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M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-Ot April 22, 2005 <br />Adequacy Memo #2 Page 2 <br />discharges be assessed and monitored? What will be the bases for modifying the <br />discharges? <br />4. Memo Item 5: Since the slope stability analyses conducted to determine mining and <br />stream offsets -minimum distances to be maintained between mining excavations and <br />neighboring structures and streams - used a ''/Z:1 slope angle, the applicant modified the <br />Mining Plan to reflect the same slope angle, instead of indicating that mining may occur <br />up to vertical. Mining at this prescribed angle will be problematic due to equipment <br />limitations and the mechanics of earthmoving. Another concern is that the requirements <br />to mine at prescribed slope angles may not be communicated from the permit <br />requirement, through company management to pit supervision to the equipment <br />operators. Although the Division has in the past approved and will approve a mine plan <br />using a prescribed pit slope angle protective of structures, the applicant must be aware <br />that if this is the compliance mechanism employed, it will be considered a violation of the <br />permit if it is determined that mining has occurred at angles steeper than those <br />prescribed. The applicant should reconsider the selection of a compliance method that <br />requires mining the pit slope at a prescribed angle. <br />If the applicant chooses to stay with this compliance method, they must describe in detail <br />and in terms of the mechanics of earthmoving how these pit slope angles will be <br />maintained in the field. Otherwise, revised setback distances should be provided based <br />on stability under a vertical highwall condition. <br />5. Memo Item 5: The applicant noted that instead of modeling surcharge loads from <br />topsoil/overburden piles, all such piles would be located outside the offset limits. There <br />are a number of oversights on the Exhibit C Mining Plan maps resulting in topsoil and <br />overburden piles outside the mapped affected areas and some still within the offsets. <br />The applicant must correct these discrepancies. <br />6. Memo Item 6: It would expedite the Division's geotechnical review to receive drill hole <br />records in electronic form, if they are already contained in a spreadsheet. If available, <br />and the applicant is amenable to this request, please a-mail the file to <br />kathleen.sullivanCa state.co.us. <br />Memo Item 8: Since much of the site is located within the 100-year flood plain of the <br />South Platte River and a flood analysis and flood control plan have not been submitted, <br />the mining operation must remain a minimum of 400 feet from the top of the riverbank. <br />Both the Exhibit C -Mining Plan maps and the Exhibit F -Reclamation Plan maps must <br />be amended to show the disturbance/affected area and "mining limits" boundaries a <br />minimum of 400 feet from the top of the bank of the river. <br />Rule 6.4.5 EXHIBIT E -Reclamation Plan <br />8. Memo Item 9: The response by the Applegate Group qualitatively addresses anticipated <br />impacts to existing wells only, and not to overall hydrologic balance (including impedance <br />to ground water flow) or other structures such as homes, including potential basement <br />flooding. The response also draws conclusions regarding the impact of the silt ponds <br />based upon the groundwater modeling by AMEC Earth & Environmental, Inc. (AMEC). <br />However, the hydrologic effects of creating silt storage cells were not modeled and <br />quantified. The only mitigation measure offered is to possibly slightly deepen a natural <br />drainage if Well Permit No. 163521 sees a ground water elevation increase "coupled with <br />a physical injury to the well." Please assess the potential impact of the silt storage ponds <br />and commit to specific mitigation measures and trigger points for implementation of the <br />