A PRELIMINARY
ASSESSMENT OF SUBAQUEOUS TAILINGS DISPOSAL IN MANDY LAKE, MANITOBA
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project
2.11.1a-d
March
1990
EXECUTIVE
SUMMARY
A preliminary
field assessment of long-term subaqueous disposal of reactive mine
wastes in Mandy Lake. Manitoba was conducted as part of the Mine
Environment Neutral Drainage (MEND) program. Mandy Lake received
approximately 73,000 metric tonnes of high-sulphur-base metal tailings
discharged from a single launder into the lake during 1943-1944.
An examination of the lake in 1975 found that natural revegetation
of tailings in the shallower areas had occurred. The water quality
was considered good. and aquatic vertebrates and invertebrates had
moved into the tailings area.
Mandy Lake
is a small lake (area = 239,000 M2) Situated in the Precambrian
Shield near Flin Flon, Manitoba. The lake, originally part of Schist
Lake, was isolated by construction of a causeway. Mandy Lake is
shallow (mean depth = 3.6 m) with a maximum depth of 5.5 m. During
the study, lake inflow and outflow was minimal. No thermocline was
observed in the water column, but low dissolved oxygen concentrations
(anaerobic conditions) were found below 2.0 m depth. The high biological
productivity (mesotrophic to eutrophic conditions) of the lake has
created sediments with high organic content and high sediment oxygen
demand.
Lake water
quality was similar at all stations and depths. The lake has moderate
hardness with conductivity, dissolved solids, and alkalinity values
comparable to other Manitoba lakes. The pH is slightly basic. Chloride,
sulphate and reactive silicate concentrations are low. Dissolved
metal concentrations are low with minimal differences between surface
and bottom samples indicating that metal release from the sediments
is minimal. Moreover, higher metal concentrations were found in
inflow waters than in the lake itself.
Elevated metal
concentrations were found in the lake sediments, particularly for
arsenic, copper, lead, mercury and zinc. However. decomposition
of the thick organic layer overlying the sediments appears to maintain
anoxic conditions, thereby reducing the potential for sulphide oxidation
and subsequent release of acid-soluble metals. Detailed petrographic,
X-ray diffraction (XRD), X-ray fluorescence (XRF), particle size
and leaching analyses were also performed to characterize the sediments.
The results
of this detailed analyses indicated an abundance of organic material
including diatem frustules. Tailings were dispersed throughout the
lake sediments with XRD scans detecting quartz, pyrite, feldspar
and chlorite in the sediments. The XRF analyses indicated a possible
mineral composition of quartz, plagioclase feldspar, micas, chlorite
and pyrite in the sediments. Trace proportions of framboidal pyrite
and rare examples of spherulitic chalcopyrite and sphalerite were
found. Leaching results using a sequential extraction technique
of sediment samples (Station 3) exposed to oxidization indicated
high release of cadmium, zinc, nickel, cobalt and manganese in the
water-soluble and exchangeable phases. As water quality results
did not indicate metals release from the sediment, the results are
more indicative of potential release under aerobic conditions. Releases
of iron, arsenic, lead and selenium occurred under moderate to strong
reducing conditions. Strongly oxidizing conditions released silver,
copper, lead, zinc and nickel and the residual phase released aluminum,
iron, manganese, arsenic, cobalt, mercury, nickel and zinc. Total
concentrations of aluminum and iron were high in the residual fraction
followed by lesser amounts of zinc and arsenic.
Lake biota
were examined in considerable detail. Benthic invertebrate densities
are low and the community is dominated by oligochetes and dipteran
larvae. Phytoplankton density and community structure characterize
the lake as mesotrophic to eutrophic with higher densities than
two oligotrophic lakes situated nearby. Diatoms are the predominant
phytoplankton. Zooplankton densities are slightly lower than other
Manitoba lakes and the community is dominated by rotifers, cladocerans
and copepods.
Aquatic vegetation
in the lake was mapped and samples collected for metals analyses.
Since 1975, the vegetation community in the tailings area has become
more diverse and comparable to other areas in the lake. Metal levels
in pondweeds (Potamogeton sp.) are higher in the tailings
area. Fish sampling revealed a healthy fish population comprised
of northern pike, white sucker, lake whitefish, yellow perch and
spottail shiner. Metal levels in fish tissues are generally low
compared to other Manitoba lakes and are within background levels
observed elsewhere in Canada.
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