1991-10-07_PERMIT FILE - M1991123 (5)

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Groundwater The project is sited in the valley fill aquifer for Boulder Creek. It is a shallow alluvial deposit approximately 10 to 20 feet deep and consists of sand and gravel ranging in size from 12 inches down to minus 200 sieve material. It is a well graded deposit between the 1-1/2 inches to 200 micron sieve. Normal transmissivities for the aquifer are expected to range between 100, 000 gpd/ft to 180, 000 gpd/ft. This is based on pump test data for wells in the general vicinity. There was no specific data examined for the wells listed in the area. Test holes drilled on the property indicated a free standing watertable at a depth of 2 .5 to 4.5 feet below the ground surface. There are 14 permitted wells located within one mile of the project site. The majority of these wells are located down-gradient from the project site. One well (No. 4 ) is shown as being located within 100 feet of the property. This well is located adjacent to Boulder Creek and with the relocation of Idaho Creek, as is proposed, would also be located adjacent to this stream. There may be some impact from dewatering operations on this particular well. The mining operation will be as hydraulically disconnected from the well as is physically possible. Mitigation of flows from this well may be necessary. The proposed mitigation plan is discussed in a later section of this exhibit. Floodplain Information The mining area lies within the floodplain of Boulder Creek as defined by the Colorado Water Conservancy Board. The high water surface elevation during a 100 year flood varies in elevation across the property from 4904 at the upstream end to 4898 at the downstream end. The Colorado Water Conservancy Board has an unpublished basin study of Boulder Creek that includes aerial photo mapping of the floodplain, HEC-2 data, and hydrologic analysis. This study adopted the following peak discharges at the specified return periods. Return Period Discharge (cfs ) 10-Year 2,000 50-Year 7 ,200 100-Year 11,900 500-Year 31,400 Boulder Creek has a drainage basin area of 440 square miles at the mouth. The streambed slopes range from 0.0035 ft/ft to 0.004 ft/ft. The stream channel is relatively stable in this area although some meander is evident in both Boulder Creek and Idaho Creek. G-2