EGL Ref: 9215 23 June 2022 Page 16 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx. As described above in Section 3.2, the rock and soils at Waihi are associated with a past geological time of volcanism. Some of the rocks are Potential Acid Forming (PAF) when exposed to air and water, and some of the rocks are Non-Acid Forming (NAF). The potential for acid generation is related to the geochemistry of each rock source and testing is undertaken to determine which rocks are PAF. Acid generated from exposed PAF can mobilise heavy metals in the rock into solution which can be transported into the wider environment if not mitigated. This is called Acid Rock Drainage (ARD). The disposal of rocks at Waihi requires special mitigation measures to limit any ARD from PAF rock and tailings. This includes: • Testing of the source rock; • Controls over rock handling and placement; • Liming of the PAF rock, zoning of the embankments to limit oxygen ingress; • Placement of NAF material on the final external surface; • Leachate and subsoil seepage collection systems; and • Ground water monitoring. 4.2. Nature of tailings at Waihi 4.2.1. General Tailings is the common by-product of processing the ore-bearing rock to extract the valuable metals and minerals. Tailings at the end of the mineral extraction process typically take the form of a slurry consisting of fine particles and water. The fine particles are typically clay to sand sized and are created by the crushing and grinding of the ore bearing rock, and processing of the grind using methods such as leaching and adsorption to extract the valuable metals and mineral. Some processing operations also use methods to remove water from the tailings slurry for the purpose of water conservation or so the tailings can be transferred and placed at a higher density in their final storage location. Water is removed from tailings using plant called thickeners and filter presses. The nature of tailings leaving the process plant is, therefore, a function of the: 1. Ore-bearing rock processed; 2. The ore processing method; 3. The water management processes; Tailings which remain suspended in water as a slurry are pumped to the storage facility via a discharge delivery pipe. The amount of water in the slurry depends on the extent of thickening undertaken at the process plant. Thicker slurries are more difficult to pump. Filtered tailings are transported by truck. Cement can also be added to a thickened slurry to form a paste which when left to harden has improved strengths and is often used for filling underground workings. Tailings which have water removed using a filter press behave as a soil and are transferred by dump trucks or conveyors to their final storage location. Tailings slurries and filter pressed tailings consolidate to form soil-like deposits. These deposits can be tens to hundreds of metres thick. The profile,
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