Dissolved barium and radium isotopes in the Chao Phraya River estuarine mixing zone in Thailand

New data on dissolved Ba, 226Ra and 228Ra were obtained in the Chao Phraya River estuarine mixing zone in 1996. Although desorption of Ba from river-suspended sediments gives rise to elevated dissolved Ba concentrations in the low salinity (S<5) zone, two types of mixing trend in the higher salinity (S>10) region have been observed depending upon the sampling periods. Under a dinofragellates (Noctiluca) red tide condition in January, significant biological removal of dissolved Ba took place in the mid-salinity (S=10–16) region, whereas under normal condition in July and November, the dissolved Ba followed a conservative linear mixing trend. 226Ra is also supplied to the coastal waters by desorption from river-transported sediments in a delayed fashion compared to Ba, at least, in the latter seasons, whereas 228Ra is predominantly supplied by diffusion from underlying sediments to the water largely in the higher salinity (S>15) region. These geochemical observations in July and November fit well to the previous studies done for other estuarine environments. riverine fluxes to the Gulf of Thailand were estimated to be 6.2×106 mol Ba/yr and 1.5×1013 dpm 226Ra/yr, about half of which is contributed by the Chao Phraya River. Although there is a sign indicating that ground water input may be contributing to the fluxes of 226Ra and Ba, we were unable to quantify them due to lack of information on the exchange rate of the coastal waters with open seawater.