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.
Flux of particulate aluminum across the southeastern U.S. continental shelf
We have determined the concentration of particulate aluminum across the coastal boundary front and the continental shelf of the South Atlantic Bight. We use a simple advection/diffusion model, incorporating particle settling, to interpret our data on the particulate aluminum distribution. By matching modelled distributions of particulate aluminum to measurements, and offshore advection velocity and offshore flux are predicted. Our conclusions are: (1) advection and settling are the major processes affecting cross-shelf particulate aluminum distributions (i.e. diffusion is insignificant); (2) less than 10 percent of the sediments delivered to the South Atlantic Bight by rivers is transported across the shelf by this mechanism; and (3) insignificant amounts of inorganic particles of diameters greater than c. 10 μm are removed from the nearshore region.