Preconcentration And Determination Of Radium Isotopes And Their Applications As Tracers To Assess Water Mixing Processes In Estuaries

Estuaries are the most productive natural habitats in the world. Water residence times and submarine groundwater discharge (SGD) variations directly influence the ecological conditions and production rates in estuaries. Natural radium isotopes have become increasingly recognized for their utility in examining mixing processes in the coastal ocean. The information obtained from radium tracers can be applied to understand the source and fate of estuarine substances from a new perspective, as well as to develop management strategies for controlling the supply of terrestrial substances to offshore areas and protect the coastal environment.In this study,"AOQI"acrylic manganese fiber was prepared as preconcentration material for radium adsorption from water samples. Radium isotopes were measured by Radium Delayed Coincidence Counter, gamma spectrometer, Radon in line and RAD7. The lab calibration system for RAD7 was particularly established. In Liaohe estuary, Yangtze River estuary and Yellow River estuary, radium isotopes were measured and used as tracers to quantify water mixing processes. Water residence times and SGD flux were calculated via"Apparent Radium Ages Model"and"Mass balance Model", respectively. Moreover, the contribution of SGD carried terrestrial material to the ocean was assessed.The author would love to highlight the main results in this thesis as follow:1)"AOQI"acrylic manganese fiber was an excellent material for radium preconcentrtion. It was very easy to prepare and had pretty low radium background which could be neglected. Its absorption efficiency was high and stable. About 95-100% radium would be collected when water flow rate was slower than 2 L min-1. When water flow rate was 2-4 L min-1, the absorption efficiency decreased, but still higher than 90%.2) The RAD7 calibration system developed in this study was an air loop circulation system with a reproducibility of about±2%. The system could maintain 6 machines at the same time. The intercomparisons with Durridge Co. and Florida State University were performed which produced comparable results within an uncertainty of about±2%.3) In Liaohe estuary, the distribution of ²²⁴Ra and ²²³Ra were not conservative along salinity. There were two peak activities at salinity 10 and 26. Combined with other parameters, such as pH, DO, COD, petroleum hydrocarbon, chlorophyll and nutrients, the peak at salinity 10 seemed to be caused by suspended particles desorption, and the one at salinity 26 was influenced by saline submarine groundwater discharge. The largest water residence time was 8.34 days which appeared at the area with salinity of 14.4) In Yangtze River estuary, the activities of ²²⁴Ra, ²²³Ra, ²²⁶Ra and 228Ra were 0.42-101.75 dpm 100L^-1, 0.13-4.39 dpm 100L^-1, 5.62-12.91 dpm 100L^-1 and 34.28-57.55 dpm 100L^-1, respectively. The highest activity was appeared at low tide near-shore area and vice versa. Within the first 30 km offshore, the water residence times gradually increased to 18 days. An anomalous low salinity point occurred at a distance of 60 km, and the water ages based on the"Apparent Age Model"decreased to about 6 days and were fairly constant until 140 km offshore. The distribution of salinity, water temperature, SPM concentration demonstrated that this anomaly was influenced by terrestrial runoff rather than benthic inputs from a submarine spring.5) In Yellow River estuary, the activities of ²²⁴Ra,²²³Ra and ²²⁶Ra were 4.17-140.35 dpm 100L^-1, 0.70-4.02 dpm 100L^-1 and 27.71-65.52 dpm 100L^-1, respectively. All the radium isotopes were non-conservative versus salinity. In September, most of the Yellow River water turned right immediately after it discharged into the estuary. Under a flow rate of 5-7 km d^-1, the diluted Yellow Rive water arrived at the top of Laizhou Bay in about one week. Small amount of river water directed to the Bohai Sea in the northeast under a lower flow rate of less than 2 km d^-1. The SGD flux ranged from 0.15-0.67 m³ m^-2 d^-1, which was about 3-13 times of Yellow River discharge. The SGD input of dissolved uranium and nutrients were significantly higher than their fluxes of Yellow River runoff.