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Mr. Thomas M. Robertson 4 June 17, 1981 <br />area are alluvial in origin, the alluvial deposits along the stream <br />channels (as mapped in Figure 2) are believed to be generally coarser <br />grained than other alluvial material in [he area, due to the higher <br />energy environment associated with flow along the stream channels. <br />n This coarser material is therefore more likely to have high permeability <br />'Lri and to be carrying seepage from the pond. Along the stream channel, <br />under the dam, the alluvial deposit is intersected by the cutoff trench <br />which extends to a depth of IS feet. <br />C The tailings dam was built from materials excavated from the pond <br />area. Approximately 5 fee[ of soil was removed from the entire pond <br />(' area, mostly the silty sand. Based on [his depth of removal, most of the <br />t soils remaining on [he surface in the pond area after excavation are <br />probably clayey sands or silty clays which are of Low permeability. <br />I The geologic cross section along the cutoff trench, provided by Homestake, <br />( maps the alluvial soils and the top of [he Creede Formation on the east <br />abutment. Near the stream channel on [he east bank, the top of the <br />Creede Formation has steep, nearly vertical faces which are overlain by a <br />relatively thin soil layer. The steep rock faces increase the potential <br />that these units may be fractured, and they may have relatively high <br />secondary permeability. <br />Portions of the pond bottom may also be covered with tailings which <br />restrict seepage by sealing the voids or creating an impermeable blanket. <br />The tailings have been discharged from the upstream face of the dam, <br />i;. resulting in a thick layer of tailings upstream of the dam. The north <br />end of the reservoir, which includes a significant amount of the alluvial <br />deposits, does not have a thick tailings layer covering it. Therefore, <br />it appears that the north end of the reservoir is a probable source of <br />seepage. The geologic cross sections developed from the boring logs <br />indicate permeable zones at the surface in this area. These zones <br />may extend under the cutoff trench and could be continuous downstream. <br />These permeable zones (silty sands) are not covered by tailings and could <br />provide an entry point for seepage under the dam. <br />2.2 POND CHARACTERISTICS <br />The tailings pond characteristics were studied in detail to determine <br />C relationships which may be affecting seepage rates. Changes in seepage <br />rates, pond water level, pond area were examined to provide in-sight into <br />the causes of the seepage. <br />Seepage rates, as estimated by Homestake personnel, are presented in <br />Table 1 and Figure 3, The most recent measurements of seepage at the <br />weir downstream of [he dam indicate a flow rate of 60 gallons per minute <br />(gpm). This seepage rate has remained fairly constant during the one and <br />one-half months of monitoring, but appears to indicate a decrease in <br />seepage from earlier this year (see Table 1). The recent value of 60 gpm <br />is consistent with the approximate magnitude of pumping rates of water <br />returned to [he reservoir, which is approximately 180 gpm on an intermit- <br />tent basis. <br />~a-3~1i V~7~1`711C-~ <br />