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3.0 Model Design and Parameters <br />This section describes the model design and parameter selection. Model parameters were <br />selected to reflect site-specific values where available, with additional data based on <br />published information and experience with the shallow aquifers of the Denver Basin <br />alluvial groundwater systems. <br />The model was based on a 26,800 foot by 26,800-foot lateral grid which was defined <br />using 200- by 200-foot grid cells in the Parsons mining area, expanded to 400- by 400- <br />foot grid cells in perimeter areas (Figure 2). The model was developed using one layer <br />representing the unconsolidated alluvium, with the base of the layer defined as the top of <br />claystone bedrock. The base elevation of the alluvial layer/top of bedrock was generated <br />based on regional bedrock elevation data (Shelton, 1975) and from lithologic data <br />obtained from 12 borings drilled on site. The alluvial aquifer thickness was based upon <br />the site borings as well as well records from the SEO for outlying wells. <br />Boundaries Conditions <br />Model hydraulic boundaries (Figure 2) were assigned based on conditions observed at the <br />site and surrounding areas, regional groundwater flow conditions from published maps, <br />and hydrogeologic principles. <br />Constant head boundaries were assigned along the east and west boundary of the model <br />domain (Figure 2) to simulate inflow and outflow from the floodplain alluvium. Constant <br />heads were also established at distance along the terraces north and south of the model <br />mine site area to simulate southerly and northerly flow through the terraces into the <br />floodplain sediments (limited due to low hydraulic conductivity). Constant heads were <br />also assigned to simulate dewatering at two active gravel mine cells east of the Parsons <br />site. Based on site observations and permitted mining plans, it was assumed, for <br />modeling purposes, that dewatering of these areas will cease by the time dewatering and <br />mining of the Parsons Mine site begins. <br />Ditch and River Leakance <br />Jones Ditch Leakance <br />Surface flow in the Jones Ditch was modeled using the MODFLOW river package <br />applied along its flow segment in the model domain (Figure 2). Note that contrary to <br />EnviroGroup Limited 4