Signature of the Mekong River plume in the western South China Sea revealed by radium isotopes

We investigated the distribution of Ra-223, Ra-228, and Ra-226 in the surface water of the western South China Sea (SCS) during summer based on a 30 day cruise conducted in August-September 2007. The activities of Ra-223 varied from almost undetectable to 0.74 disintegrations per minute (dpm)/100 L, and those of Ra-228 varied from 12.2 to 61.5 dpm/100 L. Their spatial distribution was characterized by a jet of high Ra-228 (>48 dpm/100 L) and Ra-223 (>0.4 dpm/100 L) extending eastward from the Vietnam coast along similar to 11 degrees N, curling up in the vicinity of 112 degrees E and swirling counterclockwise to form a cyclonic eddy with lower Ra-228 (21-25 dpm/100 L) and Ra-223 (0.04-0.14 dpm/100 L) at its center. High Ra-226 (10-14 dpm/100 L) appeared in the eastward jet and decreased to 6.0-8.5 dpm/100 L along the track of the jet described above. The observed distribution of Ra isotopes was consistent with the pattern of the Southeast Vietnam Offshore Current in the western SCS in summer. The higher radium activities were in all likelihood derived from the Mekong River. Using a simple two-end-member mixing model based on the Ra-228/Ra-226 activity ratio and salinity, we calculated that approximately 53% of the surface water in the western SCS was originated from the Mekong River diluted water. Note that this estimation should be regarded as an upper limit due to the lack of sampling at its immediate source, the Mekong estuary. The data revealed that more than 2 weeks were required for the transportation of freshened water from the Mekong River's mouth several hundred kilometers to the western SCS.