Hale

Post-deposition metal bioavailability in northern forests is not well understood, thus is an unknown in predicting the fate of trace metals added as a result of human activity. Understanding trace metal fate and using that knowledge to predict the concentrations of metals in soils and vegetation, as well as losses of metals from soils, is essential for ecological risk assessment. The aim of this project is to better understand the bioavailability of metals in soils after deposition to the ecosystem, and how this changes with biological and geological processes. Specifically, this project examines:

• the binding of metals to soils and living roots;
• the release and sequestration of metals during decomposition of vegetation

Chemical speciation of the metals in the soil-plant continuum is critical to understanding the potential for binding versus leaching. Chemical speciation of free metal is being investigated as a basis for predicting metal bioavailability and bioaccumulation. The solid phase of soils is omnipresent and is an important supplier of trace elements. Given that the activities of free metal in solutions are exceedingly small relative to total soil metal contents, a better understanding of the processes controlling the buffering soil solution free metal activities are critical. Diffusive gradients in thin films (DGT) is a recent tool that combines a chelating resin and a diffusive gel and allows a quantification of metal supply rates from the solid phase. It is a promising tool to enable better predictions of bioavailability through improving our understanding of the role of metal buffering in trace metal supply to vegetation. ‘Vegetation' can be a sink for metals during both growth and decomposition, and the relative sizes of these roles is dependent on plant species, metal, concentrations of the metals in soils and air, and stage of life. Studies of coniferous foliage decompositon have demonstrated that the concentration of Cu, Zn, Pb and Ni increased in needle litter, in Finland, at a site very close to a smelter (McEnroe and Helmisaari, 2001). Studies of decomposition in the field at Sudbury and Rouyn-Noranda have revealed that deciduous and coniferous foliage and fine roots can be net accumulators of some metals during decomposition, varying with location (D. Johnson, Ph.D. project in progress). It is unknown whether there is no metal loss from decomposing vegetation, with some gain from atmosphere and/or soil, or whether there is vigorous exchange, with a net gain. Laboratory studies of decomposition, under controlled hydration and temperature, would clarify this, and other, questions. The application of DGT to characterizing the speciation of metal flux to and from decomposing vegetation has the potential to answer these questions.

Ultimately, these data will contribute to ecological risk assessment of metal emitting activities (smelting of base metals, as well as other industrial or transport activities involving these metals; incineration of a wide range of wastes also results in the production of metal containing aerosols) on forest health and productivity.

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Hare (top of page)

The aim of our MITE-RN research is to develop and test in the field theoretically-based models that relate metal concentrations in animals to those in their surroundings. To develop such models we need to understand the chemical and biological processes involved in metal uptake by organisms. Thus, to estimate the exposure of burrowing animals to metal in the sediment compartment versus that in the overlying water-column compartment, we will study the manner in which invertebrates construct and irrigate their burrows. In addition, we will determine if sedimentary trace metals influence animal behavior. Secondly, we will measure the relative important of food and water as metal uptake routes. This information will aid us in building mechanistic models that will allow us to use organisms as biomonitors to estimate aqueous metal concentrations in a biologically meaningful manner.

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Chakrabarti (top of page)

We will apply the metal speciation techniques that we have developed for determination of metal speciation in freshwaters from lakes, soil pore waters, through-fall precipitation, and metal fractionation and speciation in precipitations contaminated by Cu and Zn particulates emitted by Cu and Zn smelters and refineries. This will be done by applying several electroanalytical and spectroscopic techniques that we have developed in our research laboratories which have been tested and found to provide metal speciation information which is independent of the metal speciation techniques used to obtain the results. These metal speciation techniques will provide quantitative measures of metal Speciation Parameters (Dissociation Rate Coefficient, Diffusion Coefficient, and Stability Constant of metal complexes) and are based on the analytical timescale of measurement. Hence, they are independent of the analytical techniques used to measure them and are related to fundamental properties of metal complexes in the freshwater systems. This study will provide information on the speciation of the core metals, Cu, Cd, Zn, Pb and Ni, in lakes, and soils in the Quebec and Sudbury areas. Regarding the emission of Cu and Zn particulates from Cu and Zn smelters and refineries in Quebec, this study will provide answers to the key question how speciation in the particulates effects liberation of metals as labile species when the particulates are wetted (e.g. during a storm event).

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Courchesne (top of page)

Understanding the processes that regulate the movement of trace metals through soils, their uptake by plants and their subsequent transport towards waters bodies is vital to insure the sustained development of terrestrial and aquatic ecosystems. The mobility of solutes in soils is generally considered to be controlled by interactions with reactive sites present at the surface of soil materials such as organic matter, metallic oxides, clay minerals and soil organisms. Traditionally, the bulk fraction of the soil matrix is used to characterize the reactivity of a given soil and its potential interaction with solutes. However, the soil is an extremely heterogenous environment and to focus our attention exclusively on the bulk soil might be misleading when trying to gain new knowledge on the biogeochemistry of trace metals and on the risk associated with exposure to metals. Because of the ubiquity of plants' activity on soils, the rhizosphere appears as a key component of the heterogeneity of soil materials. Due to its proximity to the site of elemental uptake by plants, the rhizosphere is a critical component of soil: plant systems. Clearly, several lines of evidence suggest that the extent of the functional role of the rhizosphere on the biogeochemical cycling of elements is much larger than the volume fraction it occupies in the field (Hinsinger, 1998). In this context, an approach that integrates the soil rhizosphere is needed to gain new insights into the processes that control the storage, cycling and bioavailability of trace metals in terrestrial ecosystems.

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Dillon (top of page)

The objective of this project is to measure the amount of various metals stored in all of the major compartments in the Plastic Lake catchment, including both the aquatic and terrestrial components, to measure the fluxes between the compartments, and to determine the factors that control the magnitude of these fluxes. These data will be used for development and testing of fate and transport models.

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Wang (top of page)

Metal speciation is the key to understanding the geochemical processes (e.g., cycling and mobility) and biological effects of metals in the aquatic environment. The overwhelming role of reduced sulfur species (RSS) in determining the speciation of class B metals (e.g., Cd, Cu, Hg, Pb and Zn) in anoxic natural waters is increasingly being recognized as more reliable stability constants for RSS-metal complexes are being determined. Recently, the possibility has been raised that metal-sulfide clusters that are resistant to oxidation could also play an important role in controlling the speciation of class B metals in oxygenated waters. Given the important consequences that the widespread occurrence of these clusters would have for determining metal speciation, we propose to determine their presence in surface waters of Shield lakes and to evaluate their importance in controlling the speciation of metals.

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