Sulphide-rich zinc-, copper-, and lead-bearing tailings have been deposited via floating
pipeline into shallow (< 8 m) Anderson Lake in north-central Manitoba since 1979. The
lake water contains elevated levels of dissolved metals which are derived mainly from
an acid-generating roadway along the north shore. Watercolumn sampling and
sediment coring were carried out through the ice on Anderson Lake in April 1993. Two
sites were occupied, one proximal to the tailings discharge (Station B, the “pure
tailings” site) and the other about 2 km away (Station A, the “natural sediments” site).
Interstitial waters were extracted from duplicate cores collected at each location. The
water column was strongly stratified at both locations, exhibiting four layers at the
natural sediments site (two of which were dysaerobic or anoxic), and two layers near
the tailings outfall, the lower being dysaerobic. The unusual multiple layering probably
reflects the influence of laterally variable advective processes. High concentrations of
dissolved iron in shallow pore waters indicate that the sediments at both locations were
anoxic at shallow subsurface depths. Concentrations of Zn, Cu, Pb and Cd were very
low or undetectable in the pore waters. There was no evidence of release of Cu, Cd or
Pb from the deposited tailings, although there was evidence of possible minor release
of Zn from surface sediments in one of the tailings cores. Metals were removed from
pore waters below the upper I 0 cm at both sites, which is attributed to precipitation of
authigenic sulphide phases.

In August, 1993, water column sampling, sediment coring and membrane dialysis array
(peeper) deployments were performed at the same two sites occupied during the
Winter Survey. An additional shallow-water tailings site was sampled for interstitial
waters alone. During the Summer Survey the water column was well mixed, containing
elevated concentrations of Zn, Cu and Pb. However, those same metals (and Cd)
displayed precipitous decreases across the sediment-water interface at all sites
indicating that the sediments of Anderson Lake (natural and tailings) were acting as a
sink rather than a source for these metals.

Metal precipitation in the interstitial waters was attributed to precipitation of authigenic
sulphides. Arsenic appeared to respond to more complex seasonal cycling being
released at depth and possibly precipitated within the surgical sediment. Dissolved As
was likely released from reduction of oxides rather than oxidative release from tailings.

Two additional studies were carried out subsequent to the work described above. The
first (Appendix G) involved observations of Anderson Lake water quality after
discharge of tailings to the lake had ceased (June to October, 1995); lake thermal and
meteorological data were used in support of the geochemical interpretation. The water
quality in Anderson Lake was found to respond to meteorological events. Increases in
precipitation were accompanied by increases in dissolved Zn and decreases in
conductivity, suggesting that the degradation in water quality resulted from the
introduction of residual acid salts associated with historic acid-rock drainage. The
second study (Appendix H) involved an overview of the mine-related inputs to
Anderson Lake which assessed the relationships between the various inputs and the
observed historical changes in lake chemistry. The limited available evidence suggests
that the primary controls on past metal and acidity enrichments stemmed from
unbuffered underground mine discharges and acidic mine site drainages. The findings
of these sister studies are consistent with those of the previous MEND assessments of
Anderson Lake.