This article attempts to understand the evolution of groundwater chemistry in the mid Gangetic floodplain through the identification of hydrogeochemical processes including the impact of surface recharge and geological features. Isotopic investigations identified that irrigation return flow is partly responsible for arsenic (As) enrichment through preferential vertical recharge. Further, the floodplain geomorphological attributes and associated As hydrogeochemical behaviour traced through isotopes tracers highlighted that meandering and ox-bow like geomorphological features owing to clay deposition leads to the anoxic condition induced reductive microbial dissolution of As-bearing minerals causing the arsenic contamination in the investigated aquifer of the mid-Gangetic plain (MGP). To achieve the objectives, 146 water samples for water chemistry and 62 samples for the isotopic study were collected from Bhojpur district, Bihar (district bounded by the river Ganges in the north and Son in the east) located in MGP during the pre-monsoon season of 2018. The chemical results revealed high arsenic concentration (BDL to 206 μg.L⁻¹, 32% samples are exceeding the 10 μg.L⁻¹ limit) in the Holocene recent alluviums which are characterized by various geomorphological features such as meander scars and oxbow lake (northern part of the district). Arsenic is more concentrated in the depth range of 15–40 m below ground surface. All other trace metals viz. Ni, Pb, Zn, Cd and Al were found in low concentration except Fe and Mn. The geochemical analyses suggest that rock-water interaction is controlling the hydro-geochemistry while the chemical constituent of the groundwater is mainly controlled by carbonate weathering with limited contribution from silicate weathering. The isotopic signatures revealed that the Son river is recharging groundwater while the groundwater is contributing to the Ganges river. A clear pattern of fast vertical recharge in the arsenic contaminated area is observed in the proximity to the river Ganges with an elevated nitrate concentration resulted from the reduced As dissolution. The origin of groundwater is local precipitation with low to high evaporation enrichment effect which is further indicating the vertical mixing of groundwater from the irrigation return flow and/or recharge from domestic discharge causing enhanced As mobilization through microbial assisted reductive dissolution of As-bearing minerals.

本文试图通过识别包括地表补给和地质特征的影响在内的水文地球化学过程来了解恒河漫滩中部地下水化学的演变。同位素研究确定灌溉回流是通过优先垂直补给富集砷 (As) 的部分原因。此外，通过同位素示踪剂追踪的漫滩地貌属性和相关的砷水文地球化学行为强调，由于粘土沉积导致的蜿蜒和牛弓状地貌特征导致缺氧条件导致含砷矿物的还原性微生物溶解，导致所研究的砷污染中恒河平原 (MGP) 的含水层。为了实现目标，^-1 , 32% 的样品超过 10 μg.L ^-1限制）在全新世最近的冲积层中，这些冲积层具有各种地貌特征，例如曲折痕和牛轭湖（该地区的北部）。砷更集中在地表以下15-40 m的深度范围内。所有其他微量金属即。除 Fe 和 Mn 外，Ni、Pb、Zn、Cd 和 Al 的浓度较低。地球化学分析表明，岩水相互作用控制着水文地球化学，而地下水的化学成分主要受碳酸盐风化作用控制，硅酸盐风化作用有限。同位素特征表明，松河正在补给地下水，而地下水则在为恒河注入水源。在恒河附近观察到砷污染区域快速垂直补给的清晰模式，由于砷溶解减少导致硝酸盐浓度升高。地下水的来源是局部降水，蒸发富集效应从低到高，这进一步表明灌溉回流和/或生活排放补给的地下水垂直混合，通过微生物辅助含砷矿物的还原溶解导致砷迁移增强。