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However, other Twentymile Sandstone wells, not directly underlain by mine workings, do not show any direct <br />impact from the mining. <br />• Major fault zones are not expected to provide significant conduits for ground water slow through the marine shale <br />in the part of the basin underlain by the proposed mining activities. This is due to the nature of the marine shale <br />and the high overburden stress in this area, which would tend to keep the fault zones closed. Appreciable ground <br />water flows through fault zones has only been observed at the basin margins where overburden stress is relieved. <br />Well BRDH-7 is located approximately one-half mile from the mine workings and has not shown any significant <br />water level decline (Figure 12a). Future mining is expected to have only localized impact on the water levels in <br />the Twentymile Sandstone. <br />The proximity of the Wadge overburden unit to the mined zone and the expectation of subsidence-induced <br />fracturing will result in dewatering of the unit immediately above and adjacent to active mine areas. Drawdown of <br />potentiometric levels in this unit will occur beyond the area of active mining as a result of lateral movement of <br />ground water toward these areas. <br />The extent of potentiometric lowering was investigated using the finite-difference simulation model described in <br />Exhibit 13, Ground Water Model Documentation. The model did not account for mine inflow derived from <br />dewatering of the overburden directly above active mining areas. However, this does not invalidate the prediction <br />of drawdown effects as this is based on the estimates of lateral movement toward mined areas which was correctly <br />assessed. The extent of potentiometric lowering is illustrated by predicted drawdown after 5 years of mining <br />(starting in 1983) and it is shown on Figure 13, Predictive Analysis After 5 Years of Mining. This figure indicate <br />that drawdown effects will extend to the outcrop areas of the overburden unit at the margin of the Twentymile Park <br />Basin. Evaluation of data from observation wells indicates that these predicted effects are probably overestimated. <br />• Recovery of potentiometric levels within the overburden unit will occur following mining through recharge of the <br />unit primarily at the basin margins. Subsidence effects will cause fracturing and rubblization of the overburden <br />unit so that its post-mining permeability will be initially much higher than typical pre-mining values. <br />Consequently, the rate of potentiometric recovery will be primarily govemed by the rate of recharge. <br />Recharge to the mine workings will be primarily from the updip spoils area of the adjacent Eckman Park surface <br />mine. During mining, and for most of the recovery period following mining, the workings and overlying <br />overburden adjacent to these spoils areas will not be saturated i.e. the effective potentiometric head will be at the <br />elevation of the base of the mined zone. The spoil at the old highwalls is expected to be fully saturated under <br />reclaimed conditions as is the case near the Foidel Creek Mine portal area. The effective potentiometric head in <br />the spoils in these areas is therefore near the land surface elevation. The head differential across the unmined <br />interval during and following mining is considerably higher than under pre-mining conditions due to the <br />dewatering of mine zone and overlying overburden unit. This situation induces recharge a[ a proportionately <br />higher rate than under baseline conditions. <br />The rate of recharge to the workings from the spoils at any given area is govemed by the distance between the two <br />areas, the transmissivity of the unmined material in this interval, the head differential across the interval, and the <br />length over which these conditions prevail. The total recharge rate is given by the standard steady state flow <br />equation. <br />4~ 1 - h2L <br />D <br />where: <br />• Q = recharge rate (ft3/day) <br />T = Transmissiviry of unmined interval (fr2/day) <br />hl = potentiometric head in spoils (ft) <br />TR99-32 2.05-139 APp]Z®'~1~~ F[tl 0 0 2000 1/3/00 <br />