FEATURE
"Scientific Uncertainty" Scuttles New Acid Rain Standard EPA study recommends no new measures to protect "sensitive areas" from continued acid deposition. REBECCA
E
PA will report to Congress this month on the controversial question of whether the Clean Air Act Amendments of 1990 (CAAA) adequately protect sensitive areas of the United States from acid rain. Although the draft report states that certain areas, especially New York's Adirondack Mountains, will continue to experience increased acidification of surface water under current emission standards, it does not call for new regulatory approaches because of scientific uncertainty primarily related to nitrogen deposition. That conclusion has raised the ire of the New York congressional delegation, which plans to take legislative action if EPA does not act to decrease acidic deposition in the region. The draft report, issued in February, does not propose or recommend a new "acid deposition standard" to protect vulnerable areas. A deposition standard would stipulate the amount of pollution, usually measured as kilograms per hectare per year, that can be tolerated by a sensitive area before some threshold of environmental harm occurs (see sidebar). Instead, it concludes that scientific uncertainty makes it "difficult to determine the appropriate level of a standard or standards at this time" (i), a conclusion endorsed by the EPA Science Advisory Board (SAB) subcommittee that reviewed the draft (2). And even though the wording of the final version may change, the conclusion will remain the same, according to project manager Rona Birnbaum. In July, 32 of New York's 33 members of Congress expressed their dissatisfaction with the draft in a letter to EPA Administrator Carol Browner calling for additional emissions reductions to protect the Adirondacks. "If the final version does nothing more, we will be drafting new legislation," said a spokesperson for Rep. Jerry Solomon (R). The New York State Department of Environmental Conservation (NYDEC) contends that by not proposing a standard, EPA has failed to fully respond to 4 6 4 A • VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the congressional directive. NYDEC argues that the need is clear on the basis of the report's own modeling, which indicates that maintaining the 1984 proportion of chronically acidic Adirondack surface waters may require reducing industrial sulfur and nitrogen deposition by 40-50% or more below levels achieved by full implementation of the CAAA. The CAAA addressed the acidic deposition problem by mandating nationwide reductions in emissions of sulfur and nitrogen oxides from electric utilities, the major contributors to acidic deposition. Title IV called for 50% reductions of S0 2 from utilities relative to 1980 levels by the year 2010; NOA. emissions from utilities were to be reduced by roughly 25%. The report to Congress, required by the CAAA, asked the Agency to report on the feasibility of setting an acid deposition standard to protect sensitive areas. EPA missed the original 1993 deadline and is under court order to issue the final report by October 15. The draft report identifies the lakes and streams of the Appalachian mountains as sensitive resources that receive damaging concentrations of acidic deposition. Three areas where sensitive water resources have been well studied were selected as providing the best available data for modeling case studies: the Adirondacks; the mid-Appalachian region, including parts of Pennsylvania, West Virginia, Maryland, and Virginia; and the Southern Blue Ridge in Tennessee, North Carolina, and Georgia.
Uncertainties about nitrogen saturation The bulk of the research on acidic deposition carried out in the 1980s focused on the effects of sulfur, the major cause of acidic deposition. The uncertainties raised by the EPA report, however, relate to the role of nitrogen in acidification processes. Nitrogen oxide emissions come primarily from vehicles (45%) and electric utilities (32%), with other forms of combustion making up the balance (3). Usually nitrogen is a fertilizer, an essential nutrient in 0013-936X/95/0929-464A$09.00/0 © 1995 American Chemical Society
high demand, so much so that it is often the limiting nutrient in forests. For this reason very little research in the past focused on ecological effects associated with nitrogen deposition in watershed acidification. However, at high levels of nitrogen deposition, watersheds and forests may become unable to utilize or store all of the nitrogen available. Once this "nitrogen saturation" occurs, excess nitrate will contribute to watershed acidification (4). Understanding what causes nitrogen saturation is one of the goals of current research, because nitrogen becomes a contributor to long-term acidification only in excess (4). Excess N0 3 ~ in watersheds can lead to depletion of basic cations and surface water acidification through similar processes to those involving excess S0 4 2 ~. Field evidence for nitrogen saturation has been largely from trends in N03~ concentrations for a few streams with long-term records or from differences in concentrations from populations of lakes sampled years apart (5). Some European forests are apparently becoming nitrogen saturated (6), and the United Nations Economic Commission for Europe is working on a new nitrogen protocol to reduce emissions of nitrogen oxides and ammonia. A field experiment that has deliberately caused nitrogen saturation to one of two catchments of the Bear Brook watershed in eastern Maine is also providing insight into this process. Since 1989, researchers at the University of Maine-Orono have been treating the West Bear Brook catchment with ammonium sulfate at levels comparable to nitrogen loading currently experienced in some parts of Europe (5). Maximum response occurred in 1992, when N0 3 ~ concentrations in West Bear Brook were chronically elevated for most of the summer and reached 80 peq/L, almost twice the pre-manipulation maximum of 43 peq/L. Current knowledge has identified several characteristics that make the Adirondacks and other parts of the Appalachians especially vulnerable to nitrogen saturation: cooler annual temperatures, shorter growing seasons, stands of old-growth forest, long histories of elevated deposition rates of sulfur and nitrogen, and thin glacial soils that offer little basic cation buffering. However, current knowledge is unable to explain N03~ trends in eastern waters over the past few years, according to Charles Driscoll of Syracuse University. During the 1980s a pattern of increasing N0 3 " concentrations was evident in some surface waters in the Adirondacks and elsewhere in the Northeast. The pattern was enough to prompt concern that this was the first sign of incipient nitrogen saturation. But the trend didn't last, Driscoll says. The pattern of increasing N0 3 " concentrations has been followed by a five-year decline, and now there is no dis-
Mixed message in an Adirondack lake A time-series analysis of more than 12 years of data from Constable Pond in the Adirondack Mountains shows a statistically significant long-term decrease in SO42' but no long-term change in pH. NO3" increases peaked in 1990 but have declined since. There are currently no significant long-term trends in NO3 concentrations for Constable Pond or other Adirondack lakes. Seasonal variations are apparent in the data. (Source: C. Driscoll, Adirondack Long-term Monitoring Program, Fifth International Conference on Acidic Deposition, Gôteborg, Sweden, June 1995.)
cemible pattern to the 12-year nitrate concentrations. Reductions are evident even in the manipulated Bear Brook watershed. "The conclusion seems inescapable," says Steve Kahl of the University of Maine research group working at Bear Brook. "The natural cycle has a more profound effect than the manipulation. We're looking for an explanation, but there are many variables—climate, temperature, seasonal wetness. Right now we don't have the answer." In fact, the overall Adirondack picture is unclear, Driscoll says. Atmospheric sulfate has dropped by almost 30% since 1970, and emissions of nitrogen oxides have decreased by 9-15% from their peak in 1978. These decreases have resulted in reductions in S042~ in precipitation and surface water, but they have not been accompanied by a widespread increase in pH or acid-neutralizing capacity of surface waters affected by acidic deposition (see figure). The decline in S042~ has corresponded to decreases in the concentrations of basic cations in Adirondack lake waters, a pattern suggesting that soil cation exchange is responding to the drop in S0 4 2 ". VOL. 29, NO. 10, 1995/ ENVIRONMENTAL SCIENCE & TECHNOLOGY • 4 6 5 A
Europeans developing "critical load" standards Although EPA has concluded that scientific uncertainty about the role of nitrogen in acidification of forested watersheds is too great to set a deposition standard to protect sensitive resources, European regulators appear to be less reticent. European policy on reducing acidification has already shifted away from imposing flat-rate emission reductions to an approach based on effects. This "critical loads" approach sets a threshold concentration of a pollutant at which harmful effects on sensitive resources, including surface water, groundwater, and forest soil, begin to be observed. Methods for calculating critical loads range from simple mass-balance calculations using empirical data to complex calculations using dynamic models; most European countries use simple calculations. Much of western Europe has adopted a system developed by the Coordination Center for Effects of the United Nations Economic Commission for Europe (UNECE) under the auspices of the UNECE Convention on Long-Range Transboundary Air Pollution. In this system, critical loads based on the potential sensitivity to acidification of forest soils and surface water are developed for individual cells of a mapping grid. The critical loads mapping grid is relatively coarse (each cell is 22,500 km2) and grid boundaries bear no relationship to boundaries between regions of different sensitivity. To get around this discrepancy, the European
Modeling efforts criticized Despite these uncertainties, EPA tried to predict the effects of sulfur and nitrogen deposition on sensitive areas using the Model of Acidification of Groundwater in Catchments (MAGIC), a large-scale regional model, and a range of arbitrary times to nitrogen saturation. This strategy was criticized by the SAB subcommittee, which recommended the use of acidic deposition models that include the biological processes controlling nitrogen cycling. But Robbins Church of EPA at Corvallis, OR, who managed the modeling effort, argues that the modeling makes a good first step. "Basically the results indicate that nitrogen could be quite important. We may not have the level of detail desired in a perfect world, but it's a good first step." Better models are also on the way, he says. Researchers led by John Aber at the University of New Hampshire have just finished work on a model that accounts for biological processes. Another EPAfunded project improving the MAGIC model is to be completed next year. But the limiting factor, according to Church, is sufficient data, not better models. "My estimate is that to do for nitrogen what we did for sulfur we will need a major regional study." As for further research on what happens to nitrogen in forested w a t e r s h e d s , a recent EPANational Science Foundation request for proposals on watershed systems is expected to fund additional work. Research groups are also active in Scandinavia and Japan.
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approach calculates a deposition level that would protect 95% of the sensitive ecological resources within the grid. Most critical loads in Europe are much lower than present deposition levels, prompting some countries to seek additional emissions reductions. UNECE is currently working on a new nitrogen protocol that will adopt the critical loads approach, looking at damage to the environment by NH3 as well as N0X. In North America, Canada and several U.S. states— including New York and Maryland—have calculated critical loads for sensitive resources. But Minnesota is the only state with a deposition standard for sensitive areas. Translating critical loads to emissions reductions that can preferentially protect a sensitive region—the task EPA evaluated in its report—raises another set of problems. Acidic deposition comes from many different areas as far as several hundred miles away, and involves a host of chemical interactions in the atmosphere. For example, not only is the Adirondacks region sensitive to acidic deposition, it also receives a major portion of its deposition from the concentration of power plants in the upper Ohio River Valley. This is why the New York State Department of Environmental Conservation advocates additional regional emissions reductions for sources upwind of the Adirondack region. —REBECCA RENNER
But perhaps it is the SAB that has the last word on what is required. The committee emphasizes the importance of environmental monitoring, suggesting that it would be a good idea to repeat the National Surface Waters Survey conducted in the mid1980s, which sampled "at risk" freshwater ecosystems using a standard protocol. "Without direct monitoring evidence for declining ecosystem quality," the SAB wrote, "it is doubtful that incremental regulations, especially if costly, would be implemented based on modeling evidence alone."
References (1) Acid Deposition Standard feasibility Study Report to Congress (Draft for Public Comment); Office of Air and Radiation, U.S. Environmental Protection Agency: Washington, DC, February 1995; EPA-430-R-95-001. (2) Review of the Acid Deposition Standard Feasibility Study Report to Congress; Science Advisory Board, U.S. Environmental Protection Agencv: Washington, DC, Julv 1995, EPA-SAB-EPEC-95-019. (3) National Air Pollutant Emissions Trends, 1900-1992; Office of Air Quality Planning and Standards, U.S. Environmental Protection Agencv: Washington, DC, October 1993, EPA-454-R-93-032. (4) Aber, J. D. et al. Bioscience, 1989, 39, 386-87. (5) Kahl, S. et al. Environ. Sci. Technol. 1993, 27, 565-68. (6) Sullivan, T. I. Environ. Sci. Technol. 1993, 27, 1482-86. Rebecca Renner is a freelance writer based in Williamsport, PA. She has written for New Scientist and covered environmental issues for The Independent newspaper in the United Kingdom.
