A sulfur-stable-isotope-based screening tool for assessing impact of acid sulfate soils on waterways

Abstract

Early warning indicators for waterways affected by acid sulfate soils (ASS) are valuable tools for water management organisations. Oxidised ASS may discharge high concentrations of metals, acid and sulfur to surrounding water. The origin of sulfate may be determined by δ34S values. δ34S values of dissolved sulfate in ~300 samples of fresh, brackish and estuarine surface water from south-west Western Australia ranged from –6.6 to 31.4‰ (Cañon Diablo Troilite). An indicator was developed based on [SO42–], [Cl–] and δ34S that categorised samples into groups with similar isotopic influences (iso-groups). Signals of disturbed ASS were identified in ~4.5% of sites. Multivariate statistical analysis showed that water quality had deteriorated at ASS-influenced sites. Although highly variable, average aluminium concentrations were higher (up to 0.12 mg L–1, compared with <0.05 mg L–1 elsewhere) in samples that are influenced by ASS disturbance. The categorisation of samples into iso-groups provides a simple tool to prioritise sites for further investigation. This study shows that δ34S values provide an early warning indicator for water affected by disturbed ASS, particularly in localities where rainfall is marine dominated with a similar δ34S to seawater.