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2.0 Conceptual Site Model <br />A conceptual site model was developed to provide a basis for simulating average <br />groundwater flow conditions in the shallow unconfined aquifer in the floodplain of the <br />Cache La Poudre River and claystone and sandstone in the adjacent bedrock terraces <br />(Figure 1). Groundwater generally flows from west to east in alluvial sands, gravels and <br />clay materials. Groundwater in weathered and unweathered claystone and sandstone <br />terraces north and south of the floodplain is expected to flow generally southeast and <br />northeast, respectively, and into the floodplain. The aquifer receives recharge from <br />precipitation, leakance from irrigation ditches, and seepage from man-made water <br />impoundments (gravel pit lakes, etc.). Alluvial groundwater that is not intercepted by <br />existing gravel pit lakes generally flows toward or along the Cache La Poudre River, <br />depending on groundwater elevations and river stage. Additional aquifer discharge <br />occurs from irrigation wells and from groundwater pumping for other nearby mine <br />dewatering operations. <br />The unconfined shallow aquifer in the area consists of an upper layer of unconsolidated <br />alluvium and a lower layer of weathered bedrock. The thickness of the alluvial sediments <br />and the weathered bedrock surface were interpreted from analysis of 12 boreholes drilled <br />on-site. Aquifer hydraulic parameters were estimated based on similar sediments <br />observed in the Denver Basin and EnviroGroup staff experience with well tests and <br />modeling of groundwater flow in Cache La Poudre and South Platte River alluvium. <br />Groundwater elevations, flow directions, and gradients were assessed from data collected <br />during on-site groundwater monitoring events, USGS topographic maps, and well records <br />from the Colorado State Engineer's Office (SEO). <br />Data for the model design were obtained from site borehole lithologic data (Appendix A) <br />and information on the regional groundwater flow system, and the inferences that can be <br />drawn from the data. For example, groundwater elevations measured in on-site wells and <br />estimated water levels based on topography and groundwater boundary conditions were <br />used to develop a potentiometric surface and estimate the directions and gradients of <br />groundwater flow. Water levels measured in on-site wells are presented in Appendix B, <br />and discussed in Section 4.0. The model also incorporated anticipated recharge <br />conditions and groundwater sinks and sources within the model domain (Section 3.0). <br />Plans for dewatering the Parsons Mine involve placement and operation of perimeter <br />drains to dewater the area of each mine phase. <br />EnviroGroup Limited 2