2008-04-15_REVISION - M1980244 (325)

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Grassy Valley hydrologic conditions 26 The footprint of the proposed ECOSA is located along the south hillside of Grassy Valley. The majority of the footprint is located above the Grassy Valley feeder of the diatreme (Section 2.3), while a small portion on the northwest end is located above granite bedrock. During the 2007 field investigation, eight piezometers were installed within this footprint at the interface between the alluvium and shallow bedrock to further investigate this zone as a potential flow path for infiltrating water draining to Grassy Creek. Six of the eight shallow piezometers were located in alluvium above the diatreme and two piezometers were located in the alluvium above granite. Recent water level data for the piezometers are shown on Figure 4.1 and presented in Table C1 (Attachment C). Water was encountered at approximately 20 to 45 feet below ground surface in two of the six piezometers sited over the diatreme. The remaining four were dry down to the diatreme-alluvial contact. In the two piezometers above the granite, water was encountered at or near the contact with the granite at approximately 42 to 44 feet bgl. This alluvial groundwater above the granite may flow towards Grassy Creek at the alluvium/granite contact. Shallow groundwater levels in Grassy Valley have been monitored in 18 wells and piezometers in the drainage. Monitoring wells with the "B" designation are generally shallow and are screened in the alluvium and the shallow bedrock. Current monitoring well and piezometer locations are shown on Figure 4.1. A list of existing wells and piezometers in Grassy Valley, with their locations, total depths and screened intervals are provided in Attachment D (Table D1). The depths of the shallow wells range from 30 to 105 feet bgl, and the thickness of the alluvial materials (interpreted as the bottom of the screened interval in the shallow wells) ranges from 30 to 70 feet bgl. Hydrographs for alluvial wells have been prepared to evaluate water levels throughout the valley including seasonal fluctuations. These hydrographs are included in Attachment D. Recent water levels in the wells are shown on Figure 4.1 and are compiled on Table D1 (Attachment D). Water levels in some alluvial wells demonstrate only minimal seasonal variation indicating infiltration to the underlying bedrock. The saturated conditions in the alluvium overlying saturated granite in other areas of Grassy Valley (GVMW-9A, -6A and -7A) indicate that the rate of infiltration info the granite is probably low. 4.3.4 Bedrock groundwater The Grassy Valley drainage is underlain by the Grassy Valley feeder of the diatreme and Precambrian granites (Section 2.3). ~ The diatreme feeder may have different hydraulic properties (possibly lower permeability) than the main diatreme, and appears to be compartmentalized as a result of fracturing and geologic structures. Some of these inferred characteristics may also be due to the influence of the underlying and intervening granites. Bedrock groundwater levels in Grassy Valley have been monitored in 22 wells in the drainage. Monitoring wells with the "A" designation are screened in deeper bedrock: Current monitoring well and piezometer locations stations are shown on Figure 4.1. A list of existing wells and piezometers in Grassy Valley, with their locations, total depths and screened intervals are provided in Attachment D (Table D1). Of the wells screened in deep bedrock, 6 wells are located in the Grassy Valley diatreme feeder, and 16 in Precambrian granites. The depths of these wells range from 185 to 825 feet bgl (9,227-10,185 feet amsl), and screened intervals range from 40 to 600 feet bgl (9,489-10,106 feet amsl). Hydrographs of bedrock wells. have been prepared to evaluate water levels throughout the valley as well as seasonal 2736 Cripple Creek & Victor Mining Company Water Management Consultants