2009-10-16_PERMIT FILE - M2009082 (10)

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4.0 Model Calibration Model "calibration" runs were made to demonstrate that the model could produce heads and flow conditions that represent the conceptual groundwater flow model in the Parsons Mine area. The primary "calibration" checks were comparisons of the modeled heads, gradients, flow directions, and saturated thickness of the alluvial aquifer to those observed in the Parsons site area and represented in the conceptual model. The potentiometric surface used for model calibration was developed from water level data observed in on-site wells and heads estimated from ground surface elevations and depth to groundwater from SEO records on private wells in the region. The calibration simulations were run for a 20 year period, at which time head conditions had stabilized. Constant head boundaries, hydraulic conductivity, and drain conductance values were varied, as necessary, to approximate the calibration targets. Calibration targets and initial steady state model results are summarized in Appendix D. The initial heads generated from the calibrated MODFLOW model (Figure 3) represent the conceptual model. The model considers water level data observed in on-site wells and incorporates boundary conditions, aerial recharge, river leakance, drain flow, and dewatering of gravel mine areas downgradient of the Parsons Mine area. The heads simulated during model calibration (pre-dewatering of Parsons Mine) were used as the starting heads to simulate dewatering of the Parsons Mine, estimate the extent and magnitude of drawdown from dewatering, and to estimate the impacts of proposed post- mining reclamation activities. Although the model is idealized, consistent with the amount of available information, the simulated initial heads, site gradients, and groundwater flow directions match the known general flow patterns and elevations sufficiently for the purpose of the model. The simulated heads in mine-site monitoring wells were within about'/2 to 2%2 feet of the "measured" heads at the piezometers with the exception of wells MW-4, MW-7, and MW-12. The simulated heads at these observation points were approximately four to five feet different than the "measured head", but it was felt that this was adequate given the idealized nature of the model, limited site and regional data, and the unknowns regarding how and where the hydraulic conductivity transitions between alluvial gravels and floodplain clays and silts. Additionally, unknown shallow perched zones of groundwater may exist at the site producing anomalous variations in the apparent groundwater EnviroGroup Limited 7