A preliminary field assessment of the effects of subaqueous disposal of tailings in
Anderson Lake, Manitoba was conducted as part of the Mine Environment Neutral
Drainage (MEND) program. The lake has received about 7.5 million tonnes of
tailings since 1979 from the Snow Lake mill operated by Hudson Bay Mining &
Smelting Co., Ltd. The mill processes copper-lead-zinc ore from four nearby
underground mines. Tailings are discharged into the lake through a floating pipe,
which is regularly moved to prevent tailings buildup.
The lake is a small, Precambrian Shield waterbody, with high biological
productivity (meso to eutrophic conditions). Historically, it has been considered a
dead lake due to the limited fisheries resources. The lake is essentially land-locked
with small inflows and outflows. A dam with a control discharge structure has
been installed at the outlet, and has increased lake area. However, due to tailings
deposition, the overall lake volume has decreased. The lake is shallow (mean depth
= 2.1 m) with a maximum depth of 6.7 m. No thermocline was observed in the
water column, but dissolved oxygen concentrations were markedly decreased
within 0.5 m of the bottom.
The lake is characterized by higher conductivity, dissolved solids, sulphate
concentrations and increased hardness since tailings discharges have begun. The
pH has generally decreased. Dissolved metal concentrations and turbidity have
increased in the area near the discharge. Inflows from the Anderson mine area are
characterized by low pH, high conductivity and dissolved solids, low alkalinity,
high sulphate and reactive silicate concentrations, higher chloride concentrations
and high dissolved metal concentrations.
Metal concentrations in the lake sediments have been greatly increased. The
sediment stations near the outfall show metal levels comparable to fresh tailings
samples. Stations where tailings have been deposited in the past are developing an
organic layer that is biologically active. Detailed petrographic, X-ray diffraction
(XRD), X-ray fluorescence (XRF), particle size and leaching analyses were
performed to characterize the sediment and tailings.
Sediments were characterized into two major groups, those predominantly
composed of tailings materials (the tailings, and Stations 1, 2, and 4) and those
primarily composed of organic material (Stations 3, 5 and 7). XRD work indicated
higher abundance of quartz, feldspar, pyrite, micas and calcite in the tailings
dominated samples. The actual tailings were composed of 55 % pyrite, 4%
pyrrhotite, 2% sphalerite and minor portions of hornblende, biotite, chlorite,
carbonate and other silicates. Minor to trace amounts of chalcopyrite, arsenopyrite
and galena were found. The amount of tailings estimated in the organic samples
varied with the method used. The organic samples contained framboidal pyrite and
fine specks of angular pyrite, while tailings dominated samples have similar
compositions and grain size to the tailings and were virtually unmodified. Mineral
compositions assumed from XRF analyses indicated the presence of various
concentrations of quartz, plagioclase feldspar, micas, chlorite, and pyrite. The
highest percentages of pyrite were found in the tailings and tailings-dominated
samples.
Sequential extractions of a tailings-dominated sample (Station 2) indicated
generally low metals availability, with most of the metals associated with the
oxidizable and residual phases. These observations are in agreement with the high
sulphide content of the sample. The results suggest that the metals are generally
non-labile and that significant metal release would require strongly oxidizing and
acidic conditions to exist. Present conditions in Anderson Lake are not conducive
to such release; hence, the potential for metal transfer from sediments to the
aqueous environment, in levels that are toxic, is considered to be minimal. With
exception, however, was the behaviour of zinc. Due to the high total Zn
concentrations, low releases of Zn occurring in the other phases indicated a
potential for detectable release under natural conditions. Cadmium, copper, lead
and nickel were also released in low quantities in phases other than the oxidizable
and residual phases. Acid-base accounting of the tailings sample indicated a
considerable potential for acid generation.
The biota of the lake were examined in considerable detail. Densities of benthic
invertebrates were low but comparable to previous studies both in number and
species composition. Phytoplankton densities and community structure
characterize the lake as mesotrophic. Examination of past phytoplankton
communities indicates that more eutrophic conditions existed prior to tailings
deposition. Zooplankton densities were highly variable, but were generally lower
near the tailings discharge. Zooplankton diversity is comparable to that observed in
other Manitoba lakes. Aquatic vegetation of the lake was mapped and samples
collected for metal analyses. Metal levels in Typha were generally highest in a
tailings seepage area, particularly for arsenic, cadmium, lead and zinc. Outside of
the seepage area, concentrations of metals were low except for nickel. Fish
sampling confirmed that the fishery resources was limited, as only brook
sticklebacks (Culaea inconstans) were caught. Tissue concentrations in the
stickleback suggest bioaccumulation of copper, lead and zinc. Metal levels in the
fish were also significantly higher in populations from the tailings deposition area
