GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 96 I.2.3 Theis drawdown analysis The Theis model supports the Cross section 2 model in predicting excavation drawdown effects on water levels. Where the cross section model was used to predict local drawdown near the excavation, the Theis model was used to predict effects to groundwater levels more distant from the excavation. It was also used to provide a comparative analysis for the SEEP/W modelling approach. The Theis model set up, outputs and sensitivity analysis are presented in this methodology section, with the outputs presented here are incorporated into the assessment results provided below (Section I.3). Theis model set-up This 1-D analytical solution allows only for dewatering from a single point, which is assumed here to be the deepest point of the undercut excavation, under unsteady state conditions. Inherent assumptions of this analysis include that the aquifer is confined, has a seemingly infinite areal extent, is homogeneous, isotropic, and of uniform thickness over the area influenced by dewatering. Prior to dewatering, the piezometric surface is assumed to be horizontal over the area that will be influenced, is pumped at a constant discharge rate, and the excavation penetrates the entire thickness of the aquifer, receiving water only from horizontal flow (i.e. does not consider recharge). Theis model input parameters The following inputs and assumptions were adopted from the information obtained from site data and the conceptual model: – A range of T values were applied to provide sensitivity analysis, with these representing the measured range of permeability values at the site (2, 20 and 200 m2/day), with rounding to account for uncertainty of the thickness of the affected saturated aquifer. – A range of nominal S values (0.1 and 0.01) were also tested to reflect unconfined fractured rock aquifer conditions to semi-confined conditions (respectively). Using the above inputs parameters, the model outputs (discharge, Q; and ZOI) were determined from the backcalculation of expected groundwater levels after 75 days continuous dewatering for the following aims: – Discharge (Q): Achieve a maximum drawdown of 20 m at 0.1 m radial distance from the abstraction point. – ZOI: Achieve a drawdown of 0.5 m (defined as the ZOI) in metres distance from the abstraction point. Theis scenarios and outputs A summary of the applied scenarios and results are in Table I.13 and Figure I.11. Values are rounded to account for uncertainty. The geomean of permeability results is considered to be represented by T = 2 m2/day and T = 20 m2/day. The results for T = 20 m2/day are generally in line with the outputs for Cross-Section 1. The analysis of T = 200 m2/day represents the upper limit of permeability measured at the site, and is consistent with Cross-section 2 (Scenario 2) outputs. Table I.13 Theis (1935) solution inputs and outputs T: 2 m2/d 20 m2/d 200 m2/d S: 0.1 0.01 0.1 0.01 0.1 0.01 Q 30 - 40 m3/d 290 - 330 m3/d 2,560 - 2,900 m3/d ZOI 70 - 210 m 210 - 610 m 610 - 1,840 m
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