贵德盆地三河流域高砷地下水中溶解性有机物三维荧光特性及其指示意义

Excitation-emission matrix spectroscopic characteristics of dissolved organic matters and the significance in high arsenic groundwater research in the Guide Basin,China

溶解性有机物(dissolved organic matter,DOM)可以通过多种方式控制含水层中砷的迁移转化。贵德盆地承压含水层地下水砷含量显著高于潜水含水层。为查明承压水中溶解性有机物对砷浓度的影响,对研究区地表水、潜水以及承压水进行吸光度和三维荧光光谱的分析,利用平行因子分析法确定了水样中有机物成分及荧光特征。结果表明,贵德盆地水体中DOM包含陆源类腐殖质(C1)、受人为影响的腐殖质(C2)、类醌化合物(C3)和微生物来源的腐殖质(C4)4种组分。陆源类腐殖质C1可在地下水中富集,占总有机质的40%~55%。相比于地下水,C2和C3则在地表水中占据较高的比例。高砷承压水中C2、C3所占比例高于低砷潜水。其中,C1可以通过络合作用促进溶解性砷浓度的提高,C3作为电子穿梭体可以促进含砷铁氧化物或氢氧化物的还原性溶解从而释放砷。微生物降解有机质生成的HCO3-可以与砷竞争吸附,促进砷的解吸附。此外,还原性溶解产生的Fe(II)与HCO3-形成FeCO3固定一部分的砷。该研究表明,地下水中的天然有机物通过络合作用和作为电子穿梭体促进铁氧化物还原导致地下水砷的富集,为分析黄河上游地区高砷地下水的成因提供理论依据。

Dissolved organic matter(DOM)controls the mobilization of arsenic(As)in several ways.Although higher As concentrations were observed in confined vs.unconfined aquifers in the Guide Basin,the roles of DOM in As mobilization are unclear.In order to evaluate DOM influence on As mobilization in confined aquifer under reducing environment in the Guide Basin,we conducted excitation-emission-matrix spectroscopic(EEMs)analysis on surface water and groundwater samples and used parallel factor analysis(PARAFAC)to determine DOM properties.The PARAFAC model identified four components in the Guide Basin and they are all humics:C1 is terrestrial humic-like;C2 is humics and affected by agriculture;C3 is semiquinone-like;and C4 is microbial humic.C1 is highly enriched in groundwater at about 40%-55%.Compared to groundwater,surface water has higher C2 and C3 contents.Meanwhile,high As confined aquifer is characterized by higher percentage of C2 and C3.The occurrence of high As groundwater in confined aquifer shows that C1 can form complex with As to increase aqueous As concentration;and C3 serves as an electron shuttle to enhance microbial reduction of iron oxides and As mobilization.The HCO3-anion generated from organic matter degradation causes Fe(II)precipitation to form FeCO3 and adsorbs some of arsenic.These results demonstrate that DOM can lead to As accumulation in groundwater by complexation or by serving as electron shuttle to promote reductive dissolution of iron oxides.This finding provides an theoretical explanation for the formation of high arsenic groundwater in the upper reaches of the Yellow River.