黄河决口扇晚更新世以来沉积微相对高砷地下水分布的控制机制研究

Albeit sound groundwater runoff conditions, the high arsenic groundwater exists in large quantity in the elevated crevasse splay of the Yellow River in the south of Hetao basin, in which the hydrogeological condition, stratum structure and depositional environment are different from those of other high arsenic areas in Hangjinhouqi and Wuyuan County. From the perspective of Hydrogeology, sedimentology and hydro-geochemistry and by dint of regional stratigraphy tracking, profile comparison, stratigraphic chronology, environmental index, and isotope method, this study focuses on the late Pleistocene aquifer in Linhe crevasse splay area and aims to elaborate the sedimentary facies into microfacies, construct the stratigraphic structure and sedimentary-environmental model, and explore the correspondence between the highly arsenic groundwater and different kinds of sedimentary microfacies. It also intends to probe into the composition and spatial distribution characteristics of “groundwater seepage - redox condition - organic matter - environmental minerals” system inside each sedimentary microfacies so as to reveal the adsorption and release mechanism under the control of sedimentary microfacies, the results of which are expected to enrich and improve the formation theory of protogenic high arsenic groundwater.

河套盆地南缘，由于黄河决口改道形成的临河决口扇，地势高、地下水径流条件好，却存在着大面积的高砷地下水，其水文地质条件、地层结构、沉积环境与盆地中部，特别是研究程度很高的杭锦后旗、五原县等高砷区有着本质区别。本项目拟以临河决口扇晚更新世含水层为研究对象，综合水文地质学、沉积学、水文地球化学，采用剖面对比、地层追踪等野外地质地貌调查手段，结合地层剖面的年代、环境指标、同位素手段，通过将决口扇沉积相细化至微相，构建地层结构及沉积环境模型，进而查明不同微相与高砷地下水的对应关系；研究各沉积微相内“地下水渗流特征-氧化还原条件-有机质-环境矿物”体系的组成及空间分布特征，从而揭示沉积微相控制下砷的吸附释放机制。其结论可以丰富和完善原生高砷地下水成因机制的理论体系。

沉积环境与高砷地下水关系的研究一直国内外专家学者关注的热点和难点。本研究针对河套盆地开展了历时多年的水文地质环境地质调查，利用采集的499组浅层地下水样品以及收集并甄选的382个水文地质钻孔记录，分析发现：河套盆地内95%的高砷地下水（As>10μg/L）集中在黄河决口、改道形成的古河道影响带及黄河决口扇裙带。其中，盆地南部首次发现了较为集中的高砷地下水，其分布范围与黄河北岸的晚更新世以来黄河决口、改道形成的四个大型决口扇，在空间上有着良好的对应关系。黄河古河道带及决口扇裙带表现为较强的还原环境及蒸发浓缩作用，以及较弱的地表水补给强度。受pH值、含铁氧化物的还原性溶解等因素影响，决口扇裙带浅层地下水中砷的含量略低于平原中部黄河古河道带。结果表明，黄河古河道带及决口扇裙带特定的沉积环境控制着地下水的水文地球化学过程，进而对高砷地下水的分布具有决定性的作用。