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ENVTRONMENTAL ENGTNEERTNG

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Microbial Mineral Respiration DIANNE K. NEWMAN Divisions of Geological and Planetary Sciences and Engineering and Applied Sciences California Institute of Technology Pasadena, California Contributions from anthropogenic sources generally dominate the discus- sion on global change. And yet, although human activities have unquestionably left their mark on the environment, when averaged over geologic time, their importance pales in comparison with changes that have been effected by the activities of unicellular microorganisms (e.g., Bacteria, Archaea, and single- celled Eucarya). Not only does the number of microorganisms on Earth signifi- cantly exceed the number of humans (Whitman et al., 1998), microorganisms, unlike humans, are also found everywhere, are remarkably efficient at catalyzing a wide range of chemical reactions through their metabolisms, and have been around for billions of years (Figure 1~. Microbial metabolisms have brought about many changes in the Earth's environment. Microorganisms have altered the chemistry of the atmosphere via oxygenic photosynthesis, nitrogen fixation, and carbon sequestration. They have modified the composition of oceans, rivers, and pore fluids by controlling min- eral weathering rates or by inducing mineral precipitation. They have changed the speciation of metals and metalloids in water, soils, and sediments by releas- ing complexing agents and/or by enzymatically catalyzing redox reactions. And they have shaped the physical world by binding sediments, precipitating ore deposits, and weathering rocks (Newman and Banfield, 2002~. MINERAL RESPIRATION Respiratory metabolisms have impacted mineral formation and/or dissolu- tion. The thought of respiring a mineral may seem suffocating, but bacteria have been doing it for billions of years (Figure 2~. Respiration is fundamentally the 3

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4 FRONTIERS OF ENGINEERING Multicellular Life M ICROBIAL Ll F 4 3 2 1 0 Billions of years ago (109) FIGURE 1 Microorganisms have dominated the history of life on Earth and have been major agents of global change. Although we will never know the exact date when micro- bial life first developed, a variety of geochemical and geobiological indicators suggest that this happened several billion years ago. FIGURE 2 A scanning electron micrograph of the metal-reducing bacterium, Shewanella oneidensis strain MR-1, attached to a steel surface. As steel corrodes, ferric iron oxides are produced. MR-1 can reduce these minerals during respiration.

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MICROBIAL MINERAL RESPIRATION s process of making energy available by transferring electrons from an electron donor to an electron acceptor. Typically, the transfer occurs down a respiratory chain embedded in the cell membrane; specific molecules hand off electrons from one end to the other. In the process, they generate a potential across the membrane that can be harnessed to do work (i.e., to store chemical energy in the form of ATP) (Mitchell, 1961~. For respiration to succeed, a terminal electron acceptor, such as oxygen, must be available to receive the electrons. Before the evolution of oxygen in the atmosphere, microorganisms had to respire with alter- native electron acceptors. Most of the terminal electron acceptors used by bacteria for respiration, such as oxygen, nitrate, and sulfate, are soluble. This means they can make their way to the cell to receive electrons from the membrane-bound molecules of the respiratory chain. The real question is how bacteria transfer electrons to solids like hematite (oc.Fe2O3) and goethite (oc.FeOOH) (Figure 3~. Because these m~n- erals are effectively insoluble under environmentally relevant conditions, simple dissolution and diffusion of fern c iron to the cell cannot be the answer (fernc iron is the constituent of the mineral that receives electrons). Therefore, bacteria must have other strategies for transferring electrons to minerals during respira- tion. The question is, what are they? The Challenge of Mineral Respiration OM Fe(OH)3` \ '- _ - _ Electron donor - ADP + ~ l ATP FIGURE 3 A schematic drawing illustrating the process of mineral respiration.

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6 FRONTIERS OF ENGINEERING Several mechanisms have been proposed (Figure 4~. Some have suggested that bacteria solubilize the minerals by producing chelators. Although the addi- tion of synthetic chelators has been shown to stimulate microbial electron trans- fer to iron minerals, no evidence has been found that bacteria use this mecha- nism in respiration. Another suggestion is that they may use soluble shuttles, such as organic compounds with quinone moieties, to transfer electrons from the cell to the mineral. These shuttles may be exogenous substances, or they may be substances produced by the organisms themselves (Lovley et al, 1998; Newman and Kolter, 2000~. Another mechanism, possibly the dominant one, is that bacteria transfer electrons directly from the cell surface to the mineral after a regulated search and attachment process. A variety of biomolecules (including cytochromes, qui- nones, and dehydrogenases) have been identified as part of this electron-transfer pathway (Schroder et al., 2003~. Several of these biomolecules are located on the outer membrane of the cell and presumably make contact with the mineral directly (Lower et al., 2001~. Given that the initial rate and long-term extent of electron transfer is correlated with their surface area and the concentration of reactive sites, this seems like a reasonable explanation (Zachara et al., 1998~. ( 11 FIGURE 4 Three models that explain how bacteria respire minerals. Chelation Shuttling Direct Contact

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MICROBIAL MINERAL RESPIRATION 7 Yet the nature of the electron-transfer event remains obscure and is a subject of active research. PUTTING MINERAL RESPIRATION TO WORK Despite uncertainties about the molecular mechanisms of mineral respira- tion, environmental microbiologists and engineers have been putting it to work for more than two decades. The best example is bioremediation; microbial me- tabolisms based on mineral respiration have been used to clean up organic and/or inorganic contaminants in groundwater (Figure 5~. In the late 1980s, researchers at the U.S. Geological Survey observed that the oxidation of aromatic hydrocarbons (e.g., benzene, xylenes, and toluene) in contaminated shallow aquifers was associated with the depletion of Fe(III) ox- ides from contaminated sediments and the accumulation of dissolved Fe(II) over time (Lovley et al., 1989~. Hypothesizing that Fe(III)-respiring microorganisms may have been responsible for this phenomenon, Derek Lovley and his cowork- ers showed that the oxidation of added toluene to CO2 was dependent on active microbial metabolism. They went on to demonstrate that Geobacter metallire- ducens strain GS-15, an Fe(III)-respiring bacterium, could oxidize a variety of Oxidation (Electron donor) Ared Aox Reduction (Electron acceptor) Box Bred FIGURE 5 Catabolic electron-transfer metabolisms (e.g., fermentation, respiration, and photosynthesis) generate energy. Microorganisms are extraordinarily versatile and can use a variety of compounds as electron donors or electron acceptors in these reactions. For example, Ared can be toxic organic compounds, such as benzene and toluene. Mi- crobes can oxidize these to CO2 under both aerobic and anaerobic conditions; in anaero- bic groundwater systems, Fe(III) is the primary oxidant (i.e., electron acceptor). In addi- tion to Fe(III), bacteria can use other metal~loids) as electron acceptors. These include As(V), Cr(VI), U(IV), Tc(VII), Se(V), and graphite electrodes. When these inorganic compounds are used as electron acceptors, their reduction may lead to their mobilization or precipitation, depending on the metal~loid). Such reactions may be important for the bioremediation of contaminated sites and/or the generation of electricity from marine sediments. Note: In a chemical reaction, Ared + Box = Aox + Bred. Ared = the electron donor for the reaction. Box = the electron acceptor for the reaction. Aox = the oxidized product of the reaction. Bred = the reduced product of the reaction.

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8 FRONTIERS OF ENGINEERING aromatic compounds. Two decades later, members of the Geobacteraceae fam- ily have been observed to account for a significant portion of the microbial population in contaminated sediments (Snoeyenbos-West et al., 2000), and the U.S. Department of Energy (DOE) is actively investing in research to get a better understanding of and to stimulate Fe(III) respiration for bioremediation (DOE, 2003a). In addition to coupling the oxidation of organic contaminants to the reduc- tion of Fe(III), microbial activity can be of value for the bioremediation of inor- ganic contaminants, uranium (U) and technetium (Tc), for example, two abun- dant radioactive metals that contaminate the subsurface environments at some DOE sites. Fe(III)-respiring bacteria have been shown to enzymatically reduce highly soluble U(VI) carbonate complexes and Tc(VII)O4- to the insoluble tet- ravalent phases, UO2 and TcO2 (Lloyd, 2003~. The theory is that if organisms with this capacity are stimulated in situ, toxic inorganic compounds may be precipitated from groundwater and immobilized in the subsurface (Barkay and Schaefer, 2001~. Although the long-term efficacy of this approach is still a subject of debate, encouraging preliminary field studies show that a significant percentage of solu- ble U can be removed rapidly from groundwater by stimulating the indigenous microbial population (Finneran et al., 2002~. Similarly, hexavalent chrominum [Cr(VI)] is a strong, highly mobile carcinogen that forms an insoluble Cr(III) precipitate when reduced. Efforts are currently under way at the Hanford DOE site to determine whether stimulation of the indigenous microbial population with lactate (a carbon source) can enhance Cr immobilization (DOE, 2003b). Besides removing toxic inorganic compounds from groundwater, microbial respiratory metabolisms also have the potential to generate electricity. In a recently reported example, members of the family Geobacteraceae were shown to grow by oxidizing organics with a graphite electrode as the sole electron acceptor (Bond et al., 2002~. When fuel cells consisting of anodes embedded in the sediment connected to cathodes positioned in the overlying seawater were deployed in two coastal marine environments (a salt marsh near Tuckerton, New Jersey, and the Yaquina Bay Estuary near Newport, Oregon), oxidation of both organic and inorganic electron donors in the sediment supported power genera- tion (Tender et al., 2002~. Although much work remains to be done before we can understand how microbial communities catalyze electron-transfer reactions to the anode, this process has the potential to sustain long-term power generation from marine sediments. Lest microbial respiratory metabolisms be considered uniformly beneficial, it is important to point out that mineral respiration does not always improve water quality. A tragic example of this can be seen today in Bangladesh, where thousands of people are dying from drinking arsenic-contaminated well water. It is now widely believed that microbial respiratory activities are contributing to this problem by mobilizing arsenic in the groundwater (Harvey et al., 2002~.

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MICROBIAL MINERAL RESPIRATION 9 Although the details have not yet been determined, evidence points to the reduc- tive dissolution of iron arsenate minerals as a likely mechanism (Oremland and Stolz, 2003). CONCLUSIONS Microbial respiratory metabolisms based on minerals are fascinating from a purely scientific standpoint because of what they can teach us about electron- transfer reactions. They are also of great interest to environmental engineers seeking novel ways to remediate contaminated environments andlor to generate electncity. How organisms evolved the capacity to transfer electrons to mineral surfaces is not well understood but meets further investigation. It is possible that horizontal gene transfer has accelerated the distribution of this capability in both time and space; if so, this mechanism could be exploited in the future to deliver useful genetic material to targeted microbial populations. ACKNOWLEDGMENTS I would like to thank the Clare Boothe Luce Foundation, the Packard Foun- dation, and the Office of Naval Research for providing generous research support. REFERENCES Barkay, T., and J. Schaefer. 2001. Metal and radionuclide bioremediation: issues, considerations and potentials. Current Opinion in Microbiology 4(3): 318-323. Bond, D.R., D.E. Holmes, L.M. Tender, and D.R. Lovley. 2002. Electrode-reducing microorgan- isms that harvest energy from marine sediments. Science 295(5554): 483-485. DOE (U.S. Department of Energy). 2003a. Natural and Accelerated Bioremediation (NABIR) Program. Available online at: . DOE. 2003b. NABIR Research Program. Available online at: . Finneran, K.T., R.T. Anderson, K.P. Nevin, and D.R. Lovley. 2002. Potential for bioremediation of uranium-contaminated aquifers with microbial U(VI) reduction. Soil and Sediment Contami- nation 11(3): 339-357. Harvey, C.F., C.H. Swartz, A.B.M. Badruzzaman, N. Keon-Blute, W. Yu, M.A. All, J. Jay, R. Beckie, V. Niedan, D. Brabander, P.M. Oates, K.N. Ashfaque, S. Islam, ELF. Hemond, and M.F. Ahmed. 2002. Arsenic mobility and groundwater extraction in Bangladesh. Science 298(5598): 1602-1606. Lloyd, J.R. 2003. Microbial reduction of metals and radionuclides. FEMS Microbiology Reviews 27(2-3): 411-425. Lovley, D.R., M.J. Baedecker, D.J. Lonergan, I.M. Cozzarelli, E.J.P. Phillips, and D.I. Siegel. 1989. Oxidation of aromatic contaminants coupled to microbial iron reduction. Nature 339(6222): 297-299. Lovley, D.R., J.D. Coates, ILL. Blunt-Harris, E.J.P. Phillips, and J.C. Woodward. 1998. Humic substances as electron acceptors for microbial respiration. Nature 382(6564): 445-448. Lower, S.K., M.F. Hochella, Jr., and T.J. Beveridge. 2001. Bacterial recognition of mineral surfac- es: nanoscale interactions between Shewanella and oc-FeOOH. Science 292(5520): 1360-1363.

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10 FRONTIERS OF ENGINEERING Mitchell, P. 1961. Coupling of phosphorylation to electron and hydrogen transfer by a chemi- osmotic type of mechanism. Nature 191: 144-148. Newman, D.K., and J.F. Banfield. 2002. Geomicrobiology: how molecular-scale interactions under- pin biogeochemical systems. Science 296(5570): 1071-1077. Newman, D.K., and R. Kolter. 2000. A role for excreted quinines in extracellular electron transfer. Nature 405(6782): 94-97. Oremland, R.S., and J.F. Stolz. 2003. The ecology of arsenic. Science 300(5621): 939-944. Schroder, I., E. Johnson, and S. de Vries. 2003. Microbial ferric iron reductases. FEMS Microbiol- ogy Reviews 27(2-3): 427-447. Snoeyenbos-West, O.L., K.P. Nevin, R.T. Anderson, and D.R. Lovley. 2000. Enrichment of Geo- bacter species in response to stimulation of Fe(III) reduction in sandy aquifer sediments. Mi- crobial Ecology 39(2): 153-167. Tender, L.M., C.E. Reimers, H.A. Stecher III, D.E. Holmes, D.R. Bond, D.A. Lowy, K. Pilobello, S.J. Fertig, and D.R. Lovley. 2002. Harnessing microbially generated power on the seafloor. Nature Biotechnology 20(8): 821-825. Whitman, W.B., D.C. Coleman, and W.J. Wiebe. 1998. Prokaryotes: the unseen majority. Proceed- ings of the National Academy of Sciences 95(12): 6578-6583. Zachara, J.M., J.K. Fredrickson, S.-M. Li, D.W. Kennedy, S.C. Smith, and P.L. Gassman. 1998. Bacterial reduction of crystalline Fe3+ oxides in single phase suspensions and subsurface mate- rials. American Mineralogist 83(11-12, Part 2): 1426-1443.

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Water-Resource Engineering, Economics, and Public Policy GREGORY W. CHARACKLIS Department of Environmental Sciences and Engineering University of North Carolina at Chapel Hill INTRODUCTION Water-resource engineering is a branch of environmental engineering that involves the analysis and manipulation of hydrologic systems, particularly in areas related to water quality and water availability (e.g., water supply and flood- ing). Since the mid-1970s, concerns about improving water quality and meeting future demands have multiplied in scope and magnitude, prompting significant changes in public policy at the federal, state, and local levels (Cech, 2003~. The strong influence of public policy on the research agenda in water-resource engi- neering (and most other environmental fields) differentiates it from other engi- neering disciplines in some important respects. In non-environmental disciplines, the primary motivation for developing a solution (e.g., a product or process) is economic doing something "faster, bet- ter, cheaper." The solution is then implemented by consumers and organizations based on their determination of how well the new product or process fits their needs. By contrast, the motivation for developing solutions to water-resource challenges is often related to meeting regulatory objectives (e.g., public health or environmental quality) that cannot be easily quantified in economic terms. A1- though attempts are made to estimate costs and benefits when evaluating and setting regulatory policy, implementation of a policy is generally dictated by centrally controlled directives that often entail high compliance costs. A number of factors can justify this command-and-control approach, but the growing number of environmental regulations, and their increasingly restrictive nature, have stimulated a growing interest in supplementing the traditional ap- proach with market principles. The development of market-based schemes for 1 1

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4 FRONTIERS OF ENGINEERING Agricultural rights* Carrizo Aquifer Simsboro Aquifer Lower Guadalupe River Colorado River Desalinated seawater * estimated $1 ,059 ,407 Cost of delivered raw water ($/AF/yr) FIGURE 1 Cost of alternative water supplies in the Edwards Aquifer region. Source: Adapted from SCTRWPG, 2000. water. Although a general policy of market-based trading has been used to allocate the newly scarce pumping capacity, critical issues are still unresolved. Policy makers would like to find a solution that minimizes the amount of water transferred out of agriculture. But if municipalities cannot achieve their supply- reliability goals under the proposed framework, then the market-based solution becomes much less valuable (McCarl et al., 1999~. An analysis that incorporates both hydrologic modeling and market simula- tion was undertaken to evaluate approaches that would maximize the average volume of water available to agriculture and still meet municipal objectives related to supply reliability (high) and cost (low). Although water transfers from agricultural to urban use are inevitable, if the only transfer mechanism available is a permanent sale, municipalities will be forced to buy rights and maintain a volume well in excess of average usage to ensure supply reliability during peri- odic droughts. These purchased rights are unlikely to be transferred back to agriculture once they are acquired. Thus, in many normal and wet years a significant fraction of the aquifer's total capacity may not be used. If more flexible types of transfer were available, such as transfers that would allow water to be acquired on an "as needed" basis, the capacity maintained by municipali- ties could be reduced and the average volume of water available to agriculture increased (Characklis et al., 1999~. Purchase decisions regarding these more flexible transfer instruments could be greatly facilitated by improved information about future supplies. In the case of the Edwards Aquifer, future shortfalls will be largely the result of the tempo- rary pumping restrictions imposed when aquifer levels drop below specified levels, conditions that can be accurately predicted through hydrologic modeling (Durbin, 2002~.

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WATER-RESOURCE ENGINEERING, ECONOMICS, AND PUBLIC POLICY 15 Flexible transfers allow for more responsive purchase decisions, particularly when supply conditions are known in advance. Annual leases provide for the temporary transfer of pumping rights over a one-year period (beginning January 1), and probabilistic information on future aquifer levels at the time of purchase could be used to estimate the volume necessary to meet supply-reliability goals. Another potential transfer instrument is the option, a transaction in which the buyer pays a small fee at the beginning of the year for the right to purchase water, or exercise the option, at a later date (June 1) at a prearranged price. The accuracy of predictions of aquifer level would have a significant bearing on decisions to purchase and exercise options. Finally, spot-market leases would provide ultimate flexibility, allowing municipalities to acquire water at any time, but also subjecting them to price volatility (i.e., dry weather = high prices). An assessment of the benefits of including all or some of these transaction types could indicate whether the costs of developing the marketing and monitor- ing framework necessary to regulate these more complex transactions is justi- fied. Because the Edwards Aquifer market is still in its infancy, little informa- tion is available on market behavior. As a result, a market simulation was developed to evaluate monthly supply and demand throughout a given year. The information was then used to calculate spot-market prices as a basis for calculat- ing the prices of the other transaction types. The monthly pumping restrictions imposed as a result of declining aquifer levels can considerably disrupt municipal water supplies, so the ability to assess the likelihood of restrictions can be valuable when formulating planning strate- gies. Therefore, a hydrologic model was developed to predict the aquifer level in the "J-17" well (the regulatory reference point) as a function of the previous month' s well level, aquifer inflows, agricultural pumping, and municipal pump- ing. A comparison of model results with empirical data measured over a 25-year period suggests that the model provides a very good estimate of the aquifer level (Figure 2~; therefore, the model was embedded in the market simulation. Market simulations are run many times for a range of initial aquifer levels, with input obtained via Monte Carlo sampling from a joint, multivariate distribu- tion created from inflow and withdrawal data. Simulated supply and demand conditions are translated into market prices for each transfer type, and the ex- pected cost and reliability of various combinations, or "portfolios," of transfer types can be computed. The transfer types are specified for each scenario, and a sequential search method is then used to identify minimum cost portfolios that meet designated supply-reliability constraints (Figure 3~. Differences in the cost of the respective portfolios indicate the value of including each transaction type in the market, as well as how the cost of market-based approaches compares to the development of the least expensive new water source (Carrizo Aquifer). Although the model shows that spot-market leasing would have consider- able economic advantages, because of the risk averse nature of municipal utili- ties, a strategy that involves exposure to spot-market price volatility, even if the

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6 710 690 - In `~ 670 Q ~ 650 - 630 610 FRONTIERS OF ENGINEERING ~ vet. .,, ~ I' '-, ill ,, .,.,i 1 .~ .; 5/0 Cutback 1 0/0 Cutback Observed Predicted 1 5/0 Cutback ,,. . / P. ... . $ .; . .f (D (D (D (D ~ ~ ~ 00 O ~ ~ ~ Year (D (D (D 00 (D (D ~ 0 ~ FIGURE 2 Predicted vs. observed aquifer level in the J-17 reference well. Source: Durbin, 2002. expected costs are lower, is not likely to appeal to municipal decision makers. They may, however, feel comfortable entering into annual leasing and option contracts, which provide much greater certainty about water availability and cost. The advantages of using these two instruments include savings of close to $1 million dollars per year relative to the cost of depending on permanent trans- fers alone. More flexible transfers would also increase the average amount of water available to agriculture by 5,000 to 25,000 acre-feet per year, a feature that is important to policy makers. Consequently, the dual objectives of maintaining municipal supply reliability and reducing the economic impacts on agriculture would appear to be furthered by the introduction of more flexible market instru- ments. Thus, an excellent argument can be made to justify an investment in institutions to monitor and regulate these types of transfers. CONCLUSIONS Integrating technical and economic analytical techniques provides a means of identifying and evaluating a range of solutions to water-resource challenges.

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17 o = . _ .e ~n o C' C~ 1 1 $ o CO XAX,XXXXXXXXXXXXXXXXXXXXXX<'<%%%%? X~ ~ ~ ~ ~ O O (D (D :- ~ ~ ~ ~.= ~ ~ ~ ~ Q ~ O C~ C~ C~ _~ ~ i~; ~//~ l l l l l l l l l l l l l l l l l l l O O O O O O O O O O O O O 10 0 10 00 ~ ~ ~ O O O O O O O O O O O O O O O O O O 10 0 10 0 10 0 U) U) ~ ~ CO CO o!~!S!nb0~ led!~!untu MaN .m _ ~ A 1 X I $ I ~ I ~ o Q 1 ~ ~o ~n I ~ I ~ 1 I .m 1 ~ I Q 1 . _ 1 ~n ~_ ~ ~ O l l l l 1 l V A 1 o CO z V _. _. _. s~ ;^ ~o C~ Ct Ct _. tn Ct Ct n, .= .= .= C~ Ct o s~ Ct o o C~ . o

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18 FRONTIERS OF ENGINEERING The example above describes the application of these methods to the develop- ment of strategies for managing regulatory limits on resource acquisition; similar approaches could be used for regulating emissions. As demands continue to increase for both water resources and the assimilative capacity these resources provide, the need for creative, interdisciplinary solutions will also increase. REFERENCES Cech, T.V. 2003. Principles of Water Resources: History, Development, Management, and Policy. New York: John Wiley and Sons. Characklis, G.W., R.C. Griffin, and P.B. Bedient. 1999. Improving the ability of a water market to efficiently manage drought. Water Resources Research 35(3): 823-832. Durbin, S.W. 2002. Utilization of a Water Market to Optimize Water Acquisition in the Edwards Aquifer Region. Unpublished M.S. thesis, University of North Carolina at Chapel Hill. Keplinger, K.O., and B.A. McCarl. 2000. An evaluation of the 1997 Edwards Aquifer irrigation suspension. Journal of the American Water Resources Association 36(4): 889-901. McCarl, B.A., C.R. Dillon, K.O. Keplinger, and R.L. Williams. 1999. Limiting pumping from the Edwards Aquifer: an economic investigation of proposals, water markets, and spring flow guarantees. Water Resources Research 35(4): 1257-1268. NRC (National Research Council). 1996. Linking Science and Technology to Society's Environ- mental Goals. Washington, D.C.: National Academy Press. SCTRWPG (South Central Texas Regional Water Planning Group). 2000. Water Supply Options for South Central Texas. Austin, Texas: Texas Water Development Board. Votteler, T.H. 1998. The little fish that roared: the Endangered Species Act, state groundwater law, and private property rights collide in the Texas Edwards Aquifer. Environmental Law 28(4): 845-904.

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Life Cycle Development: Expanding the Life Cycle Framework to Address Issues of Sustainable Development GREGORY A. NORRIS Sylvatica North Berwick, Maine Harvard School of Public Health Boston, Massachusetts The environmental movement of the late twentieth century began three decades ago amid concerns about energy depletion, water and air pollution, and mounting flows of postconsumer packaging wastes. Independently in the United States, continental Europe, and the United Kingdom, small teams of engineers and physicists were called upon by policy makers to provide a comprehensive account of the environmental and resource implications of increasingly popular disposable, plastic packaging. Each of these teams invented a methodology that came eventually (in the l990s) to be called environmental life-cycle analysis (LCA). With the increasing importance and visibility of "product policy" and "ex- tended product (or producer) responsibility" during the l990s, LCA changed from a little-known cottage industry to an internationally standardized analytical tool in support of environmental management. LCA is now used by thousands of companies, many governments, consumer and environmental groups, even the United Nations Environment Program, to shed light on the cradle-to-grave envi- ronmental consequences of product-related decisions. In 2002, the leaders of many national governments converged, along with representatives from industry and civil society, in Johannesburg at the World Summit on Sustainable Development (WSSD). Participants took stock of the successes and failures of the past 30 years and looked ahead to the promise and perils facing humanity in relation to sustainable development development that meets the needs of the present without compromising the ability of future gen- erations to meet their needs (WCED, 1987~. The WSSD issued the Plan of Implementation for Changing Unsustainable Patterns of Consumption and Pro- duction, which called on producers to "Improve the products and services pro- 19

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20 FRONTIERS OF ENGINEERING vided, while reducing environmental and health impacts, using where appropri- ate, science-based approaches, such as life-cycle analysis" (WSSD,2002~. Thus, LCA, which was originally developed to inform environmental policies, is now being called upon to assist in the search for sustainable patterns of consumption and production. The interdisciplinary frontiers of this engineering-based meth- odology are constantly evolving in response to demands in the policy arena for sustainable development. ORIGINS The field of environmental LCA originated during a period of rising concern about disposable packaging. In 1969, the packaging manager at the Coca-Cola Company was considering whether the company should begin to manufacture its own beverage containers (Hunt and Franklin, 1996~. At the same time, the previously stable world of beverage container materials and designs was shifting to new materials, such as plastic, and from returnable containers to disposable, or "one way," packaging systems. To help inform this major business decision in the context of emerging concerns about resources and the environment, the Coca-Cola packaging man- ager began to visualize a new type of study "that would attempt to quantify the energy, material, and environmental consequences of the entire life cycle of a package from the extraction of raw materials to disposal" (Hunt and Franklin, 1996~. He commissioned a study that eventually led to the development of LCA. Personnel involved in the original project migrated to the post of deputy assistant administrator for solid waste at the newly formed Environmental Protection Agency (EPA), just when EPA was considering using taxes, subsidies, or direct regulation to stem the momentum of change from returnable to disposable bever- age containers. EPA commissioned an LCA of the issue from the same people who had done the original study for Coke (Hunt and Franklin, 1974~. Contemporaneously, the German government commissioned a study of pack- aging materials, including an assessment of the potential environmental benefits of biodegradable polymers (Oberbacher et al., 1996), and the Glass Manufactur- ers Federation commissioned an LCA of returnable and nonreturnable bottles in the United Kingdom (Boustead, 1996~. Both the German and U.K. studies drew upon the newly developed methods of Hunt and Franklin. LCAs attempt to provide comprehensive analyses in two dimensions. First, the "full" set of manufacturing, transportation, and solid-waste management pro- cesses required to support the manufacture, use, and disposal of a product is considered to be potentially within the scope of the study. In practice, however, boundary rules must be adopted to keep the data requirements within feasible limits (ISO, 1998~. Second, the "full" set of relevant flows to and from the environment is potentially within the scope of the study. Rules are adopted to make the task manageable.

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LIFE CYCLE DEVELOPMENT 21 The whole LCA enterprise can only be feasible over the long term with real- world budget constraints if studies contribute to ongoing, cumulative, reusable, modular, and "generic" databases about average or representative unit processes. These radically comprehensive LCA studies provided counterintuitive, nonde- finitive results. First, the studies often showed that product properties, such as lightness or longevity-of-use, had life-cycle energy advantages that compensated for the environmental impacts of nonrenewable materials. Second, by including a broad array of pollutants and environmental impacts in LCAs, manufacturers were sometimes able to argue that the problem was much more complicated than the narrow issue that may have motivated the study or policy debate; LCAs revealed trade-offs among environmental impacts. In general, LCAs supported (and continue to support) two major results: LCAs look beyond narrow, suboptimization that shifts the environmental burden from one life-cycle stage to another or from one environmental impact category to another. LCAs can reveal leverage points or hot spots, the life cycle of a product system that provides the greatest leverage to improve its environmental profile. ATTRIBUTIONAL AND CONSEQUENTIAL PERSPECTIVES For nearly 30 years, LCA models were built by analysts responding to the (often implicit) question of how a specific product is made and disposed of. The static nature of this question, the engineering background of most LCA analysts, and the practical requirement for generic and reusable modular databases, led to static, scale-independent models of the flows of materials and energy among linear, engineering-unit processes. As LCA evolved, models describing how current inflows and outflows link these processes were built according to ac- counting conventions (such as those laid out in detail by the ISO standards for LCA). These models could allocate or attribute total economy-wide pollution and resource consumption burdens among the total economy-wide output of products (Heijungs, 1997~. Until quite recently, this engineering-based, "attributional" perspective was unquestioningly considered to lead to results valuable for identifying environ- mentally preferable decisions. But a decision-making perspective is a what-if perspective that requires a considerably different model than the model based on the "how are things made" perspective. For example, consider a product P whose manufacture is electricity inten- sive. An attributional LCA model is built by asking where the electricity comes from. The model then characterizes the fuel shares for electricity generation in the region of interest and attributes the burdens among the power plant types based on their shares of energy contribution to the regional mix (e.g., during a recent year). By contrast, a what-if perspective might ask what the environmen-

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a: a: FRONTIERS OF ENGINEERING tat consequences of making more or less of product P are. In this case, when the demand for product P goes up or down, not all power-generating plant types respond in equal proportion to their current output, either over the short term or long term. For example, if hydro power contributes 50 percent of currently generated kWh in the region of interest, increasing the demand for product P will not lead to more rain (more hydropower output) in the short or medium term. And over the long term, the plant types expected to be the most cost effective will be added to the generating mix. Thus, attributional (or pollution-cause-oriented) and consequential (effect- oriented) perspectives can lead to very different models that generate signifi- cantly different results (Ekvall, 1999~. This realization, and the fact that the usual purpose of LCAs is to help identify decisions or courses of action that will lead to beneficial environmental effects or consequences, has generated lively debate in the LCA community about fundamental criteria for model selection and database development. At the same time, LCAs are increasingly integrating information from data- bases and perspectives from beyond the engineering-unit process world. For example, to make products ranging from computers to buildings requires not only energy and materials, but also product designers, architects, engineers, mar- keters, managers, and so on. These people tend to travel as part of their work. It turns out that the environmental impacts of business travel contribute signifi- cantly to the total environmental burdens of a wide range of products (Kanzig, 2003~. By drawing upon models and databases of economic input/output analy- sis, these burdens can be addressed. SUSTAINABLE DEVELOPMENT The present application context of LCA is sustainable development, which is commonly considered to have three pillars: economic growth, ecological balance, and social progress (WBCSD, 2003~. Traditional LCA addressed only the environmental pillar. However, when economic modeling and a what-if perspective are combined, LCA can potentially address more of the sustainability agenda, thereby avoiding burden shifting among the social, environmental, and economic objectives and potentially helping to build a broader base of support for policy proposals by addressing the concerns of a wide range of stakeholders. This larger perspective is made possible in two ways. First, by integrating economic models and databases, LCA can address impacts and performance measures of groups that are routinely tracked at the level of economic sectors rather than at the level of engineering-unit processes. An example of an impact group is occupational health and safety. A recent investigation concluded that the health effects of occupational health and safety issues and incidents in prod- uct supply chains appear to be on the same order of magnitude as the expected

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LIFE CYCLE DEVELOPMENT 23 near-term human health consequences of supply chain pollution releases (Hofstetter and Noms, 2003~. Second, by integrating the economic and consequential modeling approaches and databases, LCA can show that product supply chain activities bring benefits as well as burdens for the agenda of sustainable development. Sustained in- creases in economic output among developing countries is linked to major im- provements in human health, through the mechanisms of income-poverty reduc- tion, increased investment in and access to education, and increased public investment in the public health infrastructure (World Bank, 2001~. Traditional LCA, which was focused strictly on pollution impacts and was blind to the benefits of development, is considered biased against the primary concerns of developing countries by some sustainability analysts. By addressing the benefits of economic development as well as the costs of pollution and resource degrada- tion, LCA has the potential to address these concerns and suggest truly sustain- able solutions to the challenges of consumption and production in the twenty- first century. REFERENCES Boustead, I. 1996. LCA How it came about: the beginning in U.K. International Journal of Life Cycle Assessment 1(3): 147-150. Ekvall, T. 1999. System Expansion and Allocation in Life Cycle Assessment. AFR Report 245. Goteborg, Sweden: Chalmers University of Technology. Heijungs, R. 1997. Economic Drama and the Environmental Stage. Leiden, Netherlands: Univer- sity of Leiden. Hofstetter, P., and G. Norris. 2003. Why and how should we assess occupational health impacts in integrated product policy? Environmental Science and Technology 37(10): 2025-2035. Hunt, R., and W. Franklin. 1996. Personal reflections on the origin and the development of LCA in the USA. International Journal of Life Cycle Assessment 1(1): 4-7. Hunt, R., and W. Franklin. 1974. Resource and Environmental Profile Analysis of Nine Beverage Container Alternatives. EPA 530-SW-9lc. Washington, D.C.: Environmental Protection Agency. ISO (International Standards Organization). 1998. Environmental Management-Life Cycle Assess- ment: Goal and Scope Definition and Inventory Analysis. ISO Standard 14041. Geneva: International Standards Organization. Kanzig J. 2003. Input/Output Life Cycle Assessment of Air Transportation. Masters thesis, Life Cycle Systems, Environmental Science and Engineering. Lausanne, Switzerland: Ecole Poly- technique Federale de Lausanne (EPFL). Oberbacher, B., H. Nikodem, W. Klopffer. 1996. LCA How it came about: an early systems analysis of packaging for liquids which would be called an LCA today. International Journal of Life Cycle Assessment 1(2): 62-65. WBCSD (World Business Council for Sustainable Development). 2003. Information available online at: . WCED (World Commission on Environment and Development). 1987. Our Common Future. Ox- ford, U.K.: Oxford University Press.

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24 FRONTIERS OF ENGINEERING World Bank. 2001. Attacking Poverty. World Development Report 2000/2001. Washington, D.C.: World Bank. WSSD (World Summit on Sustainable Development). 2002. Plan of Implementation for Changing Unsustainable Patterns of Consumption and Production. Johannesburg, South Africa: World Summit on Sustainable Development. Available online at: .