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-4- <br />movement of the slide on a quarterly basis in accord with their report. Based <br />on the technical evacuation, Homestake believes that the slide area will have <br />closely approximated its final configuration when the movement is determined <br />to be less than 6 to 12 inches per year and that equilibrium conditions will <br />have been achieved at this time. It is believed that not only will revegetation <br />become established at this lower rate but also that this rate often occurs <br />under natural conditions of high relief, moisture, and geologic structure and <br />is not generally detrimental to vegetation or water quality. <br />Item: The potential effect of consolidation settlement on overburden dump <br />reclamation. <br />Response: The backfilled and graded dump areas are designed to accommodate <br />consolidation settlement to the extent possible. The important facet of <br />the dumps, in terms of maintaining erosional stability, are the terraces, <br />where proper drainage is necessary. Care will be taken to account for <br />detrimental consolidation settlement along these terraces. Experience to <br />date does not indicate that differential settlement is a problem. <br />In a number of places in Dr. Pendleton's memorandum, the effect of <br />settlement on flow attentuation areas was mentioned. The gently sloping <br />flow attentuation areas will require regrading during final reclamation <br />and will also be designed to accommodate settlement to the extent possible. <br />However, the flow attentuation areas are not an essential aspect of the <br />drainage plan, and, as shown in the design drawings of Supplemental Report <br />S-2, have not been included in the flow calculations. Therefore, consolidation <br />Settlement would not compromise the intended drainage and erosion control <br />design of the various overburden dumps. further, the flow attentuation <br />areas are located either in areas of relatively shallow fill, where <br />consolidation would be Tess than in other areas of thicker fili, or in <br />areas of no fill. <br />Item: Placement of low-grade ore disposal structures and effect of settlement. <br />Dr. Pendleton notes the following concerns regarding low-grade ore storage/ <br />disposal structures. <br />Adjustment of clay cover to potential consolidation settlement. <br />Geomorphological stability of the two mounded low grade piles <br />superimposed on the top of the overburden dumps. <br />Response: With regard to the integrity of the clay cover, the clay cover <br />does not affect the projections of leachate quality since it was not taken <br />into account in these calculations. The extremely low leachate concentrations <br />were calculated assuming the absence of the clay cover (as a worst case). <br />Adjustment of the clay cover to consolidation settlement would not result in <br />higher leachate concentrations than those calculated assuming no clay cover. <br />In fact, the presence of the clay cover, even if affected by consolidation <br />settlement, would reduce the volume of leachate resulting from a precipitation- <br />infiltration event. <br />Radon flux calculations indicate that the six inch clay cap, combined with <br />the two feet of soil cover, would reduce the radpn flux to 0.57 pCi/m2-sec., <br />which is well below the EPA criteria of 20 pCi/m -sec. Since there was little <br />difference between the assumed porosities of the clay and soil (30 and 35 <br />percent respectively), the thickness of the material rather than its diffusion <br />coefficient was the determinant in reducing flux. If the clay is fractured <br />