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1 INTRODUCTION The passage of metals* through the atmosphere is integral to their biogeochemical cycling. Because of the dynamic nature of the atmos- phere, metals can be deposited in areas remote from their initial sources. With the exception of mercury, the natural rates of atmos- pheric emissions of metals are limited by their characteristically low volatilities. Since the advent of industrialization, high-temperature processes (e.g., smelting and fossil fuel combustion) have caused the rates of emission of some metals to increase substantially. With . . Increased emissions have come increased metal concentrations in the atmosphere and in atmospheric deposition. Worldwide, the known natural sources of metals in the atmosphere are soil, seawater, and volcanic dusts and Gases. The anthropogenic emissions are from industrial gases and particles, fossil fuel combustion, and tillage. Recent reviews have shown that in eastern North America and northern Europe, the rates of atmospheric deposition of silver, arsenic, cadmium, copper, mercury, manganese, nickel, lead, selenium, vanadium, and zinc are controlled by anthropogenic activities (Galloway et al., 1982; Jeffries and Snyder, 1981~. There are many other metals (beryllium, cobalt, molybdenum, tin, tellurium, and thallium) whose rates of deposition have probably increased, but data are lacking to prove it. The shift from natural control to anthropogenic control of atmospheric metal deposition represents a significant perturbation of biogeochemical cycles of these potentially toxic substances. Given this, the next obvious questions to ask are as follows: . Do these elevated rates of atmospheric metal deposition cause changes in terrestrial and aquatic ecosystems? *Of the 18 elements that are the focus of this report, 15 are true metals. Selenium, arsenic, and tellurium are not strictly classified as metals, although they exhibit some metallic properties. For the sake of simplicity in this report, the generic term "metals. is used to describe the entire set of 18, without reference to the 3 exceptions. 1

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2 What is the interaction between increased metal deposition into terrestrial and aquatic ecosystems and their concurrent acidification by acid deposition? Indeed, there has been considerable speculation on these points, and it has been suggested that perhaps some of the biological effects of acid deposition, both in the aquatic and In the terrestrial environments, may be due not only to the increase in hydrogen ion concentration per se, but also to changes in the total concentration or speciation* of certain metals (Baker and Schofield, 1980; Eriksson et al., 1980; Scheider et al., 1979; Schofield, 1980; Suns et al., 1980~. In the present study we examine these questions and attempt to identify those metals that merit further attention in this regard. We chose to focus our attention on those metals that have been classified as every toxic and relatively accessible in an environmental context (Wood, 1974~. A number of metals were deleted from this com- pilation, because very few or no environmental data were available (gold, bismuth, palladium, platinum, and antimony) , and aluminum was added because of its known implication in the response of watersheds to acid deposition. Chromium was omitted because it is considered to be normally reduced to Cr(III) in the environment. In this form chromium is less toxic than in other valence states, but more importantly, it in relatively immobile (NRCC, 1976; Sheppard et al., 19847. The final list of 18 metals considered in this report includes the following elements: silver {Ag), aluminum (Al), arsenic (As), beryllium (Be), cadmium (Cd), cobalt (Co), copper (Cu), mercury (Eg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), selenium {Se), tin (Sn), tellurium (Te), thallium (T1), vanadium (V), and zinc (fin). Since this review focuses on the interaction between increased metal deposition and freshwater acidification on a regional basis, we exclude the local impact of point sources (e.g., smelters). The environmental behavior of each of these metals has been analyzed according to a framework of 17 questions, designed to generate definitive statements about effects and/or specific recommendations for research in areas where our ignorance is profound. The questions are indicated in Figure 1.1. In Chapter 2 these questions have been grouped together and addressed in general terms. At the end of the chapter the data are summarized in condensed tabular presentations, to describe the state of knowledge for each metal. Detailed tabular summaries containing references, and indicating whether the answers to the questions are known, partially known, or unknown, can be found in the Appendix. The conclusions and recommendations are presented in Chapter 3. *The term ~speciation. is defined in this report as the partitioning among the possible physical-chemical forms of a particular trace metal that together make up its total concentration.

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3 ATMOSPHERE 1. Is deposition controlled by human processes? 2. Do we know the speciation of the metals in atmospheric deposition? (a) physical? (b) chemical? (c) dry vs. wet? TERRESTRIAL ECOSYSTEMS 3. Does the metal accumulate in the system? 4. Does the metal move in solution to the lake? 5. Is there an interaction of the metal with acidification over the pH range 7-4? 6. (a) Is the metal bioavailable? (b) Does the metal bioaccumulate? 7. Does the metal have biological effects on the terrestrial system? AQUATIC ECOSYSTEMS 8. Do we know the speciati on of the metal in solution? 9. Does the metal increase in concentration in the aquatic system? 10. Is there an interaction of the metal with acidification of the water col urn over the pH range 7-4? (a) Is the metal bioavailable in the aquatic system? (b) Does the metal bioaccumulate?- (c) I.s the metal biomagnified? (d) What is the inherent toxicity of the metal? 12. Does the metal have biological effects in the aquatic system?- SEDIMENTS , , 15. 17. 13. Do we know the partitioning of the metal in sediments? 14. Is the accumulated metal readily recycled between the sediments and the lake? Is there an interaction of the metal with acidification over the pH range 7-4? 16. (a) Is the metal bioavailable? (b) Does the metal bioaccumulate?- Does the metal have biological effects on the benthic system?- FIGURE 1.1 Framework of relevant questions pertaining to trace metals .