Tracing Coastal Water Mixing Processes And Submarine Groundwater Discharge By Radium Isotopes

Diffusion and advection are two important mechanisms of material transport in coastal and marine waters. They are significant in studying seawater solute transport, the assessment of self-purification capacity of the ocean, and the evaluation of eddy diffusion process in the ocean. This thesis focuses on the diffusion process in the southern Yellow Sea (SYS) using one-dimensional advection-diffusion model based on radium isotopes. The results reveal the effects of such process in changing the distribution of nutrient and its contribution to the primary productivity in the study area.Submaine groundwater discharge (SGD) is an important process for water and material exchange between land and ocean. It has been receiving wide academic attention in the study of hydrology and biogeochemistry, especially its role in driving nutrient transport in the biogeochemical cycles. This thesis investigates the SGD studies in the SYS, and two lagoons in the eastern coast of Hainan Island, China and Little Lagoon, Alabama, USA. The SGD-driven nutrient fluxes are quantitatively assessed to examine their impacts on the marine systems, which can help to understand the SGD processes in different geological and geomorphological environment.(1) Eddy diffusion process in the SYS (monthly scale). In the SYS, we measure the223,224,226,228Ra activities in seawater in the summer of2009. By establishing the horizontal one-dimentional advection-diffusion model (with advection indicators), the horizontal eddy diffusion coefficients are estimated as well as the corresponding nutrient fluxes. It is further interpreted that the nutrient fluxes by this process account for14%of N and2%of P required by primary productivity in the study area. Similarly, based on a vertical one-dimentional advection-diffusion model (neglecting advection), the vertical eddy diffusion coefficients below the thermocline (within the Yellow Sea Cold Water Mass, YSCWM) are estimated. The corresponding nutrient fluxes supply up to52%of N and40%of P to primary productivity. The results demonstrate that the vertical eddy diffusion process plays a significant role in supplying nutrient within the YSCWM.(2) SGD flux in the SYS and the contribution to nutrient. During the spring bloom in Febuary, March and June2009, we estimate the mean SGD into the SYS as8.4±3.9Lm-2d-1(226Ra model:6.4±4.1Lm-2d-1;228Ra model:10.4±6.7Lm-2d-1) by establishing226Ra and228Ra mass balance model. Uncertainties analysis of SGD (up to65%) shows that SGD is sensitive to the radium activities in the groundwater end-member, residence time, radium releasing from bottom sediment and the intrusion of Kuroshio water. After calculating the SGD associated nutrient fluxes and including other nutrient sources, we re-evaluate the nutrient fluxes to the Yellow Sea. The results show the SGD as an important source of dissolved inorganic nitrogen (DIN) and dissolved silicate (DSi), whereas the transfer of dissolved organic phosphorous (DOP) to inorganic forms may be a significant internal source of DIP.(3) SGD flux in the Bamen Bay and Wanquan river estuary of the Hainan Island and the associated nutrient. The SGD study is carried out in August2007and2008in the Bamen Bay (BB) and Wanquan river estuary (WQ) of the eastern Hainan Island. A three-end-member (river, groundwater and seawater end-members) mixing model based on water, salt and226Ra is applied to estimate the relative contribution of the Ra sources to two aquatic systems. The estimated SGD based on226Ra mass balance in the BB and WQ are14.5±21.5Lm-2d-1and4.87±4.47Lm-2d-1, respectively. In comparison with nutrient fluxes from the rivers, atmospheric deposition, aquaculture effluents and wastewater discharge, SGD-derived nutrient fluxes are an important nutrient supplier in the BB region. Such fluxes are non-negligiable in the calculation of nutrient budgets. In the WQ, nutrient fluxes from SGD are of the secondary importance. However, as groundwater is enriched with nutrient and high N:P ratios, the effects of groundwater discharging into lagoonal and coastal waters on water quality and eclology is worth considering.(4) SGD flux in the Little Lagoon, Alabama, USA and the implication for algal bloom. Radium (223,224,226,228Ra) and radon (222Rn) isotopes are used as tracers to assess SGD from the shallow and deep aquifers as well as the total SGD into Little Lagoon from2010through2012. The estimated lagoon-wide SGD rates based on a radium mass balance and the mixing model are1.22and1.48m3s-1(or10.0and12.2Lm-2d-1) for the shallow and deep groundwater discharges, respectively. The total SGD rates based on a radon mass balance approach are found to vary from0.60to2.87m3s-1(or4.93to23.6Lm-2d-1). We observe well-defined relationships between the concentration of nutrients and chlorophyll-a in lagoon waters during a period when there is an intense diatom bloom in April2010and when no bloom exists in March2011. A good correlation is also found between radium (groundwater-derived) and nutrients during the April2010period, while there is no clear relationship between the same parameters in March2011. Multivariate analysis for chemical and environmental factors suggests that nutrient-rich inputs during high SGD into the lagoon are a significant driver of algal blooms in April2010.In summary, this thesis presents the application of radium isotopes as tracers in quantifying submarine groundwater discharge and coastal water mixing processes. By establishing radium isotope mass balance model, we quantitatively assess the SGD flux. This helps to deepen the understanding of land-ocean interactions and to corretly evaluate the impacts of SGD on the marine ecological environment. Based on the one-dimensional advection-diffusion model, we quantitatively estimate the eddy diffusivity and the associated nutrient fluxes. The findings provide the basis for the scientific management in the coastal area.