不同冻融条件下包气带中典型石油组分迁移行为及其影响机理研究

In China, more than half of the territory belongs to frozen soil region. Soil structure, moisture content, and water transport in frozen soil region during alternating freezing and thawing action has different dynamic patterns and characteristics comparing with conditions without freezing and thawing action. What is more, the variable freezing-thawing conditions(the direction of freezing and thawing occurrence, temperature gradient) and phase transitions of water make the transport behavior of petroleum components in vadose zone much more complicated. In this project, we choices Benzene,Octane, 1, 2 and two ethyl chloride as type petroleum components, and focus on the dynamical distribution characteristics of typical petroleum components in soils changing in frozen soil under variable freezing-thawing conditions. Combining macro-distribution rules of the components and the micro-structural characterization of frozen soil and in terms of the dynamic couple characteristics of heat, moisture, and solute transport, the parameters such as temperature, moisture content of unfrozen water, concentrations of components, soil thermal conductivity, and relative permeability are dynamically monitored in frozen soil during alternating freezing and thawing action. According to the numerical simulation and mechanism analysis, we focus on the distribution, migration direction, migration rates, among others of the typical petroleum components in frozen and non-frozen layers under the dynamic changes of thaw direction and temperature gradient on the one hand, and the interface kinetics analysis of component transport on the other hand. This research will try to reveal the influencing transport mechanism of petroleum components under variable freezing-thawing conditions and the results will have theoretical and practical implications for the pollution control and remediation of petroleum in frozen soil region.

我国半数以上的国土面积为冻土区，冻土区包气带的结构、含水量变化及水分运移的特征具有不同于无冻土条件下的动态规律和特点，且不同冻融条件（冻融方向、温度梯度及温度变化速率不同）及水的相变使得冻融过程中包气带中石油组分的迁移行为变得更加复杂。本项目选择苯、辛烷和石油产品添加剂1,2-二氯乙烷为典型石油组分，围绕热量传输、水分运移、石油组分迁移耦合特征，按照组分浓度分布宏观规律分析与冻土微观结构刻画相结合的研究思路，通过冻融过程中对冻土系统的温度、未冻结水含量、土水势、组分浓度、系统导热性、相对渗透率等参数的动态监测，利用理论分析与模型计算的方法，研究不同冻融条件下冻层与非冻层界面上典型石油组分迁移方向、迁移速率等行为特征，分析组分迁移的界面动力学过程，力图揭示不同冻融条件下石油组分迁移的影响机理。研究成果对冻土区石油污染控制及修复具有理论和现实意义。

我国是世界第三冻土大国，冬季冻结、春季融冻的季节性冻土面积约占国土面积的53.50%；冻结和融冻会直接影响许多水文地质过程和生物过程，包括热状态、地下水流动模式、污染物迁移及地下生物活动等。本项目选择苯和硝酸盐为典型组分，围绕热量传输、水分运移、污染组分迁移耦合特征，通过冻融过程中对温度、未冻结水含量、组分浓度等参数的动态监测，研究不同冻融条件下冻层与非冻层界面上典型组分迁移方向、迁移速率等行为特征。. 研究结果显示：（1）温度条件的差异对于苯在冰-水两相中迁移行为的影响主要体现在冻结率0.3~0.95的区间内，这一区间温度越低,冻结前期形成的冰相浓度越高；（2）溶液初始浓度差异对苯在冰-水两相中分布迁移的影响主要体现在冻结率0.45~1的区间内，溶液初始浓度越高，未冻溶液浓缩倍数越低，苯进入冰相的比例越大。作为无机组分典型代表物硝酸盐，其在孔隙介质冻结过程中的迁移特征则明显有别于有机物苯，主要表现在：（1）冻结过程中硝酸盐在冰-液界面的再分配特征可分为：“初始过渡阶段”、“稳态阶段”和“最终过渡阶段”；（2）硝酸盐在冰-液相重新分配，部分被排入到液相，硝酸盐在冰-液相的再分配会受冻结速率和溶质向外扩散速率的影响；（3）介质孔径对饱和孔隙液冻结过程中硝酸盐再分配过程的影响显著；（4）将本研究结果扩展到野外时，可以推测，当生态湿地地表水体冻结时，由于溶质扩散受到底泥阻碍，在靠近底部饱水底泥顶面的水体中将会形成盐分高浓度层。. 本研究在理论上将有助于理解冻融过程中溶质在冰-水-土中再分配行为本质，丰富污染水文地质学理论体系；在应用价值层面，则能够深入了解溶质迁移及再分配机理，对冻土区土壤-地下水环境保护及污染修复有重要意义。