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OTETRA TECH MEMO <br />To: Connie Davis and Mike Refer, Aggregate Industries <br />Cc: Tim Flanagan <br />From: Jeffrey A. Butson, P.E., Gary Linden, P.G. <br />Date: July 17, 2015 <br />Subject: Hazeltine Mine - Orr Property Drain <br />This memorandum documents the design basis of the Orr water drain at the Hazeltine Mine. This system <br />is being installed to address an Order from the Colorado Mined Land Reclamation Board. The purpose of <br />the drain is to mitigate elevated groundwater resulting from the slurry wall surrounding the Hazeltine Mine <br />on the Orr Property. The origin of the high water conditions on the Orr property is also influenced by <br />surrounding land and water uses which are not associated with Aggregate Industries' operations. To our <br />knowledge, no site specific studies have been performed to evaluate the groundwater conditions of the <br />Orr property or to characterize the flows and sources of water (rainfall, turnouts from the Fulton Ditch and <br />ground water) in the Bull Seep. <br />Aggregate Industries proposes to construct a permanent drain to reduce groundwater levels from the Orr <br />property into the Hazeltine Mine pit. The proposed system will be a gravity system and is not intended to <br />convey all water in the Bull Seep. Some water must remain in the Bull Seep to maintain pre-existing <br />jurisdictional wetlands as well as compensatory wetlands as required by the US Army Corps of Engineers <br />permit. The compensatory wetlands were constructed when Bull Seep was relocated to facilitate mining <br />in accordance with the Corps 404 permit which was incorporated into the mining and reclamation permit <br />approved by the Division of Reclamation, Mining and Safety. The amount of water necessary to maintain <br />the wetlands and drain the Orr property is not known at this time. The drain system will include stop logs <br />to regulate water levels and flows into the mine pit in an effort to accomplish these competing objectives. <br />System Design <br />The drain, in conjunction with the existing Bull Seep, is intended to convey surface flows that are sourced <br />from rainfall, turnouts from the Fulton Ditch, and groundwater seepage. Prior to construction of the mine <br />and associated slurry wall, the flows were discharged downstream through the Bull Seep. There is <br />currently a temporary drain system that drains directly into the Hazeltine Mine pit to relieve high water <br />conditions on the Orr Property. An Agridrain is located north of the Orr property and will remain in place. <br />The final use of the gravel pit will be a water storage reservoir owned and operated by the City of <br />Thornton. The normal high water surface elevation, which is equal to riverside spillway crest elevation, is <br />planned to be at elevation 5038.0. To reduce flooding on the Orr property during normal reservoir <br />operating conditions, the pipe must have an invert elevation greater than that of the reservoir normal high <br />water surface elevation. <br />Tetra Tech <br />1900 S. Sunset Street, Suite I -E, Longmont, CO 80501 <br />Te' 303-772-5282 Fax 303-772-7039 www.tetratech.com <br />