GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 42 . Hydraulic connection to Favona The influence of the Favona mine dewatering on the Gladstone vein system has been constrained to areas that have a strong hydraulic connection with the Favona system, with this connection inferred to reduce along the strike between wells P60 and GLD04. The degree to which the Gladstone deep groundwater is currently dewatered due to Favona mine workings is not well defined, however it is expected that the vein system at the location of the proposed GOP is predominantly dewatered. Given the dry conditions recorded to at least 1,029 mRL in P60 immediately north of the proposed GOP (Figure 3.8), and 1,097 mRL at GLD01b within the southern GOP boundary, it is considered likely that the northeastern section of the proposed GOP is fully dewatered to the maximum proposed pit depth (1,005 mRL). However, the south-western section of the GOP, where the proposed maximum pit depth is shallower (approximately 1,025 – 1,040 mRL), may be dewatered to a lesser degree. The interpreted hydraulic connection between the Favona and Gladstone vein systems is supported by the data presented in Table 3.3, and is summarised as follows: – P64 well series: Three piezometers were installed to target the Favona vein system (Figure 3.8). Monitoring indicates that after dewatering of Favona commenced in 2005, water levels reduced in the two piezometers installed in the deep andesite system (P64D and P64A) and became dry in 2006. The piezometer installed in the shallow groundwater system (P64I) did not record a response to dewatering, as a result of the hydraulic separation between the shallow and deep systems. – P60 and P61 well series: These wells were installed to differing depths adjacent to one another to monitor dewatering effects to the west of the Favona vein system, and immediately to the north of the proposed GOP (Figure 3.8). P60 was recorded as dry since 2005, with no change in water levels recorded in the shallower P61. The apparent difference in response between these piezometers, and variability in hydraulic conductivity, is reflective of the highly localised influence of the dewatering on surrounding groundwater levels and potentially the shallower elevation of the P61 piezometer. – P79 well series: Deep groundwater levels at P79d have been gradually declining since 2005, with a more significant reduction in 2016 reported by OGNZL (2019) as likely due to water loss during drilling of a nearby horizontal bore (Figure 3.9). The drill hole was plugged, with no further water loss reported. However, the declining trend in water levels in P79d and shallower piezometer P79i are considered to be reflective of a minor and delayed response to dewatering effects from Favona. Although the andesite at this location is fractured, only a limited hydraulic connection is interpreted to exist between the local country rock and the vein system. – GLD01b is installed to approximately 1,097 mRL within hydrothermal breccia/weathered andesite above the andesite hosting the Gladstone vein system and has been recorded as dry since installation. The dry conditions at this piezometer may be attributed to dewatering of the underlying andesite. Groundwater levels in nearby P79d indicate a typical range between 1,077 – 1,094 mRL (Figure 3.9), approximately 50 m offset from the inferred Gladstone vein system. – GLD04d is installed adjacent to the Ohinemuri River and screened at an elevation similar to the proposed maximum GOP depth (1,005 mRL). Despite the andesite encountered in GLD04d being highly fractured and in close proximity to the inferred location of the Gladstone vein system, deep groundwater at this location has not been dewatered. An upward vertical hydraulic gradient between the deep and shallow groundwater systems has been recorded. P68d and P68s however recorded a downward vertical hydraulic gradient prior to dewatering of the Favona system. In comparison to the 1983 deep groundwater levels presented in Figure 3.10, groundwater levels in the andesite are interpreted to have reduced slightly as a result of mine dewatering, however the influence of dewatering to the southwest of the proposed Gladstone Pit may be limited. 3.3.4 Groundwater quality Detailed groundwater chemistry data recorded in the Gladstone monitoring wells between 2017 – 2020 is presented in the tables and piper plots in Appendix E. The chemistry of the groundwater in shallow monitoring wells generally changes with depth and location, with this interpreted to be a function of land-use and degree of local mineralisation. Near surface groundwater, as recorded
RkJQdWJsaXNoZXIy MjE2NDg3