Oceana Gold Project Quattro Project Quattro Geochemical Assessment – Tailings and Overburden, Treatment and Mitigation \\na.aecomnet.com\lfs\APAC\Christchurch-NZCHC1\Legacy\Projects\605X\60544823\6. Draft Docs\6.1 Reports\Waihi North\App D - Kinetic and Column Testing\WNP Draft v6_App D (Kinetic Column Testing)_20220504.docx Revision 5 – 07-Aug-2020 Prepared for – Anderson Lloyd – Co No.: 5719274 Figure D13Weekly Column Electrical Conductivity D3.3 Column Test Results Discussion In general, columns initially untreated (Col1 - Col6) oxidise rapidly and are characterised by depressed pH (pH < 3.) and elevated acidity (acidity pH 3.7 > 100 mg/L as CaCO3) by week 10. This is also associated with a proportional increase in trace element leaching. This is most noticeable in iron (from week 10), chromium (week 10), lead (week 10), and vanadium (week 15). (Figure 1 to 14 of Attachment B). In Col2 and Col5 (following saturation at week 11), pH, acidity, and sulphate production stabilise whereas Col3 and Col6 (which were limestone dosed) show no change relative to the remaining untreated columns (Col1 and Col4). At week 21, pH has reduced to ~1.5 in Col and Col4 and a concomitant increase in dissolved metals including arsenic, copper, cadmium, chromium, nickel and zinc occurs. Of note is that mercury does not appear to have mobilised with the other metals and remains at low to negligible levels in the leachate. For Col1 – Col6, the similar trends of the untreated and limestone-treated columns, and in particular their contrast with trends of the saturated columns, is indicative that the limestone treatment has had limited effect when applied after material is allowed to oxidise and produce acid. It is likely that in-part, this can be attributed to dosing of the columns after they had already reached a highly acidic state. Limestone addition can also have limited efficiency in neutralising acidity when not blended as the amount of alkalinity that will dissolve in infiltrating water is small relative to the alkalinity load. Similarly, if oxidation has resulted in the precipitation of ferric hydroxide on the limestone itself (armouring), this is also known to reduce the effectiveness of limestone addition. These, coupled with ongoing exposure to molecular oxygen (which is expected to be significantly limited in the saturated columns) is a probable cause for the limited effectiveness of limestone addition to the columns. Very high concentrations of arsenic (> 5 mg/L) are evident in the untreated (Col1 and Col4) and limestone dosed (Col3 and Col6) from week 20 onwards. This increase appears associated with several large flushing events and is likely reflective of arsenates sorbed onto iron hydroxides which have been
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