LETTERS
Acid deposition Dear Sir: The article by Cosby et al. (“Time scales of catchment acidification,” ES&T, December 1985, pp. 1144-49) has important policy implications for potential acid deposition scenarios. The thrust of the article is that waters in sensitive watersheds receiving acid deposition reach chemical equilibrium in response to changing deposition levels over the course of decades to centuries. Thus, surface water ion concentrations reach equilibrium long after atmospheric deposition chemistry stabilizes at particular values. This time lag results from two factors: the finite capacity of soils to absorb sulfur and the slow resupply of base cations leached fromSoils by weathered minerals. A direct consequence of the lag in response is a predicted time-dependent behavior for alkalinity at constant deposition rates, as vividly illustrated in Figure 2 of the article. A question of great concern from the perspective of acid rain abatement is “How much must deposition be reduced to eliminate acidification of lakes?” Attempts to answer this question from a synoptic viewpoint have been made by several investigators, who have compared the acidification status of lakes in watersheds that receive different levels of sulfate and hydrogen ion deposition or concentration (1-3). This procedure permits the selection of a target atmospheric loading level corresponding to a given minimum target alkalinity for lake water. For example, the requirement that the annual alkalinity of uncolored lakes be maintained above zero leads to the inference of target loading levels as low as 5 kg total sulfur deposition per hectare year (4). However, this procedure depends critically on the assumption of chemical steady state. Otherwise, the observed 534 Environ. Sci. Technol., Vol. 20, No. 6, 1986
acidification status of lakes is a function of time. A lake with a particular alkalinity now may exhibit a lower alkalinity over the course of decades, even at constant deposition levels. Lakes with positive alkalinity presumably may become acidified in time. The target loadings defined by current synoptic observations must therefore be regarded as upper bounds on the equilibrium state target loadings. A lower bound on the equilibrium target loading can be defined by ignoring the neutralization role of cation leaching. As soils reach sulfate equilibrium, acid deposition is neutralized only by alkalinity from primary weathering. The lower bound target loading can be derived from minimum observed alkalinities for lakes unperturbed by acid deposition. A protective target loading lies between these two bounds. The implications for policy are clear: Target loadings defined from information on the existing status of lakewater acidification should be designed with a margin of safety because the status of lake acidification will degrade with time at any level of acid deposition. Michael Oppenheimer Environmental Defense Fund New York, N.Y. 10016
References (1) Oppenheimer, M. “Thresholds for Acidification: A Framework for Policy and Research”; Environmental Defense Fund: New York, 1986. (2) Newcombe, C.P. Env. Manage. 1985, 9, 211-88. (3) Evans, L. S . et al. J . Water Air Soil Pollur. 1981,16, 469-509. (4) Acidijcation Today and Tomorrow; Swedish Ministry of Agriculture: Stockholm, Sweden, 1982.
Dear Sirs: There continues to be disagreement among scientists over the effect of acid deposition on environmental resources. There are two approaches
to dealing with whatever problem may exist: First, limit the quantity of acidic compounds deposited from the air and second, add alkaline material, such as limestone, on a local remedial basis. To date, most congressional proposals have addressed acid deposition through programs to reduce emissions. As a matter of legislative simplicity, proposed control programs have focused on emissions from coal-fired power plants, to the near exclusion of all other sources of acid-forming emissions. Such a strategy could lead to a substantial decline in total U.S. emissions of sulfur dioxide, but it is unlikely that this approach would achieve desired levels of acid deposition in those regions where this may be a problem. Would a substantial reduction in coal boiler emissions in the Midwest achieve a meaningful reduction in acid deposition in the Northeast? This is a difficult question. There are two links in the chain connecting emissions sources with remote receptors of acid deposition. The first is the process of atmospheric diffusion and transport, which accounts for concentrations of sulfur dioxide and other pollutants at remote sites. The other is the system of chemical reactions that controls the amount of acidic substances actually formed. The quantity of sulfur dioxide that will turn into acidic sulfate is strongly dependent on many things-concentrations of other pollutants, sunlight, precipitation, cloud formation, and season. All these interact in a complex fashion. The matter is far from being settled, but ongoing research promises to yield the kind of understanding policy makers will need to deal with the acid rain problem in the future. Anthony W. Moats Consolidation Coal Company Pittsburgh, Pa. 15241
