Geochemical signatures of acidic drainage recorded in estuarine sediments after an extreme drought

Abstract

Extreme prolonged drought over south-eastern Australia a decade ago (the Millennium Drought, 1996–2010) triggered extensive acid sulfate soil oxidation and associated acidification. Whilst the significant release of metal-enriched drainage has been documented during this event, the fate of these elements in receiving systems is still largely unknown.Here we investigate the spatial distribution, chemical partitioning, and potential bioavailability of S, Fe, Mn, Al, trace metals (V, Cr, Co, Ni, Cu, Pb) and rare earth elements in the surficial sediments of Lake Albert, South Australia; a system that received prolonged (2007–2011) acidic drainage during the Millennium Drought. The highest concentrations of all metals (Enrichment Factors ranging from 1.06–1.97) were observed in the sediments closest to shorelines where sulfidic material were oxidised, generating metal-enriched acidic drainage. Localised enrichment indicates metals have not been completely redistributed across the system following their initial deposition by physical (wind-driven resuspension) or chemical (diagenetic and redox changes) processes. Based on selective extraction data, these metals are likely partitioned as discrete sulfides (Pb) or are with organic material (V, Cr, Cu, Pb, REEs), Fe monosulfides (V, Co, Ni), or pyrite (Co, Ni, Pb). Lake-wide enrichment of trace metals Cr (mean concentration 57 ppm), Ni (39 ppm), Cu (36 ppm), and Pb (24 ppm) relative to other Lower Murray River sites is also evident, suggesting that metal contamination associated with acidic drainage is not restricted to near-source sites. Importantly, the mobilisation, transport, and accumulation of metals is controlled by sediment transport pathways and system hydrology and will thus function differently under varying states of geomorphology, climate, and anthropogenic modification.Our study shows that extreme drought is recorded as a lasting geochemical signature in estuarine sediments, demonstrating that metal and rare earth element geochemistry provides insights into the distribution and behaviour of contaminants mobilised into dynamic, anthropogenically altered estuaries.