2023-12-26_GENERAL DOCUMENTS - C1981044

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Page 25 of 27 Worthington Ditch provides water to the hayfields. The essential hydrologic function of this alluvial valley floor is the capacity for flood irrigation, coinciding with the regional practice. Subirrigation may also occur,but it is considered secondary to flood irrigation. In the Probable Hydrologic Consequences section of the permit application,the applicant has projected impacts that could occur to the Williams Fork alluvial water quality due to seepage from the flooded mine workings into the Williams Fork alluvium. This scenario assumes that the coal seams are in direct hydrologic communication with the alluvium of the Williams Fork River. Upon flooding of the mines, a head is expected to occur which would allow recharge of the alluvium by the mine water. The mine water from the No. 5 Mine is high in sodium content. The applicant has projected a net impact of the seepage into the alluvial valley floor to increase the SAR level from 3.6 to 9.4. The increased SAR would impact alluvial water in contact with the coal seams, specifically water available through subirrigation. The essential hydrologic function of this AVF is flood irrigation, as supplied by the surface water of the Williams Fork River. As the Williams Fork stream water is not projected to be significantly impacted by the mining operation,the essential function of flood irrigation will not be impaired. This essential hydrologic function, flood irrigation,will be reestablished upon completion of reclamation activities. The surface facilities will be removed,topsoil replaced, and the site seeded with the rangeland or the pastureland mixture,where applicable. The pasturelands will then be flood irrigated. The rangeland and marshy areas will not be irrigated. Yampa River Alluvial Valley Floors The No. 9 Mine has undermined portions of the Big Bottom alluvial valley floor(AVF). The No. 5 Mine has mined in areas beneath or hydrologically adjacent to two alluvial valley floors(AVF's) along the Yampa River: the Big Bottom and the Round Bottom alluvial valley floors. Each of these alluvial valley floors are discussed separately, along with the potential impacts of mining. Yampa River Big Bottom Alluvial Valley Floor The approximate boundary of the Big Bottom AVF is shown on Map 25 of the permit application. Hayfields and pastureland are located in the Big Bottom (AVF) area and are flood irrigated. Monitoring has shown no impacts to the alluvial valley floor due to previous undermining by the No. 5 Mine. The now abandoned No. 9 Mine has development entries under the Big Bottom AVF;however, full extraction mining was not conducted. An AVF exemption for this past mining was requested and approved by the Division and OSM. A copy of the AVF exemption request is presented in Exhibit 4, and the extent of the No. 9 Mine workings is shown on the mine plan map, Map 23. Subsidence monitoring conducted by the operator has shown no effects to the Big Bottom AVF due to past development mining. The essential hydrologic functions of this AVF have not been affected. Williams Fork Mines Prepared by: R ReiRey M.S. GISP C1981044 December 2023