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alluvi;l valley floor is the capacity for flood irrigation, coinciding with [he regional <br />practice. Subirrigation may also occur, but it is considered secondary to flood irrigation. <br />The No. 5 Mine and associated facilities were established on the Williams Fork River <br />valley floor during the early 1970s. Mine entries were developed into the F coal seam, <br />which outcropped above the valley floor. All surface facilities were constructed near the <br />portal entries on the valley floor. The No. 5 and 6 Mines utilize the original portal <br />entries and surface facilities as well as the surface facilities added to the Empire Energy <br />operation during 1981 and 1982. The No. 5 Mine mains extend across the Williams <br />Fork alluvial valley floor. The mains of the No. 6 Mine, which are developed in the <br />underlying; E Seam, will directly underlie the No. 5 Mine mains. Subsidence monitoring <br />to date has shown no impacts to the alluvial valley floor due to the development mining <br />underneath the Williams Fork alluvial valley floor. <br />Technical Revision No. 19 was submitted May 6, 1991, proposing to conduct extraction <br />mining beneath the Williams Fork alluvial valley floor and State Highway 13. The <br />Division approved construction of development mains only and additional subsidence <br />monitoring; along Highway 13. <br />Upon cess,i[ion of mining, the pumps which drain the mine of water will be shut off and <br />the mine workings allowed to fill. In the Probable Hydrologic Consequences section of <br />the permit application, the applicant has projected impacts that could occur to the . <br />Williams E~ork alluvial water quality due to seepage from the flooded mine workings <br />into the Williams Fork alluvium. This scenario assumes that the coal seams are in direct <br />hydrologic communication with the alluvium of the Williams Fork River. Upon <br />flooding of'the mines, a head is expected to occur which would allow recharge of the <br />alluvium by the mine water. The mine water presently pumped from the No. 5 Mine is <br />high in ;odium content. Using Darcy's Law, the applicant has projected a net impact of <br />the seepage into the alluvial valley floor to increase the SAR level from 1.9 to 5.2. <br />Although the SAR is projected to increase significantly, the level is still below that con- <br />sidered hazardous to crop development. The increased 5AR would impact alluvial <br />water in contact with the coal seams, specifically water available through subirrigation. <br />The essential hydrologic function of this AVF is flood irrigation, as supplied by the <br />surface water of the Williams Fork River. As the Williams Fork stream water is not <br />projected to be significantly impacted by the mining operation, the essential function of <br />flood irrigarion will not be impaired. <br />This essential hydrologic function, flood irrigation, will be reestablished upon <br />completion of reclatnation activities. The surface facilities will be removed, topsoil <br />replaced, and the site seeded with the r<~tngeland or the pastureland mixture, where <br />applicable. The pasturelands will then be flood irrigated. The rangeland and marshy <br />areas will not be irrigated. <br />Yampa River Alluvial Valley Floors <br />The Eagle No. 9 Mine has undermined portions of the Big Bottom alluvial valley floor <br />(AVF). The Eagle No. 5 Mine has mined in areas beneath or hydrologically adjacent to <br />36 <br />