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3-9 <br />Pump -back rates over the year prior to construction, from July of 1981 to July <br />i <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 flow rates (from measure- <br />ments of the old weir) with pump -back rates over the year indicate that the <br />surface flow rates have ranged from 51 to 64 percent of the average pump -back <br />rates for the same periods, or that the subsurface flow 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 in <br />the relative amount of subsurface flow which is being pumped back to the lower <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 flow 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 piezometers <br />downstream from the slurry wall are presented in Table 2. After the slurry <br />wall was constructed (July 16, 1982), and before January of 1983, these <br />piezometers did not show substantial changes in cyanide concentrations. In <br />January of 1983, all of the piezometers showed very low cyanide concentra- <br />N <br />W <br />cc <br />C7 <br />LL <br />