Mobilisation, alteration, and redistribution of monosulfidic sediments in inland river systems

The accumulation of monosulfidic sediments in inland waterways is emerging as a major environmental issue. Mobilisation and suspension of monosulfidic sediments can result in deoxygenation, acidification of the water column and mobilisation of trace metals. The controls on monosulfidic sediment mobilisation and the critical thresholds for its scour and entrainment have not been established. This study examines the effect of a minor flood event (average return interval of 5 years) on sulfidic sediment scour in the Wakool River in southern NSW, Australia. Five profiles were sampled within a small (∼300 m) reach before and after a minor flood event to determine the degree of sediment scour and transport. The results indicate substantial scour of both monosulfidic sediments and underlying bed sediments (approximately 2100 m³). Changes in the sediment geochemistry suggest large concentrations of monosulfidic sediments had been suspended in the water column, partially-oxidised and redeposited. This is supported by ²¹⁰Pb results from one of the profiles. These results suggest that these monosulfidic sediments can move as bed load during minor flood events.