北京市丰台区永定河以东浅层地下水水化学演变规律及成因分析

Analysis on Hydrochemical Evolution of Shallow Groundwater East of Yongding River in Fengtai District,Beijing

为更好支撑北京市丰台区地下水资源开发利用、污染管控与防治工作,以丰台区永定河以东区域枯水期浅层地下水长序列监测数据为基础,综合运用数理统计、Gibbs图和离子比例分析等方法探究了丰台区地下水水化学演变规律、形成机制及污染来源,结果表明:①研究区现状地下水质量整体较差,地下水中各指标平均浓度自1976年至今呈先升高后降低趋势,Cl^(−)、SO_(4)^(2-)和总硬度(TH)污染范围总体呈扩大趋势,溶解性总固体(TDS)和硝酸盐氮(NO_(3)^(-))污染范围以2005年为拐点呈先扩大后缩小趋势;②各年份地下水中优势阴、阳离子均为HCO_(3)和Ca^(2+),1976年和2021年地下水水化学类型数依次为8和17种,其主要水型依次为HCO_(3)·SO_(4)-Ca·Mg·Na(40%)和HCO_(3)·Cl·SO_(4)-Ca·Na·Mg(23.88%),各年份区域范围内与沿地下水流向上地下水水化学类型均呈现复杂化趋势,地下水水文地球化学演变过程中水化学组分受人为活动影响显著;③地下水受岩石风化和蒸发结晶双重作用,且以蒸发作用为主,地下水阳离子交替作用较弱,碳酸盐矿物的溶解为Ca^(2+)和Mg^(2+)的主要来源;④离子比例分析得出,外源输入的NO_(3)^(-)和Cl^(−)主要来源于农业活动、城市污水,且2005年前农业活动污染影响较大,与研究区历史上大量渗坑、渗井、工业和生活污水灌溉直排关系密切.

In order to enhance the support for groundwater development and utilization,as well as pollution control and prevention in Fengtai District,Beijing,a comprehensive study was conducted based on long-term monitoring data of shallow groundwater in the eastern area of Yongding River during the dry season.The mathematical statistics,Piper diagram,Gibbs diagram,and ion ratio analysis and other methods were employed to explore the pattern of groundwater hydrochemical evolution,the formation mechanism,and sources of pollution in Fengtai District.The findings were as follows:①Overall,the current groundwater quality in the study area was poor.The average concentration of each index in groundwater increased and then decreased from 1976 to the present.The pollution range of Cl^(−),SO_(4)^(2-),and TH generally expanded,whereas the pollution range of TDS and NO_(3)^(-) expanded before 2005 and then decreased with 2005 as the turning point.②The hydrochemical types of groundwater samples displayed a complex regional variation each year,as well as along the groundwater direction.The dominant anion in groundwater was HCO_(3),and the dominant cation was Ca^(2+) each year.The number of groundwater hydrochemical types in 1976 was 8,in which the predominant type was HCO_(3)·SO_(4)-Ca·Mg·Na,accounting for 40%.However,the number of groundwater hydrochemical types in 2021 was 17,in which the predominant type was HCO_(3)·Cl·SO_(4)-Ca·Na·Mg,accounting for 23.88%.The groundwater hydrochemical type showed a complex trend within the region and upstream along the flow direction each year,whereas the migration characteristics of groundwater samples,as depicted on the Piper diagram,indicated that the hydrochemical components of groundwater were significantly affected by human activities during its evolution.③The groundwater chemistry in the study area was influenced by both rock weathering and evaporative crystallization processes,with evaporation playing a major role.The alternation of groundwater cations was relatively weak,and the dissolution of carbonate minerals served as the primary source of Ca^(2+)and Mg^(2+).④The ion ratio analysis suggested that exogenous sources,mainly agricultural activities and urban sewage,contributed to the input of NO_(3)^(-) and Cl^(−).The pollution impact from agricultural activities was significant before 2005,which aligned with the historical presence of numerous seepage pits,seepage wells,and direct discharge of industrial and domestic sewage for irrigation purposes in the study area.These activities were closely associated with the high levels of pollution.However,pollution input from agricultural activities notably decreased in 2021,likely due to the effective implementation of water environmental protection programs and action plans in recent years.