FEATURE
A Cross-Media Approach to Saving the Chesapeake Bay E L A I N E L.
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n 1992, Robin Dennis and his colleagues at EPA's National Exposure Research Laboratory in Research Triangle Park, NC, began searching for a "Grand Challenge." The researchers from the Atmospheric Modeling Division were looking for a broad scientific problem that would stretch their skills in supercomputer simulations of air quality problems and be a suitable component of the National High Performance Computing and Communication "Grand Challenge" supercomputing research program. They found By linking powerful that challenge in a ground-breaking crosssupercomputer models, m e d i a study of the Chesapeake Bay that scientists are studying simulates the transfer of pollutants from one the role of atmospheric medium—in this case, the air over the bay— deposition in the to the 64,000-squaremile watershed and ulChesapeake Bay's water timately the bay itself. The cross-media quality problems. project builds on the modeling work under way at the Chesapeake Bay Program, a cooperative agreement among the jurisdictions in the bay's estuary: Maryland, Virginia, Pennsylvania, the District of Columbia, the Chesapeake Bay Commission, and EPA. Earlier this year the North Carolina air modelers joined their supercomputer model of the bay's airshed with the Bay Program's watershed model and the U.S. Army Corps of Engineers' threedimensional water quality model. The initial results are already revealing new insights into regulatory strategies needed to reduce eutrophication in the bay. By using supercomputer models to "try out" different control strategies designed to reduce nutrient flows, regional and federal regulators are testing the long-term effective-
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APPLETON
ness of potentially expensive, complex, and politically sensitive regulatory decisions. The health of the Chesapeake Bay, whose nine tributaries bring water from as far away as New York and West Virginia, has declined for decades. From 1990 to 1992 alone, the tributaries dumped 600 million lb of nitrogen and 30 million lb of phosphorus into the bay, according to the U.S. Geological Survey (i). Nutrients, which come from utilities, sewage treatment plants, farm fertilizer runoff and animal waste, and automotive exhaust, create huge algal blooms that deprive underwater grasses of sunlight and use up oxygen when they decompose, causing anoxia, or "dead water" in up to 15% of the bay's total volume (2). Since 1983, the Chesapeake Bay Program has been studying how to control these excess nutrient loadings. In 1987, the group called for phosphorus and nitrogen loading reductions of 40% below 1985 base loads by the year 2000, a goal estimated to produce a 20% reduction in dead waters. Despite studies indicating that atmospheric deposition contributes 20-35% of the millions of kilograms of nitrogen that wend their way into the bay (3), the Bay Program to date has controlled only waterborne nutrients. "The Bay Program has always determined air sources to be uncontrollable," said Lewis Linker. However, the water-only nitrogen reduction strategies have not worked. A formal evaluation of the role atmospheric deposition will play in the overall bay nutrient reduction strategies is scheduled for 1997.
Nitrogen not easily controlled As a result of implementing controls such as banning the use of phosphate-laden detergents, 48% of the phosphorus reduction goals had been met by 1992, when the program conducted its second major review. But nitrogen loading is not as easily understood or as easily controlled. By 1992 only 9% of
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the nitrogen reduction goal had been met (2), most significantly through point source reductions, such as biological nutrient removal, at sewage treatment plants affecting the West Shore basin (4). This was particularly important because nitrogen reduction is more effective than phosphorus reduction in reducing anoxic volume days (5). Today the bay's health remains in jeopardy. According to the U.S. Geological Survey, nitrogen continues to increase in the Susquehanna and Potomac Rivers, which are major bay tributaries. "Historically, there has not been much in terms of water quality controls [of nitrogen] being effective," said Dennis. Nitrogen nonpoint source controls are difficult to implement, point source nitrogen controls have only recently begun, and there are uncertainties in watershed retention of nitrogen, he said. "All we know is that nitrogen reduction is nowhere near on target, so there's a lot of interest in what air quality can do to help." Because nitrogen cycling through the atmosphere is nonlinear and far ranging, deposition at any one area comes from several sources. Thus, it is impossible to gather precise nitrogen source attribution data through monitoring alone—air quality modeling is necessary, reported Dennis (3). Once nitrogen is deposited, it moves through the watershed and various portions of the bay in different ways, depending on current movement in the tributaries and uptake by various receptors such as soil, trees, and benthic organisms. Therefore, water quality modeling is needed to transform the atmospheric loads deposited throughout the basin to nitrogen loads delivered to the bay.
A collaboration is born When Robin Dennis first began looking for a "grand challenge," environmental model coordinator Lewis Linker of the Chesapeake Bay Program had just begun to direct the linkage of two supercomputer models: one that simulates land use and its effect on water quality and one that simulates the bay itself. It was a huge task designed to provide regulators with the scientific information needed to reduce the millions of pounds of nutrients, primarily nitrogen and phosphorus, that flow into the bay yearly. Linker's objective was to pinpoint the nutrient sources and understand how they act upon receptors, including the water in the bay's main stem, its tributaries, and the land surrounding the bay. Working with the Army Corps of Engineers, Linker was combining bay and estuary modeling results, but he realized that in an area the size and complexity of the Chesapeake Bay it was impossible to get accurate estimates without including airborne sources. Little data existed, however, on the effects of atmo-
Chesapeake Bay watershed The Chesapeake Bay watershed delivers excess nutrients from as far away as New York and West Virginia to the bay, causing anoxia in up to 15% of the bay's total volume .
spheric deposition of nitrogen to the bay, a problem that the Bay Program suspected was significant. Since 1988, when Environmental Defense Fund research indicated that atmospheric nitrate deposition was a major anthropogenic source of nitrogen to the bay (3), Linker and his colleagues had wanted to add comprehensive simulations of air deposition to their water quality and land use simulations. However, to their knowledge, no one had ever attempted this kind of cross-media modeling. When Dennis heard about the Chesapeake Bay Program, he seized upon the idea of using EPA's Regional Acid Deposition Model (RADM), a supercomputer model designed to simulate the conditions that cause acid rain, to study atmospheric nitrogen deposition to the bay. RADM already included nitrogen as one of the species it could simulate. Dennis offered to link RADM to Linker's water quality models. It was auspicious timing. The 1992 bay agreement amendments mandated that the signatories "incorporate into the Nutrient Reduction Strategies an air deposition component which builds upon the 1990 Clean Air Act Amendments (CAAA) and explores additional implementation opportunities to further reduce airborne sources of nitrogen entering Chesapeake Bay and its tributaries." Linker accepted Dennis's offer. It is not surprising, suggested Dennis, that neither his office nor EPA's Office of Air Quality Planning and Standards, which had been planning NO x VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 5 5 1 A
Evaluating strategies to reduce nitrogen loads The Chesapeake Bay Program's strategies to reduce nitrogen loading from water sources have not significantly decreased overall inputs. Total nitrogen loads from the bay's three major tributaries did not change between 1985 and 1993. To meet the Bay Program's goal of a 40% reduction of controllable nutrient loads by the year 2000, the Program is now looking to atmospheric sources. Using the cross-media model, researchers are evaluating the impact of full implementation of Clean Air Act controls on emissions from stationary, mobile, and regional sources on the bay. Total nitrogen loads to the Chesapeake Bay, by basin
reduction strategies in its efforts to reduce tropospheric ozone, had not heard of Linker's work. "By taking a broader cross-media perspective," he said, "we see that we're in a potential win-win situation, where something that we do for one pollutant actually benefits another problem area. In our typical bureaucratic mind-set where we're in little cubicles, we would have ignored that. We wouldn't have known." And so the process of linking the three models— the watershed model, the Chesapeake Bay Water Quality Model (CBWQM), and the RADM airshed model—was begun. Air deposition source loads could be fed into the watershed model to determine nitrogen deposition and movement on land, and from there into the estuary model to determine their effects in the water. By the beginning of 1995, Dennis and Linker had joined all three models.
Cross-country modeling The modelers are now running different scenarios to evaluate different control strategies. What would happen, for example, in the year 2000 if regulators impose limit-of-technology control strategies to reduce nitrogen emissions from utilities that export nutrients into certain tributaries? To do a model "run," specialists in Research Tri5 5 2 A • VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
angle Park or Annapolis, MD, send these queries, or "model input files," via high-speed communication lines to a Cray C90 computer at EPA's National Environmental Supercomputing Center in Bay City, MI, where the runs are performed. Typically, the airshed model is run first so results can be used as input into the watershed and water quality models. The 3-D airshed model tracks nutrient emissions across the eastern United States by dividing the area into 20,000 square cells, each 80 km on a side. A fine grid of 60,000 20-km-by-20-km cells covers the Chesapeake Bay region. Piled one on top of another, the cells create 15 vertical layers reaching about 15 km high to the top of the free troposphere. By simulating the airshed in three dimensions, modelers can determine vertical as well as horizontal movement of nutrients. The water quality model represents the bay as 4073 computational cells that average 6 mi long, 2 mi wide, and 5 ft deep, stacked up to 15 layers in the deepest parts of the bay. The model includes a hydrodynamic model that simulates water flow rates into the bay, the mixing of the bay with coastal waters, and the mixing of water within the bay. One "what-if " scenario run on the airshed model alone requires 300 h of computer time. "RADM is a monster," said Linker. The watershed and water quality models are smaller, he said, with a typical watershed run taking 30-40 h. It is not surprising that the modelers are caught between their desire to refine the model into ever-smaller grid cells and time steps and the growing amount of computer resources such refinement requires—the developers are continually running up against the limits of computational technology. Combining air quality and water quality modeling in cross-media simulations is rife with challenges, including such problems as calibrating equivalent units of emissions in air and water and determining the sources of far-ranging atmospherically deposited nitrogen or the retention rates of nitrogen through forests. And, given the amount of data simulated in these models, data validation via traditional field monitoring is challenging if not seemingly insurmountable. For example, Linker's watershed model, designed to simulate nutrient loads delivered to the estuary under different management scenarios, includes hydrological data (temperature, precipitation, wind, solar radiation, and other factors) from 1984 to 1994, nonpoint source loads, and nine types of land uses. In river reaches, modelers calculate nonpoint source loads, point source loads, and water supply diversions on a one-hour time step. The complexity of cross-media modeling is exacerbated by emissions inventories that change as more information becomes available. Such massive complexity poses real-world problems, said Linker. "As the simulations become more and more extravagant, there's a greater and greater reliance on simple observed data to make sure we're getting the simulation right." In fact, calibrating the models is critical to "getting the right answers for the right reasons," he said. The developers calibrate the models by comparing simulation results with observed data. In addition, a panel of scientists from
Building a cross-media supercomputer model Land use, regional atmospheric deposition, and water quality of the Chesapeake Bay are joined in the cross-media model, providing regulators with detailed information on ways to combat deterioration of the bay. The three pieces of the crossmedia model developed separately. The Watershed Model and the Chesapeake Bay Water Quality Model were first linked in 1992, providing insights on the impact of point and nonpoint sources on the bay. With the added input from the Regional Acid Deposition Model in early 1995, the role of regional atmospheric deposition can also be studied. The three components will be fused into a single supercomputer model in 1997.
Chesapeake Bay Water Quality Model Created in 1992 as a linked model with the Watershed Model. Composed of 4073 computational cells that average 6 miles χ 2 miles χ 5 feet. Data inputs include tide elevation, salinity, temperature, solar radiation, bathymetry, wind speed, river flows, atmospheric inputs, watershed inputs. Lead agency: U.S. Army Corps of Engineers Waterways Experiment Station.
Watershed Model Created in 1982; current version (1992) confirmed importance of atmospheric deposition to bay water quality. The 63 model segments represent homogenous hydrologie regions that drain to a single river channel. Data inputs include precipitation, temperature, wind speed, solar radiation, cloud cover, land use, crop operations, and point sources. Lead agency: EPA Chesapeake Bay Program Office.
Regional Acid Deposition Model In development since 1983, completed in 1989. Divides eastern United States into 80 χ 80 km cells with a fine grid of 20 χ 20 km cells over the bay watershed. Computes nitrogen oxidation via a full photochemistry chemical mechanism. Data inputs include emissions and meteo rology. Lead agency: EPA National Exposure Research Laboratory.
outside the Chesapeake Bay Program reviews the models quarterly.
Trial run for future regulations Despite these challenges, the integration that to day exists among the models has given Bay Pro gram officials information about nitrogen deposi tion they did not have before. Typically, policy makers ask the modelers to create different scenarios by "tweaking" emissions sources to project the out comes of various control strategies. For instance, said Linker, they have used models to ask, "Do we need
to control both nitrogen and phosphorus, or can we control only one to meet our water quality objec tives? Or are the nutrients equal in different places, so if we reduce a kilogram of nitrogen in the Sus quehanna River at the top of the bay, would it be equivalent to reducing a kilogram in the James River at the lower end of the bay?" It turns out that geography is indeed important, and that reducing nitrogen at the northern and cen tral portions of the bay has a far greater effect on an oxic days measured in the bay's main stem than does reducing the same amount of nutrient at the south VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 5 5 3 A
The bay's "airshed "and emissions sources The cross-media model was used this summer to establish the size of the Chesapeake Bay's airshed, the geographic region that contributes 70-80% of the atmospheric nitrate deposition to the watershed. Initial results show that the airshed is more than five times the size of the bay's watershed, prompting concerns that federal rather than regional regulations may be needed to control nitrogen inputs into the bay. The modeling also indicated that mobile sources and stationary sources contribute equally to nitrogen deposition to the bay.
Modeling also has shown that, although utility and mobile source emissions are almost equally responsible for nitrogen deposition to the eastern United States (utilities contribute 37% of NO x emissions, and mobile sources 35%), they are geographically separated. The utility emissions come more from the west, and the mobile sources affecting the bay come from the more densely populated eastern seaboard (2).
Looking to ozone reduction
end of the bay. Linker warns, however, that anoxia is just one measure of health, and that nutrient reduction in southern tributaries may have other positive effects. To understand the effects of federal regulations such as the 1990 Clean Air Act Amendments on the bay, Dennis thought it was important to determine the geographical boundaries of the airshed, defined as "that contiguous region of air emissions that contributes a majority [70-80%] of the emissions to the bay watershed and main stem" (2). Although earlier runs of the combined water and estuary models had indicated that atmospheric nitrate deposition contributed at least 25% of the total nitrogen to the bay, it was still unclear exactly where it was coming from. During the summer of 1995, after first determining that the range of influence of nitrogen, especially the termination product nitric acid, is, conservatively, 700-800 km, Dennis estimated that the airshed is at least 5.5 times the size of the bay's watershed. That means the airshed extends as far as Montreal and Toronto, Canada, Detroit, MI, and Cincinnati, OH. NO x emissions from this airshed constitute about 30% of the NO x emissions in the entire eastern United States and Canada and account for approximately 70% of the anthropogenic nitrogen deposited in the bay watershed, said Dennis. He noted that emissions from west of the bay affect it the most because of prevailing winds (3). However, he cautioned that, because of bias in the model, this is a conservative estimate and, in fact, the range of influence of nitrogen could be as far as 1000 km. 5 5 4 A • VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
EPA policy makers are taking these results seriously. Until now, regulators have dealt with Chesapeake Bay problems solely on a state and regional basis. Now, said John Bachman, associate director for science policy at EPA's Office of Air Quality Planning and Standards, "If Robin Dennis's preliminary modeling turns out to be right, and his footprint for the Chesapeake Bay goes all the way to Ohio, then it turns out to be a federal issue, because you need federal regulations to convince Ohio to care about this. We're looking at that." Initially, policy makers are looking to ozone reduction plans, which include NOx reduction strategies, to help the bay. Modelers compare "milestone" scenarios—which assume either no new control strategies beyond 1985 or achievement of the Bay Program's 40% nitrogen loading reduction goal—with scenarios that could occur if federal ozone reduction strategies are implemented. The latter model run indicates that full CAAA implementation would reduce nitrogen entering the bay and provide an additional slight reduction in anoxia days over the 20% reduction projected by the Bay Agreement (4). But Bachman is also interested in the results of additional modeling runs that examine the effects of strategies devised by the Ozone Transport Commission (OTC), the 13-member regional board made up of northeastern states and the District of Columbia. OTC controls are more stringent than current CAAA strategies and would, according to Dennis, double the positive effects on the bay of the full implementation of CAAA controls alone. Preliminary modeling runs show that an even more effective strategy would be to implement OTC controls on the entire Chesapeake Bay airshed. Another issue is whether there is a need to implement ozone restrictions beyond the current summer restrictions. Model runs show that it is equally important to the bay to regulate ozone in the spring, and perhaps also in the fall and winter. Bachman said, "That's critical for us to know. I don't want to control ozone in winter as well as summer, but I have to know that as a policy maker, and these models are helpful in that regard." Bachman has asked Dennis and Linker to run more scenarios to determine if and where wintertime controls are necessary. "It certainly will be critical within the first few hundred ki-
EPA investigating cross-media emissions trading A project EPA began in August will investigate the possibility of cross-media emissions trading as a new approach to reducing nitrogen loadings to the Chesapeake Bay. Working with the Environmental Defense Fund (EDF), the Agency hopes to devise a N0X trading framework along the lines of existing sulfur dioxide trading plans to control acid rain. The Chesapeake Air Project will examine the feasibility of using emissions trading between air and water sources, including trading credits between power plants and mobile sources, to reduce the atmospheric deposition of nitrogen to the bay. EPA's Chris Knopes, who is helping to direct the project, acknowledged that cross-media trading could be a long way off. "The concern of the Bay Program is that they're not the EPA, they're not national, and so they can't say 'All of you states need to implement controls.' Their concern with multimedia trading is that it would appear to be loosening [existing] controls." Moreover, to do any kind of emissions trading requires a thorough un-
lometers of the bay," he said. "The question is where to draw that line." Another policy question that may be addressed with the aid of cross-media modeling is the cost effectiveness of different regulatory approaches. Chris Knopes, a policy analyst with EPA's Office of Policy Planning and Evaluation, would like to know what the cost differential is between implementing atmospheric controls and water controls. "The air people are saying they can reduce NO x emissions for $1000 a ton from the stack," reported Knopes, referring to selective catalytic reduction. In contrast, it can cost "a couple of hundred to a couple of hundred thousand dollars per ton" to reduce nitrogen loading from water sources. "The question is what does that [airborne reduction cost] mean in terms of the cost per ton of nitrogen not delivered to the bay? We don't know yet." Knopes is hoping that a combination of traditional cost data analysis and crossmedia air and water quality modeling will help answer this question. As the Bay Program gears up for a réévaluation of its nutrient reduction goals in 1997, Linker and Dennis are further refining the model scenarios to add to understanding of source-receptor relationships and nitrogen retention. They also hope to be able to simulate underwater grasses and benthic organisms to provide tributary-specific goals for nutrients based on habitat improvements. This adds an "ecosystem" measure of health to the "anoxic days" measure the program has so far focused upon, said Linker. "We know that the 40% nutrient reduction improves dead waters, but what does it do for the underwater grasses of the bay and the benthic organisms, both of which support the bay's fisheries?"
derstanding of source-receptor relationships, something that grid-based models such as the bay's airshed model, in contrast to plume models, aren't typically designed to do. Such an incentive program will also require more thorough understanding of the temporal nature of nitrogen loading and more knowledge of how nitrogen moves through the watershed, including forests, soil, and groundwater. Unfortunately, the science on this aspect of nitrogen is very weak. But the progress of the Bay Program nutrient reduction goals is up for réévaluation in 1997, and Knopes and EDF economist Brian Morton have high hopes that the trading plan, which would place a cap on the mass of emissions and rate of deposition allowed by all sources, will become the atmospheric deposition portion of the Chesapeake Bay Program's Nutrient Reduction Strategy. —ELAINE L. APPLETON
The complexity of the questions that the crossmedia modelers address and the urgency of finding regulatory solutions will continue to increase as population and development pressures add more nutrients into the bay. Linker and Dennis are now refining the linkages between the models and plan to complete a final integrated model by early 1997. "The nutrient reduction goal of the year 2000 really becomes a cap not to be exceeded," said Linker. "But growth will continue and sewage treatment plants will discharge more loads. We have to get smarter and more cost-effective about the work. That's driving a lot of the inquiry into air deposition."
References (1) Zynjuk, L. D. Chesapeake Bay: Measuring Pollution Reduction; U.S. Geological Survey, Washington, DC, 1995; Fact Sheet FS-055-95. (2) Dennis, R. L. "Using the Regional Acid Deposition Model to Determine the Nitrogen Deposition Airshed of the Chesapeake Bay Watershed." In Atmospheric Deposition to the Great Lakes and Coastal Waters; Baker, J., Ed.; Society of Environmental Toxicology and Chemistry: Pensacola, FL, in press. (3) Fisher, D. et al. Polluted Coastal Waters: The Role of Acid Rain; Environmental Defense Fund: New York, 1988. (4) FY 1994 National Environmental Supercomputing Center (NESC) Annual Report; U.S. Environmental Protection Agency. NESC: Bay City, MI, Feb. 1, 1995; EPA/208/ K-95/001. (5) Cerco, C. E; Cole, T. M. "Three-Dimensional Eutrophication Model of Chesapeake Bay"; U.S. Army Corps of Engineers: Vicksburg, MS, May 1994; Technical Report EL94-4. (6) Blankenship, K. Bay Journal 1992, July/August, pp. 1, 4-5.
Elaine L. Appleton is a freelance writer based in Newburyport, MA. She is former senior editor at Datamation magazine.
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