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TETRA TECH MEMO To: Connie Davis and Mike Refer,Aggregate Industries Cc: Tim Flanagan From: Jeffrey A. Butson, P.E., Gary Linden, P.G. Date: August 24, 2016 Subject: Hazeltine Mine—Orr Property Drain and Reservoir Outlet Pipes, Tetra Tech Project No. 133-23514-16002 This memorandum documents the design basis of the Orr water drain and the reservoir outlet at the Hazeltine Mine. This system will be constructed to address an Order from the Colorado Mined Land Reclamation Board. The purpose of the drain is to mitigate elevated groundwater resulting from the slurry wall surrounding the Hazeltine Mine on the Orr Property. The origin of the high water conditions on the Orr Property is also influenced by surrounding land and water uses which are not associated with Aggregate Industries' operations. To our knowledge, no site specific studies have been performed to evaluate the groundwater conditions of the Orr Property, or to characterize the flows and sources of water(rainfall, turnouts from the Fulton Ditch and groundwater) in the Bull Seep. No studies regarding riverine flooding have been performed for this area. Aggregate Industries proposes to construct a permanent drain to reduce groundwater levels on the Orr Property by allowing flow into the Hazeltine Mine.The proposed system will be a gravity system and is not intended to convey all water in the Bull Seep. Some water must remain in the Bull Seep to maintain pre-existing jurisdictional wetlands as well as compensatory wetlands as required by the US Army Corps of Engineers permit.The compensatory wetlands were constructed when the Bull Seep was relocated, to facilitate mining in accordance with the Corps 404 permit which was incorporated into the mining and reclamation permit approved by the Division of Reclamation, Mining and Safety. The amount of water necessary to maintain the wetlands and drain the Orr Property is not known at this time. The drain system will include stop logs to regulate water levels and flows into the mine pit in an effort to accomplish these competing objectives. Orr Drain System Design The drain, in conjunction with the existing Bull Seep, is intended to convey surface flows that are sourced from precipitation,turnouts from the Fulton Ditch, and groundwater seepage. Prior to mining and construction of the associated slurry wall, the flows were discharged downstream through the original Bull Seep. Subsequently the Bull Seep was relocated by Aggregate Industries and other mining entities. During mining,Aggregate Industries initially installed a temporary Agridrain system that drained water flows directly into the Hazeltine Mine pit to relieve high water conditions on the Orr Property. Subsequently two temporary 18-inch pipes were placed through the slurry wall at elevation 5,038 to further improve drainage. One of these pipes became obstructed. Based on field observation, the surface water was drained by at least 2 feet. The permanent drain system designed herein will replace both the temporary Agridrain system and the temporary pipe system. The final use of the gravel pit will be a water storage reservoir owned and operated by the City of Thornton. The normal high water surface elevation is planned to be at elevation 5036.0. To reduce flooding on the Orr Property during normal reservoir operating conditions,the drain must have an invert elevation greater than that of the reservoir normal high water surface elevation. I Tetra Tech