Spatial variations of Ra, Ra, Cs, and Cs concentrations in western and southern waters off the Korean Peninsula in July 2014.

We examined the spatial distributions of Ra, Ra, Cs, and Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and Ra) concentrations in water samples varied widely from 5 to 14 mBq/L (2-4 mBq/L), showing a negative correlation with salinity, particularly at the surface off the western Korean Peninsula. This indicates that the seawaters in this area are fundamentally comprised of Ra-poor and high-saline Kuroshio Current water and Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived Cs was below the detection limit (<0.08 mBq/L) in waters off the western Korean Peninsula, low level Cs (0.1-0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher Cs concentrations (1.6-1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.