Organic acidity characterisation of AMD in the Section Six pit of Mt Tom Price Mine, Pilbara, Western Australia.

Abstract

Temporal analyses of physical, geochemical and biological characteristics were used to define the acidity inconsistency issue identified in the Section Six (SSIX) pit of Mt Tom Price Mine, in the Pilbara region of Western Australia. The SSIX pit is used for on site storage of acidic mining water, before transfer to the on site Acid Water Treatment Plant for neutralisation. This study was initiated due to a 2007 investigation into the SSIX pit, which identified an acidity inconsistency between the calculated acidity (determined by metal ion concentrations; 45 mg/L as CaCO₃) and the total acidity (determined by acidity titration; 280 mg/L as CaCO₃). Based on this finding, it was hypothesised that the laboratory confirmed SSIX algal presence (in green coloured pit water) was potentially producing a weak/organic acidity unaccounted for in the calculated acidity. Archived geochemical raw data for the SSIX pit water and its surrounding inputs (from nearly a decade of environmental monitoring) were used in a temporal analysis. This analysis produced a trend of increased major ions in solution during seasons/periods of high rainfall. This was related to archived aerial photos of the SSIX pit, which display typical (green) algal presence during dry seasons/periods when major ions in solution are lowest (a potential lower metal phytotoxicity). A 2012 investigation into the SSIX pit determined an absence of algae, despite indication from green pit water colouration. From this, a new question was considered “what observations and measurements differ between water sampled in 2007 and 2012 that could explain either the absence of algae, or, any continued presence of acidity inconsistency”. The phytoplankton absence in 2012 was attributed to increased concentrations of major ions, causing phytotoxicity to phytoplankton growth, along with a decreased influence of phytoplankton input from source waters to the SSIX pit. These observations were opposite to the previous findings from 2007. Three pit water colour categories were determined, to decipher between the opaque green water colour witnessed during the 2007 algal presence, the translucent green water colour witnessed during the 2012 algal absence (attributed to copper and aluminium green precipitates) and the brown pit water colour during periods of high total dissolved solids (attributed to iron red precipitate and thus further metal phytotoxicity). These categories can continue to be used as an indicator for algal presence or absence before laboratory confirmation. Despite algal absence in 2012, titrations were still undertaken to characterise acid species in solution. Organic acidity was not observed, most likely due to the absence of algae. A large acidity inconsistency was also not observed, which tends to support the hypothesis that algal presence in 2007 provided cause (potential organic acidity) for acidity inconsistency. However inconclusive, it is also suggested that organic acidity is not the sole cause of acidity inconsistency, but rather an influential key player during dry seasons. Literature provides evidence that acidity inconsistencies can also occur from non-organic sources such as the presence of alkalinity in a water sample, metal sequestration, or errors in lab methods and reporting. Future research into the SSIX pit should be undertaken during a time of algal growth to further determine potential organic acidity production.