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Impact Discussion: As a result of mining, inflows from the overburden aquifer into the pit can be expected to occur. Worst-case estimates of pit inflow show maximum rates at two stages in the mine plan. The first peak infl aw of 110 gpm occurs when the trench in Pit A reaches a length of 3,000 feet. After this, the rate will decline until Pit B is opened when inflows will increase to 130 gpm. After the second peak, inflow will decline under the influence of progressively lower hydraulic gradients. All water collected in the pit will be pumped to a pond east of the pit area. The ponds have been designed to handle the maximum expected pumping rates. Normally, this water would seep through the sandy pond bottom and recharge Ennis Draw. Although this poor quality pit water would be somewhat diluted by Ennis Draw, the permittee will minimized this impact by lining the ponds with clay. The ponds will be dewatered by evaporation and by operational use of the water such as for dust control. Thus, reduction in recharge to Ennis Draw will be short-term, insignificant, and somewhat balanced by the application of irrigation water on reclaimed sites. If the quality of the water collected in the pit improves, it may be held in sumps and later used for irriyation. The drawdown of the piezometric surface due to water inflow to the pit was also assessed. Worst-case calculations shown in Appendix I indicate that drawdown in the Laramie formation will extend 1,050 feet (0,2 miles) out from the edge of the active pit. This relatively small extent of the influence of the pit is due mainly to the very low hydraulic conductivity of the Laramie overburden, causing the drawdown profile to be very steep in the vicinity of the pit. Although no quantitative measures exist to predict recovery time, the applicant predicts that water levels will recover substantially within three years following the termination of mining. The applicant's predictions of water infl ow to the pit and drawdown assume that the pit does not intercept Ennis Draw. If Ennis Draw is intercepted by the pit, the applicant proposes to: 1) Construct a compacted shale cutoff trench on the east and south edges of the pit ea that intercepts the old channel of Ennis Draw, or 2) Restrict mining activities to those areas that will not substantially influence Ennis Draw. When the application was submitted the probable location of the channel was not known. The Division, therefore, required that the applicant conduct additional studies to delineate the alluvial aquifer of Ennis Draw, The applicant responded by submitting the results of an electrical resistivity study which identified the possible and probable locations of the channel (Appendix F-1). The results show that the pit will approach within 150 feet of Ennis Draw. Although the Division is confident with the findings and with the mitigation proposals, the Division realizes the uncertainty of resistivity studies and that the effects on Ennis Draw from mining so close are hard to predict. In lieu of greater accuracy, the Division required one monitoring well located on the extreme west edge of Ennis Draw adjacent to Pit A. This well would be much more sensitive to drawdown from the pit and mining-related changes in water quality than the existing monitoring wells located farther to the east. -11-