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c�r�` August 12, 2013 Nick Michael Union Milling Company, LLC P.O. Box 620490 Littleton, CO 80162 nmichael@unionmilling.com Re: Leadville Mill TSF GCL Installation over Groundwater Seeps Dear Mr. Michael: CETCO 2870 Forbs Avenue Hoffman Estates, IL 60192 847.851.1800 800.527.9948 Fax 847.851.1339 www.cetco.com Based on our conversation last week, we understand that the Colorado Division of Reclamation Mining and Safety has expressed concern over groundwater seeps at the Leadville Mine tailings storage facility (TSF) potentially prematurely hydrating the GCL. We also understand that the seepage rate is approximately 8 gpm (gallons per minute), coming from the northeast corner of the TSF base, and that Union Milling has proposed installing a geonet underneath the GCL in this portion of the site to collect the seep. In our experience, placement of a geonet or drainage geocomposite beneath the GCL /Geomembrane composite liner is a reasonable and well - established method for collecting water underneath the liner system. This installation method has been used on multiple past projects. A brief review of the literature shows that several references on this topic are available, including Koerner (2012), Christopher and Zhao (2003), and Verwiel et al (2001). It is important to note that premature hydration of a GCL is only a concern in the short- term, during installation. If GCL is hydrated prior to the placement of at least one foot of cover material, there is potential for the bentonite in the GCL to swell excessively, resulting in reduced bearing capacity and strength. Placing geonet on the subgrade would route water away from the area, prevent prolonged direct water contact with the GCL. Additionally, we understand that at least one foot of cover material will be placed over the GCL/geomembrane liner immediately after liner placement in the hydrated areas. Installing geonet and cover material should alleviate any concerns of premature hydration. After cover material is installed over the liner system, hydration of the GCL is encouraged for bentonite activation. Because the liner system will be covered with soil or tailings soon after installation, the geonet or drainage geocomposite would only need to be sized for the short-term condition, for water that may come in contact with the GCL immediately after installation. There is no need to size a larger drainage system for future potentially higher water volumes, because as discussed above, water contact with GCL after cover placement is beneficial and encouraged. The estimated water flow rate of approximately 8 gpm across the width of the TSF base corresponds