The WSJ has a good article discussing the history (briefly) and issues surrounding the use of hydraulic fracturing. A sample: "Hydraulic fracturing and some other technology improvements have created a way to tap a domestic fuel source that has proved abundant. U.S. natural-gas production has risen about 20% since 2005 in large part because of these developments, making gas a much bigger player in energy-policy planning." And: "What most worries environmentalists isn't the water in the fracturing process—it's the chemicals mixed in the water to reduce friction, kill bacteria and prevent mineral buildup. The chemicals make up less than 1% of the overall solution, but some are hazardous in low concentrations."
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