yield, largely are unknown. The distribution and physical and chemical characteristics of saline groundwater resources need to be defined to predict the effects of extracting saline groundwater on freshwater resources and on the environment (Alley, 2003). Without such information, coastal regions may be more suitable for large-scale saltwater algal production systems than inland regions (Darzins et al., 2010).

Data on the regional availability of fresh water, salt water, and other nonfreshwater (for example, wastewater) and on the regional demand of water for agriculture and other uses are needed to assess the potential availability of different water resources for algae cultivation.

4.1.6 Potential Effect on Social Acceptability

Water security is a pressing concern globally and an emerging concern in the United States. Algal biofuel production will, to a still unspecified extent, affect consumptive use of fresh water. Freshwater availability and quality are intricately related to agricultural productivity, human health, and safety. The security impacts to this system from largescale algal biofuel production could be significant. As global weather patterns continue to become more and more extreme, resulting in harsh, prolonged drought in arid climates, uncertainty over water availability has begun to threaten geopolitical stability and represents a serious risk to human health. While the relative abundance of freshwater resources and advanced water transportation and irrigation infrastructure has insulated the United States from the immediate and severe public health and water security issues that many nations currently face, access to fresh water in multiple regions of the country is increasingly limited and likely will become a major national security concern in the future. The Ogallala Aquifer in the Midwestern United States, yielding approximately 30 percent of the U.S. groundwater used for irrigation and supplying 82 percent of the potable water for those living within the aquifer boundary, could be completely depleted in as little as two to three decades (Guru and Horne, 2001). The Southwest has been facing and will continue to face serious water shortages in the coming decades, as aquifers are drained and surface water resources become increasingly scarce.

In recent years, public concern over energy security generally has overshadowed those for water security as oil prices have fluctuated. However, in the coming years, public concerns over water availability and the associated food security and health risks could increase and override those for energy security. If the algal biofuel industry relies heavily on freshwater resources, it could face a considerable setback as the increased use of freshwater resources becomes less acceptable to the public. This will be particularly damaging if infrastructure is already in place and capital already has been deployed in facilities that are subsequently shut down over concerns for their consumptive use of fresh water. Therefore, water recycling and use of nonfreshwater resources are important to ensuring the social acceptability of the large water requirements for algal biofuel production.

4.2 NUTRIENTS

Algae require key elemental nutrients for metabolic maintenance and growth, as is true of terrestrial plants. The exact elemental stoichiometry of algal cells varies from one environment to another and among different algal species. However, photoautotrophic algae use photosynthesis to convert light energy into new algal biomass with an elemental stoichiometry that on average obeys the following equation (Stumm and Morgan, 1988):



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