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I 4-6 Average pump -back rates for the past year are plotted at the bottom of Figure 4. The rates, averaged generally over a month's time, show variation from 71 to 119 gpm prior to dam grouting. In the two months following slurry wall and grouting completion, the average pump -back rate was approximately 90 to 95 gpm. Pump -back rates increased in late November and December, 1982, to as high as 127 gpm; however, these elevated rates are believed to be largely due to significant snowmelt rather tha increased seepage rates from the pond. Now that the pump -back quantities include subsurface flow which is intercepted by the slurry wall cutoff, the pump -back rates before and after grouting activi- ties are not readily compared. However, it is encouraging that the pump -back rate with the slurry wall in place lies within the range which existed before slurry wall construction. Prior to construction, it was believed that interception of subsurface flow would substantially increase the pump -back rate. Since this is not the case, a possible explanation is that the dam grouting has been effective in reducing the .seepage rate from the tailings pond. Continuing evaluation of the pump -back rate with time will provide a better understanding of the effectiveness of the grouting. An additional possible benefit of the dam grouting relates to stability of the dam cutoff with respect to piping. The compacted earthen cutoff beneath the dam may be in direct contact with the coarse gravels and cobbles evidenced during grouting. I£ so, the potential could exist for seepage gradients to cause piping of the cutoff trench material into the coarse material, with subsequent adverse impact on the integrity of the dam Thesgrout:.vhich was injected into'-these'cones of coarsematerial-'s h6'filled much -of -the voids and reduced the;potential'�for such piping to occur. D' N