The removal of particle-reactive radionuclides in shallow water: Bottom scavenging versus particle settling of iodine-131 and beryllium-7

In pelagic waters, the removal of particle-reactive radionuclides is controlled by nuclide sorption to particles and subsequent settling by gravity. However, in shallow nearshore waters, the dominant mechanism of nuclide scavenging is not so clear. Understanding how particle-reactive radionuclides are scavenged from the water column is critical if these tracers are to be used as proxies of particle flux in shallow aquatic systems. In this study, we present evidence that the removal of particle-reactive radionuclides in nearshore and turbulent waters is primarily controlled by bottom scavenging. Specifically, we measured both water column and bottom sediment activities of sewage-sourced iodine-131 (131I, t ½ = 8.02 days) and atmospherically-sourced beryllium-7 (7Be, t ½ = 53.3 days) in a semi-enclosed harbor. We show that the water column 7Be/131I flux ratio that is required to sustain observed harbor bottom inventories of both nuclides is incongruent with 7Be/131I activity ratios on water column particles, and (2) 131I and 7Be derived mass fluxes of particulate matter to the harbor bottom are in concordance with each other and independently made estimates of river sediment loading to the harbor only when bottom scavenging of both particle-bound and dissolved (<0.7 μm) nuclide fractions are considered.