咸淡水驱替过程中的水文地球化学作用

HYDROGEOCHEMISTRY IN THE PROCESS OF SALT WATER-FRESHWATER DISPLACEMENT

在现场水文地质调查的基础上,分别采集大沽河下游地下淡水、海水和含水介质样品,并测定它们的性质和组成,然后用渗流装置模拟海水驱替淡水的过程,测定驱替过程中含水介质渗透性(渗透系数、孔隙度、弥散系数等)和主要离子(Na+、K+、Ca2+、Mg2+)的变化规律,最后,采用数值模拟方法定量研究驱替过程中的水文地球化学作用。研究结果表明,驱替过程中发生了多组分的离子交换作用,Ca2+和Mg2+最大值高于海水和淡水中的离子浓度值,Na+—Ca2+、Na+—Mg2+发生离子交换;Mg2+达到最大值后有一个降低的"台阶"阶段,此时Mg2+—Ca2+发生离子交换;K+在驱替过程中主要表现为化学吸附。离子交换过程中伴随着矿物溶解-沉淀,海水比例40%后,方解石处于过饱和状态;在驱替过程开始阶段,石膏饱和指数逐渐升高,达到峰值后又开始下降,最后饱和指数又有所升高,但始终处于非饱和状态。

On the basis of hydrogeologic investigation, groundwater,sea water and the medium are collect- ed, whose composition and nature are determined, and then the regularity of permeability（penetration coefficient, porosity and dispersion coefficient） and the main ion（Na＋ , K＋ , Ca2＋ , Mg2＋） are determined in the course of salt water-freshwater displacement with transfusion installment. Finally, hydrogeochemistry is studied for the displacement with the numerical simulation. The result indicated that ion exchanges take place in the process of salt water-freshwater displacement, the density of Ca2＋ and Mg2＋ exceeds that of seawater, and Na＋-Ca2＋ and Na＋-Mg2＋ appear. There is a smooth step for Mg2＋ when it reaches maximum value, Na＋-Mg2＋ takes place at that time,and K＋ is mainly showed as chemical absorption. Mineral disso- lution and precipitation occur with ion exchanges. Calcite reaches saturation when the percent of seawater comes up to 40%. The saturation of gypsum first becomes higher,and then drops when it reaches peak value, and finally rises slowly, but gypsum is in non-saturation state from beginning to the end.