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Regulation of groundwater arsenic concentrations in the Ravi, Beas, and Sutlej floodplains of Punjab, India
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.gca.2020.03.003
Anand Kumar , Chander Kumar Singh , Benjamin Bostick , Athena Nghiem , Brian Mailloux , Alexander van Geen

Abstract Recent testing has shown that shallow aquifers of the Ravi River floodplain are more frequently affected by groundwater arsenic (As) contamination than other floodplains of the upper Indus River basin. In this study, we explore the geochemical origin of this contrast by comparing groundwater and aquifer sand composition in the 10–30 m depth range in 11 villages along the Ravi and adjacent Beas and Sutlej rivers. The drilling was preceded by testing wells in the same villages with field kits not only for As but also for nitrate (NO3–), iron (Fe), and sulfate (SO42−). Concentrations of NO3– were ≥ 20 mg/L in a third of the wells throughout the study area, although conditions were also sufficiently reducing to maintain > 1 mg/L dissolved Fe in half of all the wells. The grey to grey-brown color of sand cuttings quantified with reflectance measurements confirms extensive reduction of Fe oxides in aquifers of the affected villages. Remarkably high levels of leachable As in the sand cuttings determined with the field kit and As concentration up to 40 mg/kg measured by X-ray fluorescence correspond to depth intervals of high As in groundwater. Anion-exchange separation in the field and synchrotron-based X-ray spectroscopy of sand cuttings preserved in glycerol indicate speciation in both groundwater and aquifer sands that is dominated by As(V) in the most enriched depth intervals. These findings and SO42− concentrations ≥ 20 mg/L in three-quarters of the sampled wells suggest that high levels of NO3–, presumably from extensive fertilizer application, may have triggered the release of As by oxidizing sulfide-bound As supplied by erosion of black shale and slate in the Himalayas. Radiocarbon dating of sub-surface clay cuttings indicates that multiple episodes of inferred As-sulfide input reached the Ravi floodplain over the past 30 kyr. Why the other river basins apparently did not receive similar inputs of As-sulfide remains unclear. High NO3– in groundwater may at the same time limit concentrations of As in groundwater to levels lower than they could have been by oxidizing both Fe(II) and As(III). In this particular setting, a kit can be used to analyze sand cuttings for As while drilling in order to target As-safe depths for installing domestic wells by avoiding intervals with high concentrations of As in aquifer sands with the well screen.

中文翻译:

印度旁遮普邦 Ravi、Beas 和 Sutlej 洪泛区地下水砷浓度的调节

摘要 最近的测试表明,与印度河流域上游的其他漫滩相比,拉维河漫滩的浅层含水层更容易受到地下水砷 (As) 污染的影响。在这项研究中,我们通过比较 Ravi 和邻近的 Beas 和 Sutlej 河沿岸 11 个村庄 10-30 m 深度范围内的地下水和含水层沙子成分来探索这种对比的地球化学起源。在钻探之前,在相同的村庄使用现场工具包测试井,不仅测试砷,还测试硝酸盐 (NO3-)、铁 (Fe) 和硫酸盐 (SO42-)。在整个研究区域的三分之一井中,NO3– 的浓度≥ 20 mg/L,尽管条件也已充分降低以在所有井中保持 > 1 mg/L 的溶解铁。用反射率测量量化的砂岩屑呈灰色至灰棕色,证实受影响村庄含水层中的氧化铁大量减少。使用现场工具包确定的砂岩屑中可浸出的砷含量非常高,并且通过 X 射线荧光测量的砷浓度高达 40 毫克/千克,对应于地下水中高砷的深度间隔。现场阴离子交换分离和甘油中保存的砂岩屑的基于同步加速器的 X 射线光谱表明,地下水和含水层砂岩中的物种形成在最富集的深度间隔中以 As(V) 为主。这些发现和四分之三的采样井中 SO42− 浓度 ≥ 20 mg/L 表明高水平的 NO3– 可能来自大量施肥,可能通过氧化喜马拉雅山黑色页岩和板岩侵蚀所提供的硫化物砷而引发砷的释放。地下粘土岩屑的放射性碳测年表明,在过去的 30 kyr 中,多次推断的硫化砷输入到达了 Ravi 洪泛区。为什么其他流域显然没有收到类似的硫化砷输入仍不清楚。地下水中的高 NO3– 可能同时将地下水中的砷浓度限制在低于通过氧化 Fe(II) 和 As(III) 所能达到的水平。在这种特殊情况下,可以使用工具包在钻井时分析砂岩屑,以便通过使用井筛避免含水层砂中砷浓度高的层段来确定安装国内井的 As 安全深度。
更新日期:2020-05-01
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