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The magnitude of the TDS increase in the overburden aquifer is small. The extent of the • <br />TDS change is limited to a close downgradient proximity to the spoil aquifer because <br />significant additional mixing, dilution, and geochemical changes will occur in a <br />downgradient direction. The duration of the impact proximate to the spoil will be a large <br />number of years. The significance of the impact to the overburden aquifer will be minimal ~ <br />because it is marginal to unsuitable for stock and irrigation use and does not yield the <br />necessary volume of water to make irrigation use feasible. ~, <br />The magnitude of the TDS increase in the Wadge Coal aquifer is large. The extent of the <br />TDS increase is limited to the immediate downgradient proximity to the spoil aquifer <br />because of additional mixing and geochemical changes, and very low hydraulic conductivity <br />values. The impact proximate to the spoil aquifer will occur over a long period of time. <br />The overall significance of the impact to the Wadge Coal aquifer will be minimal because <br />of the limited movement rate, the fact that the undisturbed aquifer is marginal to <br />unsuitable for stock and irrigation use, the TDS level will not exceed 5,000 mg/7, and the <br />aquifer will not yield the necessary volume of water to make irrigation use feasible. <br />Alluvial Aquifers. The premining flow rates for the alluvial aquifers were calculated • <br />using the equation 0 = KIA, Computational assumptions and values used to calculate the <br />alluvial aquifer and spoil aquifer flow rates are presented in Table 17-17. Some spoil <br />will be graded such that it will drain into the Sage Creek drainage. The spoil, 4,93 <br />acres in surface area, will be graded over unmined ground adjacent to the pit and will <br />drain into the 009 pond tributary. In calculating the maximum June through September <br />ground water runoff from the spoil an annual value of six inches was applied because: 1) <br />there is no potential for upward leakance from the underburden as no pit is excavated <br />beneath the spoil and 2) the highest annual spoil recharge measured at the USGS lysi meter <br />sites was six inches. The annual spoil runoff measured below the 004 pond at Seneca II <br />was six inches in 1986. Runoff during the June through September portion of that year was <br />1.9 inches (see Table 17-15). Multiplying the 1.9 inches by the clay percent ratio for <br />Seneca II and II-W yields a value of 1.64 inches, or 0.136 feet of runoff. <br />Alluvial Well CW-52W-22A was completed in the 009 tributary alluvium in late summer, 1990. <br />It was recorded that the alluvium was not saturated in the 009 tributary. It is estimated <br />that the alluvium in 009 tributary is approximately 3,200 feet long and 200 feet wide. <br />The 0.67 acre feet of spoil runoff occurring in June through September would saturate this <br />area of alluvium to a depth of 0.152 feet assuming a porosity of 0.30 for the alluvium. <br />44 <br />