Kumar Anand, Singh Chander Kumar, Bostick Benjamin, Nghiem Athena, Mailloux Brian, van Geen Alexander
Department of Energy and Environment, TERI School of Advanced Studies, New Delhi.
Lamont Doherty Earth Observatory, Columbia University, New York, USA.
Geochim Cosmochim Acta. 2020 May 1;276:384-403. doi: 10.1016/j.gca.2020.03.003. Epub 2020 Mar 12.
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 (NO ), iron (Fe), and sulfate (SO ). Concentrations of NO 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 SO concentrations ≥20 mg/L in three-quarters of the sampled wells suggest that high levels of NO , 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 NO 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.
最近的测试表明，拉维河泛滥平原的浅层含水层比印度河上游流域的其他泛滥平原更频繁地受到地下水砷污染的影响。在本研究中，我们通过比较拉维河以及相邻的比阿斯河和萨特莱杰河沿岸11个村庄10 - 30米深度范围内的地下水和含水层砂的成分，探究了这种差异的地球化学成因。在钻探之前，使用现场试剂盒对同一村庄的水井进行了测试，不仅检测了砷，还检测了硝酸盐（NO₃⁻）、铁（Fe）和硫酸盐（SO₄²⁻）。在整个研究区域，三分之一的水井中硝酸盐浓度≥20毫克/升，尽管在一半的水井中条件也充分还原，以维持溶解铁浓度>1毫克/升。通过反射率测量量化的砂屑从灰色到灰棕色的颜色证实了受影响村庄含水层中氧化铁的大量还原。用现场试剂盒测定的砂屑中可浸出砷的含量极高，以及通过X射线荧光测量的砷浓度高达40毫克/千克，与地下水中高砷的深度区间相对应。现场阴离子交换分离以及对保存在甘油中的砂屑进行基于同步加速器的X射线光谱分析表明，在最富集的深度区间，地下水和含水层砂中的砷形态均以As(V)为主。这些发现以及四分之三的采样水井中SO₄²⁻浓度≥20毫克/升表明，可能来自大量施肥的高浓度NO₃⁻，可能通过氧化喜马拉雅山黑色页岩和板岩侵蚀提供的硫化物结合态砷，从而引发了砷的释放。地下粘土岩屑的放射性碳测年表明，在过去3万年中，拉维河泛滥平原发生了多次推测的砷 - 硫化物输入事件。其他流域为何显然没有收到类似的砷 - 硫化物输入尚不清楚。地下水中的高浓度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)可能达到的水平。在这种特殊情况下，在钻探时可以使用试剂盒分析砂屑中的砷，以便通过避开井筛所在含水层砂中砷浓度高的区间，确定安装家用井的无砷安全深度。
