GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 111 Parameter Median stream flow (10,000 m3/day) Low flow (1,000 m3/day) Maximum Median predicted leachate concs. 95th%ile predicted leachate concs. Median predicted leachate concs. 95th%ile predicted leachate concs. Mn 0.0035 0.05 0.01 0.48 0.48 Ni 0.0005 0.0005 0.0008 0.0005 0.003 Pb 0.0001 0.0001 0.0001 0.0001 0.0001 Se* 0.0000010 0.000005 0.000010 0.00005 0.00005 Sb 0.0002 0.0002 0.0002 0.0002 0.0002 SO4 9 11 11 33 33 Zn 0.002 0.002 0.002 0.003 0.003 * No stream concentrations were available for Se, therefore these values are the predicted increase to the stream concentrations. J.4 Sensitivity analysis Sensitivity testing was undertaken for two key aspects influencing the model results: – The emplaced rock permeability. – The permeability of the underlying geology. The applied permeabilities are presented in Table J.6. The lower value applied for scenario Sens.1 was limited by the model’s ability to converge at such low permeabilities. A graphed comparison of the sensitivity results are presented below in Figure I.7. The results indicate: – The volume of leachate captured by the rock drain is somewhat sensitive to changes in the rock stack material permeability. The proportion of total infiltration that discharges to ground ranges averages 10% for Sens.1 scenarios (increased K) versus 28% for Sens.3 (decreased K), compared with 16% for the base model. These results are considered appropriate, where a greater permeability would allow for more direct flow to the rock drain under Sens.1. – The volume of leachate captured by the rock drain is very sensitive to changes in the underlying geology permeability. In the first instance, the base model boundary conditions were required to be updated, where increased K of the geology resulted in a lowered water table and vice versa for the decreased K (elevated water table). After adjustment to achieve a groundwater level similar to the base model, the results indicated that: All rock stack leachate drains to the ground with an increase of the geological K values. A greater proportion of groundwater was captured by the drains (>94%) with lowered geological K values. The results of the K values are consistent with the basis for placing the WRS within this gully, where the naturally low K of the soils combined with the steep slopes is expected to direct groundwater predominantly to the rock drain at the base of the WRS gully. Table J.6 WRS sensitivity testing scenarios (K values in m/s) Material: Rock stack Residual soil/tuff Andesite Base model K: 1 x 10-5 1 x 10-8 3 x 10-8 Sensitivity test no. Change to model: Sens.1 Rock stack K 1 x 10-4 Sens.2 Rock stack K 1 x 10-6 Sens.3 Geology K 1 x 10-7 3 x 10-7 Sens.4 Geology K 8 x 10-9 1 x 10-9
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