point source pollution by states or the federal government—something like a Total Maximum Daily Load (TMDL) regulation for a multi-state region or large river system. TMDLs are calculations of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. A paucity of data and difficulty in assigning responsibility for non-point source pollution raise technical and political challenges. Use of the best available science can help make TMDL programs more equitable and effective (NRC, 2001).
Although cellulosic crops in general hold soil better than corn, they can also pose problems of nutrient leaching and erosion. The imminent expansion of biomass production raises the urgency of this concern.
Process water for corn ethanol production raises both quantity and quality concerns. However, both the impacts and the regulatory opportunity for mitigation are likely to be at the local or state level. With the rapid expansion of ethanol production, some local communities and governments have not anticipated withdrawal levels or discharge volumes and have suffered the resulting water draw-downs and water treatment requirements. Mitigation will require effective withdrawal rules and enforcement and/or enhancement of existing state/federal rules on point discharge.
Many different metrics can be used to assess real-world consequences of different crop choices. For example, measuring greenhouse gas emissions per unit of energy produced can be a useful metric when attempting to capture some of the environmental consequences of biofuels production. Or, measuring petroleum displacement per unit of energy produced can be useful when assessing strategies that are driven by a policy leading to greater energy independence for the United States. The choice of metric is important because different feedstocks will be ranked differently and will have varying strengths and weaknesses depending on the choice of metric.
One possible metric to compare the impact of biofuels on water quality, as discussed in Chapter 3, is to compare crops based on inputs of fertilizers and pesticides per unit of the net energy gain captured in a biofuel. Similar metrics could be developed for water quantity; water application rates or consumptive water use could be used depending on the kinds of impacts