GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 46 . URS (2003) reported that monitoring of groundwater levels at the Black Hill Orchard bore (72_9336) had not identified any effects from dewatering of the Martha Mine. Testing of groundwater from the bore and completion of a pumping test in 2017 (GWS, 2018b) suggests that the Black Hill Orchard bore is unlikely to be in hydraulic connection with the Gladstone area, with this indicated by: – A four-hour pumping test provided a transmissivity (T) for the andesite aquifer at this location of 53 m2/day for early time testing (the first hour). A boundary effect is obvious in later test time, resulting in a lower T of 18 m2/day. This boundary is inferred to be a lower permeability unit estimated within 500 m of the bore. The inferred transmissivity and boundary effect suggest that the aquifer is of limited extent and with impaired hydraulic connection. – A comparison of groundwater quality from the Black Hill Orchard Bore with that of deep groundwater from monitoring wells installed near Gladstone indicates that the water type is different with the Black Hill Orchard Bore being relatively uninfluenced by mineralisation. The Black Hill Orchard Bore is a sodium bicarbonate type water with low dissolved solids and a neutral to high pH (7.6), with elevated alkalinity. The water has low sulphate (<5 mg/L), with comparatively higher sodium and chloride. In general, dissolved mineral content is low and water chemistry does not show a notable influence of mineralisation. By comparison, groundwater from monitoring bore GLD04d has a mixed water type that corresponds most closely with a calcium sulphate water type. Groundwater from the andesite at this location has a slightly lower pH (6.8), elevated sulphate of 78 mg/L, lower chloride, and elevated dissolved zinc (0.17 mg/l), indicating some influence of mineralisation that is not present in the groundwater at Black Hill Orchard. 3.4 Gladstone Open Pit and Tailings Storage Facility assessment methodology Potential impacts on groundwater and surface water associated with the proposed GOP and TSF were assessed using conceptual interpretation, supported by the use of 2D numerical groundwater models and analytical models. The assessment comprised: 1. Conceptual groundwater model development (Section 3.3): Understanding of the current geological and hydrogeological environment using site investigation data (Appendix A) and OGNZL leapfrog model (2021). 2. GOP and TSF assessment: a. Development of two steady-state cross-section models (Geostudio 2021 SEEP/W finite element numerical modelling software) representing existing shallow groundwater flow paths that extend radially from Gladstone Hill. These models were used to predict changes in groundwater levels and flow and estimate discharge of tailings porewater to the TSF drainage system and the receiving environment. The model set-up considers a situation where the shallow groundwater system is currently under-drained as a result of existing mine dewatering (Section 3.3.3). The assessment modelled the following sequential scenarios: i. Current conditions: Calibrated base model representing the existing environment. ii. GOP excavation: Excavation of the GOP to maximum proposed depth. iii. Operational TSF: Pit backfill with rock and tailings, presence of a tailings pond and TSF drainage system prior to rewatering of the deep groundwater system. iv. TSF closure: Drainage of the tailings pond, placement of a capping layer and continued operation of TSF drainage system, subsequent to rewatering of the deep groundwater system v. Long-term TSF: No further operation of the TSF drainage system. b. Development of a groundwater recharge model to estimate changes in groundwater recharge area for each of the assessment scenarios. This analytical solution was used as a comparative analysis to provide a degree of further confidence in the numerical model calibration. 3. Gladstone vein dewatering assessment: a. Development of a steady-state cross-sectional model (Geostudio 2021 SEEP/W finite element numerical modelling software) representing existing shallow and deep groundwater systems and the Gladstone vein system. The model was orientated perpendicular to the strike of the Gladstone vein system where it
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