2010-06-04_GENERAL DOCUMENTS - C1982056

Home Browse Search

Effects of Mining on Identified Alluvial Valley Floors Due to elevated levels of salinity documented in current and future mine discharge water, and to potential inhibitory effects of increased soil and alluvial ground water salinity on flood irrigated and sub-irrigated crops, the potential for material damage caused by salt loading was a concern which had to be evaluated for each of the AVF's identified above. Material damage projections were calculated in both a sub-irrigation and a flood irrigation case analysis, using data obtained during low-flow-of-record years. The sub-irrigation scenario assumed that the stream would recharge the alluvial aquifer in the spring during peak flows. The flood irrigation case utilized the period between June 1 and August 31. Assumptions and equations used in projecting water quality impacts are discussed in the Yampa River CHIA, which is on file at the Division office. Salt loading projections were arrived at by stream flow modeling and analysis of past monitoring results and projected dewatering activities. In a supplemental information package dated September 22, 1986, the applicant submitted information on vegetation cover and production by species for flood irrigated and sub-irrigated croplands on the affected alluvial valley floors. Alluvial bodies, irrigated fields and vegetation sample locations are depicted on supplemental Map No. 1 "Twentymile Park Salt Loading Study Land Use and Hydrologic - Boundaries." This and other information was used by the Division to make the required findings with regard to material damage. Required findings for identified alluvial valley floors on Fish Creek, Trout Creek, and Foidel Creek/Middle Creek confluence are presented separately below. Fish Creek The proposed mining operations will minimally interrupt, discontinue or preclude farming on a portion of the alluvial valley floor. The boundaries of the Fish Creek AVF are as delineated on Map No. 15 of the PAP. The AVF includes all alluvial deposits within and downstream from the permit area as delineated on supplemental Map No. 1. Information discussed below has reaffirmed the Division's original determination that alluvial deposits on Fish Creek meet AVF criteria based on existing flood irrigation, potential for flood irrigation and sub- irrigation. Map 15 shows the following features in the Fish Creek AVF: areas having potential for flood irrigation, currently flood-irrigated fields, division structures, and irrigation ditches. Several of the soils in the AVF exhibit late growing season ground water elevations within six feet of the surface (Exhibit 16, Soil Mapping Unit Descriptions and backhoe pit data provided on Page 2.06-9 of the PAP). Hay crop Foidel Creek Mine 39 June 4, 2010