大同盆地地下水中砷的形态、分布及其富集过程研究

Spatial Variation,Speciation and Enrichment of Arsenic in Groundwater from the Datong Basin,Northern China

为查明大同盆地高砷地下水的分布规律及其主要控制因素，对大同盆地典型高砷区35件地下水样进行了水化学特征及形态分析研究。结果表明，高砷地下水b（As）≥50,μg／L]要存在于20～50m的浅层地下水中，息砷质量浓度为0．56～927μg／L，主要以As（V）形态存在。该区高砷地下水以Na—HCO2型水为主，具有明显的高pH值，高HCOf、Fe2＋、HS-质量浓度及低Eh值，低SO2质量浓度特征。这可能与微生物催化氧化有机碳的同时还原含铁矿物和硫酸盐的过程有关。PHREEQC模拟矿物饱和指数结果表明，高砷地下水[．D（As）≥50／μg／L]中菱铁矿均为过饱和，而低砷地下水[P（As）〈50／μg／L]中均不饱和，且菱铁矿饱和指数与地下水中总砷质量浓度呈显著正相关性，该现象表明微生物还原含铁矿物生成FeEO2（菱铁矿）的过程可能是控制本区地下水中砷富集的主要因素。

In order to understand the distribution, enrichment of As and their controt tactor e, Basin groundwater, a total of 35 groundwater samples were collected for hydrochemica[ and speciation a- nalysis. The results indicate that high As groundwater （As〉50 /μg/L） mainly occurred in the shallow groundwater with the depth ranging from 20 to 50 m. The concentration of As in the Datong Basin ground water ranged from 0.56 to 927 μg/L and mainly presented as arsenate. The high As groundwater is typical Na-HCOa type water and is characterized by high pH values, HCO3 , Fez＋ , and HS concentration and low Eh values and SOZ4- concentrations. This may be related to the reductive dissolution of iron oxy- hydroxides driven by microbial degradation of organic carbon. The PHREEQC modeling results indicate that siderite is oversaturated in the high As groundwater （As〉50 μg/L）, while it is undersaturated in the low As groundwater （As〉50 μg/L）. A positive correlation can be observed between and dissolved As in groundwater, indicating microbial transformation of ferric minerals to ferrous carbonate minerals such as siderite may account for the enrichment of As in groundwater in the study area.