Acid Rains Essay, Research Paper

Acid Rains

Scientific evidence has shown that atmospheric deposition of sulfur and nitrogen compounds can harm ecosystems. Title IV of the Clean Air Act Amendments of 1990 (CAAA or the Act) addresses the problem of such effects by mandating reductions in emissions of sulfur and nitrogen oxides, the major precursors of acidic deposition. Coupled with Titles I and II of the Act, which address new and existing stationary and mobile sources of sulfur and nitrogen oxides, implementation of Title IV is expected to provide significant benefits to the United States and Canada. These benefits include decreases in the acidity of lakes and streams, concomitant improvements in fish population diversity and health, decreases in soil degradation and forest stress, improvements in visibility (especially to scenic vistas), decreases in damage to materials and cultural resources, and a reduction in human health effects. Congress included Section 404 in Title IV (Appendix B of the Act) requiring the Environmental Protection Agency (EPA or the Agency) to provide a report to Congress on the feasibility and effectiveness of an acid deposition standard or standards to protect sensitive and critically sensitive aquatic and terrestrial resources. Specifically, Congress listed six areas to be addressed in the report:

+ Identification of sensitive and critically sensitive aquatic and terrestrial resources in the U.S. and Canada which may be affected by the deposition of acidic compounds;

+ Description and specification of a numeric value for an acid deposition standard sufficient to protect such resources;

+ Description of the use of such standard or standards in other Nations or by any of the several States in acidic deposition control programs;

+ Description of measures that would be needed to integrate such standard or standards with the control program required by Title IV of the Clean Air Act;

+ Description of the state of knowledge with respect to source-receptor relationships necessary to develop a control program on such standard or standards and additional research that is on-going or would be needed to make such a control program feasible;

+ Description of impediments to implementation of such control program and the cost-effectiveness of deposition standards compared to other control strategies including ambient air quality standards, new source performance standards and the requirements of Title IV of the Clean Air Act.

This report fulfills the requirement of Section 404 by integrating state-of-the-art ecological effects research, emissions and source-receptor modeling work, and evaluation of implementation and cost issues to address the six areas and other issues related to the feasibility of establishing and implementing an acid deposition standard or standards. Congress also requires the National Acid Precipitation Assessment Program (NAPAP) to conduct a study similar to the technical portions of this report, on the reduction in deposition rates needed to prevent adverse ecological effects. NAPAP is required to submit its report to Congress in 1996 (Section 901[j] of the Clean Air Act Amendments).

Developing a Standard to Protect Sensitive Resources

An acid deposition standard is a level of deposition (most likely in units of kilograms of pollutant per hectare per year) that provides a predetermined level of protection to specific ecological resources. The natural resources most at risk from acidic deposition and those most amenable to a quantitative assessment are aquatic systems. Therefore, a standard designed to protect against the ecological effects of acidic deposition would most likely be developed based on effects to aquatic systems. Other ecological resources such as high elevation red spruce forests in the eastern United States and Canada may also be at risk, but less is known about the effects process, and the rate and extent of impacts on those resources. Research conducted under the auspices of the National Acid Precipitation Assessment Program (NAPAP) concluded that regions in the United States most at risk from continued acidic deposition are located along the Appalachian Mountain chain stretching from the Adirondacks to the Southern Blue Ridge. Although many surface waters in western North America are as sensitive as, or more sensitive than, aquatic systems in the East, deposition levels in the West are sufficiently low that the risk of chronic (longterm) acidification to resources in most of the West is low at present and is expected to remain low in the foreseeable future. Episodic acidification (from spring snowmelts) adversely affects some eastern surface waters. It may be affecting high-elevation western surface waters, as well.

An acid deposition standard or standards could be designed to achieve a variety of environmental protection goals. For example, the goal of a standard may be to (a) maintain specific conditions as observed at a particular point in time (e.g., conditions observed in 1984 during the National Surface Water Survey); (b) protect all systems from any harmful anthropogenic effects (i.e., return to preindustrial conditions); or (c) balance effects, costs, and other societal values. A standard can be designed to address chronic or episodic acidification and could vary by region based on the regional variability of ecological sensitivity. No guidance is provided in the statutory language regarding the degree of protection desired by a standard or standards.

Target populations of Adirondack lakes, Mid-Appalachian streams, and Southern Blue Ridge streams were selected for detailed analysis in this study because they represent areas that receive fairly high levels of acidic deposition, are sensitive to acidic deposition, have the best historical data, and have been extensively studied by scientists. Potential future impacts of acidic deposition are estimated by modeling the response of a target population of aquatic systems (lakes and streams) in these areas to various levels of deposition. Target populations are selected to represent sensitive surface waters over broad geographic regions. Individual target populations of sensitive surface waters used for various acidic deposition studies have become progressively smaller over time as investigators have refined their work to study more intensively the acidification processes influencing the most sensitive surface waters. For example, of the total population of lakes in the eastern United States, 18,156 of these lakes potentially most sensitive to acidification were included within the target population of the National Surface Water Survey (NSWS, 1984). In turn, further refinement of sensitivity characteristics for the NSWS lakes led to targeting a total population of 3,227 lakes in the Northeast during the Direct/Delayed Response Project (DDRP, 1988) and 703 lakes in the Adirondack Mountains during the Nitrogen Bounding Study (NBS, 1994). Similar refinements also occurred for lakes and stream reaches in other regions. This approach is illustrated diagrammatically in Exhibit I (not to scale). In this report, analyses of risks from acidic deposition to sensitive lakes and streams focuses extensively on extrapolations regarding the highly sensitive, but limited, target populations used during the NBS studies.

Selecting an Acid Deposition Standard or Standards

The analyses presented in this study provide model projections of direction and magnitude responses for modeled target watersheds under various sulfur and nitrogen deposition scenarios. Based on the remaining scientific uncertainties, particularly regarding the effects of nitrogen and the rate of those effects on the watershed, selection of an appropriate level or levels for a standard to achieve any particular environmental goal is very difficult. Therefore, this study does not recommend an acid deposition standard or standards at this time. The modeling analyses in this report however, do indicate important watershed responses to emissions reductions in the Clean Air Act and provide estimates of deposition reductions that would be needed to achieve a range of environmental goals within the uncertainty of the modeling results.

Impact of the CAAA on Sensitive Surface Waters

Model projections from the Nitrogen Bounding Study (NBS) indicate that sulfur deposition reductions mandated by Title IV of the Clean Air Act Amendments would benefit sensitive surface waters by the year 2040. Exhibits II-IV show ranges of percent target population lakes or streams in each sensitive region projected to be chronically acidic (acid neutralizing capacity [ANC]