Shallow
Water Covers - Equity Silver Base Information: Physical Variables
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report 2.11.5ab
May 1996
EXECUTIVE
SUMMARY
A field program
to collect baseline physical data for an existing tailings pond
was completed between September 11th and 16th, 1995, at the Equity
Silver Mine located near Houston, British Columbia. The program
was undertaken as part of a multi-disciplinary study to evaluate
the effectiveness of using shallow water covers to control acid
generation from mine tailings. A specific objective of the study
outlined in this report was to collect data from the Equity pond
that could be used to evaluate a calculations procedure used to
determine the depth of water cover required for bed stability in
a man-made tailings pond.
The field program
consisted of 3 components:
- a bathymetric
survey of the pond;
- the collection
of 13 surficial sediment samples and 2 sediment cores from each
of 3 separate locations in the pond; and,
- the observation
of bed forms within the pond at different locations.
The median
(D50) grain size of the sediment samples, based on the
average of the 15 samples from each location, were 0.0512 mm at
the shallowest location (1.3 m water depth), 0.0075 mm at the middle
location (2.3 m water depth) and 0.0040 mm at the deepest location
(3.4 m water depth). The sediment size distributions at each location,
and hence water depth, were statistically distinguishable. Variation
in median size between the 15 samples collected at each location
was small. One standard deviation around the median grain size yielded
a size range of 0.0248 - 0.0775 mm at the 1.3 m water depth location,
0.0049 - 0.0101 mm at the 2.3 m water depth location and 0.0036
- 0.0044 mm at the 3.4 m water depth location. These data are consistent
with the transport of sediment away from the tailings outfall during
discharge into the pond.
Sediment submerged
weight density was averaged for the 15 samples collected at each
water depth location. This density decreased with increasing water
depth. Submerged weight density of the sediments was 1.645 ± 0.299
tonnes/m3 at the 1.3 m water depth location, 1.220 ±
0.116 tonnes/m3 at the 2.3 m water depth location and
1.068 ± 0.193 tonnes/m3 at the 3.4 m water depth location.
The decrease in in situ submerged weight density with increasing
depth is consistent with the dewatering of tailings around the pond
periphery following discharge. The average submerged weight density
for the sediments in the pond as a whole is 1.318 ± 0.328 tonnes/m3
regardless of covering water depth.
The 2 cores
from each location revealed similar stratigraphy between cores.
The cores collected at 1.3
m depth exhibited
some stratigraphic differentiation with transitions evident between
layers of
unconsolidated
and semi-consolidated tailings to a depth of approximately 0.70
m. The cores from the
two deeper
sites obtained approximately 0.65 m of core but only exhibited one
transition from
unconsolidated
tailings to semi-consolidated tailings at approximately 0.35 m below
the bed.
Using the presence
of bed forms on the bottom of the pond as evidence of movement of
bed sediments, indicated that wave-induced mobilization of sediments
on the bed has occurred to a water depth of 1.3 to 1.4 m since tailings
were deposited. The maximum open-water wind speed recorded at the
site since cessation of tailings deposition was 18.6 m/s. Using
the procedure outlined by Hay & Company Consultants Inc. (1995)
indicated the depth of water cover required for no movement of Equity
tailings during a wind speed of 18.6 m/s is 1.4 m. The close comparison
between the disappearance of bed forms at 1.3 to 1.4 m depth and
the prediction of a maximum depth of 1.4 m for the historic wind
conditions
provides confidence in the predictive equation. At present, the
percentage of the Equity pond with water depths shallower than 1.4
m is 7%.
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