充气法防治华北平原区咸水下移的初步研究

The saline water descending occurs when deep fresh groundwater is over extracted in the coastal region of the North China Plain, which makes deep fresh groundwater salinization. In this study, a compressed air injection method was proposed to control the problem of saline water descending. A three-dimensional gas-liquid-solid multi-component coupling numerical model was first established for simulating subsurface saline-fresh water transport, the process of air injection and soil deformation, which was first verified by simulating the classical cases with analytical solution. The laboratory experiments related to the process of saline water descending with compressed air injection were further performed, and simulated with the proposed numerical model, and the model will be revised through the comparisons of simulating results to measured data. Then, the proposed numerical model will be used to investigate the impacts of the possible factors on the movements of saltwater and freshwater interface and to simulate the effects of compressed air injection for mitigating the saline water descending. Based on the deep analysis of application conditions and feasibilities of the compresses air injection, combining with the numerical simulation analysis, the field experiments were conducted, to further check the efficiency of the compresses air injection to control the saline water descending, thereby to contribute to exploring new methods suitable for the characteristics of water resources in the North China Plain for mitigating the saline water descending.

针对华北平原滨海地区由于深层淡水的过量开采，引起上覆咸水体的界面向下移动，对深层淡水水质造成威胁的问题，提出了人工充气法防治咸水下移的新思路。本研究首先建立模拟地下咸-淡水运移、充气过程及土体变形相互作用的三维气-液-固多组分耦合数学模型，并通过模拟分析经典算例对模型进行初步验证；进行充气作用下咸-淡水运移过程的室内模型试验，利用所建立的数学模型对室内试验过程进行模拟，通过实测值与模拟值的对比分析，进一步对模型进行修正；然后以该模型为工具定量研究各种可能的影响因素对咸-淡水界面运移规律的影响，模拟预测充气法防治咸水下移的有效性；在深入分析充气法的适用条件及可行性分析的基础上，结合数值模拟分析，进行野外现场试验，进一步对充气法防治咸水下移的有效性进行检验，从而为探索适合华北平原水资源特点的新型咸水下移防治对策做出贡献。

华北平原地区的咸水下移问题是由于在浅层咸水分布区过度开采深层承压淡水时，承压水位大幅下降，改变了原有的水动力平衡，引起浅层咸水体界面向下移动，导致深层淡水水质咸化，从而对深层淡水水质造成威胁。根据减小深层淡水与浅层咸水之间的压力差可防治咸水下移的机理分析可知，除了人工回灌淡水外，人工充气法可以将高压空气注入到咸-淡水界面下层的含水层中，形成具有向上顶托作用的空气帷幕，从而达到防治咸水下移的目的。本项目提出了人工充气法防治华北地区地下咸水下移的新思路，通过数值模拟、室内试验和野外试验相结合的方法，探索研究了人工充气法的可行性。首先对TOUGH2/T2DM进行三维扩展，建立三维条件下考虑水动力弥散的气-液二相流多组分运移模型；然后应用FLAC3D软件来数值分析地下水三维应力场，通过FISH语言开发接口程序实现3DT2DM与FLAC3D耦合，从而建立模拟地下咸-淡水运移、充气过程及土体变形相互作用的三维气-液-固多组分耦合数学模型。然后采用经典的变异Henry问题对3DT2DM进行验证；然后借助一对称的三维理想算例，进一步验证3DT2DM在三维水动力弥散问题上的适用性。开展了咸水运移的砂箱模型试验，进行了抽水引起的咸水锥进试验，利用所建立的数学模型模拟了咸水锥进过程，将数值模拟结果与试验结果进行了对比，进一步验证了模型的准确性和有效性。进而以所建立的模型为工具，定量分析了抽水速率、含水层介质特性等特性参数对咸水运移的影响；模拟分析了人工充气法对抽水引起的咸水下移过程的影响，模拟结果表明在充气作用下，所形成的空气帷幕有效减缓了上层咸水体界面的向下移动。最后在宝坻黄白桥水文站进行野外现场试验，获得了地下充气作用对各个土层的饱和度、渗压力、地下水盐度及地表变形的测量数据，结果表明充气作用形成了有效的空气帷幕，并且充气作用停止后空气的消散过程比较缓慢。该研究成果为进一步充气法防治咸水下移的应用研究提供了一定的理论依据和实践经验。