基于人工降水入渗的热带海岛表面砂土层水质变化特征

珊瑚砂是热带海岛表层土的重要物质组成,开展大气降水入渗通过表面砂土层的水化学研究是正确认识和评估海岛地下水质的前提。本研究于2017年8月采集了我国热带海岛3种不同样地的典型表层砂土(编号D1、D2和D3),基于人工降雨模拟试验,分析了其基本理化参数变化特征。在此基础上,综合水化学相图、Visual MINTEQ水文地球化学模拟、碱基交换指数(IBE)计算和Gibbs图解等方法,揭示了渗流通过不同砂土层的水样化学变化特征及主要控制过程。结果表明:①表面砂土盐碱化程度高,入渗通过D1、D2和D3的水样pH范围分别为7.92~8.81、7.86~9.30和7.82~9.35,明显高于大气降水的均值6.0,其电导率分别从48.50、89.90、77.60 mS/cm降到了131.4、111.6、98.4μS/cm;②入渗通过D1、D2和D3的水样中许多微量元素浓度值随降雨淋洗会逐渐减小,其中Al、As、B(D1和D2)、Cr、Fe、Pb较难溶于水,应该引起重视;③D1、D2和D3储存的水中主要离子浓度大小为:Na⁺>Mg²⁺>Ca²⁺>K⁺和Cl^–>SO^2–₄>HCO^–₃>NO^–₃>CO^2–₃>Br^–>F–,其水化学类型随降雨淋洗均由Cl-Na水变为HCO₃^-Na?Ca?Mg水;④控制通过砂土层水样水质的水化学机制包括:碳酸盐、氯化物和硫酸盐矿物的溶解和沉淀,以及降水入渗过程中Na⁺与K⁺、Ca²⁺与Mg²⁺之间的交换作用。该研究对于指导热带海岛地下水资源的合理开发与利用,保障海岛居民用水安全及生态系统维护具有重要的科学意义。

Water Quality Variation Characteristics of Sandy Layer on Surface of Tropical Islands Based on Simulated Precipitation Infiltration

Coral sand is an important composition of tropical islands topsoil,and it is the premise of correctly understanding and evaluating the groundwater quality of islands to carry out the hydrochemical study of atmospheric precipitation infiltrated through sand layer.In this study,the surface sand soil layers(No.D1,D2 and D3)from three different plots of tropical islands in China were collected in August 2017.Based on the simulated rainfall experiment,the characteristics of the basic physiochemical parameters of water infiltration were analyzed.At the same time,the characteristics of hydrochemical change and main controlling process infiltrated through different sand soil layers were revealed by using hydrochemical phase diagram,visual MINTEQ hydrogeochemical simulation,indices of base exchange(IBE)calculation and Gibbs diagram.The results showed that the degree of salinization of coral sands was high.The ranges of pH values of water samples through D1,D2 and D3 were 7.92–8.81,7.86–9.30 and 7.82–9.35,respectively,which were obviously higher than the mean value of atmospheric precipitation(6.0),and EC decreased from 48.50,89.90 and 77.60 mS/cm to 131.4,111.6 and 98.4μS/cm,respectively.The concentrations of many trace elements in the water infiltrated through D1,D2 and D3 gradually decreased with rainfall washing.Among them,Al,As,B(D1 and D2)and Cr,Fe,Pb were more difficult to be dissolved in water,which should be paid more attention.The concentrations of main ions in water stored at D1,D2 and D3 were Na⁺>Mg²⁺>Ca²⁺>K⁺and Cl^–>SO^2–₄>HCO^–₃>NO^–₃>CO^2–₃>Br^–>F^–,and their chemical types of water were changed from Cl-Na to HCO3-Na?Ca?Mg water with rainfall elution.The controlling mechanisms of the hydrochemistry penetrating the coral sand layer included the dissolution and precipitation of carbonate,chloride and sulfate minerals,and the exchange of between(Na⁺,K⁺)and(Ca⁺,Mg⁺)ions during precipitation infiltration.This study is of great scientific significance to guide the rational development and utilization of groundwater resources,to ensure the safety of water of residents and to maintain the ecosystem in the tropical island.