GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 65 G.2.4 Water quality assessment results All groundwater that discharges into the GOP during excavation, and any surface water runoff within the pit, will be treated at the water treatment plant prior to discharge to the Ohinemuri River. There are therefore no notable water quality effects to the receiving environment predicted during GOP excavation. The TSF drainage system is predicted to provide a high degree of hydraulic containment of the tailings porewater during the operational TSF and TSF closure scenarios, with seepage through the backfilled rock and tailings not predicted to migrate to the shallow groundwater system or the surface water receiving environment. While discharge of tailings impacted porewater to the deep groundwater system is predicted to occur prior to rewatering of the deep system, this discharge is predicted to migrate towards other mine dewatering systems that control deep groundwater flow directions. There are therefore no notable water quality effects to the receiving environment predicted during the operational TSF and TSF closure scenarios. After an improvement in water quality is observed within the discharge from the TSF drainage system, operation of the drainage will be discontinued (long-term TSF scenario). The water quality assessment has therefore assessed the potential impact of tailings porewater discharge on shallow groundwater and the surface water receiving environment. As presented in Table G.14, approximately 4.5 m3/day of tailings porewater is predicted to discharge west to the Ohinemuri (OH6) catchment in the long-term TSF scenario. Table G.26 presents the predicted water quality of the total groundwater discharge (64.8 m3/day) to this groundwater catchment from the TSF, and subsequent mixing within the Ohinemuri River. Table G.26 Predicted receiving water quality in the long-term TSF scenario (values in mg/L) Parameter Existing shallow groundwater quality (GLD04s) Predicted values after equilibration of TSF discharge with shallow groundwater Existing Ohinemuri River quality (OH6 median) Predicted values after mixing with Ohinemuri River Receiving water quality criteria pH 6.8 7.7 7.1 - 6.5 – 9.0 Al 0.088 0.07 0.016 0.016 As 0.001 0.0012 0.0010 0.0010 0.19 Ba 0.14 0.109 0.038 0.038 Total Alkalinity 65 42 15.4 15.6 Ca 10.5 79 43.5 43.8 Cd <0.00005 0.0001 <0.00005 <0.00005 0.0003 Cl 9 11 16.4 16.4 Co 0.017 0.04 0.0009 0.0011 Cr <0.0005 <0.0005 <0.00050 <0.00050 0.01 Cu 0.0014 0.0008 0.0005 0.00050 0.003 Fe 0.03 2.2 0.09 0.098 1 Hg <0.00008* 0.00011 <0.00008* <0.00008* 0.000012 K 3.5 6 3.6 3.6 Mg 2.8 12 3.1 3.1 Mn 0.43 1.5 0.019 0.024 2 Na 24 27 17.1 17.2 Ni 0.0089 0.007 0.0005 0.00053 0.04 Pb <0.0001 <0.0001 <0.0001 <0.0001 0.0004
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