Biogeochemical Cycling in the Boreal Forest - Metal Concentrations in Woody Tissues - Phase II

Bégin, C., M. Savard, M. Parent, J. Marion and A. Smirnov

Dendrogeochemical investigation of a series of six forest sites in the Rouyn-Noranda region have shown that part of metals emitted from the Horne smelter accumulate in woody tissues. As an example, concentrations of Cd range between 0.02 to 1.05 ppm for contaminated sites near Rouyn (GSC MITE 1998-2001) while they vary from 0.01 and 0.03 ppm for our control site in the Hudson Bay region. What does this represent at the scale of the forest> How will these metals be incorporated and cycled in the forest system? The quantitative assessment of the total metal pool in woody vegetation is the first step to calculate the long term return of these metals to forest soils. Such investigations will directly contribute to the global understanding of metal biogeochemical cycles in the boreal ecosystem.

As a first phase in addressing this long term objective we have focussed on two boreal forest sites exposed to contrasting levels of airborne pollutants in the Rouyn region and calculate the total metal content in black spruce trees using a method that takes into account metal variations along tree stem (MITE-RN 2000-2001). In fact, for this species, it was shown that lead concentration gradually decrease from 0.27 ppm at ground level to 0.01 ppm at the apex in response to distinct assimilation processes (MITE-RN 1999-2000). Our results indicate that for spruce trees growing at the polluted site, the total Cd load of standing wood can reach 1.2 kg/km2, which is about 10 times higher than our estimation for the unpolluted site. However, in terms of wood volume, black spruce trees count for 30% to 50% of a typical boreal forest stand. To provide a quantitative estimation for a fuller spectrum of forest constituents at the two selected sites, co-dominant tree species were investigated this year.

We have adapted the field sampling strategy and the biomass evaluation methodology developed last year (MITE-RN 2000-2001) to the study of the balsam fir, paper birch and aspen. At each site, four trees per species, each representing a different growth stage, were selected and sampled at one meter intervals along the main stem and an average of 6 branches were sampled on each tree using a stratified sampling protocol. Detailed measurements were made to calculate the biomass of every single tree. At our tree-ring laboratory, branches were re-sampled (3 samples/branch) and all wood samples were prepared for metal analysis. Over 600 samples were submitted to the geochemistry laboratory for the analysis of metal concentrations by ICP-MS. Metal concentrations, along with individual tree biomass will provide an accurate estimation of the total metal content in spruce trees. Demographic data for boreal spruce populations provided by provincial forest services will serve to extend the total metal content to a given surface unit.

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Quantification and Modelling of Metal Mobility in Lakes

Diamond, M.L^1,3., L.J. Evans², S. Bhavsar³, P. Cypas², K. Rudnitski^2
1 Department of Geography, University of Toronto, Toronto, Ontario M5S 3G3
² Department of Land Resource Science, University of Guelph, Guelph, Ontario N1G 2W1
³ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario

The aim of our research is to develop a general model of metal chemistry and fate in an aquatic system. The research derives from the need to link metal emissions and loadings to lakes with resultant concentrations, including the bioavailable fraction of metal, and metal distribution within a lake. This linkage provides the pathways analysis component necessary to estimate metal concentrations and hence exposure within an ecological risk assessment. For metals, achieving this goal demands consideration of metal speciation as a function of ambient chemistry.

We have developed a general chemistry-fate model named TRANSPEC (Transport and Speciation Model). The model is applicable to most metals and most aquatic systems. The chemistry component of the model is an adaptation of the equilibrium speciation model MICROQL while the fate model is based on the multi-species version of the aquivalence-based QWASI model (Quantitative Water Air Sediment Interaction) of Mackay (1991) and Diamond et al. (1992) and co-workers. We have developed steady- and unsteady-state versions of TRANSPEC that estimates metal fate in dissolved, colloidal (DOC-bound) and particulate phases within a stratified water column and two vertical sediment layers.

To calibrate and evaluate model performance, we used the model to estimate the fate of zinc and copper in Ross Lake, Manitoba, in which these metals have accumulated to high concentrations in the organic-rich sediments. This system has provided a rigorous test of various aspects of the model due to its complexity of fluctuating redox conditions in surface sediments and the remobilization of sedimentary metals. The model is also applicable to simpler systems such as those with minimal historically-accumulated metal and with organic-poor sediments.

For the application of TRANSPEC to Ross Lake we sampled and analysed the following: weekly zinc and copper concentrations in the lake's inflow and water column, year-round estimates of sediment deposition and resuspension using sediment traps, pore water chemistry using peepers, sediment accumulation rate by means of Pb-210 dating of a vertical sediment profiles, mass transfer coefficient for the diffusive release of metal between pore water and the water column by means of a diffusive release experiment, and finally depth profiles of acid volatile sulphides and simultaneously extracted metals.

The results of the model indicate that zinc mobility is controlled by redox-sensitive diffusive fluxes from sediment-to-water and the resuspension of metal-rich sediments. The main route of zinc mobilization changes seasonally in response to changing sediment redox conditions.

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The fate and speciation of zinc in Ross Lake: a mine-impacted lake in Flin Flon, Manitoba

Evans, L.J.¹, K. D. Rudnitski¹, P. Cypas¹, M.L. Diamond², S. Bhavsar²

¹University of Guelph, Guelph, ON
²Univeristy of Toronto, Toronto, ON

The North basin of Ross Lake receives effluent discharged from the Hudson Bay Mining and Smelting Company (HBM&S) tailings ponds. The primary objective of this research was to monitor the concentration of zinc in the inflow and outflow of the North basin of Ross Lake using weekly surface water samples, seasonal porewater samples, bulk sediment traps, sediment core samples, and bulk sediment core samples.

The purpose of this monitoring program was to determine if the concentration of zinc was seasonally dependent, and possibly related to changes in the redox status of the lake. The results from the monitoring program were used to interpret the speciation of zinc in the North basin and to provide inputs for a coupled model combining an aqueous equilibrium speciation/complexation component and the QWASI (Quantitative Water Air Sediment Interaction) multispecies fate model.

The average concentrations of zinc in the surface waters of the outflow of the North basin were higher than that of the inflow, 857 g L-1 and 434 g L-1, respectively. Linear regression suggested that processes within the North basin contributed to the concentration of zinc in the outflow as opposed to the concentration of zinc in the inflow. The annual maximum concentration of zinc occurred during the fall (high concentrations were also present during the late spring and early summer) whereas the minimum occurred during the late winter (and to a lesser extent, low concentrations followed the early summer maximum). The annual high concentrations of filtered iron present during the winter may have indicated that the basin experienced a period of seasonal anoxia. This suggests that the basin may act as a sink for zinc when the conditions tend towards anoxia and switches to become a source when the conditions are oxic.

Under anoxic conditions, zinc is likely to precipitate as sphalerite (ZnS) in the presence of sulfur. Based upon the porewater concentrations of zinc and sulfur, the saturation index for sphalerite was 0.97. Therefore, sphalerite may control the solubility of zinc and sulfur in the porewater.

The average and maximum concentrations of zinc in the sediment of the North basin were 4.4 % and 15%, respectively. The X-ray diffraction traces and scanning electron microscope images did not provide interpretable data. Synchrotron X-ray absorption spectroscopy (XAS) may provide higher quality results. The dates obtained from Pb-210 data indicate that the concentration of zinc in the sediment rose after the mine began operations and rose to a maximum in the 1960s, the period preceding the implementation of abatement measures. Therefore, the profile of zinc in the sediment core corresponds well with the historical record. The total organic carbon (TOC) content of the sediment core also corresponds well to the historical record. The maximum TOC concentration occurred during the 1950s, a period in time just before the City of Flin Flon halted the discharge of raw sewage into the North basin of Ross Lake.

Processes within the North basin, such as a seasonal release of zinc from the sediment, may contribute to the elevated concentrations in the outflow from Ross Lake. The understanding gained from the monitoring program at Ross Lake has been instrumental in the development of a coupled speciation and fate model that may be used to assess the impact of other metals at other sites.

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Metal uptake routes for the biomonitor Sialis

Croisetière, L., L. Hare (PI) & A. Tessier.

Institut National de la Recherche Scientifique - Eau, Terre et Environnement (INRS-ETE), Université du Québec, Sainte-Foy, QC.

Measurements of trace metals in biomonitors allow us to quantify metal exposure and, using an appropriate model, estimate metal concentrations in water or sediment. To be effective, such models should be based on chemical and biological knowledge about the organism and the metal. Our previous research suggests that larvae of the phantom midge Chaoborus can be an effective monitor for Cd but not for Cu or Zn. We are currently evaluating the potential of the alderfly Sialis as a biomonitor for several other trace metals. In our quest to understand how this animal accumulates metals, and thus to put our biomonitor on a solid mechanistic foundation, we are conducting experiments to determine the relative importance of food and water as metal sources for Sialis. Understanding how animals accumulate metals is an important aspect of Ecological Risk Assessment. In an earlier laboratory study we showed that Cd is largely accumulated from food. We now will determine if this is the case in nature and if the majority of other metals (Cu, Pb and Zn) are also accumulated from this source. To achieve our goal, we exposed uncontaminated Sialis larvae to metal-poor or metal-rich prey (Chironomus riparius larvae). Metal-rich prey were contaminated for two weeks in net-covered containers placed at the bottom of Lake Dufault (Rouyn-Noranda). The predator was thus offered metals in either water only, in food only, or in both water and food. However, predators in all treatments were fed; in the water only and control treatments Sialis were fed metal-poor Chironomus larvae raised in our laboratory. Unfortunately these metal-poor prey had higher concentrations of Cd, Cu and Zn than expected but were low in Pb as planned. Predators contaminated only by exposure to Pb-contaminated water from Lake Dufault had much lower Pb concentrations than Sialis exposed to this metal in prey, suggesting that food is their major Pb source. These field results are consistent with our previous laboratory and field experiments in suggesting that animals can take up an important part of their metal from food. As a consequence, the results of water only toxicity tests could be misleading and models of metal dynamics and fates in aquatic systems should consider metal transfer along food chains.

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Increases in food web cadmium following reductions in atmospheric inputs to some lakes: value of a biomonitor to ERA

M.N. Croteau, L. Hare (PI) & A. Tessier.

Institut National de la Recherche Scientifique - Eau, Terre et Environnement (INRS-ETE), Université du Québec, Sainte-Foy, QC.

Substantial expenditures at many metal smelters have resulted in reductions in atmospheric emissions from these important pollution point sources. We show for the first time that concentrations of the toxic metal cadmium have decreased over the last decade in nine lakes located near two large Canadian smelters. Because the exposure of animals to pollutants is controlled by many factors, we measured cadmium in a biomonitor (the insect Chaoborus) to determine if declines in dissolved cadmium have reduced the exposure of aquatic organisms to this pollutant. Although biomonitor Cd declined in several lakes, it actually increased in a few acidic lakes. We explained this environmentally worrisome and counterintuitive result using the precepts of a theoretically based construct, the free-ion activity model; increases in organism cadmium were explained by reductions in hydrogen ions that compete with cadmium ions at biological uptake sites. The net result of simultaneous declines in aqueous cadmium and hydrogen ions was an increase in the Cd accumulated by animals in some lakes. The large number of lakes in northern Europe and North America influenced by acidic and metal-rich atmospheric fallout suggests that the phenomenon has occurred, and will continue to occur, over large geographical areas. Furthermore, because our biomonitor takes up its cadmium from prey (based on previous studies of our research group), it is likely that increases in cadmium have occurred throughout the food web in such lakes. Our results suggest that assuming declines in metal exposure from measurements in water alone could lead to the erroneous conclusion that the concentrations of toxic metals in organisms are declining when the opposite is actually the case. As such, this study demonstrates the value of biomonitors as a tool in ecological risk assessments for metals.

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Burrowing behaviour of aquatic insect larvae as a determinant of metal exposure and effects

A. Gosselin, L. Hare (PI) & A. Tessier. 

nstitut National de la Recherche Scientifique - Eau, Terre et Environnement (INRS-ETE), Université du Québec, Sainte-Foy, QC.

Ecological risk assessments should by definition consider the behaviour of target animals, which is a key aspect of their ecology. Thus a benthic invertebrate's relative exposure to trace metals in sedimentary and overlying water is likely to be influenced by its burrowing behaviour. In return, trace metals accumulated in a benthic animal can affect its behaviour. We are studying these interactions using two kinds of insects. First, we measured the burrowing depth of larvae of the phantom midge Chaoborus flavicans to determine if they remain in surface oxic sediment or if they penetrate into deeper sediment where they could be exposed to metals in anoxic interstitial waters. We chose this animal for study because it is a keystone planktonic animal in most Canadian lakes and because it has been successfully used as a Cd biomonitor. We used X-ray images of larvae in thin aquaria to determine the depth to which they burrow during their daily vertical migrations. Our observations indicate that larvae burrow to depths between 10 and 25 mm, that is, well below the surface oxic layer (usually only a few mm). Furthermore, larvae do not create a burrow connected to the overlying water through which they could pump oxygenated water. These observations suggest that Chaoborus larvae are exposed to metals in anoxic interstitial waters. Study of larval movements in sediment should allow us to determine if they remain immobile or hunt while in anoxic sediment. Secondly, we conducted preliminary experiments to determine the effect of sedimentary cadmium on the behaviour of larvae of the mayfly Hexagenia limbata. We chose this insect because it is a major taxon responsible for sediment bioturbation and an important source of metals for fish. We asked the questions: 1) is burrowing behaviour modified by the presence of Cd in sediment? and 2) can behavioral measurements serve as precocious indicators of adverse effects? We first exposed Hexagenia larvae to sedimentary Cd concentrations found in lakes exposed to smelter emissions. The levels of Cd in contaminated larvae were representative of those measured in mayflies found in mining areas. We then used X-ray images to visualize the burrows dug by uncontaminated and Cd-contaminated larvae as well as to measure their cumulative burrow lengths after 24 hours of activity. Preliminary results with a limited number of larvae suggest that there is no significant difference in the activity of control and contaminated individuals. If this trend holds for larger numbers of individuals (experiments underway), then we can say either that cadmium has no effect on the burrowing activity of larvae or that the type of behavioral measurement used is an inappropriate indicator of adverse effects. We will continue these behavioral studies on Cd contaminated and uncontaminated larvae using an infrared video technique to compare the irrigation, burrowing and walking activities of Hexagenia larvae as well as testing a micro-current meter to measure current speeds in burrows. Our studies should contribute to ecological risk assessments through the integration of information on animal behaviour into present ecotoxicological paradigms that generally consider only extreme responses such as lethality and reductions in growth and reproductive success.

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Can metal speciation be predicted in freshwaters with low nickel concentrations using the competing ligand exchange method and adsorptive cathodic stripping voltammetry?

Guthrie, J.W. and C.L. Chakrabarti

Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, ON, K1S 5B6

The competing ligand exchange method relies on determining the total metal concentration of the original sample, and that this metal concentration is sufficient to be measured by the kinetic speciation technique throughout the experiment. Previously, speciation in freshwaters with low metal concentrations has been reported to be the speciation after the sample was spiked with a known concentration of metal. Since any perturbation of the sample inevitably changes its characteristics, this cannot possibly be its true speciation. Pre-concentration by gentle evaporation has also been an option; however, this becomes impractical since large sample volumes are required. A new method must be developed to predict the metal speciation in these samples.

This study tries to address the issue by applying several small spikes to aliquots of an "original" model solution and observing how the system behaves as the nickel to humic acid mole ratio changes. The kinetic speciation of each spiked sample was determined using the Competing Ligand Exchange Method with Adsorptive Cathodic Stripping Voltammetry using dimethylglyoxime as the competing ligand.

All data was fitted to the kinetic model with two components using non-linear regression. The results showed that as the concentration of nickel increased, the first kinetically distinguishable component increased while the second decreased. The first rate coefficient (weakly bound nickel complexes) showed slight increases with increasing Ni:HA mole ratio, and the second rate coefficient (strongly bound nickel complexes) stayed relatively constant. The behaviour of the both the rate coefficients and the concentration components suggest that the model has chemical significance. The trends shown in these results suggest that it may be possible to predict the chemical speciation of nickel in freshwaters with low concentrations of nickel.
The significance of this study is that it will help in the determination of the speciation of waters with very low concentrations of nickel ions. This is important since the toxicity of nickel does not depend on the total nickel concentration, but on its chemical speciation.
The speciation technique provides speciation data needed for assessing the risk to ecosystems from nickel. This Project is directly applicable to the other Projects (B1, B2, B3, C1, C3, C9) for which it provides speciation data; for other Projects, the application is indirect. This Project is also directly applicable to PSL-2 Risk Assessment (SOURCES domain) because the speciation data provided by this Project will build on and improve the PSL-2 Risk Assessment.

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Relative fluxes of metals from foliage and fine roots to soils

Hale, B. and D. Johnson

University of Guelph, Guelph, ON

Plant-associated metals are semi-conserved in ecosystems through decomposition, mineralization and re-uptake from soil. Metals accumulate in all plant vegetation compartments (foliage, branch, trunk, bark and root), but not all plant compartments contribute equally to annual fluxes of metals to soils due to differences in metal concentrations, biomass production and longevity on the plant. Under normal circumstances, annual metal return to the soil environment from forest plant communities is chiefly through the short-lived, non-lignified tissues of foliage and fine roots. The purpose of our investigation was to compare the relative inputs of metals (Cu, Ni, Pb and Zn) from these two compartments to soils at study sites located along transects following soil metal gradients near smelters in the regions of Rouyn-Noranda, QC and Sudbury, ON.

Metal pools in fine roots and foliage were quantified by multiplying metal concentrations of vegetation samples with species biomass estimates, obtained directly for fine roots by extrapolating from biomass extracted from soil cores and indirectly for foliage through the use of species-specific allometric equations. Transfer of fine root metals to soils was calculated as the sum of metals pooled in fine roots that were turned-over (mortality) during the course of the year; foliar input to soils was calculated as the sum of metals pooled in deciduous species' foliage and the metals pooled in the fraction of coniferous species' foliage deemed to be returned to the forest floor in that year. Results confirmed that fine roots played the dominant role in Cu and Pb transfer to soils at all study sites, a result consistent with the strong binding properties of these elements to root tissues. Fine roots also played the greater role in Ni transfer at most study sites, but not to the same degree. Zinc flux to soils appeared to be linked proportionally with biomass turnover, and therefore transfer to soils was more evenly distributed between the two compartments. These results indicate that annual metal fluxes to soils depend on relative metal biomobility within the plant and underline the exaggerated role played by fine roots in the biogeochemical cycling of some metals.

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Foliar trace metal (Cu, Ni, Cd, Pb and Zn) input to surficial soils in contaminated northern forest ecosystems

Hale, B. and D. Johnson

University of Guelph, ON

Mass estimates of phytoaccumulated trace metals in contaminated forest communities, and of subsequent biogenic input to surficial soils through vegetative cycling, are necessary to properly evaluate the impact of continued anthropogenic metal deposition. The specific objective of our study was to determine the rate of trace metal (Cu, Ni, Pb and Zn) return to soils through the decomposition of foliage, at sites located along transects following decreasing gradients of atmospheric metal deposition near the smelting regions of Sudbury, Ontario and Rouyn-Noranda, Quebec.

In a litterbag experiment, leaves of both deciduous and coniferous tree species were gathered, placed into mesh bags, and then returned to the forest floor from which they were collected periodically and analyzed for loss of trace metals. In order to give an overall perspective to biogenic cycling of metals through foliage, data derived from this experiment were compared with estimates of atmospheric metal input to open-field collectors placed near the study sites. Results from year one of our study showed that, despite an average loss of dry litter biomass in the range of 30-40%, Cu and Pb content actually increased in leaf litter, even at those sites representing areas considered to receive background concentrations. In contrast, Zn showed a net loss from litter at almost every study site. Ni demonstrated characteristics midway between these extremes; accumulating at the most heavily contaminated sites, but differing from Cu and Pb in showing a net loss from litter at the uncontaminated sites. Atmospheric deposition of metals was sufficient to account for accumulation in most cases, except for Pb at two sites in Rouyn-Noranda, where litter content was higher than could be explained from this source alone.

It is apparent from our investigation that litter decomposition is a poor general predictor of metal return to soil. The relative role of litter as a short-term source or sink is strongly dependent on the binding characteristics of the trace metal studied.

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Speciation parameters for characterization of Ni, Cu, Pb and Zn species in lake waters, soil pore waters and throughfall samples.

Hassan, N.M., J. Murimboh, F. Raoufi, A. Jamaluddin, L. Si and C.L. Chakrabarti

Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada

Although a variety of analytical techniques have been developed to characterize metal speciation, few have been used successfully in the complex matrices encountered in samples of lake waters and soil pore waters. Here, we present the results of our studies of the Competing Ligand Exchange Method (CLEM)/Adsorptive Cathodic Stripping Voltammetry (AdCSV) and CLEM/Graphite Furnace Atomic Absorption Spectrometry (GFAAS) to determine dissociation rate coefficients. The CLEM-AdCSV technique was also used to determine free metal ions of Ni, Cu, Pb and Zn in the above samples. The CLEM-AdCSV can measure the faster dissociation rate coefficients (kd = 10-1 s-1); the CLEM-GFAAS can measure the slower dissociation rate coefficients (kd 10-3 s-1). Square Wave Anodic Stripping Voltammetry with a Thin Mercury Film Rotating Disk Electrode was used to determine dissociation rate coefficient, diffusion coefficient, stability constant of metal complexes, concentrations of free metal ions and ratios of free metal ions to metal complexes. DOC forms soluble complexes with these metals, and the lability of these complexes depends critically on the [DOC]/[metal] mole ratio being low. Most of the metal complexes in lake waters were labile with kd 10-3 s-1, or moderately labile with kd 10-3 s-1 because of relatively low DOC concentration (about 5 mg/L), resulting in low [DOC]/[metal] mole ratios. However, metal complexes in the soil pore waters and throughfall samples were much less labile with kd 10-3 s-1, or inert with kd 10-6 s-1 because the concentration of DOC was relatively high (10 - 50 mg/L), resulting in high [DOC]/[metal] mole ratios. The main advantage of the electrochemical technique is that only a small sample volume is required; this is especially important when only small sample volumes are available, as for soil pore waters.

The speciation parameters provide speciation data needed for assessing the risk to ecosystems from these metals. This Project is directly applicable to the other Projects (B1, B2, B3, C1, C3, C9) for which it provides speciation data; for other Projects, the application is indirect. This Project is also directly applicable to PSL-2 Risk Assessment (SOURCES domain) because the speciation data provided by this Project will build on and improve the PSL-2 Risk Assessment.

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Evaluation of soil surface charge using back-titration technique

Hendershot, W. and Y. Ge

Dept. of Natural Resource Sciences, Macdonald Campus of McGill University

Soil surface charge is considered to be responsible for many soil reactions such as ion exchange and complexation. In addition to the permanent charge (Qp), soil oxides and organic matter, which contain either inorganic hydroxyl groups or organic functional groups, contribute to the total charge of the surface. Thus, these reactive surface materials often provide a number of sites with pH-dependent charge (Qv) properties. This study was designed to find out the distribution of surface charge among the binding sites and, more importantly, the surface components responsible for the binding reactions.

Surface charge of soils with a wide range of properties was determined using a back-titration technique. The procedure defined the surface charge as the OH consumption by surface reactions corrected for dissolution of the solid phase and other solution reactions (e.g., metal hydrolysis). Four proton dissociation constants (pK i, i =1-4) were derived with a graphical method for each soil. With the surface charge data, pKi values and a multi-site adsorption equation, we optimized maximum surface charge of functional groups (Qmi). The four-site model predicted soil surface charge well with values of Qv varying from 10 to 100 cmol (+)/kg. When the pH was below 8, the surface charge was mainly distributed to the 1st and 2nd sites for organic soils and to the 1st, 2nd and 3rd sites for the mineral soils. The last site seemed to have a strong contribution to the total surface charge at pH>8. Statistical analyses also showed that the charge maxima of four sites were related to soil pH, CEC, organic carbon, Fe and Al oxides, indicating both inorganic and organic soil components may control the charge properties of each site. The charge properties of the surfaces may be used to explain metal adsorption, which is one of the soil processes responsible for solubility and bioavailability of metals.

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Column soil solution extraction method produces solutions comparable to zero-tension lysimeter solutions. Applications to trace metals.

Hendershot, WH. and MacDonald, JD.

Department of Natural Resource Sciences, Macdonald Campus, McGill University, Montreal, QC.

Lysimeter solutions are accepted to be representative of the mobile fraction of soil solutions. Laboratory methods of soil solution extraction, particularly for dry soils, result in solutions that are not comparable to solutions extracted in the field through zero-tension lysimetry. The lack of correspondence between field results and laboratory results questions the validity and value of processes noted in laboratory experiments. It is difficult to differentiate between processes representative of the chemical properties of the soil itself, artefacts of extraction methods and pre-extraction sample treatment. This study was carried out to develop a laboratory procedure capable of reproducing soil solutions obtained through lysimetry in terms of major cations, trace metals concentrations (Cu, Ni, Cd, Pb and Zn), pH, ionic strength and dissolved organic carbon (DOC) using dried homogenised soils.
Soils examined were organic horizons of podzolic soils taken from metal contaminated sites, as well as sites with low levels of anthropogenic metal contamination. Sampling was carried out on transects originating from metal smelters in Sudbury and Rouyn Noranda, Canada at sites in which lysimeters were installed in the spring and summer of 1999. The laboratory extraction method involves two steps. First, a pre-extraction wash of soil columns to remove the effects of sample treatment and drying (i.e., products of cell lysis and desiccation). Washing is followed by slow leaching of a CaCl2/SO4 solution of low ionic strength through soil columns. Total dissolved trace metal concentrations were measured using ICP-AES and GF-AAS.
The pre-extraction washing of soils eliminates the confounding effects of ionic strength and, elevated concentrations of DOC and divalent cations not typically found in lysimeter solutions. Soil extractions produced soil solutions that showed no significant differences from seasonal variations in lysimeter solutions for ionic strength, DOC, Ca, Mg, K and pH in most soils. Partitioning of Cu, Cd, Zn, Pb and Ni between the solid and solution phases demonstrated the same relationship between DOC, pH and metal concentrations in both lysimeter and in laboratory extraction solutions. This result suggests that the exchange processes resulting in the extracted solutions are consistent with exchange as it occurs in the field and sampling artefacts have been eliminated.
The ability to reproduce lysimeter solutions in the laboratory insures that in laboratory experimentation with soil solutions is occurring at environmentally relevant concentrations and that data produced in the laboratory is relevant to processes observed in the field.

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Medium–scale variations in trace metal concentrations in podzolic soils of northern forests exposed to smelter emissions.

Hendershot, WH and MacDonald, JD

Department of Natural Resource Sciences, Macdonald Campus, McGill University, Montreal, Canada.

Regional surveys of trace metals originating from point sources, such as metal smelters, have indicated that accumulations occur principally in organic horizons of acidic forest soils. Few studies have examined the degree of variability of metal accumulation within single forest stands. We calculated the number of samples required per m2 to produce a mean within the 10% confidence interval, examined if there was a pattern of variability associated with the point source and examined the relationship between concentrations of deposited metals and soil characteristics. Since trace metals are strongly bound to organic material and aerially deposited particulates are slowly dissolved it was proposed that accumulations would be dominantly controlled by physical factors such as canopy effects and micro-topography within forest stands. Consequently, accumulation of all smelter metals would occur proportionally with points of higher accumulation indicating higher input of deposited metals to organic horizons.

Small samples were gathered at 5 points on a 5 m radius around a central soil pit, in which large bulk samples were gathered, at 3 study sites on two separate transects from point sources of metal emission in Sudbury ON, and Rouyn Noranda, QC. Soil samples were analysed for pH and organic C and hot-acid soluble metal concentrations of Cu, Cd, Pb, Zn and Ni.

A maximum of one sample per square metre was observed. Coefficients of variation ranged from 50% in organic horizons to as high as 70% in mineral horizons but with the exception of Cu, did not appear to be related to distance from the smelter. The two transects demonstrated different trends with respect to metal accumulation. In the Sudbury region, significant correlations between concentrations of smelter related metals were observed, i.e., concentrations of all smelter metals in individual samples increased proportionally. In the Rouyn Noranda region there were not strong relationships between concentrations of smelter metals in organic samples, however mineral horizons show strong relationships between concentrations of all smelter metals. Concentrations of smelter metals are related to concentrations of organic carbon in mineral horizons in Rouyn Noranda.

The degree of variability associated with trace metals in forest soils identified in this study emphasises the need for extensive sampling of soils in close proximity to the organisms being studied. Understanding variability of metals in soils may prevent errors in risk assessment of sites due to inadequate sampling strategies and provide information with respect to the movement of metals within the ecosystem.

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Fluxes of trace metals between ecosystem pools, the influence of soil pH, organic matter, and species composition. Model design.

Hendershot, WH¹, MacDonald, JD¹ and Johnson, D².

¹Department of Natural Resource Sciences, Macdonald Campus, McGill University, Montreal, Quebec.
²Department of Land Resource Development, University of Guelph, Guelph, Ontario.

Transects with distance from point source emitters provide natural dose/accumulation relationships for trace metals in ecosystem compartments. In forest ecosystems both the forest floor and the vegetation act as indicators of historical metal input. Semi-empirical model parameters may be developed by observing these natural dose/accumulation relationships. This poster describes the fundamental relationships used in developing such a model and how such a model may reproduce historical metal emissions.
Two transects were established with distance from point source metal emitters. Experimental sites were established at 7, 15 and 41 km from Rouyn Noranda, QC and 10, 25 and 51 km from Sudbury ON. Metal inputs to the sites were monitored during the years 2000 and 2001 using open-field precipitation collectors, and throughfall collectors. Extensive vegetation and soil sampling was carried out to characterise the total concentration of hot-acid soluble metals in the various ecosystem compartments. Metal outputs were also monitored through the use of zero-tension lysimeters installed below the forest floor and at 30 cm.
The simple box model uses a series of regression equations developed from the data from our experimental sites to calculate input and output of metals from forest soils at increasing metal concentrations. Regression equations were developed using the bound metal ratio (ratio of metals per unit carbon or BMR); pH and DOC reproduce lysimeter concentrations. Both vegetation uptake from and return to the forest floor is also calculated from vegetation monitoring and is divided between coniferous and deciduous species. Finally output from mineral soils may be calculated using existing regression equations that relate total metals, SOM and pH to dissolved metals. The model reproduces the pattern of metal accumulation in monitored sites in relation to historical metal emissions from metal smelters.
This semi-empirical model is intended to identify the relative impact of different factors in the cycling and retention of metals in northern forests. This knowledge is essential in evaluating the environmental impact on forest ecosystems. The factors integrated into the model include, soil pH, SOM, and the solubility of organic carbon, as well as vegetation species influences.
By evaluating the concentrations of metals in different ecosystem compartments, the fluxes between these ecosystem compartments and the factors that influence these fluxes, in ecosystems with increasing concentrations of total metals, semi-empirical models may be developed that calculate changes in ecosystem compartments with time.

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Alleviation of trace metal rhizotoxicity

Hendershot WH¹, A Voigt1, G Sunahara²

¹ Department of Natural Resource Sciences, McGill University, Ste-Anne-de-Bellevue, QC
² Biotechnology Research Institute (CNRC), Ecotoxicology Group, Montreal, QC

We are still not able to predict unambiguously the amounts of metals that are available to plants in soils. Short-term toxicity tests using higher plants, such as the root elongation test, could be practical to assess the bioavailability of trace metals in soil solutions and furthermore to link toxicological effects directly to the bioavailable trace element status of soils.

The toxicity of metals is influenced by the speciation of the metals in solution and by the presence of other ions. Calcium is capable of ameliorating trace metal toxicity. However, quantitative data for the effect on the toxicity of Cd, Cu, Ni, Pb and Zn for plants is lacking. The effect of pH can be seen as twofold. The pH is influencing the speciation of metals in solution and H+ ions themselves can become rhizotoxic at higher concentration. The 2-day root elongation test of lettuce as a sensitive assay has been used to examine the effect of pH and Ca on the rhizotoxicity of these metals. The inhibition of root elongation has been related to metal ion activities. These activities have been incorporated into non-linear growth equations for root elongation versus toxic trace metals and alleviating ions.

All metals led to significant inhibitions of root elongation at ion activities that have been reported in the literature for soil solutions. The toxicant strength increased in the following order: Cu > Ni > Pb > Cd > Zn. The rhizotoxic effect of the metals decreased with increasing H+ concentration in solution. Root growth was severely inhibited at pH's lower than 6. Calcium alleviated the rhizotoxic effect of all metals and of H+. The lettuce root elongation bioassay is sensitive and reproducible at environmentally relevant activities in simple solutions. Rhizotoxicity scores are a useful tool to study processes that are influencing the bioavailability of trace metals in soil solutions.

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Speciation parameters of copper in freshwaters by Square Wave Adsorptive Cathodic Stripping Voltammetry using the Competing Ligand Exchange Method

Raoufi, F., N.M. Hassan, A. Jamaludin, J. Murimboh and C.L. Chakrabarti

Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada

Copper is an essential element at low concentrations, but it may be toxic at elevated concentrations. The Free Ion Activity Model (FIAM) is generally accepted as a good indicator of metal bioavailability for organisms in the aquatic environment, but FIAM requires determination of free metal ion activity (or concentration) which may be very low in lake waters. Free cupric ion concentrations in freshwater samples from Lake Opasatica, Lake Vaudray, Lake Osisko and Lake Dufault were determined by Square Wave Adsorptive Cathodic Stripping Voltammetry using catechol as a competing ligand. The method is based on competition for the complexation of Cu(II) between the naturally-occurring organic ligands in the samples and the added competing ligand, catechol. The concentration of free cupric ions was determined indirectly from equilibrium calculations. The free cupric ion concentrations in the lake water samples were pCu 14-17. The conditional stability constants of the copper complexes and the concentration of naturally-occurring ligands in the lake water samples were calculated from the titration data using the FITEQL program. A one-ligand model was found to fit the data well. The results suggest the presence of very strong ligands in the water samples. The working concentration of catechol (2 ´ 10-5 – 8 ´ 10-5 mol/L) was optimized for each sample. The method allows freshwater samples to be studied at the natural total copper concentrations (0.4 ´ 10-7 – 2.2 ´ 10-7 mol/L) and indirect determination of very low free cupric ion concentrations.

The speciation parameters provide speciation data needed for assessing the risk to ecosystems from copper. This Project is directly applicable to the other Projects (B1, B2, B3, C1, C3, C9) for which it provides speciation data; for other Projects, the application is indirect. This Project is also directly applicable to PSL-2 Risk Assessment (SOURCES domain) because the speciation data provided by this Project will build on and improve the PSL-2 Risk Assessment.

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Speciation parameters of cadmium species in lake waters by the Competing Ligand Exchange Method using Square Wave Anodic Stripping Voltammetry

Salam, M.S.A., N.M. Hassan, A. Jamaludin, F. Raoufi, J. Murimboh and C.L. Chakrabarti

Ottawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K1S 5B6

Cadmium is a toxic metal. In the aquatic environment, the Free Ion Activity Model (FIAM) is generally a good predictor of metal bioavailability. However, FIAM requires determination of the free cadmium ion activity (or the concentration) which may be very low in these lake waters. The Competing Ligand Exchange Method (CLEM) with Square Wave Anodic Stripping Voltammetry (SWASV) at a Static Mercury Drop Electrode was used to determine free cadmium ion concentrations and Cd(II) complexation, i.e. naturally-occurring ligand concentrations and the conditional stability constant of the Cd(II) complexes. The method is based on the competition of cadmium between the naturally-occurring ligands in the samples and the added competing ligand, and also on the competition between cadmium (trace metal) and other co-present trace metals, such as Cu(II), Pb(II), and Zn(II), for both the naturally-occurirng ligands in the samples and the added competing ligand, resulting in both Ligand-Exchange and Metal-Exchange (i.e. Double-Exchange Reactions). The method consists of titration of the naturally-occurring ligands in the water sample with cadmium in the presence of an added competing ligand, ethylenediamine (en), which forms Cd-en complexes that are fully labile relative to the electrochemical timescale of measurement of SWASV. At any point during the titration, the total dissolved cadmium distributes between the ethylenediamine and the naturally-occurring ligands. As expected, the concentration of free cadmium ions was found to depend on the mole ratio of cadmium to Dissolved Organic Carbon, and also on the co-presence of the other metals such as Cu(II), Pb(II) and Zn(II).

The speciation parameters provide speciation data needed for assessing the risk to ecosystems from cadmium. This Project is directly applicable to the other Projects (B1, B2, B3, C1, C3, C9) for which it provides speciation data; for other Projects, the application is indirect. This Project is also directly applicable to PSL-2 Risk Assessment (SOURCES domain) because the speciation data provided by this Project will build on and improve the PSL-2 Risk Assessment.

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In situ two-dimensional high-resolution profiling of sulfide in sediment interstitial waters

Wang, F., C. DeVries

Environmental Science Program and Department of Chemistry, University of Manitoba, Winnipeg, MB.

Sulfide is one of the major controls of the cycling, speciation and bioavailability of many trace metals (e.g., "Class B metals" such as Cd, Cu, Pb, Zn, Hg) in sediment interstitial waters. A potential toxicant to all aerobic organisms, sulfide is also one of the major factors affecting the distribution of aquatic organisms. Although the vertical profiles of sulfide in sediment interstitial waters have been well reported at a spatial resolution ranging from micrometers to centimeters, few studies have been conducted on its lateral distribution at a comparable spatial resolution.

We have modified a recently developed diffusive-gradients-in-thin-films (DGT) technique with computer-imaging densitometry to study the two-dimensional distribution of sulfide in sediment interstitial waters. By inserting Ag+ into a binding gel followed by submersing it in a KI solution we were able to produce a very stable, evenly dispersed, AgI binding gel. We also significantly reduced the darkening of the AgI gel by setting the gels at room temperature and by using black Plexiglas plates as the gel holders. Once inserted into sediments, the sulfide in the interstitial water diffuses through a diffusive gel layer and is then bound to the AgI in the binding gel. From the color changes of the binding gel (AgI is pale yellow and Ag2S is black) the sulfide concentrations can be determined by computer-imaging densitometry. The improved technique was then used to profile the distribution of sulfide in a highly productive Prairie wetland at a vertical and lateral resolution of 0.2 mm. The in situ high-resolution microprofiles revealed unprecedented two-dimensional heterogeneity of sulfide concentrations in the sediment interstitial waters. The mosaic distribution of oxic and sulfidic microenvironments indicated not only the complexity and heterogeneity of the biogeochemistry of Class B metals in sediments, but also the capability of a variety of aquatic organisms for coping with the sulfidic environment. The technique will be used in the field season of 2002 for sulfide profiling in several Canadian Shield lakes.

Our results clearly demonstrate that ecological risk assessment (ERA) of metals in sediments needs to take into account in situ vertical and lateral gradients of metals and sulfide. The current practice of ERA based on the bulk sediment or porewater measurements ignores and destroys such natural heterogeneities. The sulfide profiles also suggest that the current practice of porewater toxicity testing is not an appropriate tool for effects assessment for ERA of metals in sediments, because of the unpredictable changes in sulfide concentrations and metal speciation during the testing.

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A preliminary study on the stability of metal-sulfide complexes in oxic waters

Wang, F.¹, K. Sukola¹, and A. Tessier²

1. Environmental Science Program and Department of Chemistry, University of Manitoba, Winnipeg, MB.

2. INRS-Eau, Université du Quebec, Sainte-Foy, QC.

Most metal ions of environmental concern (e.g., Cd, Cu, Pb, Zn, Hg) are soft Lewis acids and tend to form strong complexes with sulfide, which is a soft Lewis base. Although sulfide is often the major control of the speciation of these metals in anoxic waters, its role in oxic surface waters is poorly known. In this study, the stability of metal-sulfide complexes in oxic waters was investigated over the period of several weeks. Aqueous Mn-, Pb-, and Zn-sulfide complexes were synthesized in the laboratory by titrating the metal solution with sulfide until conditions of slight undersaturation of the metal sulfide solid were reached. Although there was measurable sulfide (by the colorimetric methylene blue method) in the solution immediately after the synthesis, most metal-sulfide complexes rapidly destabilized after 1 day (e.g., 30% loss after 1 day for a 10-mM Zn-sulfide solution), and the sulfide was not measurable after the solution was exposed to air for 3 days. These results suggest limited stability of metal-sulfide complexes in oxic waters.

We have also measured the total dissolved sulfide concentrations (by an acid purging and trapping method) in the oxic epilimnion waters of 2 Canadian Shield lakes. The average sulfide concentration was 1.8 nM in Lake Tantaré and 1.2 nM in Lake St-Augustin. Since free sulfide is not stable in oxic waters, the sulfide measured is likely in the forms of metal-sulfide complexes. The very low concentrations of the dissolved sulfide in these oxic waters are probably due to their limited stability. Direct identification of metal-sulfide complexes in both laboratory solutions and natural waters is currently undergoing.

Our preliminary results suggest that metal-sulfide complexes can be present in oxic surface waters, although they may not be stable for a prolonged period. As a result, the role of sulfide in controlling metal speciation and bioavailability must be taken into account in ecological risk assessment (ERA) of metals in surface waters, particularly where there is a continuous supply of metal-sulfide complexes (e.g., from effluent discharges).

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