内蒙古河套平原高砷地下水中稀土元素含量及分异特征

Concentrations and patterns of rare earth elements in high arsenic groundwaters from the Hetao Plain,Inner Mongolia

高砷地下水是世界范围内的一个重大环境问题。尽管已有工作在高砷地下水的地球化学特征、形成条件和富砷机理等方面取得了很大进展,但其稀土元素(REE)含量及分异特征却一直不清楚,这就限制了稀土方法在评价此类地下水中的应用。在对内蒙古河套盆地地下水样品进行现场测试、实验室分析检测的基础上,我们发现高砷地下水的REE浓度较低。对地下水REE浓度依照北美页岩平均组分(NASC)进行归一化处理后,我们发现大部分水样富集重稀土元素(HREEs),而贫化轻稀土元素(LREEs)。HREEs的富集与地下水中碳酸根对于REE的络合作用密切相关,而弱碱性环境下REE的再吸附可导致LREEs的贫化。所有地下水样表现为Eu正异常,其中大部分表现为Ce负异常。Eu正异常似乎与铁锰氧化物的还原性溶解和解吸附有关,地下水As浓度随[Eu]NASC的升高而升高。从某种意义上说,还原性含水系统中Eu正异常的程度也许可以反映地下水As的富集程度。

High arsenic groundwater is a world-wide environmental issue.Although the progress has been made concerning the geochemistry,formation environment and As enrichment mechanism of high As groundwa-ters,the concentrations and patterns of rare earth element（REE） in these groundwaters are unclear.Groundwater samples from the Hetao Basin,a typical high As groundwater basin in China,are investigated through in-situ measurements and laboratory analyses.We found that the concentration of REE is relatively low in the high As groundwater samples.The data normalized according to Northern American Shale Composite（NASC） exhibit a REE pattern with enriched HREEs and depleted LREEs for all groundwater samples.HREEs enrichment can be explained by carbonate complexation,while readsorption in slightly weak alkaline environment could be the cause of LREEs depletion.All groundwater samples show positive Eu anomalies,while most of them show negative Ce anomalies.The positive Eu anomalies seem to have resulted from reductive dissolution of Fe oxyhydroxides and/or reductive desorption.Arsenic concentrations increase with increasing NASC.Generally speaking,positive Eu anomalies would reflect As enrichment in reducing groundwater environments.