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predictive simulation and conclusions. The groundwater modeling was conducted in general <br />conformance with ASTM Standards for groundwater modeling. <br />CONCEPTUAL MODEL <br />The conceptual model of the aquifer system analyzed for this investigation consists of one layer; <br />the unconfined alluvial aquifer of the Arkansas River. Hydrologically the aquifer is bounded on <br />the sides and bottom by bedrock consisting of low permeability Pierre Shale. These boundaries <br />act as no-flow boundaries because the permeability of the rock is much lower than that of the <br />alluvial aquifer. The aquifer is comprised of alluvial deposits,which consist primarily of well <br />graded to poorly graded gravelly sand with minor mud lenses. The hydraulic conductivity(K) of <br />the aquifer is generally around 500 feet per day(Blue Earth Solutions, 2013). The saturated <br />thickness typically averages about 25 to 30 feet. The principal source of water for the aquifer is <br />the Arkansas River. Other minor sources include precipitation recharge, irrigation recharge and <br />seepage from irrigation ditches. In the project area, the Arkansas River acts as a source because <br />it is a losing stream, that is, the groundwater heads are lower than the stage of the river. Sinks <br />include well pumping, and evapotranspiration. Steady-state conditions are simulated because the <br />maximum water level rise is of principal interest and the time required to reach steady state is not <br />of concern. <br />GROUNDWATER MODELING SOFTWARE <br />The MODFLOW-2005 computer code was used to simulate ground water flow by solving the 3- <br />dimensional groundwater flow equation using a finite-difference method where the model <br />domain is subdivided into a grid of cells, and the hydraulic head is calculated at the centroid of <br />each cell (Harbaugh, et al., 2000). Groundwater flows into and out of the model via constant <br />head, head-dependent flux, and constant flux boundaries. These flows are calculated in the same <br />manner for each simulation. Pre- and post-processing of MODFLOW-2005 files were <br />completed using Groundwater Vistas Version 7.11 Build 5 (Rumbaugh&Rumbaugh, 2017), a <br />graphical design system for MODFLOW. Groundwater Vistas facilitates model construction, <br />data analysis and data presentation. It summarizes results as contours, shaded contours, velocity <br />vectors and detailed mass balance analyses. This section discusses the modeling assumptions, <br />limitations, solution techniques, and the way that they affect the models. <br />When analyzing the groundwater flows in the model, as implemented, MODFLOW-2005 <br />simulates the system as unconfined with isotropic hydraulic conductivity. One limitation is that <br />cells can go"dry" or"flood". If the calculated head is above the top of the aquifer(ground <br />surface) at any model cell,then that cell is flooded and will be treated as if the aquifer is <br />confined(i.e., the saturated thickness will equal the top-elevation minus the bottom-elevation). <br />If the calculated head falls below the bottom of the aquifer, that cell is dry and will be assigned a <br />zero value for hydraulic conductivity. <br />The preconditioned conjugate-gradient(PCG2) solver package of MODFLOW-2005 was used to <br />solve the groundwater flow equations for the model. This package defines the number of outer <br />and inner solver iterations, as well as criteria for both maximum head and residual change <br />between iterations before allowing convergence. Tolerances for the maximum change in head <br />E-2