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.
