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Grassy Valley hydrologic conditions 29 <br />the purposes of this study, deterministic modeling was chosen for both closure and <br />operational scenarios. <br />The Grassy Valley water balance considers the entire upper reach of Grassy Valle from the <br />drainage divides to the north, west, and south down to surface water flow measurement <br />location GV-03 at the eastern edge. Figure 4.1 illustrates the drainage boundary together <br />with the diatreme boundary and the locations of surface water monitoring locations GV-01 <br />through GV-03. <br />4.4.1 Objectives <br />The objective of the water balance modeling was to develop a model, calibrated to current <br />hydrologic and hydrogeologic processes operating in Grassy Valley drainage, which would <br />provide the basis for simulating mining and post-mining conditions. The objective of the <br />mining and post-mining water balance simulations was to evaluate any potential impacts to <br />the existing hydrologic and hydrogeologic systems in Grassy Valley as a result of proposed <br />activities. <br />4.4.2 Conceptual model -current conditions <br />Figure 4.8 provides a conceptual illustration of the current system in Grassy Valley. The <br />valley is underlain by the Grassy Valley feeder of the diatreme and Precambrian granites. <br />These rocks are overlain by alluvium which varies in thickness and composition both laterally <br />and vertically. The thickest alluvium is located in the valley floor. Some of the alluvium has <br />low permeability. This has contributed to the development of riparian areas in the flatter parts <br />of the valley floor. <br />The valley is recharged by direct precipitation in the form of rain and snow. For the water <br />balance conceptual model, precipitation falling onto the valley either: (1) runs off as surface <br />water in riparian areas; (2) evaporates/evapotranspires/sublimates; or (3) infiltrates into the <br />shallow alluvium, where it either reports back to the surface flow regime as interflow or <br />infiltrates into the deep bedrock. Most of the water that infiltrates the alluvium flows towards <br />the valley floor as interflow. Infiltration of precipitation from the alluvium to deep bedrock <br />occurs, but appears to be limited based on -the lower permeability of the granites and the <br />Grassy Valley feeder of the diatreme. Flow paths, rates and volumes within the alluvium are <br />controlled by its highly variable composition. Some interflow ultimately becomes surface flow <br />in Grassy Creek and some exits the drainage as alluvial groundwater flow. Alluvial <br />groundwater flow and surface water flow leave the (modeled) drainage at GV-03. These flow <br />paths and processes are shown on Figure 4.8. <br />4.4.3 Conceptual model -mining conditions <br />The mining conditions water balance model is used to evaluate the potential for short term <br />impacts to Grassy Valley during the mining period. Figure 4.9 illustrates the conceptual <br />model for the mining period. <br />The surface and groundwater flow systems described under current conditions are expected <br />to function in much the same way during mining. The proposed MLE activities which may <br />affect Grassy Valley hydrology include: <br />2736 Cripple Creek & Victor Gold Mining Company <br />Water Management Consultants <br />