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3 CONCLUSIONS AND RECoMMENDATIONS Analysis of the effects of atmospheric deposition on the geochemical cycling and biological availability of trace metals has revealed certain areas where knowledge is relatively strong and others where it is rather weak. The former are summarized below as conclusions, the latter as research recommendations. - CONCLUSIONS 1. In most of eastern North America, anthropogenic sources determine the concentrations in atmospheric deposition of AS, Cd, Cu. Pb, Me, Hg, Ni, Se, Ag, V, and Zn. The level of other metals may also be so controlled (se. co. Ma. An . ~ - . and ~1 ~ hut If i rmi no Beta Ar" lacking. ~ _ _ ~ _ _ ~ _ ~ _ ~ _ _ ~ ~ ~ ~ ~ _ ~ _ ~ ~ ~ - 2. With the exception of A1, all metals for which data are available {Cd, Mn, Ni, Pb, and Zn) are present in precipitation pr imar fly in dissolved rather than particulate form. 3. Although data are sparse, studies indicate that dry deposition is an important component of the total deposition. 4. The acidification of terrestrial environments will result in increased rates of mobilization of metals from soils in the order: A1, Mn, Zn, Cd > Cu. Ni > Co, V, Pb (high) (low) Insufficient data are available to rank Ag, AS, Be, Hg, Ho, Se, Sn, Te, and T1. 5. In the terrestr ial system, A1 toxicity to perennial plants is a real possibility, given the demonstrated toxicity of A1 to certain plant species in acid soils. To date, it is the only metal that has been shown to present a risk for plants in acid-stressed terrestrial systems . 6. Based on the results of chemical equilibrium calculations for simple aquatic systems containing no adsorbing solid surfaces, only A1, Mn, Be, Cu. and Hg should exhibit significant changes in speciation and hence reactivity as the pH decreases from 7 to 4. Metals forming oxyanions (AS, Mo, Se, and V) will change their degree of protonation over this pa range. 44

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45 7. The concentrations of A1, Mn, and Zn in aquatic systems increase in response to acid deposition. 8. If present in the noncrystalline fraction of the sediments, acidification of an overlying water column to a critical or threshold pH value will increase the geochemical mobility in the sediments of the following trace metals: A1, Mn, and Zn, and perhaps Cd, Co, and Ni. 9. Bioaccumulation of Hg, Pb, and Cd in fish has been shown in some studies to increase with decreasing pH of lake water. This bioaccumulation poses a potential health hazard (e.g., He in some fish, particularly piscivorous species, has exceeded the Canadian guideline of O.5 ppm, and, in some instances, the U.S. guideline of 1.O ppm). 10. Food chain transfer of A1 may result when passerine birds feed in acidified systems. A1 appears to interfere with calcium deposition in eggshells with subsequent effects on clutch size and eggshell permeability. 11. Aluminum toxicity to fish is related to depressions in ambient pH, but the respective roles and interactions of hydrogen and A1 ions have not been completely elucidated. 12. Metal bioaccumulation from sediments by the benthic biota has not been evaluated, nor has metal toxicity to benthic biota been demonstrated. The sediment-dwelling biota accumulate metals, but the source of these metals has not been elucidated. It may be the sediment proper, but it also may be the water column, suspended sediment, or the benthic organisms that are consumed. RESEARCH RECOMMENDATIONS 1. In evaluating where future research efforts might be directed, the authors of the present report have considered the following criteria (see Table 2.14~: relative importance of anthropogenic inputs to the atmosphere; geochemical mobility; sensitivity of chemical speciation to pH changes in the critical pH range (7 ~ 43; intrinsic toxicity; and potential for bioaccumulation. Without invoking an arbitrary weighting scheme for these various criteria, we suggest that future research on the interactions among metals, acid deposition, and biological receptors be concentrated on the following metals: A1, Cd, Cu. Hg, Mn, Ph. and Zn. In the context of acid deposition, the environmental behavior of AS, Be, Mo, Sn, and Te does not warrant particular attention; available knowledge is as yet insufficient to determine whether Ag, Co, Ni, Se, T1, and V merit concern. 2. In order to relate metal deposition to ecosystem effects, better data are needed on the spatial and temporal trends of trace metal deposition in North America. We recommend that the National Trends Network, the National Atmospheric Deposition Program and the Canadian Atmospheric Sampling Program establish a program to obtain such data. During the development phase of this program, ecologists must determine whether wet and dry deposition data need to be obtained separately in order to relate deposition to ecosystem effects, or whether total deposition data would be sufficient. In addition, they must determine whether metal speciation in atmospheric deposition is an

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46 important determinant of ecosystem effects. As a first attempt we recommend that the proposed national metals deposition network measure only total deposition of acid-soluble metals, i.e., that the precipita- tion samples be filtered to separate dissolved from particulate forms. As part of this program, methods to measure deposition of metals via dry deposition, fog, dew, frost, and cloud water must be initiated. These types of atmospheric deposition are probably quite important for mountainous forested ecosystems. 3. Extensive and intensive investigations in the field and laboratory should be made to relate soil pa and available Al, Me, and an concentrations to plant uptake and physiology, with special emphasis given to plant species of economic or ecological significance. Accumulation and toxicity of metals other than Al have not been demonstrated for plants in soils receiving acid deposition except in reg ions where known sources produce greatly elevated levels of metal deposition or contamination. Tree ring analysts or metals reveals trends toward increasing uptake of metals in more recent years, and yields information on the response of trees to historical episodes of pollution. Thus, uptake of metals is a possibility and is likely to increase as Ca is depleted. Terrestr ial systems with soils of very law cation exchange capacity and low base saturation would be suitable study sites to test this hypothesis. Since there is a potential effect of metals on soil microbial processes, including mycorrhizae and their functions, and since there are contradictory results in the literature regarding the effects of mycorrhizal fungi in protecting their hosts from metal toxicity, it is recommended that studies on metal-mycorrhizae interactions be done at metal levels that are realistic in the context of atmospheric deposition away from point sources. 4. The suspected mobilization of Cd and Ni from lake sediments below a threshold pH of about 5 should be confirmed (e.g., in experimentally manipulated systems, by the use of biological monitors in natural field situations, and/or by paleolimnological technique 5. The observed negative correlation between lake pH and Hg, Cd, and Pb concentrations in fish muscle should be tested on a wider range of lakes, especially in the United States. There is a need to standardize the age, weight, and species of the f ish collected, and to extend the study to other biota. 6. In order to develop a more predictive model for metal bioaccumulation, experimental work should be designed to differentiate between the role of the hydrogen ion in determining Al, Cu. Be, Hg, and Pb speciation, and its role per se on the physiological processes of the receptor organism. These include metal uptake, but also other physiological processes such as transport of nutrients. These two aspects have not been separated in field measurements. 1 1