北京南郊平原地下水化学特征及成因分析

HYDROCHEMICAL CHARACTERISTICS AND GENESIS OF GROUNDWATER IN SOUTHERN SUBURB OF BEIJING PLAIN

地下水是旱区重要的供水水源,水化学组成是决定地下水可利用性的关键因素之一。以北京南郊平原为研究对象,通过系统调查浅层地下水和深层地下水化学组成,利用水文地球化学图解、多元统计及空间插值分析等多种方法,探讨了快速城镇化及剧烈农业活动条件下旱区地下水化学空间演变特征及其成因机制。结果表明:研究区地下水均为弱碱性低矿化水,水化学类型主要为HCO_(3)-Ca和Cl-Mg·Ca,并有少量HCO_(3)-Na·Ca型。浅层地下水和深层地下水均存在不同程度的硝酸盐浓度偏高现象,二者水化学演变与NO^(-)_(3)浓度升高密切相关,随着NO^(-)_(3)浓度升高优势阴离子逐渐由HCO^(-)_(3)转变为SO_(4)^(2-)、Cl^(-)。浅层地下水和深层地下水化学组成受自然因素和人为因素双重作用控制,含水层结构控制着水化学主导因素的空间分布,人为因素主导的水化学区主要分布在单一含水层结构的西北部城镇地区,而自然因素主导的水化学区主要分布于多层含水结构的东南部农业区。水化学组分的自然来源机制包括岩石风化溶解和阳离子交替吸附作用等;人为来源主要为城镇地区地表污染物输入,再生水灌溉对水质影响较小。

Groundwater is an important water resource for water supply, and the hydrochemistry is one of the key factors determining its availability. In this study, the southern near-suburb plain of Beijing was taken as an example, multiple approaches including hydrogeochemical diagrams, multiple statistics analysis and spatial interpolation model were performed, to reveal groundwater chemistry and its genesis in arid and semiarid regions undergone rapid urbanization and densely agricultural practices. The results demonstrated that groundwater was generally slightly alkaline fresh water in the study area. The hydrochemical type of groundwater were dominantly HCO_(3)-Ca and Cl-Mg·Ca type, along with a few of HCO_(3)-Na·Ca type. Elevation of NO-3 concentrations were observed in both shallow and deep aquifers. The hydrogeochemical faces evolution of shallow and deep groundwater was significantly related to the elevated NO-3 concentrations. The dominant anions evolved gradually from HCO-3 to SO^(2-)_(4) and Cl^(-) with the increase of NO^(-)_(3) concentration. Hydrogeochemical components of shallow and deep groundwater were controlled by both natural and anthropogenic factors. The spatial distribution of dominant controlling factors of groundwater chemistry was governed by the aquifer structures. The hydrochemical compositions of groundwater in the northwestern urban areas with single aquifer structure were dominantly controlled by anthropogenic factors, while that in the southeastern agricultural areas with multiple aquifer structure were mainly governed by natural factors. The natural factors controlling groundwater chemistry included rock weathering, minerals dissolution and ions exchange. Ground surface pollutants input was the main anthropogenic factor controlling groundwater chemistry. While the reclaimed water used for agricultural irrigation was found to have little impact on groundwater chemical evolution.