Building on their prior studies (earlier post, earlier post), researchers at the University of Texas at Austin have assessed the lifecycle water intensity in “gallons of water per mile traveled” for light duty vehicle (LDV) travel using selected fuels based upon petroleum, natural gas, unconventional fossil fuels, hydrogen, electricity, and two biofuels (ethanol from corn and biodiesel from soy).
Carey King and Michael Webber analyzed the amount of water withdrawn (used and returned directly to its source) and consumed (not directly returned to its source) during the production and use of different fuels. Their findings suggest that producing alternative fuels could strain already limited water supplies in some regions of the country.
They found that fuels more directly derived from fossil fuels are less water-intensive than those derived either indirectly from fossil fuels (e.g., through electricity generation) or directly from biomass.
The lowest water consumptive (<0.15 gal H2O/mile) and withdrawal (<1 gal H2O/mile) rates are for LDVs using conventional petroleum-based gasoline and diesel; non-irrigated biofuels; hydrogen derived from methane or electrolysis via non-thermal renewable electricity; and electricity derived from non-thermal renewable sources.
LDVs running on electricity and hydrogen derived from the aggregate US grid (heavily based upon fossil fuel and nuclear steam-electric power generation) withdraw 5-20 times and consume nearly 2-5 times more water than by using petroleum gasoline.
The water intensities (gal H2O/mile) of LDVs operating on biofuels derived from crops irrigated in the United States at average rates is 28 and 36 gal H2O/mile for corn ethanol (E85) for consumption and withdrawal, respectively. Ethanol processed from corn grain from non-irrigated fields results in water consumption and withdrawal intensities of 0.15-0.35 gal H2O/mile and 0.33-0.56 gal H2O/mile, respectively.
If ethanol is processed from corn stover in irrigated fields, then water consumption is 2.6-46 gal H2O/mile (average of 19 gal H2O/mile) and withdrawal is 5.6-63 gal H2O/mile (average of 23 gal H2O/mile). Ethanol processed from corn stover from non-irrigated fields results in water consumption and withdrawal intensities comparable to corn grain at 0.25 gal H2O/ mile and 0.41 gal H2O/mile, respectively.
For biodiesel derived from soy in irrigated fields, the average consumption and withdrawal rates are 8 and 10 gal H2O/mile. If the soy fields are not irrigated, the consumption and withdrawal are 2 orders of magnitude less at 0.01-0.02 gal H2O/mile and 0.03-0.12 gal H2O/mile, respectively.
The difference in water intensity between irrigated and non-irrigated biofuel feedstocks (up to 3 orders of magnitude in gallons per mile) shows the tremendous amount of need to properly plan for their incorporation. Due to water resource limitations at aquifers that are already being used intensively for food crop production, using those same aquifers for fuel production may exceed existing limits. The enhanced use of biofuel crops that need less water and the organized planting of crops in water and rain rich areas can lessen the water impact of biofuels.—King and Webber (2008)
Moving to other fossil resources (coal, shale oil, tar sands), other than natural gas, to make liquid fuels approximately doubles the water consumption intensity compared to petroleum fuels, and the water used will likely be from inland sources where fresh water is already scarce, the researchers note.
Water consumption—without considering proposed technological reductions in water consumption—for converting oil shale to gasoline for use in LDVs is 0.15-0.37 gal H2O/mile. For oil sands the water consumption is calculated a little higher, at 0.20-0.46 gal H2O/mile. Water withdrawal rates are 0.71-0.86 gal H2O/mile for oil shale and 0.76-0.95 gal H2O/mile for tar sands.
The water consumption for converting coal and natural gas to Fischer-Tropsch diesel is 0.19-0.58 gal H2O/mile and 0.12-0.43 gal H2O/mile, respectively. Water withdrawal for coal and natural gas to F-T diesel is essentially the same as consumption because most water is used for processing the syngas, they said.
Making decisions while only considering aggregate water consumed and withdrawn on the basis of a region as large as the United States is too simplified. In practice regional impacts will dictate the successful implementation of any of the discussed fuels for LDV travel. Example regional impacts range from relatively localized around shale oil mining and coal to liquids refining to larger agricultural regions used to grow biofuel crops.
Future work needs to show the viable areas of the US where each fuel can be mined, farmed, refined, and consumed to minimize the regional impacts while maximizing broad economic and policy objectives that include water resource and energy sustainability. Where possible, the use of low-quality water sources, such as saline or reclaimed waters, can minimize the quantity of fresh water impact from most of the fuels included in this study. Policy makers should be aware that, due to the inherent distribution of water (through geology and weather), fossil, and natural resources, each state or region may not be able to contribute to the production of future transportation fuels in the same manner.King and Webber (2008)
Carey W. King and Michael E. Webber (2008) Water Intensity of Transportation. ASAP Environ. Sci. Technol., doi: 10.1021/es800367m