Groundwater security is a pressing environmental and societal issue, particularly due to significantly increasing stressors on water resources, including rapid urbanization and climate change. Groundwater arsenic is a major water security and public health challenge impacting millions of people in the Gangetic Basin of India and elsewhere globally. In the rapidly developing city of Patna (Bihar) in northern India, we have studied the evolution of groundwater chemistry under the city following a three-dimensional sampling framework of multi-depth wells spanning the central urban zone in close proximity to the River Ganges (Ganga) and transition into peri-urban and rural areas outside city boundaries and further away from the river. Using inorganic geochemical tracers (including arsenic, iron, manganese, nitrate, nitrite, ammonium, sulfate, sulfide and others) and residence time indicators (CFCs and SF₆), we have evaluated the dominant hydrogeochemical processes occurring and spatial patterns in redox conditions across the study area. The distribution of arsenic and other redox-sensitive parameters is spatially heterogenous, and elevated arsenic in some locations is consistent with arsenic mobilization via reductive dissolution of iron hydroxides. Residence time indicators evidence modern (<~60–70 years) groundwater and suggest important vertical and lateral flow controls across the study area, including an apparent seasonal reversal in flow regimes near the urban center. An overall arsenic accumulation rate is estimated to be ~0.003 ± 0.003 μM.yr⁻¹ (equivalent to ~0.3 ± 0.2 μg.yr⁻¹), based on an average of CFC-11, CFC-12 and SF₆-derived models, with the highest rates of arsenic accumulation observed in shallow, near-river groundwaters also exhibiting elevated concentrations of nutrients including ammonium. Our findings have implications on groundwater management in Patna and other rapidly developing cities, including potential future increased groundwater vulnerability associated with surface-derived ingress from large-scale urban abstraction or in higher permeability zones of river-groundwater connectivity.

地下水安全是一个紧迫的环境和社会问题，尤其是由于水资源压力显着增加，包括快速城市化和气候变化。地下水砷是一项重大的水安全和公共卫生挑战，影响着印度恒河盆地和全球其他地区的数百万人。在印度北部快速发展的巴特那市（比哈尔邦），我们按照跨越恒河中心城区的多深度井的三维采样框架，研究了该市地下水化学的演变（恒河）并过渡到城市边界以外和远离河流的城郊和农村地区。使用无机地球化学示踪剂（包括砷、铁、锰、硝酸盐、亚硝酸盐、铵、硫酸盐、₆），我们评估了整个研究区域氧化还原条件下发生的主要水文地球化学过程和空间格局。砷和其他氧化还原敏感参数的分布在空间上是不均匀的，并且某些位置的砷升高与通过铁氢氧化物的还原溶解引起的砷动员一致。停留时间指标证明了现代（<~60-70 年）地下水，并表明整个研究区域的重要垂直和横向流动控制，包括城市中心附近流动状态的明显季节性逆转。基于 CFC-11、CFC-12 和 SF ₆的平均值，总砷积累速率估计为 ~0.003 ± 0.003 μM.yr ^-1（相当于 ~0.3 ± 0.2 μg.yr ^{-1 ）}衍生模型，在浅水、近河流地下水中观察到的砷积累率最高，也显示出包括铵在内的营养物质浓度升高。我们的研究结果对巴特那和其他快速发展的城市的地下水管理产生了影响，包括未来潜在的地下水脆弱性增加，这与大规模城市抽水或河流-地下水连通性较高的渗透性区域的地表侵入有关。