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A Mr. Thomas M. Robertson 4 June 17, 1981 area are alluvial in origin, the alluvial deposits along the stream channels (as mapped in Figure 2) are believed to be generally coarser grained than other alluvial material in the area, due to the higher energy environment associated with flow along the stream channels. �y This coarser material is therefore more likely to have high permeability llb3 and to be carrying seepage from the pond. Along the stream channel, under the dam, the alluvial deposit is intersected by the cutoff trench which extends to a depth of 15 feet. L The tailings dam was built from materials excavated from the pond area. Approximately 5 feet of soil was removed from the entire pond area, mostly the silty sand. Based on this depth of removal, most of the soils,remaining on the surface in the pond area after excavation are probably clayey sands or silty clays which are of low permeability. The geologic cross section along the cutoff trench, provided by Homestake, maps the alluvial soils and the top of the Creede Formation on the east abutment. Near the stream channel on the east bank, the top of the (� Creede Formation has steep, nearly vertical faces which are overlain by a r relatively thin soil layer. The steep rock faces increase the potential that these units may be fractured, and they may have relatively high secondary permeability. Portions of the pond bottom may also be covered with tailings which restrict seepage by sealing the voids or creating an impermeable blanket. The tailings have been discharged from the upstream face of the dam, resulting in a thick layer of tailings upstream of the dam. The north end of the reservoir, which includes a significant amount of the alluvial deposits, does not have a thick tailings layer covering it. Therefore, it appears that the north end of the reservoir is a probable source of seepage. The geologic cross sections developed from the boring logs indicate permeable zones at the surface in this area. These zones may extend under the cutoff trench and could be continuous downstream. These permeable zones (silty sands) are not covered by tailings and could. provide an entry point for seepage under the dam. ( 2.2 POND CHARACTERISTICS The tailings pond characteristics were studied in detail to determine C relationships which may be affecting seepage rates. Changes in seepage rates, pond water level, pond area were examined to provide in—sight into the causes of the seepage. CSeepage rates, as estimated by Homestake personnel, are presented in Table 1 and Figure 3. The most recent measurements of seepage at the T weir downstream of the dam indicate a flow rate of 60 gallons per minute (gpm). This seepage rate has remained fairly constant during the one and one—half months of monitoring, but appears to indicate a decrease in seepage from earlier this year (see Table 1). The recent value of 60 gpm is consistent with the approximate magnitude of pumping rates of water returned to the reservoir, which is approximately 180 gpm on an intermit— tent basis.