Acid Deposition Effects on Geochemical Cycling and Biological Availability of Trace Elements (1985) / Chapter Skim
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2. State of Knowledge of Metal-pH-Ecosystem Interactions
2. State of Knowledge of Metal-pH-Ecosystem Interactions
Pages 4-43
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From page 4... ...
by comparing the ratios of metal concentrations in the atmosphere to the ratios of metal concentrations in the natural sources, 3. by determining historical trends of metal concentrations in atmospheric deposition, 4.
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There are insufficient data from which to draw conclusions on atmospheric deposition rates for Be, Co, Mo, Sn, Te, and T1. Of the 18 elements considered in this report, the sole one for which the atmospheric deposition rate is known to be controlled by natural sources is A1, due to its prevalence in geological materials (Galloway et al., 1982; Jeffries and Snyder, 1981~.
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There are two methods to estimate the chemical speciation of metals in precipitation: direct measurement and thermodynamic modeling. Both approaches have disadvantages.
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The data used are from the annual volume weighted concentrations of major inorganic ions in precipitation at the Hubbard Brook Exper imental Forest for the period 1963 to 1974 (Likens et al., 1977) , and the median concentrations of trace metals measured in rural locations (from Table 2.1~.
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For the most part, the mean dry fraction lies between 0.3 and 0.6. The further the site is away from a source area, the less important dry deposition will be compared to wet deposition.
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3 gel.) These laboratory studies are useful in illustrating the potential effects of a relatively small change in pH on trace metal mobility and demonstrating the selectivity of certain solid phases for different trace metal cations.
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In the second type of study, involving the leaching of soil columns, it in not surpr ising that trace metals have been found to migrate at different rates and to exhibit different sensitivities to pa changes. Three representative studies are discussed here.
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Pb, Ni, V, A1, and Fe) exhibited maximum concentrations In late summer and autumn and minimum values in winter and early spring; considerable losses of these elements occurred under conditions favoring the leaching of organic matter (high soil temperature and moisture content)
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From page 12... ...
The moss Hylocomium spl endens was also collected from the sites used for soil analysis, and its metal content showed very large geographical differences for As, Pb, and Sb, with much higher values in southern Norway. A similar but less pronounced pattern of elevation was shown for Ag, Cd, Se, V, and Zn (Rambaek and Steinnes, 19801.
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From page 13... ...
The authors suggest that Zn is also accumulated in soils where the pH values for precipitation and soil solution are generally Greater than 5.0-5.5~; at lower pH values Zn and other Chemically similar elements are leached at an accelerated rate. Recent studies of trace metal profiles in forest floors in remote regions showed similar vertical profiles for Pb, Cu.
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From page 14... ...
Based on an analysis of the available experimental results (types 1, 2, and 3) , the 18 trace metals of current concern are tentatively classified according to their potential mobility in a terrestrial environment subject to acid precipitation: high mobility: A1, Cd, Me, Zn moderate mobility: Cu.
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From page 15... ...
Airborne particles may also be incorporated into plant tissue; e.g., particulate Pb has been shown to accumulate in plant tissue close to point sources (Roberts et al., 1974~. While there is potential for forage plants or edible vegetables to accumulate metals in acid soils, normal soil amendment practices such as liming and addition of organic matter will counteract the tendency for metals to be taken up from soils by vegetation (e.g., see gingham et al., 1979~.
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From page 16... ...
However, microorganisms may be affected, as suggested by increased organic matter paralleling increases in metal" in the forest soils in eastern North America {Friedland et al., 1984b; Siccama et al., 19807. These studies provide circumstantial evidence for toxicity to decomposers in soils at concentrations of metals that result from long-range transport.
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From page 17... ...
? Examples of possible physicochemical forms of trace metals in natural waters are shown in Table 2.5; note that the operational distinction between dissolved, colloidal, and particulate forms is convenient {particularly to the analytical chemist)
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18 0 X to : A In 0 £ so En o A o V · s 0 S V)
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Factors influencing the speciation of a particular trace metal in the water column will include the following: 1. the nature of the ligands (L1, L2, L3, ..., Ln)
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21 an A ,0 ~ o o .
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Question 10: IS There an Interaction of the Metal with Acidification of the Water Column over the pa Range 7 to 4? aqua Ion: Regional surveys of water quality in areas affected by acid precipitation together with results from the experimental acidification of lakes (Schindler and Turner, 1982; Schindler et al., 1980a,b)
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From page 23... ...
Trace metal concentrations were assigned either average observed values (Al and Mn) or arbitrary but representative values (Table 2e8 ~ column 2~; provided that ligands are present in excess (i.e., [L]
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25 O O ~ In +-o~ :t O o o o o o o o o v o o o o o o o o v Q o o ~ o o ~ o .,' V In V V O C~ - - ' 1 - ' 1~0 ~ ~ 1~0 O o, 1 - 1~r 0 0 ~ ~ #¢ 0 0 0 cn 1 ~ ~ ~ ~ ~ ~ g o U~ o o o o o o o o ~ o o ~ ~ ~ · e ~ · · · · · ~ O O U~ ~ O O O ~ ~ ~ ¢: ~: £ Z P. u, c~ O~ ~ U)
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From page 26... ...
as aqua ions throughout the studied pa range -- Ag, Cd, Co, Hi, and Zn; 2. trace metals existing as oxyanions, for which the pH change merely affects the degree of protonation of the anion -- As, Ho, Se, and V; 3.
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From page 27... ...
Essential trace elements from the list include Co, Cu.
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From page 28... ...
, which only relate metal toxicity to various ion-specific physicochemical parameters and ignore other possible modifying factors. These equations take the following form: AN pT = aO + al log _p 2 0 where aO ~ a1, a2 = pT = negative logarithm of the metal concentration causing a particular toxic response; AN = atomic number; TOP = difference between the fonts ionization potential with oxidation number n, and the ionization potential with the next lower oxidation number, n - 1; AEo = absolute value of the difference in electrochemical potential between the ion and the first stable reduced state; constants obtained by regression analysis, values of which depend on the ion group, on the biota considered, and on the particular toxic effect determined.
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From page 29... ...
Furthermore, the levels of zinc in acid waters are approaching those that have resulted in toxicity to plankton in field experiments (Marshall et al., 1981~. The complex effects that have been shown in laboratory experiments when the toxicity of mixtures of metals are determined illustrates the potential for synergistic or antagonistic effects in surface waters susceptible to increased concentrations of more than one trace metal.
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From page 30... ...
In principle, the speciation of sediment-bound trace metals could be determined both by thermodynamic calculations (provided equilibrium conditions prevail) and by exper imental techniques .
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From page 31... ...
the interstitial water; · the redox potential; · the pa; · the nature and concentration of the solid phases present. The experimentally observed distribution of a trace metal obtained with any extraction scheme will not necessarily reflect the relative scavenging action of discrete sediment phases; hence the partitioning should be considered as operationally defined by the method of extraction (Rendell et al., 1980; Tessier et al., 1979~.
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From page 32... ...
Typical sediment components acting as trace metal sinks (e.g., clays, iron and manganese oxides, and humic acids) are, of course, also found in soils, and laboratory studies of interactions between trace metals and these components are pertinent both to soil systems and to aquatic sediments.
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From page 34... ...
A different laboratory approach has been to determine the effect of a change in pH on trace metal partitioning in the sediment, an determined experimentally by sequential selective extractions. Several such studies on river sediments have been carried out by Gambrell et al.
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From page 35... ...
Regional Sediment Surveys/Experimental Acidification Experiments. Both regional sediment surveys in areas affected by acid precipitation and the results from the experimental acidification of lakes and streams suggest that the acidification of an overlying water column will increase the geochemical mobility in the sediments of the following trace metals: Al, Cd, Co, Fe, Mn, Ni, and Zn.
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From page 36... ...
Pb, and Zn were also influenced by the protection or competitive effect of other sediment constituents, notably amorphous iron oxyhydroxides and, to a lesser extent, organic matter. Benthic organisms can assimilate metals from dissolved forms in pore water or from particles ingested into the gut.
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From page 37... ...
Studies on the geochemical characteristics of metals in sediments and on the relationship between pa and metal release from sediments suggest that there is a reasonable basis for expecting some metals in sediments to show increased bioavailability as pH declines. The literature is, however, quite deficient in definitive experiments on biological uptake made in conjunction with geochemical studies; those that exist tend to relate to heavily polluted sediments .
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From page 38... ...
The dilemma faced by the researcher is quite obvious: biota can only assimilate metals from solution, which they can either obtain by ingesting solid material and absorbing weakly bound metals from the solid, or by absorbing metal directly from the water or from the water that is ingested with the sediment. Since there is a flux of metals between sediment and water, it may be in fact Impossible to determine whether sediment-dwelling organisms are accumulating metal from water or sediment.
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From page 39... ...
Are metal concentrations in the atmosphere controlled by anthropogenic activities?
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