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.