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We analyzed a total of 25 seawater samples collected from 2 columns within the Sea of Japan for Ra (half life 1,600 years) and Ra (half life 5.75 years) by employing low-background γ-spectrometry. The Ra activity level in the deep proper water (DPW) (~0.13 mBq/L) was found to be higher than that of deep waters from the Pacific and Atlantic Oceans (<0.1 mBq/L), indicating its continuous delivery to the DPW by active water circulation before complete radioactive decay. Using a steady state box model of Ra and Ra, the turn-over time in the DPW was calculated to be ~80 years. [ABSTRACT FROM AUTHOR]