承压含水层中放射性示踪剂的非常规径向弥散机理

Mechanism of unconventional radial dispersion of radiotracer in confined aquifers

采用放射性示踪剂评价承压含水层对污染的敏感性和再现污染物补给历史时,示踪剂在承压含水层中的迁移机理尤为重要。采用两区模型建立考虑混合效应的径向弥散数学模型来研究放射性示踪剂在承压含水层中的非常规径向弥散机理,采用拉普拉斯变换法推导模型的解析解,采用Stehfest法实现拉普拉斯数值逆变换;为了检验模型中假定条件的精度,利用COMSOL软件建立有限元数值模型进行模拟验证。结果表明:①混合效应越明显时,放射性示踪剂在井筒中的浓度越低;②放射性示踪剂的衰变常数越大,其在承压含水层中的迁移越缓慢;③当质量传输速率小于0.001 a^(-1)时,可以忽略非常规径向弥散对放射性示踪剂的影响,反之,质量传输速率越大,放射性示踪剂在承压含水层中的浓度越低。

The mechanism of migration of radiotracer in confined aquifers is particularly important when it is used to evaluate the sensitivity of confined aquifers to pollution and reconstruct the history of pollutant vechange.In this study,a new mobile-immobile model was used to establish a radial dispersion mathematical model considering the mixing effect to study the unconventional radial dispersion mechanism of radiotracer in confined aquifers.Analytical solution was derived using the Laplace transform,and the Stehfest method was used to facilitate the numerical inverse Laplace transform.To test the assumptions used in the mathematical model,a numerical simulation was executed by the finite element method developed by the COMSOL Multiphysics.The results are as follows:①A larger mixing effect can result in a smaller concentration of radiotracer in the wellbore;②The greater the decay constant of the radiotracer,the slower the migration of the radiotracer in the aquifer;③The unconventional radial dispersion in an aquifer can be neglected when the mass transfer coefficient is less than 0.001 a^(-1).Otherwise,the greater the mass transfer coefficient,the lower the concentration of the radiotracer in the aquifer.