Metal Mobility Among Biotic and
Abiotic Compartments of A Shield Watershed
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Research
Team |
| M. Diamond
(University of Toronto); L. Evans (University of Guelph); B. Hale ( University of Guelph;
L. Hare (INRS-Eau); W. Hendershot (McGill University); R. Martin (University of Western
Ontario); W. Rauser (University of Guelph); A. Tessier (INRS-Eau) |
|
Summary |
| The mini-MITE for Processes
consisted of three projects that focused on the mobility of metals among the biotic and
abiotic compartments of the terrestrial and aquatic environment. The studies addressed:
cycling of metals in the terrestrial environment, including soils, plants and aquatic
biota; metal mobility in Shield watersheds; and, looking for biomarkers of metal exposure
in terrestrial woody plants. These studies will improve estimates of risk to ecosystems
from metals, as it is critical that movement and transformation of the metals after
deposition be included in the process of estimating the metal exposures that various biota
receive, and that exposure concentrations are not simply calculated from total metal
deposition rates without regard for the forms of metal being deposited or the
post-depositional transformations undergone by these metals (de Vries and Bakker 1996). |
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Scientific
Background |
| After deposition from the
atmosphere to soils and surface waters, metals undergo transformations in speciation and
consequent bioavailability; the distribution of metal inputs among various dissolved and
particulate fractions of soils and surface waters is dependent on prevailing conditions in
these receiving media, is not well predicted in complex matrices and is probably
time-dependent. Following these transformation processes, the bioavailable metals are
further distributed among biotic components of the ecosystem according to patterns of
metal uptake that are biotic species specific. In terrestrial systems, vegetation plays a
very important role in the biogeochemical cycling of many nutrient elements. Decomposition
of plant material that has been deposited to soils as part of deciduous growth cycles
contributes to the metal enrichment of surface soils. This contribution is currently not
well distinguished from the enrichment that occurs as a result of atmospheric deposition. Water movement is the driving force behind the transport of metals within the
soil column and from soils to receiving waters; it follows that the partitioning of the
metal between the soil solution and the solid phase is of crucial importance. Natural
organic matter is a key component in these soil water equilibria, both as a sorbent (e.g.,
solid humic acid) and as a metal complexing/metal-reducing ligand in the mobile phase
(e.g., dissolved fulvic acid). Current estimates of partitioning constants need to be
improved and incorporated into models designed to predict the behaviour of metals in the
terrestrial environment and their movement into the aquatic system. Once in the aquatic
ecosystem, estimates of biologically available trace metal concentrations, and their
effects on aquatic life require models that can relate the bioaccumulation fn toxicity of
metals to metal concentrations and metal speciation in the environment, as bioaccumulation
can be viewed as the logical link between metal that is available for uptake by organisms
and toxicity.
The long-term objectives of the <Processes> projects
are: characterize the distribution of essential and non-essential metals, post-deposition,
among soils and vegetation of the boreal forest; determine the relative contributions of
host sediment, overlying water, or sediment-water interface to metal accumulation by
benthic aquatic animals; quantify the role of organic and mineral surfaces in metal
binding to soil particles; and how this binding affects metal mobility in watersheds.
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Practical
Background |
| Over the time frame 1999-2004 we
propose to expand the studies that were initiated under mini-MITE (1998-99), specifically:
post-depositional mobility of trace metals in boreal forest ecosystems (Hale, Hendershot
& Martin); metal accumulation processes in aquatic invertebrates (Hare & Tessier);
quantification and modelling of metal mobility in lakes and watersheds (Evans &
diamond); chemical speciation of metals in fresh waters and in atmospheric precipitation
(Chakrabarti). Funding for these expanded studies of <Processes> will be secured
from some combination of NSERC (Research Network Grant or Collaborative Research and
Development Grants) and industrial support (MAC/Ontario Power Generation Inc.). The
1998-99 studies were conducted with a combined budget of $89,000; the budget for 1999-2004
is expected to be approximately three-fold larger than that for 1998-99. |
|
Overall Goals |
| A goal of the 1998-99 research
season (in addition to gathering preliminary data that would help each of us focus the
larger work planned for subsequent years) was to develop working relationships among the
<Processes> scientists. This was well demonstrated by a joint field trip for
Hendershot, Hale and Martin, during which we collaborated on choosing sampling sites that
met most of the criteria that each of us had for our individual projects, and allowed us
to gather data in a co-ordinated, cost-effective manner. Such relationships will continue
to be a key feature of <Processes> (and indeed, MITE as a whole) as we believe that
data which are integrated at the field level of development will be of greater value to
the processes of risk assessment than data which are integrated after the individual
studies are completed. |
|
References |
| de Vries, W. and D.J. Bakker. 1996.
Manual for calculating critical loads of heavy metals for soils and surface waters. DLO
Winand Staring Centre, Report 114. |