Benthic fluxes of metals into the Pearl River Estuary based on 224 Ra/ 228 Th disequilibrium: From alkaline earth (Ba) to redox sensitive elements (U, Mn, Fe)

We extended the 224Ra/228Th disequilibrium approach to examine benthic fluxes of a variety of metals, ranging from alkaline earth (Ba) to redox sensitive elements (U, Mn, and Fe), into the Pearl River Estuary (PRE), China. Depth profiles of 224Ra and 228Th in bulk sediment, as well as dissolved 224Ra and trace metals in porewater were measured along a transect within the estuary in July 2015. Significant deficit of 224Ra relative to 228Th was commonly observed in the upper 0–15 cm sediment. We took advantage of the 224Ra/228Th disequilibrium in the bottom sediments to construct a full mass balance of 224Ra in the overlying water column. We demonstrated that porewater exchange (PEX) processes with scale lengths of several centimeters are the predominant mechanism for solute transport between sediments and overlying waters in the PRE. In contrast, deep porewater flow or submarine groundwater discharge (SGD) with scale lengths of “meters to kilometers” are a negligible component in the water column budget of 224Ra.