GENERAL41075 - Laserfiche WebLink

Home Browse Search

VIII. Alluvial Valley Floors (2.06.8, 4. Description of Existing Environment The only alluvial valley floor (AVF) which exists in the mine and adjacent area is Hay Gulch (see below for a more detailed discussion of the AVF). An aerial photograph survey delineated several small deposits of unconsolidated stream- laid materials in other adjacent drainages. However, these areas are considered too small for agricultural development. The valley bottom landform in which Hay Gulch exists was determined to be an AVF because: 1) the valley contains unconsolidated stream-laid deposits, and 2) there is sufficient water available for agricultural development as evidenced by the existence of flood irrigation practices and subirrigation. An irrigation ditch exists in the drainage and is actively used for flood irri- gation purposes adjacent to the mine. The applicant determined that depths to the water table range from 1 to 3 feet. This is a shallow enough depth to sub- irrigate the native hay crop being grown on the AVF. For a more detailed discussion of the Hay Gulch alluvial aquifer, see L', Ground Water Hydrology, of this document. The extent of the AVF is delineated on the Regional Geology map supplied in the application (excluding the tributary drainages). Land use on the AVF is primarily pasture land with occasional hay production. The essential hydrologic functions that supply water to the AVF are both flood irrigation and subirrigation. The diversion structures for the flood irrigation activities occur upstream from the proposed permit area. The applicant has not presented any information regarding the significance of the AVF to the individual farming units. Therefore, the AVF must be considered significant for purposes of this findings document. Description of Applicant's Proposal The applicant does not propose any additional disturbance of the AVF. To date, the mine has constructed a haul/accres road across the AVF and installed a water-supply well into the alluvial aquifer. In addition, no mining is proposed beneath the AVF. Several potential impacts to the AVF are possible, as discussed below. Discussion of Potential Impacts The Hay Gulch AVF has the potential to be impacted due to proposed mining operations in two ways. First, leachate from the mine bench could infiltrate into the alluvial aquifer and degrade the quality of the water. Second, pumping of the mine's water-supply well could lower water levels such that the sub- irrigation functions of the AVF would be affected. These potential impacts are discussed in greater detail in V, Ground Water Hydrology, of this document. Neither of these impacts are considered significant enough to warrant changes in the permit application. Additionally, the monitoring program stipulated in V, Ground water Hydrology, of this document is adequate to ensure that continued mining operations will not impact the AVF. Therefore, the Division finds that: