| Project Title |  |
| Project
A4: Geochemical mobility of metals in surface sediments:
Influence of sediment diagenesis.
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| Research Team | |
| R. Carignan,
Univ. de Montréal. Email: carignan@ere.umontreal.ca Tel: 514-343-7239 Co-Investigators:
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| Project Summary | |
| The objective of the project is to
determine to what extent and under what circumstances lake sediments can provide useful
historical records of trace metal loading from the atmosphere. The chemical behaviour and
post-depositional mobility of trace will be examined in recent lake sediments of boreal
Shield lakes exposed to high (Rouyn-Noranda-Sudbury region), medium (Haute-Mauricie) and
low (sept-Iles) levels of atmospheric pollution. We will also investigate techniques to
improve vertical resolution of pore water profiles and our understanding of diagenetic
processes. Site pre-selection at the remote Sept-Iles area has been completed. Vegetation and topographic maps provided by local forest companies were used to select 11 headwater lakes among over 200 possible candidates. The lakes could not be cores as planned in October 1998 because of ice cover formation. They will be sampled through the ice cover in April 1999. Final selection of the three lakes needed to perform the study in this area will be made as field data are obtained in April. We have been investigating two techniques to obtain porewater profiles at high vertical resolution: i) a Hg coated Ir voltammetric microelectrode; ii) DGT (Diffusion Gradient in Thin films). We have constructed functional Ir microelectrodes and have started to calibrate them and to optimize the operating conditions. We have also explored the DGT technique in depth and have deployed DGT samplers and peepers in several lakes. Surprisingly, we found greater concentrations of metals measured by DGT than by peepers. We hypothesize that this overestimation of labile metals measured by DGT is due to the fact that the diffusion coefficients of trace metals entering the DGT are significantly increased by the strong efflux of Na from the DGT system; this effect is presently not taken into account in the original DGT technique. We are currently investigating this problem.
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