江汉平原地下水流系统演化与劣质水的形成

In sedimentary basin, from piedmont plains to the basin center, groundwater flows in the hierarchically nested groundwater flow system. The natural inferior groundwater which is widespread in basin is a significant water quality problem to be solved. Usually, natural inferior groundwater is located in groundwater stagnation zone and the harmful elements are often associated with the components of adjacent clay layer. Based on this hypothesis, a new idea about water quality evolution study through reconstructing groundwater flow system evolution is adopted. This project will take Jianghan plain as the study area. The cross-section from piedmont to basin and until the inferior groundwater area will be studied as the typical profiles. Deep borehole will be used to reflect the sedimentary environmental evolution through the analysis of core lithology, particle size, mineral and sediment age. Shallow boreholes will be drilled to collect clayey cores and porewater. Groundwater samples in different hierarchical flow systems will also be collected. Based on the geochemical, isotopic and age characterization for both porewater and groundwater, the cause of the inferior aquifer water and its relationship with adjacent aquitard porewater will be elaborated and the groundwater “stratigraphy” records will be gotten. Groundwater numeral models will be established by stages based on the sea level fluctuation and cutting depth variation of Yangtze river. Aquitard water releasing and solute diffusion will be considered in the model. Through analyzing groundwater runoff condition and evolution process of groundwater flow system, the water quality evolution and distribution pattern of natural inferior groundwater will be revealed.

平原区从山前至盆地腹地地下水流呈现出多级次嵌套式模式。盆地中分布不均且普遍存在的天然劣质地下水（如高氟、高砷、高碘等）是亟待解决的原生水质问题。天然劣质水通常处于地下水弱径流带（或滞流区），其有害组分往往与相邻粘土层组分密切相关。我们提出通过恢复地下水流系统演变史揭示水质演化规律的新思路。项目拟在江汉平原选取从山前至盆地到劣质水分布区的典型剖面，利用控制性深孔的岩性、矿物及沉积物年龄资料刻画江汉平原地层特征，分析沉积环境演变史。用浅层钻孔采集粘性土柱获取孔隙水，采集不同水流系统地下水，对比粘性土孔隙水与地下水水化学（同位素）和水年龄特征，阐述劣质水的形成与相邻粘性土孔隙水的关系，获取江汉平原地下水地层学档案。基于晚更新世以来海平面波动与长江切割变化，考虑粘性土释水和扩散影响，分阶段进行典型剖面地下水流数值模拟；展示地下水流-水化学-质点运移演化过程，揭示劣质水分布与地下水质演变规律。

江汉平原原生劣质地下水广布，多聚集在水流封闭的滞缓区或径流排泄区。地下水流系统的演化是影响劣质水时空分布的重要因素。本课题以江汉平原第四纪弱透水-含水层系统地下水为研究对象，通过恢复地下水流系统的演变揭示劣质水的形成机制与分布规律。.沿汉江纵剖面和长江横剖面采集了15个钻孔沉积物，提取了沉积物孔隙水；采集了典型区域含水层地下水。根据沉积物粒度、矿物和化学元素，分析了江汉平原河湖交替及水动力条件演变，得出全新世以来沉积细粒相为主；由孔隙水、地下水的化学-同位素-年龄的空间分布特征，识别了区域地下水的补给来源与水流模式；精细刻画了人类活动影响下，河间地块局部水流的循环特征，量化了不同区域地下水的更新能力。平原区劣质水富集氮、砷及重金属组分；综合分析沉积物-孔隙水化学及氮氧同位素，得出浅层劣质水的高氮主要来源于农业与其他人为污染，深层来源于有机氮矿化和大气降水。综合沉积环境、水化学同位素指标和长江切割深度变化，重建了末次盛冰期以来水流系统三个阶段的演变过程；全新世粘性土沉积后，含水层氧化还原环境改变，沉积物中的砷、铁、锰向地下水释放；典型剖面的对流-扩散数值模拟得出，地下水流的演变是劣质水聚集在晚更新世含水岩组顶部主要原因。.本课题从地质环境演变角度出发，深入分析地下水流系统演变模式对劣质地下水分布的影响，为平原区原生劣质水的形成、赋存规律，以及水质演化预测提供了新的理论和方法。