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The geochemistry of Fe, Mn, Ni, Cu, Zn and As in the water column, sediments and porewaters in a seasonally anoxic lake

University of British Columbia

The distributions of Fe, Mn, Ni, Cu, Zn and As in the water column, interstitial waters and associated solid phases in Balmer Lake, Ontario, were determined from samples collected in July and October of 1993, and March and May of 1994, in order to assess the seasonal biogeochemical controls governing trace metal behaviour and mobility. The basin has served as a repository for tailings pond effluents since 1967, and as a result, hosts elevated levels of contaminants in the sediments and lake waters. During the ice-free periods of summer, fall and spring, the water column is characterized by fully oxygenated bottom waters and homogeneous distributions of all measured parameters. However, reducing conditions develop in deeper areas during the period of ice cover in response to the high biological demand of the organic-rich sediments coupled with restricted atmospheric exchange. Trace metal profiles collected during winter exhibit considerable lakewide variation and appear to reflect variability in the duration and extent of bottom water anoxia, and the relative influence of metal-rich lateral inputs from the tailings circuits. Solid-phase profiles indicate that the top decimetre of the sediment column has received variable contributions of both organic matter- and feldspar-rich natural detritus, and carbonate-, chlorite- and metal-rich tailings inputs. High resolution profiles of porewater NO₃⁻, Fe, Mn and SO₄²- illustrate that the sediments in Balmer Lake become anoxic within a few centimetres of the sediment-water interface during well-mixed periods. Evidence for the precipitation of a Cu-bearing sulphide phase in the lowermost winter bottom waters suggests that the sulphate redox-cline migrates above the sediment-water interface at some point during ice cover. The seasonal porewater distributions of dissolved Ni, Cu and Zn exhibit pronounced consumption profiles at depths consistent with sulphate reduction zones, suggesting they are sequestered as metal-sulphide phases. Enrichments of dissolved Ni and Zn in the surficial sediments at most sites reflect the dissolution of labile particulates at the sediment-water interface. Diffusive influxes calculated for Zn, Ni and Cu suggest that diffusion mechanisms contribute insignificantly to the accumulation rates of these metals. Arsenic is remobilized at deeper sediment depths and appears to be largely governed by the redox geochemistry of Mn. The data collectively demonstrate that trace metal mobility in the Balmer Lake water column/sediment system varies significantly over the course of the four seasons. With the exception of arsenic, however, dissolved metal fluxes indicate that the underlying contaminated sediments are providing a significant and permanent sink for dissolved metals.

Thesis/Dissertation

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2009-03-11

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http://hdl.handle.net/2429/5882

Earth and Ocean Sciences

University of British Columbia

UBCV

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