| Abstract
| - The post-depositional behavior of Cu in a gold-miningpolishing pond (East Lake, Canada) was assessed aftermine closure by examination of porewater chemistry andmineralogy. The near-surface (upper 1.5 cm) sediments areenriched in Cu, with values ranging from 0.4 to 2 wt %.Mineralogical examination revealed that the bulk of the Cuinventory is present as authigenic copper sulfides.Optical microscopy, energy-dispersion spectra, and X-raydata indicate that the main Cu sulfide is covellite (CuS).The formation of authigenic Cu−S phases is supported bythe porewater data, which demonstrate that the sedimentsare serving as a sink for dissolved Cu below sub-bottomdepths of 1−2 cm. The zone of Cu removal is consistentwith the occurrence of detectable sulfide and the consumptionof sulfate. The sediments can be viewed as a passivebioreactor that permanently removes Cu as insoluble coppersulfides. This process is not unlike that which occurs inother forms of bioremediation, such as wetlands andpermeable reactive barriers. Above the zone of Cu removal,dissolved Cu maxima in the interfacial porewaters rangefrom 150 to 450 μg L-1 and reflect the dissolution of a Cu-bearing phase in the surface sediments. The reactivephase is thought to be a component of treatment sludgesdelivered to the lake as part of cyanide treatment. Fluxcalculations indicate that the efflux of dissolved Cu fromthe sediments to the water column (14−51 μg cm-2 yr-1) canaccount for the elevated levels of dissolved Cu in lakewaters (∼50 μg L-1). Implications for lake recovery arediscussed.
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