The water footprint of biofuels: a drink or drive issue?

Ensuring inexpensive and clean water is an overriding global challenge noted as one of the Millennium Development Goals of the United Nations. This challenge will likely be intensified by the increasing demand for biomass-derived fuels (i.e., biofuels) for transportation biofuel needs, because (1) large quantities of water are needed to grow the fuel crops, and (2) water pollution is exacerbated by agricultural drainage containing fertilizers, pesticides, and sediment. These potential drawbacks are balanced by biofuels’ significant potential to ease dependence on foreign oil and improve trade balance(s) while mitigating air pollution and reducing fossil carbon emissions to the atmosphere. In the United States, the Energy Independence and Security Act of 2007 (EISA) mandated the annual production of 56.8 billion L of ethanol (15 billion gal/yr [BGY]) from corn by 2015 and an additional 60.6 billion L (16 BGY) of biofuels from cellulosic crops by 2022 (1), a total that represents 15% of the gasoline used in the U.S. in 2006 on an energy basis. The EISA requirements virtually guarantee a large increase in biofuel production. Furthermore, this mandated and subsidized change will occur largely free from the market pressures and environmental constraints that would normally apply. Although the growth rate of ethanol production in the current economic recession is uncertain, it vastly outpaced most U.S. industries in 2008, with record amounts of ethanol produced (>9 billion gallons) (2) and a corn harvest only slightly behind the 2007 record production (3). Continued growth could have far-reaching environmental and economic repercussions and it will likely highlight the interdependence and growing tension between energy and water security. Developing a sustainable national biofuels program requires careful consideration of logistical concerns (e.g., suitable production and distribution infrastructure) and of unintended environmental impacts. Numerous recent studies have considered the latter, with a primary focus on air quality (4-6), land use (7-9), and net energy value (10-15). These studies generally reflect beneficial environmental trade-offs for biofuels compared to fossil fuels, with a few notable exceptions that recently considered greater CO2 emissions associated with massive deforestation in tropical regions (8, 10, 16). However, the effect of increased biofuel production on water security has not been subjected to the same scrutiny (17). As biofuel production increases, a growing need exists to understand and mitigate potential impacts to water resources, primarily those associated with the agricultural stages of the biofuel life cycle (e.g., water shortages and water pollution)sherein referred to as the water footprint.