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CLOSE CONSULTING GROUP Fnvironme nt.1l Sc enc:e& Fn jweenn,a March 17,2016 Mr.Adam DeVoe Lewis Roca.Rothgerber Christie LLP 1200 17th Street,Suite 3000 Denver,CO 80202-5855 RE: Groundwater Monitoring Evaluation for the GCC Pueblo Plant and Quarry Dear Mr.Devoe: Close Consulting Group LLC is pleased to provide this letter report for the above-referenced project. The Colorado Division or Reclamation,Mining and Safety(DRMS)is requesting additional assessment and justification regarding the technical feasibility and scope of groundwater monitoring. The purpose of this report is to review the hydrogeology and past and current groundwater monitoring at the GCC Rio Grande, Inc.Pueblo Plant(GCC)and develop findings,alternatives and recommendations to satisfy regulatory and risk management decisions. SITE DESCRIPTION AND BACKGROUND The attached Figure 1 illustrates the site features and mine plan,and Attachment A provides available local and generalized stratigraphic sections. GCC quarries the Fort Hayes Limestone Member of the Niobrara Formation,and began extraction and processing in 2007 and 2008,respectively. GCC is permitted to quarry the Fort Hayes Limestone and approximately 5 feet into the Codell Sandstone,which has also been described as hard,brown sandy petroliferous limestone and a platy and shaly sandstone(Attachment A). Site drilling logs indicate that the Codell also contains multiple shale or sandy shale lenses. Initially,GCC planned to remove and processed Codell Sandstone,but that was eliminated near the onset of operations when further testing determined the material was not beneficial to process. GCC extracts only the Fort Hayes, leaving the bottom one foot of the limestone. Regionally,the Juana Lopez Shale Member separates the Fort Hayes and Codell members,but is only about 2 feet thick. Locally,the Juana Lopez Member is largely absent due to an erosional unconformity, such that the Fort Hayes directly overlies the Codell(Collum,2000). Underlying the Codell is approximately 400 feet of upper Cretaceous members,principally composed of dense shales,which provides an effective hydraulic barrier from the underlying Dakota Sandstone aquifer. The Fort Hayes and underlying strata strike essentially perpendicular to the mine panels and dip approximately 2.7 degrees to the northeast parallel to the mine panels(Figure 1). The approximate current extent of mining is also shown on Figure 1. During recent mining a fault was encountered near the middle of the Years 6- 15 mine panel. This fault is a normal dip-slip fault,trending southwest to northeast,with approximately 10 or more feet of vertical displacement that currently appears to increase to the northeast.