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Groundwater Evaluation For Proposed Heit Aggregate Operation <br />Weld County, Colorado <br />3.0 PROPOSED MINE PLAN <br />It is proposed to mine the approximately 80 acres of the property that is the agricultural field. <br />The limit of proposed mining is shown on Figure 1. Prior to mining, a slurry wall will be <br />constructed to bedrock around the mine area. The slurry wall will limit inflow into the pit during <br />mining, but some pumping will be needed to keep the excavation dry. It is assumed that the <br />water pumped from the pit will be discharged off the site. The lined vessel will be used for water <br />storage at the cessation of mining. <br />In addition, several acres in the northwest corner of the property may be mined. If this is done, <br />no slurry wall or lining will occur at the excavation. <br />4.0 STUDY METHODS <br />A two-dimensional, steady state, finite-difference model of the study area was developed using <br />the U.S. Geological Survey's (USGS) MODFLOW-2000 program to represent the groundwater <br />flow through the shallow unconfined alluvial aquifer in the vicinity of the proposed gravel pit. <br />The goal of the modeling was to determine the potential impacts to the groundwater flow system <br />from construction of the slurry walls surrounding the pit. There is little information available <br />regarding the existing groundwater flow system, and the resulting model should be considered to <br />be at a conceptual level only. Further information is needed to develop a more refined model of <br />the system, as discussed below. <br />4.1 Model Setup <br />The horizontal extent of the model was determined by the definition of boundary conditions <br />using the data available from the USGS (Hun, et al 1972). This report shows the extent and <br />thickness of the alluvial material and the elevation of the water table in 1969. While it is <br />recognized that the water table of the alluvium varies in response to climatic conditions, the data <br />in Hurr, et al (1972) was considered appropriate for this level of study. <br />021-110.030 Wright Water Engineers, Inc. Page 3 <br />March 2003 <br />