2009-10-30_REVISION - C1982056 - Laserfiche WebLink

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within the typical range reported in USDA Handbook 60. Based on leaching calculations included in the TCC permit application, and on soil sampling conducted on the Fish Creek AVF by the Division, the 1.5:1 relationship (soil salinity/irrigation water salinity) is considered conservative. However, the applicant previously submitted vegetation data for irrigated areas on Trout Creek (Exhibit 42) that demonstrate that moderately sensitive species comprise a very small portion of the total vegetative production on those areas irrigated downstream of the Foidel Creek mine. The applicant also points out that irrigation water is applied in excess of what is necessary for the species grown, and this excess leaches the soil profile, thus making the assumed relationship between soil salinity and irrigation water salinity a worst case assumption. The applicant proposes that no decrease in production of any of the species currently grown on the Trout Creek AVF would be expected unless the conductivity of the irrigation water were to exceed 1 mmhos/cm. Based on species composition data submitted, the applicant predicts that significant production decreases (>3% for a farm unit) would not occur unless irrigation water salinity were to exceed 2.0 mmhos/cm. Given the factors involved, the Division finds it is appropriate to propose an irrigation water material damage suspect level of 1.5 mmhos/cm for Trout Creek, from the confluence of Middle Creek to the Yampa River. Establishing the suspect level at 1.5 mmhos/cm acknowledges the evidence submitted by the applicant while maintaining a reasonable suspect level established through research and application. As a result of TR-32, the applicant has proposed a plan to discharge mine water only when the stream has the dilution capacity to maintain waters at or below the discharge points on Fish Creek and Trout Creek to 1.5 mmhos/cm. In summary, projected flood irrigation water quality is 1.230 mmhos/cm and material damage in terms of productivity declines would not be expected unless water salinity were to exceed 1.5 mmhos/cm. Given the minor effect of mine discharge on Trout Creek surface water quality, a specific projection of alluvial ground water impact was not made. Since alluvial recharge would occur in late spring and early summer when dilution due to snowmelt runoff is at its peak, it follows that alluvial waters (and thus subirrigation) would be less affected by mine discharge than Trout Creek surface water (and flood irrigation). 3. The proposed operation would preserve the essential hydrologic functions of the alluvial valley floor. The essential hydrologic functions of the Trout Creek alluvial valley floor have been identified as the capacity to support subirrigated and flood irrigated agricultural activities. The proposed plan would not result in any surface disturbance or undermining of the Trout Creek AVF. Also, as discussed previously, no material damage would result from changes in the quality of surface and ground water that supply the alluvial valley floor. Thus, the essential hydrologic functions of the alluvial valley floor would not be affected by the proposed operation. Foidel Creek/Middle Creek Confluence Area Map 15 of the life of mine application delineates previously designated alluvial valley floors in the vicinity of the Foidel Creek/Middle Creek confluence, downstream along Middle Creek to the Trout Creek confluence and downstream on Trout Creek to the Yampa River. Map 15 shows a flood irrigated field at the confluence of Foidel and Middle Creek and a number of flood irrigated fields along the Trout Creek AVF downstream from the Middle Creek confluence. In the 1983 Findings for the Foidel Creek Mine, the Division concluded that lower Foidel Creek, lower Middle Creek and lower Trout Creek were alluvial valley floors because of a presumed capability to support flood irrigated and subirrigated agricultural activities. The Trout Creek AVF was addressed 33