Identifying resuspended sediment in an estuary using the 228 Th/ 232 Th activity ratio: the fate of lagoon sediment in the Bega River estuary, Australia

Thorium-series nuclides (228Th and 232Th) have been used to identify resuspended sediment in the Bega River estuary, south-eastern Australia. A non-conservative increase in concentration of suspended sediment of water in the vicinity of mid-estuary back-flow lagoons was associated with a decrease in the 228Th/232Th activity ratio (AR) of the suspended sediment. The lagoon sediment is characterized by a low estuarine 228Th/232Th signature, distinguishing it from freshwater suspended sediment recently delivered to the estuary, and identifying it as the likely source of the additional suspended sediment. Sediment-core 210Pb profiles show that the lagoons are accumulating sediment, presumably during high river-flow events. However this study indicates that during intervening periods of low flow, 40% of sediment deposited in the lagoons is subsequently resuspended and exported to the lower estuary, and possibly to the ocean. The utility of the 228Th/232Th AR to quantify sediment resuspension in estuaries is likely to be estuary-dependent, and is controlled by the extent of scavenging of dissolved 228Th by suspended particles.