health depends on whether the pollutants are emitted close to highly populated areas and exposure.
An assessment of the effects of producing biofuels to achieve the RFS2 consumption mandate on water quality requires detailed information on where the bioenergy feedstocks would be grown and how they would be integrated into the existing landscape. The increase in corn production has contributed to environmental effects on surface and ground water, including hypoxia, harmful algal blooms, and eutrophication. Additional increases in corn production under RFS2 likely will have additional negative environmental effects (though production of corn-grain ethanol in 2010 was only 1 billion gallons less than the consumption mandate for years 2015 to 2022). Perennial and short-rotation woody crops for cellulosic feedstocks with low agrichemical inputs and high nutrient uptake efficiency hold promise for improving water quality under RFS2, particularly if integrated with food-based crops. Use of residues would not require much additional inputs so that they are not likely to incur much negative effects on water quality as long as enough residues are left in field to prevent soil erosion. The site-specific details of the implementation of RFS2, and particularly the balance of feedstocks and levels of inputs, will determine whether or not RFS2 will lead to improved or diminished water quality.
Water Quantity and Consumptive Water Use
Published estimates of consumptive water use over the life cycle of corn-grain ethanol (15-1,500 gallons per gallon of gasoline equivalent) and cellulosic biofuels (2.9-1,300 gallons per gallon of gasoline equivalent) are higher than petroleum-based fuels (1.9-6.6 gallons per gallon of gasoline equivalent), but the effects of water use depend on regional availability. An individual refinery might not pose much stress on a water resource, but multiple refineries could alter the hydrology in a region. In particular, biorefineries are most likely situated close to sources of bioenergy feedstock production, both of which draw upon local water resources. Yet, regional water availability was not always taken into account in the models that project cellulosic biorefinery locations.
Soil Quality and Biodiversity
Effects of biofuel production on soil quality and biodiversity primarily result from the feedstock production and removal stages, particularly on the rates of biological inputs and outputs and the levels of removal. The effects of achieving RFS2 on biodiversity currently cannot be readily quantified or qualified largely because of the uncertainty in the future. Bioenergy feedstock production can reduce or enhance biodiversity depending on the compatibility of feedstock type, management practices, timing of harvest, and input use with plants and animals in the area of production and its surroundings. Precise regional assessments at each site of feedstock production for biofuels are needed to assess the collective effects of achieving RFS2 on biodiversity.
BARRIERS TO ACHIEVING RFS2
FINDING: Key barriers to achieving RFS2 are the high cost of producing cellulosic biofuels compared to petroleum-based fuels and uncertainties in future biofuel markets.