A Quantitative Study On The Sources And Sinks Of Radium Isotopes In Near-shore Waters

In recent years, concerns have been expressed over the evaluation of ecosystem health of near-shore waters and sustainable development by means of natural radioactive radium isotopes to trace the path and rate of input, transfer and conversion of land-origin materials in coastal waters. There are two main aspects applying radium isotopes, the submarine groundwater discharge (SGD) and the mixing process of near-shore waters. However, accuracy of oceanographic information revealed by radium isotopes depends on the cognition for sources and sinks of radium isotopes in near-shore waters. The thesis returns to the scientific essence from the trace application of radium isotopes, focus to sources and sinks of radium isotopes in near-shore waters, and compares their various influence to the four radium isotopes. These foundations can be expected to supply some significant information for the discrimination and evaluation the source, sink and biogeochemical behavior of continental materials in near-shore waters.(1) Analysis methods for radium isotopes. Based on over two years experience in measurement 224Ra and 223Ra using the radium delayed coincidence counter (RaDeCC), this thesis discusses the systematic uncertainties, efficiencies, background values, critical and detective limits of RaDeCC. The results show that the system efficiencies of 224Ra and 223Ra are in the ranges of 0.47～0.59 and 0.45～0.57. The detective limits are 0.02 dpm 100L-1 for 224Ra and 0.07 dpm 100L-1 for 223Ra, with a precision of 5% for 224Ra activity and 15% for 223Ra in routine analyses. However, the uncertainties of low activities samples will increase to 10%-30% for lower than 5 dpm 100L-1 of 224Ra and 20%-80% for lower than 0.3 dpm 100L-1 of 223Ra. This thesis also discusses the systematic uncertainties of 226Ra and 228Ra measured by Alpha-spectrometry. The results show that a precision of 4.5% for 225Ra activity and 5.1% for 228Ra in routine analyses, which has no variation with the 226Ra or 228Ra activity. Through the comparison with Gamma-spectrometry, Alpha-spectrometry can give more accurate data for 226Ra and 228Ra isotopes to oceanographers.(2) Radium desorption experiment. The desorption behavior of radium isotopes on coastal sediments is a key factor impacting their land-ocean interaction in near-shore waters. By using laboratory desorption experiment and field observation of 224Ra activities in pore water of sediments, this thesis discusses the effect of water salinity on radium desorption from bottom sediments in the mangrove wetland, Bamen Bay, Hainan. The results show that the sediment contains 0.44 dpm g-1 of exchangeable 224Ra, which corresponds to approximately 35% of the total 228Th activity. On the basis of the field data of 224Ra activities in pore water, it is found that there is a inversely proportional relationship between the distribution coefficient (Kd) and the water salinity (S), Kd= 8.4×102 S-1, which is more representative of the real behavior of radium desorption from sediments, as compared with the results of the laboratory desorption experiments. It deduces that the 224Ra-228Th secular equilibrium was approached in the upper 25～40 cm sediment column. By conducting laboratory desorption experiment, this thesis also finds that the suspended sediment of Changjiang contains 1.4 dpm g-1 of exchangeable 224Ra, which corresponds to approximately 38% of the total 228Th activity.(3) Radium sources and sinks in Changjiang estuary and its adjacent offshore area. This thesis reports the initial result of the flux of submarine groundwater discharge (SGD) into the Changjiang diluted water (CDW). A radium mass balance model and a 224Ra/223Ra activity ratio (AR) model were applied to estimate the residence time of water above the pycnocline in the CDW. These two approaches gave similar results, yielding residence times of 5.4 d and 7.0 d, respectively, in the inner and outer plume zones. The non-conservative inventory of 226Ra was established in the CDW, and converted to a 226Ra flux of 5.5×1011 dpm d-1 by dividing by the water residence time. The excess 226Ra flux into the CDW was mainly contributed by the desorption of 226Ra from suspended sediment and the releasing from SGD, which accounted for 48% and 49%, respectively. When the 226Ra activity of groundwater in Chongming Island and Shengsi Islands was used as the end-member, the SGD flux into the CDW was estimated to be (0.2～1.0)×109 m3 d-1, which was equivalent to 6%-30% of the Changjiang water discharge during flood season.(4) Benthic fluxes released from the mangrove wetland, Bamen Bay, Hainan. Based on the 224,223,228,226Ra activities of pore-waters and the 228Th,228,226Ra activities of sediments in the mangrove wetland during April-May 2010 and August 2011, this thesis utilizes four mathematical models to estimate the benthic fluxes for all of radium isotopes, including the Hancock model, the Nozaki model, the S&T model and Webster model. The controlling processes described by these four model include molecular diffusion from the sediment, sedimentary mixing by bioturbation, the ion-exchange equilibrium between sediment and pore water, sediment accumulation, and the radioactive decay and production. The results of sensitivity analysis show that the relative importance of these individual transport processes should be carefully assessed with respect to each radium isotope. Through the comparison among the four models, we choose the Nozaki model to calculate the benthic fluxes released from the bottom and intertidal sediments of the entire lagoon.(5) Radium sources and sinks in the lagoon water of Bamen Bay, Hainan. Based on the 224,223,228,226Ra activities of the lagoon water and two time-series samplings of four radium isotopes conducted simultaneously at the Qinglan Tidal Inlet and Wenchang River Estuary during April-May 2010 and August 2011, the steady state mass balance model employed in this thesis considers all of the processes potentially controlling the temporal and spatial variability of radium. The results show that the residence time of 11±5.9 d led by the lagoon exchanging with the offshore water. Of these radium sources, the rivers and suspended sediments inputs accounted for 1%～ 5% of the total radium flux in Bamen Bay. The benthic inputs from intertidal and bottom sediments were significant for short-lived radium isotopes and accounted for 68%-100% of the total 224Ra and 223Ra fluxes. The groundwater discharges were dominated for the long-lived radium isotopes and accounted for 76%-96% of the total 228Ra and 226Ra fluxes, respectively. The distinct radium (228Ra/226Ra) isotopic signature for fresh and brackish groundwater offered a means of distinguishing these components. After constructing the binary mixing model, we estimated the fluxes of fresh and brackish groundwater, which ranged from (3.0±2.3)×105 m3 d"1 (wet season) and from (4.8±13)×005 m3 d-1(dry season) for the fresh groundwater and from (5.0±2.4)×105 m3 d-1 (wet season) and from (1.2±1.5)×106 m3 d-1 (dry season) for the brackish groundwater.(6) The mixing process of coastal waters in Gaolong Bay and Boao Bay, Hainan. The 224Ra/223Ra AR model was utilized to calculate the apparent age of waters based on the offshore distribution of 224,223,228,226Ra in Gaolong Bay and Boao Bay in May 2010 and August 2011. The dissolved radium are mainly released from the coast of Gaolong Bay. The water apparent ages were 4.4 d in dry season and 2.2 d in wet seasons in the distance of 10 km from coast to seaward, which was consistent with the water residence time. The Boao Harbor’s output and coastal release were both the main sources of the dissolved radium in Boao Bay, which maintained in the same order of magnitude. The water in Boao Bay migrated from near shore to approximate 2.8 km away from coast with around 3.3 d in dry season. In wet season, the water transfer to approximate 6.9 km away from coast and spent around 2.2 d. The water residence time of Boao Bay was roughly equal or slightly less than the apparent age calculated by the 224Ra/223Ra AR model. When the apparent age is less than 2.5 d, the uncertainty analysis shows that the uncertainties are always higher than 100% and exponentially increased accompanying with the decrease of the apparent age. This situation weakens the reliability of the apparent age, which is evaluated for coastal mixing process by using 224Ra/223Ra AR model.Therefore, in order to take advantages of the tracing application of radium isotopes in near-shore biogeochemistry, it is urgently needed to the quantitatively assess significances of radium source and sink terms in different water types. Only if the researchers start from the hydrographic features of the objects, build and apply these mathematical models fitting to practical situation, reasonable information could be offered to these research questions that release, migration and conversion of the terrestrial materials, such as nutrients, carbon and trace metal elements, in near-shore environment.