Hydrochemical and stable isotope indicators of pyrite oxidation in carbonate-rich environment; the Hamersley Basin, Western Australia

Sulphur (S) is a commonly occurring element in most aquifers, primarily in oxidised (sulphates) and reduced (sulphides) forms. Sulphides often constitute a risk to groundwater quality due to acid rock drainage, especially in catchments that are subject to mining excavations or groundwater injection. However, in semi-arid regions detection of the acid rock drainage risk can be challenging and traditional methods based on observations of increasing SO₄ concentrations or SO₄/Cl ratios in surface and groundwater, are not necessarily applicable. In addition, decreasing pH, usually accompanying pyrite oxidation, can be masked by the high pH-neutralisation capacity of carbonate and silicate minerals. Analysis of 73 surface and groundwater samples from different water bodies and aquifers located in the Hamersley Basin, Western Australia found that most of the samples are characterised by neutral pH but there was also a large spatial variability in the dissolved sulphate (SO₄) concentrations that ranged from 1 mg/L to 15,000 mg/L. Not surprisingly, groundwater in aquifers that contained pyrite had high sulphate concentrations (>1000 mg/L). This was associated with low δ³⁴S_SO4 values (+1.2‰ to +4.6‰) and was consistent with the values obtained from aquifer matrix pyritic rock samples (−1.9‰ to +4.4‰). It was also found that the SO₄ concentrations and acidity levels were not only dependent on δ³⁴S_SO4 values and existence of pyrite but also on the presence of carbonate minerals in the aquifer matrix. The groundwater in aquifers containing both pyrite and carbonate minerals had a neutral pH and was also saturated with respect to gypsum and had high magnesium concentrations of up to 2200 mg/L suggesting de-dolomitisation as the process buffering the acidity generated by pyrite oxidation. Based on the findings from this study, a classification scheme has been developed for identification of the acid rock drainage contribution to groundwater that encompasses a myriad of geochemical processes that occur in aqueous systems. The classification uses five proxies (SO₄, SO₄/Cl, SI of calcite, δ³⁴S_SO4 and δ¹⁸O_SO4) to improve assessment of the oxidation of sulphide potential contribution to overall sulphate ion concentrations regardless of acidity levels of the aqueous system.