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%.
