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.
