Spatial variations of 226 Ra, 228 Ra, 134 Cs, and 137 Cs concentrations in western and southern waters off the Korean Peninsula in July 2014.

We examined the spatial distributions of226 Ra,228 Ra,134 Cs, and137 Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and226 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 of228 Ra-poor and high-saline Kuroshio Current water and228 Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived134 Cs was below the detection limit (<0.08 mBq/L) in waters off the western Korean Peninsula, low level134 Cs (0.1-0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher137 Cs concentrations (1.6-1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of134 Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.