Interactive comment on “ Radium-based estimates of cesium isotope transport and total direct ocean discharges from the Fukushima Nuclear Power Plant accident ” by M . A
The paper presents an interesting estimation of the offshore and vertical 134Cs fluxes from the Fukushima Nuclear Power Plant that followed the earthquake and resulting tsnunami of 11 March 2011. The radium-based methodology is appropriate to attain the objectives of the study, allowing to estimating water apparent ages as well. The manuscript is well written and structured and the covered issue is of undoubtedly relevance within the scope of BG. However, there are some general and specific comments that should be addressed before publication.
On the calculation of eddy diffusivity in the shelf water from radium isotopes: High sensitivity to advection
Abstract The concentrations of the radium isotopes have been used in previous studies to estimate eddy diffusivity on the continental shelf. These studies assume that the advective transport of the radium isotopes is negligible. A theoretical investigation using an analytic model with advection indicates, however, that the eddy diffusivity thus estimated is highly sensitive to the advection. It is shown that the error can be very large even for an advective velocity of the order of 1 mm/s–1 cm/s. The sensitivity increases with the increase of the half life of the isotope. For a 1 mm/s advective velocity, the estimated eddy diffusivity for the radium isotope with the shortest half life (i.e. 224Ra) is almost doubled. In addition, we also conclude that (1) advection has more important effects on smaller values of eddy diffusivity; (2) the effect of advection increases rapidly as advection increases; and (3) offshore advection tends to increase the apparent eddy diffusivity and inshore advection tends to decrease the apparent eddy diffusivity, if the advection is ignored. Based on these facts, an improved model is presented to calculate both advection and eddy diffusivity using the activities of two isotopes, which yields consistent diffusivity and advection from different isotope pairs.
Determining coastal mixing rates using radium isotopes
Abstract Coastal waters contain elevated dissolved activities of four radium isotopes. These elevated activities arise through desorption of Ra from particle surfaces and the input of submarine groundwaters enriched in Ra. The input of Ra near the coast is balanced by a flux of each Ra isotope toward the open ocean. Two of the Ra isotopes decay almost completely before they reach the edge of the continental shelf; the other two decay hardly at all. These differences in decay rates provide a powerful constraint on models of water movement and mixing on the shelf. These models are used to assess the factors that control the export of radium. Understanding the factors that control the export of radium allows an assessment of physical processes that regulate fluxes of other dissolved constituents in the coastal ocean. In this paper I use observations that were made during vertically stratified conditions in the South Atlantic Bight. Offshore transects of the long-lived 226Ra and 228Ra indicate that eddy diffusion controls their distributions within 50 km of shore. The short-lived 223Ra and 224Ra distributions in this region yield an eddy diffusion coefficient of 360–420 m2 s−1. The offshore fluxes of 226Ra and 228Ra derived from their across-shelf activity gradients and the eddy diffusion coefficient require a substantial volume of groundwater discharge to balance Ra removal.