GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 21 A water balance model was used to assess the pumping rate required to meet groundwater level targets over the LOM taking into account treated water discharge constraints and variable rainfall conditions. This analysis is summarised in section 6.3.1. 2.7 Tailings Storage Facilities 2.7.1 General The current TSF embankments (TSF1A and TSF2) are constructed in stages making use of rock from the Martha pit. Careful planning of material quantities is undertaken to ensure that the storage available in the active TSF (i.e., embankment height above tailings level) is sufficient to contain planned tailings deposition. In addition, a freeboard approaching 3 m above the tailings is provided at all times to conservatively provide for storage of an extreme rainfall event without overflow. The freeboard provides for the Probable Maximum Precipitation (PMP) plus 1 m contingency. There have been no overflows from an active TSF since the mine has been in operation. Decant water is pumped to the processing plant for re-use or to the WTP. As part of the WTP operation, the storage available in the active TSF is monitored and if necessary, decant water treatment is prioritised to ensure freeboard is maintained. 2.7.2 Process Water Recycle Process water requirements are based on production rates. Tailings slurry consists of approximately 16% tailings solids by volume and the water component is made up primarily of recycled decant, with small amounts of river water (elution water) and seepage water. Collection pond or minewater is used for top-up when needed. 2.7.3 Seepage An extensive seepage collection system exists beneath both TSFs. This system is designed to capture upwelling groundwater, seepage from tailings, and leachate from the rock used to construct the TSF embankments. The term “seepage” describes the combined flows from these sources. The characteristics of seepage depend on the source, quality and quantity of the individual flows as follows: Tailings underdrains collect seepage from the tailings as well as upwelling groundwater. During the initial period of tailings placement in TSF2 and TSF1A, the flow from tailings was relatively high and contained elevated levels of cyanide and soluble trace elements. As the tailings volume has increased, the permeability of the tailings mass has reduced significantly, resulting in a decrease in flow and an improvement in quality. The characteristics of the underdrain flows now approaches those of natural groundwater. Upstream cutoff drains collect tailings seepage and groundwater at the upstream toe of the TSF embankments. Experience has shown that tailings liquor concentrations in these drains are highest when decant pond water is standing against the embankment, and reduce as tailings levels rise and consolidate to provide a low permeability barrier. The source of tailings liquors in the drains is typically from the areas adjacent to the embankment abutments where water levels are highest and tailings levels are low. Leachate drains collect seepage from rock placed in the TSF embankments, much of which is PAF and contains elevated concentrations of metal sulphides. Flows depend on the rainfall volumes that fall on the embankment between the time that the rock is placed and capped. As the embankments are completed, and capping is constructed
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