华北平原弱透水层化学渗透效应的试验研究及其数值模拟

发布时间:2019-01-10 14:50
【摘要】:低渗透粘性土或沉积物中的地下水流和溶质运移问题是地下水领域内的一项重要研究课题。与传统的高渗透性介质中地下水流问题不同的是,低渗透介质会表现出与生物膜类似的"半透膜"性质,其内的地下水流不仅会受到水力梯度的驱动,而且还会受到浓度梯度(化学渗透)、电压(电渗)以及温差(热渗)等组成的共轭梯度场的驱动,其中,化学渗透的驱动作用最为明显。近年来,这种由不同梯度驱动的低渗透介质耦合流问题被越来越多不同领域的研究所关注。然而,我国关于粘土半透膜效应的化学渗透研究鲜有涉及。为此,本文从化学渗透的角度着力讨论华北平原典型咸淡水界面的下移机理。论文首先综述了研究背景、意义以及粘土半透膜效应的化学渗透研究现状,包括室内试验、场地试验和化学渗透现象的野外证据等。针对粘土的"双电层"理论总结了评估粘土膜性能的多个不连续模型。通过对比验证发现,化学渗透率系数(σ)随着阳离子交换容量(CEC)的增大而增大;而随着孔隙度、溶质浓度的增加,σ减小。为了更加深入了解粘土的半透膜效应,本文利用自主设计的硬壁式渗透仪对华北平原典型咸淡水含水层间弱透水层的原状土及利用原状土加工制作的重塑土分别进行了渗透率与化学渗透率系数σ的测定。通过渗透试验,在浓度差为0.152M的浓度梯度下,测得原状土的σ=0.013;重塑土 σ=0.089。对两种土样来说,虽然具有同样的成分及孔隙度,但原状土的膜性能明显小于重塑土。这是由于重塑土较原状土的粘土颗粒排列更加规则,因而表现出更高的膜性能。通过反渗透(超滤)试验,在10m水头差以及0.1092M浓度差的作用下,原状土在水头升高时发生破裂未测得相应的σ,重塑土的σ=0.28,在20m的水头差下,重塑土的σ=0.238,远大于化学渗透法测得的σ。同时,讨论了基于不同不连续模型估测的σ,与试验结果对比表明,Bresler模型与实际结果相对比较匹配。为拟合试验中的压力差及浓度差随时间瞬时变化的关系,引入一种连续模型对室内试验结果进行模拟,结果表明,如果参数赋值合理,连续模型可以很好地拟合试验过程,尤其该连续模型还能对Noy et al.(2004)的场地试验结果进行拟合。此外,对模型的各个参数进行敏感性分析,由此确定了不同参数对压力曲线进化过程的影响程度。其中,σ主要影响压力峰值的大小,弥散度D主要影响压力曲线到达峰值后的衰减速率,而粘土的渗透率k与多孔石的弹性释(贮)水系数Ss对压力曲线到达峰值的时间影响很大。最后,选择华北平原咸淡水典型区的衡水市桃城区,将本文确定的咸淡水界面下移情况与前人的研究进行了对比,发现下移速度较缓。考虑到该区的粘性土层具有一定的膜性能,认为该区含水层系统形成了一个典型的化学渗透系统,产生的化学渗透压力一定程度的抵消了由于过采深层淡水导致的向下的水头差,从而减缓了咸淡水界面的下移速率。同时,在华北平原典型咸淡水剖面上利用连续模型对剖面上渗透压力的演化进行模拟,剖面东部由于浓度差较大而具有更高的向上的渗透压力,第Ⅲ、Ⅳ含水层由于厚度大较第Ⅱ含水层更晚的到达压力峰值。在不考虑人为因素以及其他地质构造作用的情况下,一些地区的渗透压力峰值可以达到近60m。因此,在华北平原咸-淡含水层间的弱透水层普遍存在着化学渗透现象。显然,在研究该区咸淡水界面下移机理时,不能忽略咸-淡含水层间粘性土的化学渗透效应对咸淡水界面下移的影响。
[Abstract]:The problem of groundwater flow and solute transport in low-permeability cohesive soil or sediment is an important research subject in the field of groundwater. in contrast to the problem of the flow of groundwater in a conventional high-permeability medium, the low-permeability medium will exhibit similar "semi-permeable membrane" properties as the bio-film, with the groundwater flow in it not only being driven by a hydraulic gradient, but also by a concentration gradient (chemical penetration), The electro-osmotic pressure (electroosmosis) and temperature difference (heat-infiltration) are the driving of the co-static gradient field, and the driving effect of the chemical penetration is the most obvious. In recent years, this problem of coupled flow of low-permeability media driven by different gradients is becoming more and more important in many different fields. However, there are few chemical permeation studies on the semi-permeable membrane effect of clay. In this paper, the mechanism of the downward movement of the typical salty fresh water interface in the North China Plain is discussed from the angle of chemical penetration. The paper first reviews the research background, significance and the research status of the chemical penetration of the clay semi-permeable membrane effect, including the indoor test, the field test and the field evidence of the chemical penetration. A number of discrete models for evaluating the performance of the clay film are summarized for the "double electric layer" theory of the clay. By contrast, the chemical permeability coefficient (CP) was increased with the increase of the cation exchange capacity (CEC), and with the increase of the porosity and the concentration of the solute, the concentration of the solute decreased. In order to get a better understanding of the semi-permeable membrane effect of the clay, this paper makes use of the self-designed hard-wall permeameter to measure the permeability and the chemical permeability coefficient of the undisturbed soil of the weak water-permeable layer between the typical saline and fresh water-bearing layers in the plain and the remolded soil made from the original soil. Through the penetration test, under the concentration gradient of 0.152M, the density of the undisturbed soil was measured to be 0.013, and the remodeled soil pressure was 0.089. For both soil samples, although the same component and porosity, the film performance of the undisturbed soil is obviously less than that of the remolded soil. This is due to the more regular arrangement of the clay particles in the undisturbed soil of the remolded soil, and thus exhibits a higher film performance. By means of reverse osmosis (ultrafiltration) test, under the action of the head difference of 10m and the difference of the concentration of 0.1092M, the undisturbed soil has not been measured at the elevation of the head, the pressure of the remodeled soil is 0.28, under the head difference of 20m, the pressure of the remodeled soil is 0.238, which is much larger than that measured by the chemical penetration method. At the same time, the results of the estimation of different discontinuous models are discussed, and the comparison between the results of the test and the test results shows that the Bresler model is relatively matched with the actual results. In order to simulate the pressure difference and the difference of the concentration difference over time in the fitting experiment, a continuous model is introduced to simulate the indoor test results. The results show that, if the parameter assignment is reasonable, the continuous model can fit the test process well. In particular, the continuous model can also fit the site test results of Noy et al. (2004). In addition, the sensitivity analysis of each parameter of the model is carried out, and the influence degree of different parameters on the evolution process of the pressure curve is determined. In this paper, the peak value of pressure is mainly affected by the pressure curve, and the dispersion degree D mainly affects the decay rate after the pressure curve reaches the peak value, and the time that the permeability k of the clay and the elastic release (storage) water system number Ss of the porous stone reach the peak value of the pressure curve is very large. In the end, we select the Taocheng District of Hengshui City, which is the typical area of the salt and fresh water in the North China Plain, and compare the downward movement of the salty fresh water interface determined in this paper with the previous research, and find that the moving speed is slower. Considering that the cohesive soil layer in this area has a certain membrane property, it is considered that the aquifer system of this area has formed a typical chemical permeation system, and the resulting chemical penetration pressure is partly offset by the downward head difference due to overproduction of deep fresh water, so as to slow down the down-down rate of the salty fresh water interface. At the same time, the evolution of the permeability pressure on the section is simulated by the continuous model in the typical salty fresh water section of the North China Plain, and the eastern section of the section has a higher upward permeation pressure due to the large concentration difference, and the third, The fourth aquifer reaches the pressure peak more late due to the larger thickness than the second aquifer. In the absence of human factors and other geological structures, the peak of the osmotic pressure in some areas can reach nearly 60m. Therefore, the phenomenon of chemical penetration is common in the weak water-permeable layer between the saline-fresh water-bearing layers in the North China Plain. It is clear that the effect of the chemical penetration effect of the cohesive soil between the brackish-water and the brackish fresh water is not neglected in the study of the down-moving mechanism of the brackish fresh water interface in this area.
【学位授予单位】:南京大学
【学位级别】:博士
【学位授予年份】:2017
【分类号】:P641.3

【参考文献】

相关期刊论文 前3条

1 费宇红;张兆吉;宋海波;钱永;陈京生;孟素花;;华北平原地下咸水垂向变化及机理探讨[J];水资源保护;2009年06期

2 席永慧;冯世进;;粘质土壤的膜性能研究进展[J];同济大学学报(自然科学版);2009年02期

3 宋海波;张兆吉;费宇红;王振昌;钱永;卫文;;开采条件下河北平原中部咸淡水界面下移[J];水文地质工程地质;2007年01期



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