近海海域镭同位素周期性变化的解析研究

发布时间：2017-12-31 21:31

本文关键词：近海海域镭同位素周期性变化的解析研究　出处：《中国地质大学(北京)》2016年硕士论文　论文类型：学位论文

【摘要】：海底地下水排泄(SGD)是全球水循环的一个重要组成部分,近年来成为陆-海相互作用的研究热点。在评估SGD、水体滞留时间和混合速率等方面的各种方法中,镭同位素方法的应用是最为广泛和成熟的。当前对镭同位素近海分布的解析研究都建立在稳态模型的基础上,并且通常不考虑水深和洋流速度。研究者通常利用稳态平流扩散方程估计涡动扩散系数,进而求得SGD。而实际中镭同位素在近海的放射性浓度随高低潮、大小潮和季节变化非常明显,此外,因为大陆架坡度和洋流的影响,水深的变化和流速对镭同位素浓度分布的影响也不能忽略。这样,上述常用模型就显示出其不足之处。本研究建立考虑水深变化、流速影响和周期性浓度变化的镭同位素平流扩散模型,讨论这三种影响因素在估计相关参数时所造成的影响,根据Moore在不同季节的观测数据,用新的模型估计涡动扩散系数,并与稳态模型求得的值进行比较。本研究发现新的模型在估计近海涡动扩散系数和SGD等方面有重要改进。反映近海镭同位素分布时空变化的平流扩散方程由四个项控制,分别是涡动扩散项、对流项、衰变项和时变项。前两项表示的是由于海水的涡流和平流所造成的扩散,衰变项表示同位素放射性衰变造成的影响,时变项表示镭同位素放射性浓度随时间变化所造成的影响。根据数据分析,近海镭同位素放射性浓度往往在夏天最高,冬天最低,这是由于SGD等的季节性变化所导致的。Moore在1998到2000年观测到Winyah Bay附近海域,223Ra浓度在3dpm/100L上下变化,年变化量在2~3dpm/100L,224Ra和226Ra则在25dpm/100L上下浮动,季节性变化达到了约15dpm/100L,228Ra在35dpm/100L上下浮动,其变化的范围则高达约25dpm/100L。根据这个数值进行计算,对于长半衰期的同位素,即226Ra和228Ra,其衰变项很小,这时时变项对方程的影响要比衰变项大2~4个数量级,因此如果不考虑时变项,那么利用平流扩散方程估计参数会造成很大的误差。对223Ra,不考虑季节性变化估计参数的误差高达30%,对224Ra这个误差是8%。因此,对于短半衰期的镭同位素,季节性变化也不能忽略。利用镭的四种同位素的数据,根据新模型估计出Winyah Bay的涡动扩散系数是98.5km2/d,而稳态解求出的平均值则为87.7km2/d,新的模型求得的涡动扩散系数是稳态模型的1.12倍,这个结果表明了新模型与稳态模型的明显差别,说明了在模型中考虑镭同位素季节性变化的重要性。
[Abstract]:Submarine groundwater discharge (SGD) is an important part of the global water cycle, in recent years become a hot research topic in land ocean interaction. In the assessment of SGD, various methods of water retention time and mixing rate etc. in the application of radium isotope method is the most widely used and mature. The analytical study the distribution of offshore radium isotopes are based on the steady state model, and are not usually considered the water depth and current velocity. Researchers usually use the steady advection diffusion equation to estimate the eddy diffusion coefficient, and then obtain the SGD. and actual radium isotopes in radioactive concentration near the sea with high tide, tidal and seasonal variation in size is very obvious. In addition, because of the impact of the continental shelf slope and ocean currents, the changes of water depth and flow rate on the impact of radium isotope concentration distribution also cannot be ignored. In this way, the common model shows its disadvantages of this research. In considering the change of water depth, velocity and effect of radium isotope flat periodic concentration flow diffusion model, discuss the impact of these three factors in the estimation of the relevant parameters, according to the Moore observation data in different seasons, estimation of eddy diffusion coefficient with the new model, and compared with the steady-state model is obtained value. This study found that the new model has significant improvement in the estimation of offshore eddy diffusion coefficient and SGD. Reflect the temporal and spatial variation of isotopic distribution of radium offshore advection diffusion equation is controlled by four, respectively is the eddy diffusion, the flow, decay and time varying. Before two is due to the diffusion caused by the vortex flow of the seawater of peace, said the impact of radioactive isotope decay term caused by the time-varying term represents the influence radium isotope radioactive concentration changes with time caused. According to the data analysis, offshore The concentration of radioactive radium isotopes in the summer are the highest, lowest in the winter, this is caused by the seasonal change of SGD and.Moore in 1998 near the sea observed in 2000 Winyah Bay, the 223Ra concentration in 3dpm/100L, annual variation in 2~3dpm/100L, 224Ra and 226Ra were floating down in the 25dpm/100L, seasonal changes to about 15dpm/100L, 228Ra floating down in the 35dpm/100L, the change range is up to about 25dpm/100L. calculated according to this value, the isotope half-life, namely 226Ra and 228Ra, the decay is small, then the time-varying term of equations than decay term 2~4 order of magnitude, so if you don't considering the variable, then the advection diffusion equation of parameter estimation will cause great error. On 223Ra, without considering the seasonal variation of the parameter estimation error of up to 30% of 224Ra this error is 8. Therefore, for short Radium isotope half-life, seasonal changes can not be ignored. By using four kinds of radium isotope data, according to the new Winyah Bay model to estimate the eddy diffusion coefficient is 98.5km2/d, and the average value for the steady-state solution for 87.7km2/d, a new model to obtain the eddy diffusion coefficient is 1.12 times higher than the steady-state model. The results show that the difference between the new model and steady-state model, illustrates the importance of model in senior high school entrance examination into seasonal changes. Radium isotopes

【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：P734;P641.3

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