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3-9 <br />• <br />Pump-back rates over the year prior to construction, from July of 1981 to July <br />of 1982, are presented In Figure 4. The rates, which have been averaged over <br />approximately one month periods, range from a low of 71 gpm in September 1981 <br />(during the dry season) to 119 gpm in March of 1982 and a brief period of 126 <br />gpm in December of 1982. A comparison of surface floe rates (from measure- <br />ments of the old weir) with pump-back rates over the year indicate that the <br />surface floe rates have ranged from 51 to 64 percent of the average pump-back <br />rates for the same periods, or that the subsurface floe rates have ranged from <br />36 to 49 percent of the pump-back rate. <br />Therefore, it appears at this date that there has been a slight increase 1n <br />the relative amount of subsurface floe which is being pumped back to the lover <br />tailings pond. The actual increase is somewhat greater than the above <br />figures, considering that the flow through the new weir partially reflects <br />subsurface flow since it is supplied by the buried drainage pipe near the toe <br />• of the dam. Further observation of weir floe rates and pump-back rates is <br />needed to substantiate the above conclusion. <br />A comparison of total pump-back rates before and after slurry wall construc- <br />tion is inconclusive at this time. Pump-back rates since construction, 88-126 <br />gpm, generally lie within the range of pump-back rates prior to construction, <br />71 to 119 gpm, although 126 gpm is slightly greater than the maximum previous <br />rate. Interpretation of these rates is difficult because the tailings pond <br />level dropped during construction activities (see Figure 4) and because <br />grouting activities may have influenced seepage rates from the pond. This is <br />discussed further in Section 4.3. <br />Measurements of cyanide concentrations in water samples from plezometers <br />downstream from the slurry wall are presented in Table 2. After the slurry <br />wall vas constructed (July 16, 1982), and before January of 1983, these <br />plezometers did not show substantial changes 1n cyanide concentrations. In <br />January of 1983, all of the plezometers shoved very low cyanide concentra- <br />~J <br />,:~: <br />y <br />w <br />J <br />m <br />Q <br />r <br />N <br />w <br />C7 <br />LL <br />