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species grown, and this excess leaches the soil profile, thus making the assumed relationship <br />between soil salinity and irrigation water salinity a worst case assumption. The applicant <br />proposes that no decrease in production of any of the species currently grown on the Trout <br />Creek AVF would be expected unless the conductivity of the irrigation water were to exceed <br />1 mmhos/cm. Based on species composition data submitted, the applicant predicts that <br />significant production decreases (>3% for a farm unit) would not occur unless irrigation <br />water salinity were to exceed 2.0 mmhos/cm. <br />Given the factors involved, the Division finds it is appropriate to propose an irrigation water <br />material damage suspect level of 1.5 mmhos/cm for Trout Creek, from the confluence of <br />Middle Creek to the Yampa River. Establishing the suspect level at 1.5 mmhos/cm <br />acknowledges the evidence submitted by the applicant while maintaining a reasonable <br />suspect level established through reseazch and application. <br />As a result of TR-32, the applicant received approval of a plan to dischazge mine water only <br />when the stream has the dilution capacity to maintain waters at or below the discharge points <br />on Fish Creek and Trout Creek to 1.5 mmhos/cm. In summary, projected flood irrigation <br />water quality is 1.230 mmhos/cm and material damage in terms of productivity declines <br />would not be expected unless water salinity were to exceed 1.5 mmhos/cm. <br />Given the minor effect of mine discharge on Trout Creek surface water quality, a specific <br />projection of alluvial ground water impact was not made. Since alluvial rechazge would occur <br />in late spring and early summer when dilution due to snowmelt runoff is at its peak, it follows <br />that alluvial waters (and thus subirrigation) would be less affected by mine dischazge than <br />Trout Creek surface water (and flood irrigation). <br />3. The proposed operation would preserve the essential hydrologic functions of the alluvial <br />valley floor. <br />The essential hydrologic functions of the Trout Creek alluvial valley floor have been <br />identified as the capacity to support subin•igated and flood irrigated agricultural activities. <br />The proposed plan would not result in any surface disturbance or undermining of the AVF. <br />Also, as discussed previously, no material damage would result from changes in the quality <br />of surface and ground water that supply the alluvial valley floor. Thus, the essential <br />hydrologic functions of the alluvial valley floor would not be affected by the proposed <br />operation. <br />Foidel Creek/Middle Creek Confluence Area <br />Map I S of the PAP delineates previously designated alluvial valley floors in the vicinity of <br />the Foidel Creek/Middle Creek confluence, downstream along Middle Creek to the Trout <br />Creek confluence and downstream on Trout Creek to the Yampa River. Map 15 shows a <br />flood irrigated field at the confluence of Foidel and Middle Creek and a number of flood <br />irrigated fields along the Trout Creek AVF downstream from the Middle Creek confluence. <br />37 <br />