Biofuels have come under a lot of scrutiny lately do to their environmental and social consequences. Most studies of the impacts are on air quality, land use, and net energy yield. Biofuels such as ethanol burn cleaner than traditional fossil fuels, and normally have positive environmental trade-offs; however, a recent study in the journal Environmental Science and Technology reports that we have overlooked an input that may be the key to the success of biofuels: water.
Biofuels are made from plants, and plants require water—but how much? In rainy agricultural states such as Ohio or Iowa irrigation only provides 1% of the water, but drier states like Nebraska must irrigate 61% of the water for their crops. The amount of water needed varies on the crop, but the rates are still very high: in Nebraska it takes 800 gallons of water to produce just one gallon of corn-derived ethanol, after irrigation and processing. That total gallon amount, divided by the mile per gallon average for ethanol-powered cars (16 mpg), comes out to a whopping 50 gallons of water needed for just one mile. Biofuel made from sorghum irrigated in Texas works out to 115 gallons of water per mile. Energy production and oil extraction processes also require large amounts of water, but biofuel consumption is much greater by comparison.
| Process | L/MWh |
|---|---|
| Petroleum extraction | 10−40 |
| Oil refining | 80−150 |
| Oil shale surface retort | 170−681 |
| NGCC power plant, closed loop cooling | 230−30,300 |
| Coal IGCC | 900 |
| Nuclear power plant, closed loop cooling | 950 |
| Geothermal power plant, closed loop tower | 1900−4200 |
| EOR | 7600 |
| NGCC, open loop cooling | 28,400−75,700 |
| Nuclear power plant, open loop cooling | 94,600−227,100 |
| Corn ethanol irrigation | 2,270,000−8,670,000 |
| Soybean biodiesel irrigation | 13,900,000−27,900,000 |
Increasing the supply of biofuels means increasing plant yields to meet federal mandates, which is done through agrichemical practices and farm land expansion. Tilling more land causes groundwater degradation as well as raises the potential for soil erosion and nutrient runoff. Pesticides and fertilizers used to increase crop production then flow into and pollute costal waters, causing hypoxia such as that in the Gulf of Mexico.
The Energy Independence and Security Act will require the United States to produce 15 billion gallons of corn-derived ethanol per year by 2015, and 16 billion gallons of fuel from cellulosic crops by 2016. Scientists are wary of a dependence entirely on biofuel, and some say we need to include biofuel technology in an eclectic array of alternative energy sources to sustainably reduce our dependence on foreign oil. The biofuel industry should not be discarded outright. It has taken 100 years to get to where it is today, and more advancements could make it very successful: crops such as switchgrass that are very drought-tolerant can help reduce the dependency on water; the perennial miscanthus grass can grow ten feet in a year—there just is no fuel conversion technology for it yet. But if biofuel energy is heavily pursued as our only option we may have to choose between water for drinking and water for driving.
