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An analysis done by the Division showed that a maximum drawdown of 1.6 feet could occur where <br />the alluvium was closest to the West Ridge box cut. Data from the alluvial wells did not indicate <br />that drawdown to this degree occurred, only a seasonal fluctuation in water level. <br />The slightreductioninsub-irrigationpotential was predicted in the irrigatedcroplandneaztheWest <br />Ridge mining azea due to temporary alluvial aquifer drawdovvn. This reduction did not significantly <br />affect hay production. The irrigated croplands on the west bank of the Trout Creek AVF were not <br />affected, since the lowering of the alluvial water table was confined to the east side of Trout Creek. <br />Leaching of Dissolved Solids from Spoil Material <br />In considering the impact on AVFs, irrigation water from surface water sources and sub-irrigation <br />from alluvial groundwater must be considered. The operator prepazed an estimate of changes in the <br />alluvial water quality resulting from the percolation of water through the spoil material and into the <br />alluvium (Section 2.5.3 of the permit application). The analysis of TDS impact was done by <br />estimating the concentration of Trout Creek water downstream of the mine in the month of April. <br />(At the elevation of the mine, run-off begins at the mine site several weeks ahead of the onset of <br />heavy runofffrom the upper reaches of Trout Creek. Thus, high-TDS water from the mine entering <br />Trout Creek experiences less dilution in April than occurs in May, June, and July, when low-TDS <br />water in high volumes reaches the mine site from Trout Creek headwaters in the Flattop Range. <br />Later in the summer, when snowmelt run-off reaching the Edna Mine is much lower, the flow of <br />water from mine site is also very low, so lower amounts ofhigh-TDS water enter Trout Creek. The <br />end result of these flow conditions is that the period of greatest TDS influence from the mine on <br />Trout Creek occurs in April.) <br />In the original permit application, a relationship was developed to relate total dissolved salts to <br />electrical conductivity using data specific to the Edna Mine (permit pages 2.5-95b). The equation <br />is <br />TDS=O.3 6 7(EC) EE1.107. <br />This relationshiphas been used in this analysis to calculate potential effects of the mine effluent on <br />the alluvial valley floor. Data aze available from recent yeazs for Trout Creek and for effluent water <br />from the permit area, including both the hydrochemistry and flow volumes. Data used aze from <br />NPDES dischazge point 007, a point that was created by the combination of three individual points <br />002, 003, and 006 during the last permit term. <br />Using these data, the equation and the method described in MLRD directive "A Description of the <br />Material Damage Assessment Process Pertaining to Alluvial Valley Floors, Surface Water, <br />Groundwater and Subsidence at Coal Mines," potential impacts to the AVF were assessed by DMG. <br />An equation from Maas and Hoffman (1977) describes the "relative yield" of a crop as a function <br />of its tolerance to salinity. The equation <br />Edna Mine 27 Permit Renewal No J <br />