Novel insights into the mechanisms controlling the heterogeneous distribution of arsenic-rich groundwater in the Yangtze River Basin from evolutionary analysis of sedimentary environment and groundwater flow systems

In the alluvial-lacustrine Jianghan Plain (JHP), the distribution of widespread but heterogenous arsenic(As)-contamination of groundwater is determined by the evolution of the groundwater flow system (GFS) since the Last Glacial Maximum (LGM), under the influence of progressively changing climate. Whereas, as a critical driving factor for As enrichment, the evolution of GFS has been largely overlooked. Based on the systematical integration of basin sedimentology, paleohydrogeology, and modern hydrogeochemical environment, this study establish a macro-scale theoretical framework linking “sedimentary evolution–GFS driving–geochemical field formation” in the JHP. Sedimentary environment of the aquifer system shaped by paleo-climatic changes has significantly influenced both GFS evolution and As-rich groundwater enrichment since the LGM. Beyond the reductive dissolution of amorphous As-rich minerals under favorable sedimentary conditions, the spatio-temporal GFS evolution plays a crucial role in As heterogeneity. Deep incision of the ancient Yangtze River channel leads to the most potent GFSs driving during the LGM. Rapid burial and weathering As-rich minerals provide the ample provenance during the late deglaciation period (LDP). The formation of an overlying clay cover during the Holocene warm period (HWP) ensures a reducing environment for underlying aquifer. These factors collectively lay the foundation for As-rich groundwater formation. Additionally, the continuous convergence of As-rich groundwater towards the regional discharge are is driven by the multiple-hierarchical GFS pattern alongside a gradually sluggish flow and upward discharge under a sustained hydraulic gradient, which contributed to the stable occurrence of As.在冲积湖积成因的江汉平原（JHP）中，分布广泛却具有显著非均质性的地下水砷（As）污染格局，受控于末次盛冰期（LGM）以来的地下水流系统（GFS）演化与持续变化的气候条件。而作为砷富集的关键驱动因素，地下水流系统的演化长期以来却被极大地忽视。本研究通过系统整合盆地沉积学、古水文地质学与现代水文地球化学环境的相关数据，构建了江汉平原“沉积演化—地下水流系统驱动—地球化学场形成”的宏观理论框架。末次盛冰期以来，由古气候变化塑造的含水层沉积环境，对地下水流系统演化与富砷地下水的富集均产生了显著影响。除了有利沉积环境下富砷无定形矿物的还原溶解作用外，时空尺度下的地下水流系统演化同样是造成砷污染非均质性的关键因素。末次盛冰期古长江河道的深切作用，形成了最强劲的地下水流系统驱动作用；冰消期晚期（LDP）阶段，快速埋藏与风化作用形成的富砷矿物为砷富集提供了充足物源；全新世暖期（HWP）期间上覆黏土层的形成，为下伏含水层营造了还原环境。上述要素共同奠定了富砷地下水形成的基础。此外，受多层级地下水流系统格局驱动，富砷地下水持续向区域排泄区汇聚，且在持续存在的水力梯度下呈现水流逐渐迟滞、向上排泄的特征，这进一步促成了砷污染的稳定赋存。