HYDRO23317 - Laserfiche WebLink

Home Browse Search

Temporary Substitute Supply Plan -Line Camp, Montezuma County, SEP 2004, Page 11 4. Wafer Management. a. Exposed Water Surface. The amount of water surface to be exposed varies based on the phase of mining, status of pumping, groundwater level, and the time of year." There are three types of open water which can be potentially exposed on the site: (1) water in small depressions or ditches which is generally on the surface for less than 24 hours before it evaporates or inf Itrates, (2) water in (relatively) shallow catch basins which is used for processing and washing, and is pumped into equipment, then flows into sediment ponds before being recirculated, and (3) water in the: open pit where the ground water is exposed. For purposes of calculating evaporation consumption, both types (2) and (3) are considered to be open water, even though the catch basins and sediment basins may not actually be exposing ground water. As mining proceeds and ditches are constructed to improve surface flow, if the ditch exposes ground water (so that there is standing water in the ditch for long periods), the area of the ditch with such standing water will be included in the open water used for calculations. As mining proceeds, the f nal floor of the pit is expected to be above the water table and therefore there would be no open water exposed and causing evaporation losses. However, this plan assumes (for a conservative estimate and plan) that all excavation of greater than 10 feet below the original surface will expose ground water. The mining plan allows for dewatering, and the mine holds a CDPHE permit for discharge of dewatering and process water. It will be determined as mining progresses whether mining will be done "in the water" (using backhoes, buckets, etc. and removing sand and gravel through the water), or dewatering will be used to allow the excavation to be done with front-end loaders on dry ground. If dewatering is used, the amount of open water from which evaporation occurs will be reduced. However, this plan assumes (for a conservative estimate and plan) that dewatering will be limited and that there will always be some ground water exposed. Up to 15 acres of open water may therefore exist at some time during the mining phase. Following mining, the reclamation into three separate ponds will leave 11.09 acres of exposed water surface. This is the maximum planned {and expected) to be exposed, for an extended period. b. Death to Ground Water. The estimated average depth of the pit during mining will be 20 feet, with floor depths ranging from 14 to 23 feet below the original ground level. During the first two and one half years, the estimated average depth of the pit, not including ditches and sumps, will be approximately 10 feet, ranging from 7 to 14 feet below the original ground level. The "worst-case" estimate for depth to ground water is the most shallow expected at the southwest corner of the proposed pit. That ground water level is expected to be approximately 10 feet below the surface in the spring and early summer. During the later summer, fall and winter months, the depth will increase to as much as 20 feet. It is also assumed that as the land (and bedrock) rises to the west and northwest away from the river, the depth to groundwater will increase. This means that the water will be shallower in the north end of the pit, and the bedrock in the north end will be exposed, with no standing exposed water surface. c. Surface Water Management Concept: Temporary structures will be constructed on the floor of the pit and in stripped areas to divert water from active mining areas and to reclamation and long-term stockpile areas, or to a discharge point. During the mining of the lower level, a Swale about one foot deep and with its base about fifteen feet back from the toe of the working face will be established to reduce water at the face of the excavation, if it is decided to dewater and excavate under "dry" conditions. Sumps (as well as sediment and wash basins and ditches) may be dug during the mining of the upper level, when necessary to allow better drainage, both surface and subsurface. The drainage from the active areas will be controlled and treated as required by water quality regulations and permits to ensure that no sediment from the pit is discharged into the Dolores River. The pit floor is below the surrounding land and there will be no discharge under normal conditions from the pit areas during mining, except by drainage through the pipe or by pumping of water through surface-laid pipes to