Radium Isotopes In Changjiang Estuary/ East China Sea And Their Application In Analysis Of Mixing Among Multiple Water Masses

Methods were established for extraction, purification and determination of natural radium isotopes in sea water. Both of the Alpha Spectrometry and Gamma Spectrometry were used to determine the activity of radium isotopes. Although the different results between the two instruments were proved, it could be ignored to some degree.In August 2005, the activities of ²²⁶Ra and ²²⁸Ra in dissolved, particulate and sedimentary phases were measured in Changjiang Estuary. Dissolved radium had a strong source in the mixing zone with a maximum activity (4.85±0.19 Bq/m³ for ²²⁶Ra and 13.01±0.46 Bq/m³ for ²²⁸Ra) at the salinity～17 psu. In October 2006, the activities of dissolved ²²⁴Ra and ²²⁶Ra indicated the non-conservative behaviour either. By using a mass-balance model, the added radium isotopes had been estimated. Using the dissolved ²²⁴Ra, ²²⁸Ra and ²²⁶Ra, based on a series of equations, the water mean residence time in Changjiang Estuary could be calculated. In the mouth of Changjiang, the residence time ranged from 1.1 d to 24.9 d, which is lower than the residence time off Changjiang Estuary (average 0.40±0.06 year). In the Estuary, the residence time had a linear relationship with the salinity gradients (ΔS/ΔX).There was a clear hypoxic zone adjacent to the Changjiang Estuary, which only appeared in summer (August 2006). The minimum of dissolved oxygen concentration in hypoxic zone (DO=0.94 mg/L) was lower than that in previous reports, which indicated the environment faced the more serious menace. The formation and maintenance of hypoxia is due to the compositive function of respiration, mineralization and stratification, which are denoted byΔρ(density difference between surface and near-bottom waters) and Apparent Oxygen Utility (AOU). TheΔ²²⁶Ra (activities difference of ²²⁶Ra between surface and near-bottom waters) has some relationship withΔρ,ΔDO and the mixing among water masses, which can be one of the indications. To further understand, the hydrographic data and mixing proportions are analyzed.To compare the analysis of mixing proportions between traditional Temperature-Salinity diagram method and multiparameter matrix method, an ideal case of mixing among four-water masses was created. It is obvious that the latter method can give the more confident solutions, but the advantage of traditional method is the wide application due to the validity and sufficient of the data. Then the latter method was applied in East China Sea to evaluate the mixing among four water masses and to test the feasibility of the third tracer-²²⁶Ra.East China Sea is bounded by the continent where the fourth largest river of Changjiang discharges large amounts of freshwater to the west and by the Kuroshio in the East and connected to the South China Sea via Taiwan Strait, therefore water characteristics are very complex and undergo great seasonal changes. The dominant source waters in the East China Sea are found to be Kuroshio Surface Water (KSW), Kuroshio Sub-surface Water (KSSW), Changjiang Diluted Water (CDW), and Taiwan Strait Warm Water (TSWW). Multiparameter analysis using temperature, salinity and ²²⁶Ra were applied to quantify the contribution of individual source water to the surface water of the East China Sea in summer and winter. The successful application of radium isotope in analysis demonstrates the usefulness of ²²⁶Ra in the discrimination of mixing among multiple water sources. In surface layer, the mixing proportion of KSW and CDW change obviously season to season, while the KSSW and TSWW have no qualitative change. In bottom layer, the KSSW flowed underwater primarily after intruding the continental shelf and entered the surface with the upwelling in shore, which is one of possible impetuses for the complementary nutrients from exterior regions. The Taiwan Strait Warm Water extended northward continuously up to the Changjiang Estuary.