000 m, then injection of a fluid composed of water, sand and detergents under pressure (600 bar), are required to fracture the rock and release the gas. These operations have already caused groundwater contaminations across the Atlantic, mainly as a result of hydrocarbon leakage along the casings. In addition, between 2% and 8% of the extracted fuel would be released to the atmosphere at wells (still in the United States). However, it is mainly composed of methane (CH4), a greenhouse gas that is considerably more powerful than CO2. Surface installations must be based on concrete or paved soils connected to the road network. A gas pipeline is also required to evacuate production. In total, each farm would occupy an average area of 3.6 ha. However, the gas fields are relatively small. Exploitation of shale gas could therefore lead to fragmentation of landscapes. Finally, a borehole requires about 20 million liters of water, the daily consumption of about 100,000 inhabitants. Economics Although shale gas has been produced for more than 100 years in the Appalachian Basin and the Illinois Basin of the United States, the wells were often marginally economic. Advances in hydraulic fracturing and horizontal completions have made shale-gas wells more profitable. Improvements in moving drilling rigs between nearby locations, and the use of single well pads for multiple wells have increased the productivity of drilling shale gas wells. As of June 2011, the validity of the claims of economic viability of these wells has begun to be publicly questioned. Shale gas tends to cost more to produce than gas from conventional wells, because of the expense of the massive hydraulic fracturing treatments required to produce shale gas, and of horizontal drilling. The cost of extracting offshore shale gas in the UK were