Evaluation of excess Th-228 in surface bottom sediment of shallow marine ecosystem: a case study from Dongshan Bay

Naturally-occurring thorium isotopes and corresponding Ra-Th disequilibrium in aquatic system are good tracers for particle dynamic processes. Here we reported a case study on the occurrence of ²²⁸Th_ex-²²⁸Ra disequilibrium in sediments of a shallow estuary-coastal bay system (Zhangjiang Estuary-Dongshan Bay). ²²⁸Ra activity concentration in saline water and ²²⁸Ra and ²²⁸Th activities in the surface sediments were measured and we found ²²⁸Ra’s non-conservative behavior in saline water and the existence of excess ²²⁸Th (²²⁸Th_ex = ²²⁸Th-²²⁸Ra) in surficial sediments at all the stations. Spatially, ²²⁸Th_ex gradually decreased from the Zhangjiang Estuary (16 Bq/kg) to the inner Dongshan Bay (8–11 Bq/kg), which is consistent with the variation trend of ²²⁸Ra activity concentration in seawater. This consistency explained the reason for the occurrence of ²²⁸Ra-²²⁸Th disequilibrium (or called “²²⁸Th_ex occurrence”) in the surface sediments. Because ²²⁸Ra activity concentration is very high in estuarine water, and ²²⁸Th continues to decay from ²²⁸Ra, and the newly formed ²²⁸Th is quickly cleared by suspended particles in the water and subsequently settles down to the seabed sediments. Based on a constructed simple irreversible scavenging model, the residence time of ²²⁸Th_ex was constrained to be 15–30 days in the estuary and to be 5–11 days in the inner bay. These results show that Th and other similarly particle-reactive trace metals will be scavenged rapidly from the water column of shallow estuary-bay systems.