2008-03-05_REVISION - M1997054 (24)

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Solving for the drawdown at a distance, rz, away from the pumping well as it relates to the total saturated thickness, H, the drawdown, S is equal to: T2 S=H - FT-In r +h1 1 t 1 1 1 fl t This is the basic equation used in the development of the analytical model for the Parkdale Sand and Gravel Pit. MODEL DEVELOPMENT - ASSUMPTIONS AND CONCEPTUALIZATION The use of the equation above necessarily requires certain assumptions to remain valid. These assumptions include: • The well completely penetrates the aquifer; • The aquifer is homogeneous and isotropic; • The pumping rate is steady; • Flow towards the well is radial and uniform; • Initially the piezometric surface is horizontal, and • Water is released from the aquifer in immediate response to a drop in the piezometic surface. Alone, the equation does not take into account the effects of boundary conditions such as the existence of a constant head source of water, such as a river, in the vicinity of the well. This type of boundary condition is often simulated by the introduction of 'image wells' into the analysis. By placing an image well opposite the river and treating it as a recharge well it simulates the boundary condition that the drawdown at the river is negligible. Conceptualization of the site as a single pumping well, however, is not appropriate because the drawdown will occur throughout the pit. To simulate this, the site was conceptualized as a series of three pumping wells. To simulate the effects of the Arkansas River, pumping wells were mirrored onto the south side of the river to effectuate the necessary boundary condition that the drawdown at the river be negligible (i.e., constant head boundary condition). Figure-3 of Appendix E shows the conceptualization of the model. Note that the conceptualization of the model does not account for the no-flow boundary condition of bedrock outcroppings that form the horizontal limits along the east and west of the outwash aquifer. Therefore, the results discussed below extend beyond the limits of the aquifer, but in reality the effects are necessarily confined to the extent of the aquifer. Tables 2 and 3 of Appendix D display summary results from the analytical model, and Figures 3, 4 and 5 of Appendix E show the approximate extent and/or the estimated drawdown contours of the alluvial aquifer, based upon geologic conditions defined in the geologic maps provided in Appendix F. Parkdale Aggregate Mine I Parkdale Mine Analytical Ground Water Model