2006-12-19_REVISION - M2001107 - Laserfiche WebLink

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J&T Consulting, Inc. ~ ~~~ December 18, 2006 ' ,,R~rECEIVED Erica Crosby '~ U[~ ~ 9106 Environmental Protection Specialist State of Colorado ivision of Reclamation, Division of Reclamation, Mining, & Safety ;~ Mining and Safety 1313 Sherman Street -Room 215 Denver, CO 80203 t~ ~ RE: NCCI Pit #1, Technical Re ision No. 3 Adequacy Review Response File No. M-2001-107 Dear Ms. Crosby, C/~' ~ ~'i NCCI offers the following response to the six items that the Division requires be addressed for the NCCI Pit #1 Technical Revision No. 3 request to be approved: 1. In accordance with Rule ~4.7(2)(b). the Division requests that NCCI identify and characterize the aquifer to be mined in or through. The Division also requests that the applicant define or predict the cone of depression expected for any areas to be dewatered and dry mined during the life of the operation. The cone of depression assessment should deftne, at a minimum the horizontal and vertical extent of drawdown expected. Ij during the assessment, the applicant determines that a drawdown will occur offsi[e that may impact a current groundwater user, the applicant should explain any mitigation measures to be implemented and trigger points that would put mitigation measures into effecx The mining activities will extract material from the South Platte aquifer. This is an unconfined alluvial aquifer which on this site consists mainly of fine to coarse sand with some fine to coarse gravel intermixed throughout. The thickness of the alluvium is approximately 30 feet and it is bounded on the top by one to five feet of relatively impervious overburden and bounded on the bottom by relatively impervious claystone bedrock. The historic groundwater depth was assumed to be three to five feet based on water level measurements done during the geotechnical investigations of the site and from limited data gathered from monitoring wells before the dewatering activities at the mine started. Boring logs from the geotechnical investigations have been included in the slope stability report that is attached for your reference. The monitoring welt data and a map showing the monitoring well locations have been attached for your reference. The methodology presented in "Groundwater and Wells -Second Edition" by Fletcher G. Driscoll, for wells in an unconfined aquifer that are pumped at a constant rate and have reached equilibrium conditions, was used to estimate the limits of the cone of depression/radius of influence (Equation 9.1 ). Attached are excerpts of this reference that give the details of the methodology used and a figure showing the typical hydraulic conductivity values for soils of this type in an unconfined aquifer (Figure 5.14). 1400 W 122'" Avenue- Suite 120 • Westminster CO 80234 • Ph: 30357-0735 • Fax: 303-920-0343