2010-01-21_PERMIT FILE - M2009082 (11)

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f The dewatering system would discharge to the Cache la Poudre River. Dewatering of the mine would lower the groundwater levels to a limited extent in the surrounding alluvial aquifer. Effects on groundwater levels are projected to be limited in extent due to natural and manmade hydrologic and hydrogeologic characteristics and boundaries, principally including the transmissive nature of the alluvial aquifer, the Cache la Poudre River system, and the mining operation and reclamation ponds on the adjacent property. Results of groundwater flow modeling (described below) indicate that the Parson Mine should not have any significant adverse effects on groundwater uses in the adjacent areas (Attachment G-2). Note that the groundwater modeling evaluation in Attachment G-2 was prepared as part of an earlier submittal to the DRMS that was subsequently withdrawn. It remains presented in its original form but additional interpretations of the modeling results, in response to questions and feedback from the DRMS, are included below as part of this document. Historic Use The site has been managed for agricultural uses (i.e., farming and grazing) and oil and gas production. There are irrigated alfalfa and corn fields and non-irrigated hay and wheat fields within the area of the site and surroundings. There is an adjacent active sand and gravel mine located to the east of the site that has been operated by Hall-Irwin Construction Company since 1996 (Firestein Pit; DMG Permit #M-1996-060). Sand and gravel have also been extracted southeast of the site, where reclamation has included several ponds/reservoirs. Monitoring Well Installation In April 2007, twelve (12) monitoring wells were installed around the perimeter of the site as shown in Figure 2. In September 2009 and in response to feedback from the DRMS, two additional monitoring wells (MW-13 and MW-14) were installed further enhance the ability to monitor potential effects on riparian vegetation along the Cache la Poudre River. The monitoring wells were installed outside the limits of contemplated mining, but within the proposed permit boundary, so that groundwater levels can be monitored during and after mining. The monitoring wells were constructed of 2-inch Schedule 40 PVC casing and screen. Silica sand was placed from approximately two feet above the top of the screen to the bottom of the borehole (bedrock). Above the silica sand, a bentonite seal was placed in the borehole annulus to restrict infiltration of surface water. Each of the monitoring wells was finished at the surface with a locking, aboveground, steel protective casing set in concrete. Additional details of the monitoring well installations are provided in Table 1. Monitoring well permits from the Office of the State Engineer, Colorado Division of Water Resources, are included in this attachment along with the borehole logs and well completion information. The objectives of the well installation and monitoring program are to monitor the current groundwater conditions and to provide a basis for assessing potential effects to groundwater levels during and after the proposed mining. Some of the wells are located to also monitor Lafarge West, Inc. - Parsons Mine 2 Groundwater Monitoring and Mitigation Plan