Sujathan Surya, Singh Abhas
Department of Civil Engineering, Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India.
Environ Sci Technol. 2023 Feb 7;57(5):1970-1986. doi: 10.1021/acs.est.2c08524. Epub 2023 Jan 24.
Elevated uranium (U) (>WHO limit of 30 μg L) in Indian groundwaters is primarily considered geogenic, but the specific mineralogical sources and mechanisms for U mobilization are poorly understood. In this contribution, statistical and geochemical analyses of well-constrained metadata of Indian groundwater quality ( = 342 of 8543) were performed to identify key parameters and processes that influence U concentrations. For geochemical predictions, a unified speciation model was developed from a carefully compiled and updated thermodynamic database of inorganic, organic (Stockholm Humic model), and surface complexation reactions and associated constants. Critical U contamination was found at shallow depths (<100 m) within the Indo-Gangetic plain, as determined by bivariate nonparametric Kendall's Tau and probability-based association tests. Analysis of aquifer redox states, multivariate hierarchical clusters, and principal components indicated that U contamination was predominant not just in oxic but mixed (oxic-anoxic) aquifers under high Fe, Mn, and SO concentrations, presumably due to U release from dissolution of Fe/Mn oxides or Fe sulfides and silicate weathering. Most groundwaters were undersaturated with respect to relevant U-bearing solids despite being supersaturated with respect to atmospheric CO (average CO of reported dissolved inorganic carbonate (DIC) data = 10 atm). Yet, dissolved U did not appear to be mass limited, as predicted solubilities from reported sediment concentrations of U were ∼3 orders of magnitude higher. Integration of surface complexation models of U on typical aquifer adsorbents, ferrihydrite, goethite, and manganese dioxide, was necessary to explain dissolved U concentrations. Uranium contamination probabilities with increasing dissolved Ca and Mn exhibited minima at equilibrium solubilities of calcite [∼50 mg L] and rhodochrosite [∼0.14 mg L], respectively, at an average groundwater pH of ∼7.5. A potential indirect control of such U-free carbonate solids on U mobilization was suggested. For locations ( = 37) where dissolved organic carbon was also reported, organic complexes of U contributed negligibly to dominant U speciation at the groundwater pH. Overall, the unified model suggested competitive dissolution-precipitation and adsorption-desorption controls on U speciation. The model provides a quantitative framework that can be extended to understand dominant mobilization mechanisms of geogenic U in aquifers worldwide after suitable modifications to the relevant aquifer parameters.
印度地下水中铀（U）含量升高（>世界卫生组织规定的30μg/L限值）主要被认为是地质成因的，但铀迁移的具体矿物学来源和机制尚不清楚。在本研究中，对印度地下水水质的严格元数据（8543个中的342个）进行了统计和地球化学分析，以确定影响铀浓度的关键参数和过程。为了进行地球化学预测，从精心编制和更新的无机、有机（斯德哥尔摩腐殖质模型）以及表面络合反应及其相关常数的热力学数据库中开发了一个统一的物种形成模型。通过双变量非参数肯德尔 Tau 检验和基于概率的关联检验确定，在印度河-恒河平原浅层深度（<100米）发现了严重的铀污染。对含水层氧化还原状态、多元层次聚类和主成分的分析表明，铀污染不仅在高铁、锰和硫酸根浓度下的有氧含水层中占主导地位，在混合（有氧-缺氧）含水层中也占主导地位，这可能是由于铁/锰氧化物或硫化铁溶解以及硅酸盐风化导致铀释放。尽管相对于大气二氧化碳（报告的溶解无机碳酸盐（DIC）数据的平均二氧化碳分压 = 10 atm）大多数地下水处于过饱和状态，但相对于相关含铀固体而言，大多数地下水是不饱和的。然而，溶解铀似乎不受质量限制，因为根据报告的沉积物铀浓度预测的溶解度要高约3个数量级。有必要整合铀在典型含水层吸附剂（水铁矿、针铁矿和二氧化锰）上的表面络合模型来解释溶解铀浓度。在平均地下水pH值约为7.5时，随着溶解钙和锰的增加，铀污染概率分别在方解石[约50mg/L]和菱锰矿[约0.14mg/L]的平衡溶解度处出现最小值。有人提出这种不含铀的碳酸盐固体对铀迁移可能存在间接控制作用。对于也报告了溶解有机碳的地点（37个），在地下水pH值下，铀的有机络合物对主要铀形态的贡献可忽略不计。总体而言，统一模型表明竞争溶解-沉淀和吸附-解吸对铀形态有控制作用。该模型提供了一个定量框架，在对相关含水层参数进行适当修改后，可扩展用于理解全球含水层中地质成因铀的主要迁移机制。
