Ostensibly, Obama made the decision to block the pipeline because of concern over contamination of the Ogallala Reservoir, a vast underground water source that makes much of Midwestern agriculture possible.

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In arid regions, in places feeling a need for the lack of water, underground water resources are often the only source of fresh water.

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To put that figure in context, Iannelli says that the energy required to pump freshwater from underground (which, he says, is the most common source of drinking water in the West) typically produces just over 1.5 tonnes of CO2 per million gallons.

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