REP35589 - Laserfiche WebLink

Home Browse Search

Bu[[dop Mine Tailings Ponds Evaluation some parts of a layered soil profile, the vertical hydraulic conductivity of the finer grained layers • is less than the infiltration rate. If saturated conditions aze observed in the finer-grained layers but perched water is not prevalent in the coazser-grained layers, the vertical hydraulic conductivity of finer-grained layers is similaz to the infiltration rate. The laboratory-based value of hydraulic conductivity for the tailings is approximately 5 x 10-~ cm/sec or 0.5 fUyr (from Section 4.3). This value is supported by observations made during the field investigations, where perched water conditions were noted above finer-grained layers in the upper portion of the tailings profile, with more continuously saturated tailings at greater depths. As mentioned above, perched water conditions aze found when the effective vertical hydraulic conductivity is equal to or less than the infiltration rate. Conversely, if the vertical hydraulic conductivity is much greater than the infiltration rate, drained conditions would exist and the tailings would desaturate. Assuming that roughly 40 percent of the 15 inches of average annual precipitation at the site • infiltrates into the tailings, a net infiltration into the tailings is about six inches per yeaz. The fraction of precipitation that infiltrates is based on water balance calculations from other mine sites for a relatively flat surface and spazse vegetation (and low evapotranspiration). For vegetated areas in the western United States, the fraction of annual precipitation that infiltrates (and is not removed by evapotranspiration) is approximately 5 to l0 percent. In the site area, an infiltration fraction of 10 percent is equivalent to about 1.5 inches per yeaz. The observed perched conditions in the tailings indicate that the vertical hydraulic conductivity is approximately equal to the infiltration rate into the tailings. Thus the observation of perched conditions implies that the vertical hydraulic conductivity is about six inches per year or SxlO-~ cm/sec, which is consistent with the hydraulic conductivity value measured in the laboratory. 4.6 Flow and Dilution Modeling Using the tailings flow conditions discussed in Section 4.5, some basic dilution calculations can be made for tailings seepage (if it were to seep from the tailings mass) mixing with underlying . ground water. These calculations are made based on the aerial extent of the tailings ponds and the average infiltration rate of precipitation into the tailings. HomesraM A9ining Company $deplrerd .tfiller. lnt enr-~r+ia~dra,~..n~ I.] ,apri(!1. 199"