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i <br />2-3 <br />Inc. (1975). Additional laboratory testing and analysis were performed <br />by the reviewers. Agreement concerning dam stability was not reached by <br />these two parties and a third party review was conducted by Dames and <br />Moore (1975). Dames 'and Moore drilled additional borings, performed <br />laboratory tests, and performed new slope stability analysis. They <br />P' concluded that the dam exhibited a minimum factor of sa._fety4pf4.,on <br />C� the downstream slope. Minimal tailings liquid was impounded behind the <br />dam at that time. Seepage had, however, been noted in downstream areas, <br />but was not considered in the initial stability analysis. <br />The collection and pump -back system installed by Homestake in 1978 <br />consisted of a small earthen dam across the natural drainage downstream <br />of the dam. The dam was designed to collect surface water or seepage <br />exiting at the surface and running downstream. Water was pumped from <br />the pond below the dam back to the lower tailings pond through a <br />pipeline. The system operated continuously with only minor interrup- <br />tions until it was replaced with a new pump -back system in July 1982. <br />Seepage has been observed in the downstream toe area since some time <br />after the pond began operating in 1975. The first visual estimate of <br />seepage flow rate by Homestake personnel was made in 1978, when 10 to 15 <br />gallons per minute (gpm) was estimated with some seasonal variation. In <br />1980, flow rate was estimated (again visually) to have increased to <br />approximately 45 to 50 gpm. In the spring of 1981, the rate was <br />estimated to be more than 100 gpm. <br />Measurements between July 1981 and July 1982 of the pump -back quantities <br />indicated that the rate varied from 80 to 120 gpm as shown in Figure <br />2. The rate was highest in the months of February and March 1982. The <br />seasonal fluctuations are probably the result of precipitation, infil- <br />tration, and runoff. Natural springs had been observed in the area <br />downstream from the dam toe prior to dam construction, however, the <br />extent or possible contribution by these springs to the pump -back rate <br />is unknown. <br />�,'a � l I I'�' ms`s. <br />