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>~ CIVIL RES~URCES,LLC ENGINEERS $ PLANNERS March 9, 2007 ~ C~ ,/ Mr. Carl Mount Division of Reclamation ,Mines, and Safety 1313 Sherman Street, Room 215 Denver. Colorado 80203 RE: SW Chambers, LLC, Permit No. M-2003-016 - Heit Pit Resource; Underdrain Design Memo to Prevent Groundwater "Mounding" , Dear Mr. Mount: c~ In accordance with Permit M-2003-016, SW Chambers, LLC, is submitting the following:l) $188 application fee; 2) Technical Revision #3 (TR-3) Underdrain Design Memo; and 3) construction drawings for the proposed Heit Pit underdrain. This memo is based on prior work and submittals by Wright Water Engineers (WWE) and requirements per the DBMS: 1. Groundwater Evaluation For Proposed Heit Aggregate Operation, Weld County, Colorado, Prevention Plah, dated March 2003- (WWE) 2. Groundwater Monitoring for Heit Aggregate Resource Operation, Weld County, Aquatic and Wetland Nursery and Coyle Properties, dated July 2003. (WWE) 3. Heit Pit Groundwater Monitoring Data, March 2004 through January 2007. 4. Monitoring Well boring Logs from March 2, 3, and 11, 2004. (WWE) 5. Request For Technical Revision for Mine Permit #M2003-016, SW Chambers, LLC, dated September 14, 2005. (WWE). Slurry Wall -Groundwater Upslope Parameters The Heit Pit area prevailing groundwater flow direction isnorth-northeast toward the South Platte River. Locally the groundwater levels and flow directions are also impacted by;1)the groundwater "shadow effect" from the slurry walled Koenig Pit immediately to the south of Weld County Road (WCR) 20 and the proposed Heit Pit, and 2) the Little Dry Creek drainage which roughly parallels the west side of the Heit Pit at distances ranging from 230 feet to 450 feet to the low flow channel. The slurry wall around the Heit Pit could cause "mounding" of groundwater on the upstream side of the proposed slurry wall based on modeling completed as part of TR-2, dated September 14, 2005, by Wright Water Engineers, as stated below. "A potential impact to groundwater will have occurred when post-mining groundwater monitoring data show: l) a change in the average (growing ornon-growing season) groundwater by more than 2-feet, or 2) a change in three or more consecutive readings by more than 3-feet compared to data taken the same time of year prior to mining " Per TR-2, an uderdrain system will eliminate upstream mounding. The underdrain design installs a perforated pipe within the ovedapping zones of the permitted two foot rise of average groundwater elevation for the growing and non-growing seasons. The capacity of each underdrain was sized to convey the -„~ r-u,~ ~ ^ 2aa~ ~ Division n; p".) . Minin ° pion, 9 `n~ Seteiy 323 FlFrri STREET • P.O. Box 6B^ • FREDERICK, CO 8O53^ • Pr-lor.le (303) 833-1 41 6 • Fiix (303) 833-285^