珠江三角洲弱透水层孔隙水水化学及运移模拟

Hydrochemistry and numerical simulation of pore water solute of the Quaternary aquitard in the Pearl River Delta

珠江三角洲第四纪弱透水层上覆于底部承压含水层之上,其水文地球化学演化过程对底部含水层有决定性影响。为认识珠江三角洲弱透水层孔隙水的形成及演化,本文对中山市民众镇ZK01钻孔黏性土孔隙水化学特征、水文地球化学参数、矿物饱和指数等进行了分析,结果显示孔隙水盐度较高,TDS=4.42~30.47 g/L。孔隙水水化学类型为Cl^(-)Na型,与海水类型一致,且主要水化学组分具有浓度随深度增加而上升的趋势。Cl/Br、γ(Na/Cl)系数和Gibbs分析显示孔隙水具有海水起源的特征,地下水盐度很可能来源于全新世海水。SO_(4)^(2-)主要受黏性土中硫酸根还原作用的控制,而不受石膏沉淀作用的影响。浅层孔隙水中Ca^(2+)、Na^(+)在孔隙水演化过程中受到阳离子交替吸附作用影响,表现为Na^(+)的亏损和Ca^(2+)的富集。NH_(4)^(+)主要与Mg^(2+)发生交换作用,致使孔隙水中NH_(4)^(+)增加。K^(+)在孔隙水演化过程中受混合作用和吸附作用控制,表现出衰减特征。结合14C测年数据,在考虑沉积环境、沉积过程与沉积速率的基础上,以水文地球化学模拟软件PHREEQC构建了弱透水层Cl^(-)一维溶质运移模型,模拟结果表明该区域弱透水层中存在垂向上的扩散作用,垂向对流速度十分缓慢,弱透水层的有效扩散系数约为6.0×10^(-11)m^(2)/s。

The Quaternary aquitard of the Pearl River Delta(PRD)overlies the basal confined aqui‐fer,and its hydrogeochemical evolution process has a decisive influence on the basal aquifer.In order to understand the formation and evolution of pore water in the aquitard of the Pearl River Delta,this study analyzes the major ions(K^(+),Na^(+),Ca^(2+)、Mg^(2+),Cl^(-),SO_(4)^(2-),Br^(-)),hydrogeochemical parameters and mineral saturation index of pore water samples of the ZK01 borehole drilled in Zhongshan City.The results show that the salinity of pore water is relatively high and the TDS is between 4.42 and30.47 g/L.The water chemistry of pore water is generally Cl^(-)Na type,which is similar to seawater,and the concentrations of major ions tend to increase with the borehole depth.The Cl/Br,γ(Na/Cl)co‐efficients and Gibbs analysis show that the salinity of the aquitard pore water is mainly originated from seawater,and probably originated from Holocene seawater.Concentration of SO_(4)^(2-)is mainly con‐trolled by sulfate reduction in the aquitard and not affected by the precipitation of gypsum.Ca^(2+)and Na^(+)in shallow pore water are affected by cation exchange,which causes Na^(+)depletion and Ca^(2+)enrich‐ment.NH_(4)^(+)mainly exchanges with Mg^(2+),resulting in the increase of NH_(4)^(+)in pore water.K^(+)is mainly controlled by mixing and adsorption during pore water evolution,showing depletion characteristics.Based on the14C dating data and taking the sedimentary environments,processes and rates under con‐sideration,an one-dimensional solute transport model of the aquitard Cl^(-)was established by using the hydrogeochemical simulation software PHREEQC.The simulation results show that the effective diffu‐sion coefficient of the aquitard is about 6.0×10^(-11)m^(2)/s,the vertical convection velocity can be neglected,and diffusion is the key mechanism for vertical solute transport in the PRD Quaternary aquitard.