Laserfiche WebLink
Physical. Disruption of AVF by Mining <br />Mining operations did not extend into the Trout Creek alluvial valley floor; consequently, <br />the .AVF was not physically disturbed. <br />Dewatering of Trout Creek Alluvium <br />An analysis done by the Division demonstrated that a maximum . drawdown of 1:6 feet <br />could have occurred where Trout Creek alluvium is nearest the West Ridge box cut. Data <br />from the, alluvial wells did not indicate, that drawdown to this degree had occurred; only .a <br />seasonal fluctuation in water, level was apparent. <br />The slight reduction in sub - irrigation potential was predicted in the irrigated cropland <br />near the. West Ridge mining area due to temporary alluvial aquifer drawdown. This <br />reduction did not significantly affect hay production, and drawdown has recovered since <br />mining ceased.. The irrigated croplands on the west bank of the Trout Creek AVF were <br />not affected, since the predicted lowering of the alluvial water table was confined to the <br />east side of Trout Creek. . <br />Spoil Leachate Impacts on AVF <br />Material damage to the AVF would be suspected if the specific conductivity of Trout <br />Creek stream water or alluvial water significantly exceeded 2,300 mmhos /cm during the <br />July- August irrigation season. This material damage suspect level is based on Table 1 of <br />the Division's 1988 guideline, "A Description of the Material Damage Assessment . <br />Process Pertaining to Alluvial Valley Floors", assuming an Orchardgrass crop is grown <br />with calcium sulfate irrigation water. Trout Creek stream and alluvial water, <br />downgradient from the mine have not approached the suspect level. The data indicate this <br />trend can be expected to continue. <br />A major discharge point for Edna's leachate is NPDES discharge outfall 007: Much of <br />this outfall's discharge comes from the Koll irrigation ditch whose flow is largely <br />composed of leachate from Spoil Spring 1. That spoil spring's concentration of total <br />dissolved solids commonly exceeds 3000 mg/1, which equates to specific conductivity <br />greater than the material .damage suspect level for Trout Creek stream and alluvial water;. <br />therefore, the Division has imposed the following permit stipulation: <br />WA_ TER FROM NPDES.POINT.007 WILL NOT BE USED AS SURFACE IRRIGATION <br />WATER UNLESS THE OPERATOR FIRST DEMONSTRATES AND THE DIVISION FINDS <br />THAT IRRIGATION WITH THAT WATER WILL NOT RESULT IN MATERIAL DAMAGE. <br />AVF Findings <br />Based on the preceding discussions of impacts from leachate, physical disruption, and <br />dewatering, the Division finds: <br />8 2006 Annual Reclamation Report <br />C1980 -001 Edna Mine Permit Renewal No. 6 30 <br />