A PRELIMINARY
ASSESSMENT OF SUBAQUEOUS TAILINGS DISPOSAL IN ANDERSON LAKE, MANITOBA
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project 2.11.1a-c
March 1990
EXECUTIVE
SUMMARY
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.
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