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3 Water Quality T hroughout the nation, standard agricultural practices for most crops involve the application of fertilizers such as nitrogen (N) and phos- phorus (P) along with herbicides, fungicides, insecticides, and other pesticides. The amount of a fertilizer nutrient that is captured in a crop, and the amount of pesticide that remains in the soil, depend on the crop, the amount, timing, and method of application, the methods of soil cultivation (see next chapter), and other variables. A certain amount inevitably moves offsite by various pathways. Nitrogen in forms such as nitrate (NO3) is highly soluble, and along with some pesticides infiltrates downwards toward the water table (Figure 1-1). From there it can migrate to drinking water wells, or slowly find its way to rivers and streams. Another pathway is surface runoff, which transports N and P to streams either in solution or attached to eroding soil particles. A third pathway is wind erosion (or volatilization to the atmosphere in the case of nitrogen) followed by atmospheric transport and deposition over a broad area downwind. Surface runoff and infiltra- tion to groundwater both have significant impacts on water quality, as is discussed below. HOW MIGHT INCREASED BIOMASS PRODUCTION AFFECT THE WATER QUALITY OF OUR RIVERS? Biomass feedstocks such as corn grain, soybeans, and mixed-species grassland biomass differ in current or proposed application rates of fertilizers and of pesticides. Of these three potential feedstocks, the greatest applica- tion rates of both fertilizer and pesticides per hectare are for corn (Figure 3-1). Phosphorus application rates are somewhat lower for soybeans than for corn. Nitrogen application rates are much lower for soybeans than for corn because soybeans, which are legumes, fix their own nitrogen from the atmosphere. Pesticide application rates for soybean are about half those for 27
28 Water Implications of Biofuels Production in the United States FIGURE 3-1â Comparison of fertilizer (top) and pesticide (bottom) application rates for corn, soybean, and low-input high-diversity (LIHD; âbiomassâ in the figure) mixtures of native grassland perennials. Fertilizer and pesticide application rates are U.S. averages. SOURCE: Tilman et al. (2006). Reprinted, with permission, from American Association for the Advancement of Science. Â© 2006 by the American Association for the Advancement in Science. 3-1 corn. The native grasses compare highly favorably to corn and soy for both fertilizers and pesticides, with order-of-magnitude lower application rates. The impacts of these differences in inputs can be visualized nationally by comparing N inputs (such as fertilizer and manure) and the concentra- tions of nitrate in stream water (Figure 3-2, top). There are similar patterns for stream concentrations of atrazine, a major herbicide used in corn cultiva- tion (Figure 3-2, bottom), although the environmental effects of pesticides in current use are difficult to decipher. Both of these maps show that regionally the highest stream concentrations occur where the rates of application are highest, and that these rates are highest in the U.S. âCorn Belt.â These stream flows of nitrate mainly represent application to corn, which is already the major source of total N loading to the Mississippi River.
Water Quality 29 FIGURE 3-2â (top) N fertilization rates and stream concentrations of nitrate. (bottom) Atrazine application rates and stream concentrations of atrazine. FIGURE SOURCE: J. Ward, U.S. Geological Survey, personal commun., July 12, 2007. DATA SOURCES: stream N, Mueller and Spahr (2007); N inputs, Ruddy et al. (2006); stream atrazine and atrazine inputs, Gilliom et al. (2006). 3-2
30 Water Implications of Biofuels Production in the United States Increased sediment runoff is the most important environmental effect in many regions, such as the upper Mississippi River (UMRBA, 2004). High sedimentation rates increase the cost of often-mandatory dredging for trans- portation and recreation. They also have consequences for ecosystems and sport fishermen; many of the backwater areas along major streams, which are important in the lifecycles of fish and their prey, are slowly filling in with sediment. One of the most likely causes of increased erosion in the near term may be the withdrawal of lands from the U.S. Department of Agricultureâs (USDA) voluntary Conservation Reserve Program (CRP; see Chapter 6), as well as expansion of biomass production on non-CRP marginal land, due to increases in food and energy prices. The CRP makes annual rental payments to farmers to convert environmentally sensitive or highly erodible acreage to native grasses, wildlife plantings, trees, filter strips, and riparian buffers. It also provides cost-share assistance for up to 50 percent of the costs in establishing approved conservation practices. In exchange, participants enroll in CRP contracts for 10 to 15 years. High rates of withdrawal from the program in favor of growing biomass will have the effect of converting lands that may be helping to ameliorate water pollution into lands that are additional sources of water pollution. In the longer term, the use of crop residues such as corn stover either as feedstock for cellulosic ethanol or as fuel for conventional biorefineries has the potential to greatly increase ero- sion, as described in the next chapter. WHAT MAY BE THE IMPACTS OF BIOMASS PRODUCTION ON THE NATIONâS COASTAL AND OFFSHORE WATERS? The effects of biomass production on the nationâs coastal and offshore waters may be considerable. Nitrogen in the Mississippi River system is known to be the major cause of an oxygen-starved âdead zoneâ in the Gulf of Mexico (Figure 3-3), which in 2007 was the third largest ever mapped (http://www.gulfhypoxia.net). The condition known as hypoxia (low dis- solved oxygen) occurs because elevated N (and, to a lesser extent, P) load- ing into the Gulf leads to algal blooms over a large area. Upon the death of these algae, they fall to the bottom and their decomposition consumes nearly all of the oxygen in the bottom water. This is lethal for most fish and other species that live there. The Chesapeake Bay and other coastal waterbodies also experience the same phenomenon. Over the last 40 years, the volume of the Chesapeake Bayâs dead zone has more than tripled, and in many summers comprises
Water Quality 31 FIGURE 3-3â Dissolved oxygen contours (in milligrams per liter) in the Gulf of Mexico, July 21-28, 2007. SOURCE: Slightly modified from http://www.gulfhypoxia.net/shelfwide07/PressRelease07.pdf. Re- printed, with permission, from N. Rabalais, Louisiana Universities Marine Consortium. 3-3 almost a quarter of the water in the mainstem Bay (Chesapeake Bay Foun- dation, 2006). All else being equal, the conversion of other crops or non-crop plants to corn will likely lead to much higher application rates of nitrogen (Figure 3-1). Given the correlation of nitrogen application rates to stream concentrations of total nitrogen, and of the latter to the increase in hypoxia in the nationâs waterbodies, the potential for additional corn-based ethanol production to increase the extent of these hypoxic regions is considerable. Since the dead zone in the Gulf of Mexico is already on the order of 10,000 square kilometers, the economic stakes are high. WHAT ARE SOME LIKELY EFFECTS ON GROUNDWATER QUALITY? Groundwater quality is directly impacted by the high levels of nitrate and nitriteâthe products of nitrogen fertilizersâthat leach into the ground- water from corn fields. Independent of the form of fertilizer N applied to
32 Water Implications of Biofuels Production in the United States FIGURE 3-4â Probability that nitrate exceeds 4 milligrams per liter in shallow groundwaters of the United States, based on a logistic regression model. SOURCE: Reprinted, with permission, from Nolan et al. (2002). Â© 2002 by The American Chemical Society. 3-4 agricultural fields, soil microorganisms convert much of the excess fertil- izer N into nitrate, which, under anaerobic conditions in the soil or the groundwater, is converted into nitrite. U.S. Environmental Protection Agency (EPA) water quality standards classify wells that have nitrate+nitrite levels greater than 10 milligrams per liter as impaired and recommend that water be treated to remove the nitrate and nitrite before consumption. Failure to do so can have significant health impacts, including causing âblue baby syndromeâ in infants, when ingested nitrite binds with hemoglobin thus preventing oxygen transport. Nolan et al. (2002) demonstrate convincingly that the probability of
Water Quality 33 nitrate contamination of shallow groundwater correlates strongly with in- creased N fertilizer loading, as well as with well-drained surficial soils over unconsolidated sand and gravels along with various other factors. This is shown visually in Figure 3-4. The probability of encountering N levels above 4 milligrams per liter is greatest in the High Plains, which is characterized by both high N fertilizer loading and well-drained soils overlying uncon- solidated, coarse-grained deposits. Some pesticides may also leach to groundwater. In a national study, pesticides were detected in 61 percent of shallow wells sampled in agri- cultural areas (Gilliom et al., 2006). However, in only 1 percent of these cases did any pesticide occur at concentrations greater than water quality benchmarks for human health. As with nitrate, pesticide contamination in groundwater is correlated with moderate to high application rates where soils are permeable and drainage practices do not divert recharge to surface waters, such as in parts of Iowa, Minnesota, Wisconsin, and Pennsylvania (Gilliom et al., 2006). It is reasonable to believe that groundwater quality issues associated with increased biofuels production may also be focused in these areas, and in others identified in Figure 3-4. Groundwater contamination problems take longer to develop and longer to fix than surface water problems. However, over time the proportion of affected wells would increase if a common practice of year-to-year rotation of corn to soybeans to corn to soybeans, etc., were replaced by continuous corn or by corn grown with a higher frequency than half of the years. The area of the nation subject to having elevated groundwater nitrate and nitrite levels would also increase if corn were grown in new areas. HOW CAN ENVIRONMENTAL EFFECTS OF DIFFERENT BIOMASS TYPES BE COMPARED? There are many possible metrics, but an index that builds on the work shown in Figure 3-1 is inputs of fertilizers and pesticides per unit of the net energy gain captured in a biofuel. To estimate this first requires calculation of a biofuelâs net energy balance (NEB), that is, the energy content of the biofuel divided by the total fossil energy used throughout the full lifecycle of the production of the feedstock, its conversion to biofuel, and transport. U.S. corn ethanol is most commonly estimated to have a NEB of 1.25 to 1.3, that is, to return about 25-30 percent more energy, as ethanol, than the total fossil energy used throughout its production lifecycle (Farrell et al., 2006; Hill et al., 2006; Wang et al., 1997; Shapouri et al., 2004). The NEB estimated for U.S. soybean biodiesel is about 1.8 to 2.0, or about a 100
34 Water Implications of Biofuels Production in the United States a b Application per NEB (g/MJ) Application per NEB (g/MJ) Otger Glyphosate Metolachlor Acetochlor Atrazine Other Glyphosate N P Pesticide Fertilizer Corn grain ethanol Soybean biodiesel FIGURE 3-5â Environmental effects from the complete production and combustion life cycles of corn grain ethanol and soybean biodiesel. The figure shows the application of both (a) fertilizers and (b) pesticides, per unit of net energy gained from biofuel production. SOURCE: Hill et al. (2006). 3-5 percent net energy gain (Hill et al., 2006; Sheehan et al., 1998). Switchgrass ethanol via fermentation is projected to be much higherâbetween 4 and 15 (Farrell et al., 2006). Similarly high are the estimates for (a) cellulosic ethanol and (b) synthetic gasoline and diesel from certain mixtures of pe- rennial prairie grasses, forbs, and legumes (NEB=5.5 and 8.1, respectively; Tilman et al., 2006). Per unit of energy gained, corn ethanol and soybean biodiesel have dramatically different impacts on water quality (Hill et al., 2006). When fertilizer and pesticide application rates (Figure 3-1) are scaled relative to the NEB values of these two biofuels, they are seen to differ dramatically (Figure 3-5). Per unit of energy gained, biodiesel requires just 2 percent of the N and 8 percent of the P needed for corn ethanol. Pesticide use per NEB differs similarly. Low-input high-diversity prairie biomass and other native species would also compare favorably relative to corn using this metric. This is just one possible metric of biofuelsâ impact on water quality.
Water Quality 35 Other measures might incorporate land requirements per unit of biofuel, soil erosion, or impacts of the associated biorefinery (Chapter 5). The large recent increases in U.S. corn acreage have already led to in- creased rates of N and P loading into surface and groundwaters. If projected future increases in use of corn for ethanol production do occur, the increase in harm to water quality could be considerable. REFERENCES Chesapeake Bay Foundation. 2006. The Chesapeake Bayâs Dead Zone: Increased Nutrient Runoff Leaves Too Little Oxygen in 40 Percent of the Bayâs Mainstem in July. On- line fact sheet available at http://www.cbf.org/site/DocServer/DeadZoneFactSheet_ May06. pdf?docID=5583. Accessed on July 13, 2007. Farrell, A. E., R. J. Plevin, B. T. Turner, A. D. Jones, M. OâHare, D. M. Kammen. 2006. Ethanol can contribute to energy and environmental goals. Science 311:506-508. Gilliom, R. J., J. E. Barbash, C. G. Crawford, P. A. Hamilton, J. D. Martin, N. Nakagaki, L. H. Nowell, J. C. Scott, P. E. Stackelberg, G. P. Thelin, and D. M. Wolock. 2006. The Quality of Our Nationâs WatersâPesticides in the Nationâs Streams and Ground Water, 1992â2001. U.S. Geological Survey Circular 1291. Reston, VA: U.S. Geological Survey. Hill, J., E. Nelson, D. Tilman, S. Polasky, and D. Tiffany. 2006. Environmental, economic, and ener- getic costs and benefits of biodiesel and ethanol biofuels. Proceedings of the National Academy of Sciences 103(30):11206-11210. Mueller, D. K., and N. E. Spahr. ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ 2007. Nutrients in Streams and Rivers Across the Nationâ1992â 2001. U.S. Geological Survey Scientific Investigations Report 2006-5107. Reston, VA: U.S. Geological Survey. Nolan, B. T., K. J. Hitt, and B. C. Ruddy. 2002. Probability of nitrate contamination of recently re- charged groundwaters in the conterminous United States. Environmental Science & Technology 36(10):2138-2145. Ruddy, B. C., D. L. Lorenz, and D. K. Mueller. 2006. County-level Estimates of Nutrient Inputs from Fertilizer, Manure, and Atmospheric-deposition Sources in the Counterminous United States. U.S. Geological Survey Scientific Investigations Report 2006-5012. Reston, VA: U.S. Geological Survey. Shapouri, H., J. Duffield, A. McAloon, and M. Wang. 2004. The 2001 Net Energy Balance of Corn- Ethanol. Proceedings of the Fourth Corn Utilization & Technology Conference, Indianapolis, Indiana, June 7-9, 2004. Chesterfield, MO: National Corn Growers Association. Sheehan, J., V. Camobreco, J. Duffield, M. Graboski, and H. Shapouri. 1998. Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus Final Report. Report by the Na- tional Renewable Energy Laboratory (NREL) prepared for the Department of Energy Office of Fuels Development and the U.S. Department of Agriculture Office of Energy. NREL: Golden, CO. Available online at http://www.nrel.gov/docs/legosti/fy98/24089.pdf. Accessed on July 13, 2007. Tilman, D., J. Hill, and C. Lehman. 2006. Carbon-negative biofuels from low-input high-diversity grassland biomass. Science 314:1598-1600. Upper Mississippi River Basin Association (UMRBA). 2004. Upper Mississippi River Water Quality: The Statesâ Approaches to Clean Water Act Monitoring, Assessment, and Impairment Decisions. St. Paul, MN: UMRBA. Wang, M. Q., C. L. Saricks, and M. Wu. 1997. Fuel-Cycle Fossil Energy Use and Greenhouse Gas Emissions of Fuel Ethanol Produced from U.S. Midwest Corn. Report prepared for Illinois De- partment of Commerce and Community Affairs. Argonne, IL: Argonne National Laboratory.
Courtesy of the Natural Resources Conservation Service Center, U.S. Department of Agriculture