2008-04-15_REVISION - M1980244 (325)

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22 Grassy Valley hydrologic conditions The Grassy Valley hydrogeology and hydrology has been studied extensively. Previous studies include numerous field programs during which a combined total of over 70 borings and 40 monitoring wells were completed. Field programs included: • Pre 2000 - 22 monitoring wells installed, predominantly in upper Grassy Valley bedrock. • 2000 -Seven monitoring wells were installed in 2000 and tested in 2001. Water levels were monitored monthly after installation. • 2001 - 32 probe holes (four lines with eight boreholes each) were drilled averaging 36 feet deep to investigate the shallow hydrogeology in the valley bottom and riparian areas. Four of the borings were converted to shallow piezometers (one per line) to monitor shallow groundwater conditions. • 2006 -Twelve shallow borings were installed in the riparian areas of Grassy Valley to assess shallow (< 5 feet) and seasonal saturated conditions. These piezometers are monitored during the growing season by CC&V. • 2007a -Eight shallow borings and piezometers were installed for the purpose of defining the shallow subsurface conditions in the footprint of the proposed ECOSA (described in Section 4.2). • 2007b -Five bedrock monitoring wells were installed in the northern flank and downgradient valley bottom of Grassy Valley (described in Section 4.2). Data from previous field investigations and ongoing monitoring, together with data from the WMC 2007 field investigation (Sections 4.2 and 4.3) were used in the development of the hydrogeological conceptual model for Grassy Valley which forms the basis of the water balance (Section 4.4) 4.2 2007 Field investigation activities This section summarizes the field program activities that occurred in 2007. New well and piezometer locations installed as part of these activities are shown on Figure 4.1. A description of the supplemental field program including borehole logs and field notes is provided in Attachment B. 4.2.1 Investigation scope and objectives A supplemental field program in Grassy Valley was conducted during July and August 2007. The field program was designed to further characterize the shallow hydrogeology in the ECOSA footprint and the granite bedrock system north and west of Grassy Creek. The data collected were also used to support development of the hydrogeological conceptual model used as the basis of the Grassy Valley water balance (Section 4.4). 4.2.2 Drilling and monitoring well and piezometer installation Drilling and well installation were conducted during two phases. The first phase of the field program was conducted between July 20 and July 25, 2007, during which eight piezometers were installed within the footprint of the proposed ECOSA. The locations of 2736 Cripple Creek & Victor Gold Mining Company Water Management Consultants