2004-05-20_REVISION - M2000158 (2)

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Platte Sand & Gravel LLC P.O. Box 180 Littleton, CO 80160 (303) 996-8071 May 20, 2004 Ms. Kate Pickford Reclamation Specialist Division of Minerals and Geology 1313 Sherman Street RM 215 Denver, CO 80203 Dear Ms. Pickford; RE: Platte Sand and Gravel LLC S&H Mine - M-2000-158 Technical Revision RF^ ~... ,,~D .MAY ~ ~ 2004 DiaiaioyptMine,,, , •var J '~ This Technical Revision is to address some minor changes necessary at this mine involving the method of mining and how the lake area will be left when mining is done. Included is a new bond estimate to cover the changes and installation of the slurry wall. TECHNICAL REVISION #1 In this proposed revision we want to change the mining method from wet to dry mining. Currently the approved plan calls for mining the site as a wet operation using a dredge. Dewatering is permitted on a limited basis to allow installation of the dredge in each lake area. We have found that it will be far more practical to mine the area as a dry operation. To avoid impacting surrounding wells we propose installing a bentonite slurry wall around each lake area prior to mining it. Since the mining area will be sealed, dewatering will not adversely impact surrounding wells. This does two things for our company, it eliminated the need to make up evaporative loss from the lake area surface and reduces mining costs. Within the lake areas the water elevation may increase and decrease because it is a reservoir, so the slopes around each lake will be mined 3:1 from top to bottom to allow for the changing lake elevation. The proposed end use remains unchanged as Recreation for the lakes and wildlife on the areas along the river. When mining is complete the sealed lake area will be filled with water and the above water area will be revegetated using the approved plan. At this time the exact design for the slurry wall is not complete. As soon as it is, we will submit it to the Division for review. The first area to be lined will be the Longhorn Lake area. When the lining of the Sharkey's Lake area is done it will be tied into the south side of the first liner so that each lake is indepen- dent from the other. The bentonite slurry wall will be installed using large trackhoes to dig thru the gravel and 3 feet into the shale that underlies it. This will create a seal that blocks ground- water from entering the lined area. Once the slurry wall is in- stalled, mining will begin and the water trapped in the sealed area will be pumped into the river. GEI Consultants, Inc. prepared the attached engineering analy- sis of the hydrologic balance impacts due to lining the lake areas.