I'm surprised no one has commented on the amount of fresh water we use to produce ethanol, particularly ethanol derived from corn.

Water is needed to grow the corn and also in processing it.

"The amount of water it takes to produce ethanol varies according to how much irrigation is needed for the corn, particularly since row crop agriculture for corn is the most water consuming stage of ethanol production. In Ohio, because of sufficient rainfall, only 1% of the corn is irrigated while in Nebraska 72% of the crop is irrigated. It takes 19 gallons of water to produce a bushel of corn in Region 5, 38 gallons in Region 6, and 865 gallons in Region 7. The Baker Institute estimates that producing the corn to meet the ethanol mandate for 2015 will require 2.9 trillion gallons of water.

Most of this irrigation water is drawn from groundwater aquifers in a region that is already water stressed. Conflicts over water allotments have occurred in Kansas and Nebraska, and the Ogallala Aquifer, which lies under the Great Plains and supplies 30% of the nation’s groundwater for irrigation, is in danger of running dry.

Growing corn also requires a great deal of fertilizer, and extensive use of nitrogen fertilizer and pesticides is having severe impacts on water quality now. Fertilizer laden runoff into streams in the Midwest makes its way to the Mississippi River, and eventually contributes to the eutrophication (when algae bloom, then die, depleting the area of oxygen and suffocating plants and animals) in the Gulf of Mexico. In 2010, this dead zone in the Gulf of Mexico was estimated to be almost 8000 square miles and it continues to grow. 2.39 million additional tons of nitrogen fertilizer will be needed to meet the 2015 mandate.

Easily erodible and less productive land set aside by the conservation reserve program, that paid farmers to retire inferior land, is now being pressed back into service due to the lure of high corn prices. This land will likely need additional fertilizer and irrigation to be productive, which will result in more polluted runoff and water consumption.

Although biofuels are promoted because they are said to reduce greenhouse gas emissions, some scientists argue that when the life cycle of ethanol production is compared to that of conventional gasoline, there may be no reduction in greenhouse gas emissions at all. More corn is being planted on more land fertilized with nitrogen, resulting in additional air and water pollution from nitrous oxide, a chemical 300 times more potent than carbon dioxide as a greenhouse gas.

Processing corn to make ethanol requires substantial amounts of water for the grinding, liquefaction, fermentation, separation and drying procedures. Over the last ten years, however, some of the nation’s 200 biorefineries have been able to reduce water use from 6.8 gallons of water per gallon of ethanol to 3 gallons by boosting the efficiency of their equipment. South Dakota based Poet,the world’s largest ethanol producer is aiming to cut its overall water intake by 22% by 2014, and use only 2.33 gallons of water per gallon of ethanol produced. This would reduce its annual water use by 1 billion gallons."

http://blogs.ei.columbia.edu/2011/03/21/ethanol%E2%80%99s-im...