Groundwater contamination with fluoride (F) and uranium (U) has been reported in many parts of India. However, the sources and mobilization mechanisms of these contaminants remain poorly understood. The present study aimed to identify the processes governing the coexistence of elevated F and U in groundwater at a typical site in India’s middle Gangetic plain. Sustained groundwater sampling at 21 locations over two years indicated persistence of high F and U in a shallow aquifer (12 m depth), but absence of these pollutants in a deeper aquifer (30 m depth). For both the aquifers, Mn exhibited strong inverse correlations with F (−0.587; p < 0.01) and U (−0.581; p < 0.01). X-ray diffraction analysis of representative sediment cores indicated few differences in the mineralogy of the two aquifers, which consisted of fluorite (CaF_2(s)) and calcite (CaCO_3(s)), among others. Analysis of groundwater speciation and saturation state and sequential extraction on aquifer sediments suggested that elevated F in shallow groundwater occurred due to calcite precipitation-induced fluorite dissolution. The conditions in both the aquifers were oxidizing with respect to U, but reducing with respect to Mn. Elevated U was attributed to carbonate-promoted mobilization from iron–manganese (Mn) and residual sediment fractions. In the deeper groundwater, elevated Mn and lower pH levels persisted with conditions at saturation with respect to rhodochrosite (MnCO_3(s)). Furthermore, medium (30 d) to long-term (300 d) batch experiments were performed to systematically evaluate the role of variable Mn on calcite precipitation under approximate in situ conditions. Precipitation of rhodochrosite outcompeted calcite precipitation and resulted in lower pH compared to pH of Mn-free systems, which (a) inhibited calcite precipitation and associated fluorite dissolution and (b) constrained pH and alkalinity in the deeper groundwater. These findings have implications for understanding F and U mobilization in comparable Mn-deficient sites and development of appropriate Mn-based amendments for in situ remediation.

中文翻译：

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碳酸锰沉淀在控制地下水中氟化物和铀迁移中的作用

据报道，印度许多地区的地下水被氟化物 (F) 和铀 (U) 污染。然而，这些污染物的来源和动员机制仍然知之甚少。本研究旨在确定控制印度恒河中部平原典型地点地下水中 F 和 U 升高共存的过程。两年内在 21 个地点的持续地下水采样表明，浅层含水层（12 m 深）中的高 F 和 U 持续存在，但在较深的含水层（30 m 深）中没有这些污染物。对于两个含水层，Mn 与 F (-0.587; p < 0.01) 和 U (-0.581; p< 0.01)。代表性沉积物岩心的 X 射线衍射分析表明，这两个含水层的矿物学几乎没有差异，其中包括萤石 (CaF _2(s) ) 和方解石 (CaCO _3(s) ) 等。对地下水形态和饱和状态的分析以及对含水层沉积物的连续提取表明，浅层地下水中 F 升高是由于方解石沉淀引起的萤石溶解。两个含水层中的条件相对于 U 是氧化性的，但相对于 Mn 是还原性的。升高的 U 归因于碳酸盐促进铁锰 (Mn) 和残留沉积物部分的动员。在较深的地下水中，锰的升高和 pH 值的降低在菱锰矿 (MnCO_3(s) )。此外，进行了中期（30 天）到长期（300 天）批次实验，以系统地评估变量 Mn 在近似原位条件下对方解石沉淀的作用。菱锰矿的沉淀胜过方解石沉淀，导致与无锰系统的 pH 值相比较低的 pH 值，这 (a) 抑制了方解石沉淀和相关的萤石溶解，以及 (b) 限制了深层地下水的 pH 值和碱度。这些发现对理解可比较的缺锰位点中 F 和 U 的动员以及开发用于原位修复的适当的 Mn 基修正物具有重要意义。