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OCR for page 44
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.
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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
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Representative terms from entire chapter:
ecosystem effects
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