4.
Model results
With the daily pumping of water from the SSG to the OCP, the simulated median result for the SSG is emptied within approximately the first year and remains relatively the same through Phases 2 – 4, following typical rise and fall associated with the seasonality of the area. The newly proposed TSF, namely the UMG TSF shows, general seasonal increases through the wet season in Phases 1 and 2 (see Figure 4.2), assuming that the UMG outlet to the Dam 8 catchment is sealed from the first day (see Section 3.2, which confirms that this structure is not being used for water storage, nor receiving any operational inflows). Once the UMG becomes the receiving TSF (from Phase 3 onwards), the base levels begin to rise with the tailings deposited, with only minor fluctuations in water levels due to the reuse of the water being released form the deposited tailings (see Figure 3.2). The results presented in this figure also confirm that the TSF was not simulated to spill based on the maximum level as simulated in this study, using the historical climatic data and assumptions defined here-in. The results presented for OCP water levels Figure 4.3 were assessed for the following key levels in OCP as specified by Heritage Minerals:
– 267.3 m AHD – 263.0 m AHD – 259.3 m AHD to sustain a 1% AEP buffer volume.
Table 4.1 presents the simulated key water levels along with their corresponding dates. All simulated dates fall within Phase 2, with the exception of the date associated with a relative level of 259.3 m AHD in OCP, which occurred during Phase 3.
Simulated dates for at OCP levels of 267.3, 263.0, and 259.3 m AHD for the 50 th and 75 th percentiles
Table 4.1
July Simulations
Relative Level (m AHD)
50 th
75 th
267.3
21/06/2026
05/10/2026
263
25/01/2028
16/10/2028
18/08/2029 1
18/11/2029 1
259.3
1 Phase 3
During Phase 3, late 2029 (UMG as TSF), OCP water levels were simulated to continue dropping further at an increased rate. The spill level of the OCP is at 273.52 m AHD and the simulations show that there is negligible spill risk as shown in Figure 4.3, due to the declining water level. However, that is based on the starting storage level applied at the commencement of the modelling, which was input as 271.0 m AHD and all the net loss assumptions assumed here-in. It is noted that Appendix A presents the simulated water balances for each of the four phases, all of which show a net loss of water from the mine water inventory, which is as a result of the treatment (through the WTP) and discharge of water from the site at 4.8 ML/d. Should these values differ, these results should be reassessed to confirm the revised spill risk. It is also noted that these results are based on historical climatic inputs and, as such, do not make any allowance for climate change impacts.
12624987 | Heritage Minerals EA Amendment Application – Updates to Water Balance Model July 2025 (Final) 8
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