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<br />-4- <br />The third aouifer studied was the Laramie overburden. It is located immediately <br />above the coal seam and ranges 20-60 feet below the ground surface, Zt <br />consists of 0-280 feet of interbedded shales and sandstones. Overburden around <br />water f1ow5 to the northeast and discharges into Ennis Draw, Recovery tests <br />indicate transmissivities of 1.1x10-2 and 1.8 x 10-3 ft2/min. Discharge from <br />this aquifer to Ennis Draw was es tinated to be 5.4 acre-feet/year from the <br />mine site. Observation well data indicate that this acuifer is locally confined. <br />The water quality data in Table 3 of Appendix I-1 indicates that the overburden <br />aquifer would be unsuitable for many irrigation uses. Total dissolved solids <br />concentrations are above 7,000 mg/1 wieth 5odi'um ranginc from 1400 to 1500 mg/1 <br />and Sulfate ranging from 3500 to 3700 mg/1. <br />The final aquifer considered is Ennis Draw located on the east edge of the permit <br />area. Ennis Draw is a relatively broad, flat, topographic low that is covered <br />by wind blown sands. There is no stream channel nor is there any evidence of <br />present or past surface water flow. Flow in this draw is entirely sub=urface <br />to the north with an eventual discharge to Box Elder Creek, four miles north <br />of the permit area, Dispite the discharge of poor overburden water into Ennis <br />Draw, the water quality data indicates that Ennis Draw is not being significantly. <br />affected by the overburden aquifer. This can be attributed to dilution. Ennis <br />Draw exhibits good water quality with TLIS concentrations E times lower than the <br />overburden aquifer (1200-1700 mg/1). <br />i <br /> <br />Impact Discussion: <br />Rs a result of mining, inflows from the overburden aquifer to the pit can be <br />expected to occur. Worst-case estimates of pit inflow show that the rates <br />peak at two times. The first peak inflow rate of 110 gpm occurs when the trench <br />in F,it A reaches a length of 3000 feet. F.fter this, the rate will decline <br />until Pit B is opened when inflows will again increase to 130 gpm.- After the <br />second peak, inflow rates will decline under the influence of progressively <br />smaller hydraulic gradients. Any water collected in the pit will be pumped to <br />a pond east of the pit area. The ponds have been designed to handle the maximur.. <br />expected pumping rates. Normally, this water would seep through the sandy pond <br />bottom and recharge Ennis Draw. Althouch this poor cuality pit water would be <br />somewhat diluted by Ennis Draw, the applicant is proposing to minimize this <br />impact by lining the ponds with clay. The ponds will be dewatered by evapora- <br />tion and operation use such as dust control. This reduction in recharge to <br />Ennis Draw will be short-term, insignificant, and somewhat balanced by the use <br />of irrigation, water on reclaimed sites. <br />The drawdown of the piezometric surface due to pit inflow was also assessed. <br />Worst-case calculations shown in Appendix I indicate that drawdown in the <br />Laramie formation will extend ID50 feet (0.2 miles) adjacent to the pit. This <br />relatively small extent of the influence of the pit is due mainly to the very <br />low hydraulic conductivity of the Laramie overburden, causing the.drawdown <br />profile to be very steep in the vicinity of the pit. klthough no cuantitative <br />measures exist to predict recovery time, the applicant predicts that water <br />levels will recover substantially within three years following the termination <br />of mining. <br />The applicant's predictions of pit inflow rates and drawdown assume that the pi <br />does not intercept Ennis Draw. Zf Ennis Draw is intercepted by .the pit, the <br />applicant proposes to: I) Construct a compacted shale cutoff trench on the east <br />and south edges of the trench area that intercepts the old channel of Ennis <br />