EXECUTIVE SUMMARY

Available evidence suggests that the post-depositional (diagenetic) chemical reactivity of mine tailings is inhibited by storage under water, and implies that such storage may provide a preferred long-term disposal option for such wastes. To assess this, we examine in this report the chemistry of interstitial waters and the associated solid phases in tailings and natural sediments in Mandy Lake. near Flin Flon, Manitoba. Mandy Lake was used as a receiving basin for tailings in the period 1943-1944. Tailings are areally widespread in the small lake and still occur in significant concentrations in surface sediments (top 5 mm) despite 46 years having elapsed since discharged ceased.

Two cores were collected in June 1990. and processed under nitrogen to extract interstitial waters. Solid-phase chemical measurements indicate that the core raised from the central basin of the lake penetrated through the tailings-rich zone into underlying. premine. organic-rich (up to 15 wt. % C) natural deposits; the other core, collected near the former discharge outfall, consisted essentially of a mixture of tailings and organic-rich natural detritus- At the central basin site, dissolved iron measurements made on the pore water samples indicate that the deposits are suboxic or anoxic at very shallow depths, probably within several millimetres of the sediment-water interface. This reflects a high benthic oxygen demand at that location. An enrichment of solid-phase Mn at the top of the near shore core, coupled with a low Fe concentration in near-surface pore waters. indicates that the surface deposits at this location are oxic.

Relative to overlying bottom water, dissolved Zn. Cu and Pb concentrations decrease with depth in the upper decimetre at the near shore site. These distributions are attributed to precipitation of the metals as sulphide phases at depths on the order of 7 or 8 cm. Sulphate reduction with concomitant production of HS- is expected, given the evidence for shallow anoxia in the deposits. although the rate of sulphide production may be limited by the low sulphate concentration in Mandy Lake water. At the central basin site. the pore water data indicate that Zn is diffusing into the sediments, as is the case at the other location. However, near-surface dissolved Cu and Pb concentrations, although very low, are slightly higher than in the overlying bottom water, indicating that their must be a small benthic efflux of these two metals at this location. Diffusion calculations indicate that the efflux is so small as to have no measurable impact on the dissolved metal inventory in the overlying lake water.

Last Modified: 2003-11-26		Important Notices

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