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<br /> <br />measured at one foot from July 1983 to October 1990. Measurements taken <br />downstream of the Moffat pits over the same period show a drawdown of less <br />than one foot. <br />The sub-irrigation potential may be slightly reduced in the irrigated cropland <br />near the West Ridge mining area due to temporary alluvial aquifer drawdown. <br />This reduction is not expected to significantly affect hay production. The <br />irrigated croplands on the western bank of the Trout Creek AVF would probably <br />not be affected, since the potential lowering of the alluvial water table is <br />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 <br />and sub-irrigation from alluvial ground water need to be considered. The <br />operator has prepared an estimate of changes in the alluvial water quality <br />resulting from the percolation of water through the spoil material and into <br />the alluvium (Section 2.5.3 of the permit application). The maximum TDS <br />concentration of 609 mg/1 in Trout Creek, is predicted to occur in April, <br />prior to the irrigation season. <br />Research conducted by Maas and Hoffman (1977) determined salinity threshold <br />levels and response rates for selected agricultural species, and grouped those <br />species as well as a number of additional crops into four categories based on <br />relative tolerance to salinity (sensitive, moderately sensitive, moderately <br />tolerant and tolerant). <br />The maximum TDS concentration predicted to occur in Trout Creek (607 mg/1> was <br />used in the analysis although it occurs prior to the irrigation season. This <br />value corresponds to an electroconductivity (EC> of 0.8 mmhos/cm, using an <br />equation developed from data specific to the Edna Mine (pg. 2.5-95>. <br />USDA Handbook No. 60 (Richards, 1969) reports that a factor of 1.0 to 1.5 is a <br />conservative estimate to determine the relationship between root zone soil <br />salinity and the salinity of irrigation water applied. Assuming a maximum <br />root zone soil salinity of 1.2 mmhos/cm would result from flood irrigation <br />water diverted from Trout Creek below the Edna Mine, no significant decrease <br />in crop production would occur based on the above criteria. No decrease in <br />production of any of the species currently grown on the Trout Creek AVF would <br />be expected unless the conductivity of the irrigation water were to exceed 1 <br />mmhos/cm. Based on species composition data submitted, significant production <br />decreases (3% for a farm unit) would not occur unless irrigation water <br />salinity were to exceed 1.5 mmhos/cm. <br />As previously mentioned, Division concerns with monitoring data submitted by <br />the applicant have not yet been addressed. Therefore, projections of impacts <br />to irrigation water are not included in this document. Upon compliance with <br />Stipulation No. 32, impacts to irrigation water can better be determined. <br />Given the small effect of mine discharge on Trout Creek surface water quality, <br />a specific projection of alluvial ground water impact was not made. Since <br />alluvial recharge would occur in late spring and early summer when dilution <br />due to snowmelt runoff is at its peak, it follows that alluvial waters (and <br />thus sub-irrigation) would be less affected by mine discharge than Trout Creek <br />surface water. <br />-32- <br />