Methylated Arsenic Enrichment in Groundwater of Jianghan Plain: Insights from Carbon Isotope and DOM EEMs Analysis
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摘要: 高砷地下水造成的砷中毒对人体健康产生巨大威胁，砷甲基化过程可将毒性更高的无机砷转化为毒性低的甲基砷，在一定程度上可以降低砷的环境风险从而减轻对人体的毒害，有望成为调控地下水砷污染问题的有效途径.然而目前高砷地下水的研究主要侧重于无机砷的迁移转化，对于甲基砷富集的关键过程与控制因素研究还十分有限.本研究选取江汉平原长江与汉江沿岸的浅层地下水开展无机碳同位素示踪与溶解性有机质三维荧光光谱分析，解析江汉平原地下水中控制甲基砷富集的有机质降解途径，识别关键生物地球化学过程.研究发现江汉平原的甲基砷浓度范围为 < 0.01~444 μg/L（平均值为30 μg/L），长江沿岸和汉江沿岸地下水甲基砷的富集主控过程有明显差异：长江沿岸地下水中，以高分子量芳香族有机化合物降解驱动的产甲烷过程为主导，促进砷的生物甲基化过程；此外，伴随有机质发酵的硫酸盐还原过程也可导致地下水中甲基砷的富集.汉江沿岸地下水中，以小分子活性有机质发酵过程为主导，促进了砷生物甲基化过程.Abstract: Arsenic poisoning caused by high-arsenic groundwater represents a significant threat to human health. The arsenic methylation process, which converts highly toxic inorganic arsenic into less toxic methylated arsenic species, has the potential to mitigate the environmental risk of arsenic contamination and reduce its toxicity to humans. This process may provide an effective approach to managing arsenic pollution in groundwater. However, current research predominantly focuses on the migration of inorganic arsenic, with limited understanding of the key processes and controlling factors that govern the enrichment of methylated arsenic. In this study, it analyzed shallow groundwater from the Jianghan plain, located along the Yangtze and Han rivers, using inorganic carbon isotope tracing and EEMs of dissolved organic matter. The aim of this study is to elucidate the organic matter degradation pathways that regulate the enrichment of methylated arsenic and to identify the key biogeochemical processes involved. It found that methylated arsenic concentrations in the Jianghan plain ranged from < 0.01 to 444 μg/L, with an average concentration of 30 μg/L. The processes controlling methylated arsenic enrichment differed significantly between groundwater from the Yangtze River and the Han River. Along the Yangtze River, the degradation of high-molecular-weight aromatic organic compounds drives methanogenesis, which in turn promotes arsenic biomethylation and the subsequent enrichment of methylated arsenic in groundwater. Additionally, the sulfate reduction process, associated with the fermentation of organic matter, also supports arsenic biomethylation. In contrast, in the Han River region, the arsenic biomethylation process is primarily driven by the fermentation of small-molecule reactive organic matter.
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图 1 研究区地下水采样点及甲基砷浓度分布
Fig. 1. Distribution of groundwater sampling sites and methylarsenic concentrations in the study area
图 2 江汉平原地下水中MeAs与As(III)、Fe2+、NH4+、DOC质量浓度的关系
Fig. 2. Relationship between MeAs and mass concentrations of As(III), Fe2+, NH4+, and DOC in groundwater of Jianghan plain
图 3 长江、汉江沿岸地下水中无机碳碳同位素δ13C-DIC箱线图
Fig. 3. Box plot of δ 13C-DIC characteristics in groundwater along the Yangtze River and Han River banks
图 4 平行因子分析鉴别出地下水中溶解性有机质的3个荧光组分及其荧光特征
Fig. 4. Spectral characteristics of the three components identified by EEM-PARAFAC model
图 5 长江、汉江沿岸地下水中甲基砷与δ13C-DIC的关系
气泡大小分别为Fe2+浓度、NH4+浓度
Fig. 5. Relationship between methylated arsenic and δ13C-DIC in groundwater along the Yangtze River and Han River
图 6 长江与汉江沿岸地下水溶解性有机质荧光指数箱线图
Fig. 6. Box plot of the relationship between MeAs and fluorescence index
图 7 长江沿岸和汉江沿岸地下水中天然有机质组分的相对含量与甲基砷浓度的关系
Fig. 7. The relationship between the relative content of natural organic components and MeAs in groundwater with different concentrations of MeAs
图 8 长江与汉江沿岸地下水中有机物组分相对含量间的关系
Fig. 8. The relationship between the relative content of natural organic components in groundwater
表 1 江汉平原长江汉江沿岸地下水砷形态及水化学特征统计
Table 1. Hydrochemical characteristics of groundwater from Jianghan plain
指标 长江沿岸地下水 汉江沿岸地下水 最小值 最大值 平均值 标准差 变异系数 最小值 最大值 平均值 标准差 变异系数 pH 6.70 7.57 7.13 0.22 0.03 6.50 7.73 6.89 0.38 0.06 Eh(mV) ‒171.10 ‒62.20 ‒130.47 28.72 ‒0.22 ‒149.90 85.50 ‒103.69 79.40 ‒0.77 DO (mg/L) 0.15 4.75 1.04 0.92 0.88 0.94 7.20 2.56 2.06 0.80 EC (μs/cm) 108 2 023 997 268 0.27 302 1 422 859 307 0.36 DOC (mg/L) 3.52 28.17 9.57 5.07 0.53 0.00 14.90 4.14 4.13 1.00 δ13C‒DIC ‒16.39 10.50 ‒6.28 7.05 ‒1.12 ‒17.53 0.85 ‒8.79 5.84 ‒0.67 NH4‒N(mg/L) 0.44 35.50 8.31 9.52 1.15 0.45 45.30 6.76 14.08 2.08 K (mg/L) 0.44 9.28 2.87 1.96 0.68 0.46 3.71 1.99 0.99 0.50 Na (mg/L) 9.71 33.45 19.37 7.00 0.36 6.40 31.51 17.39 8.06 0.46 Ca (mg/L) 84.26 275.11 155.54 34.96 0.22 47.85 261.09 148.39 61.41 0.41 Mg (mg/L) 24.00 103.10 41.62 12.79 0.31 7.78 55.01 33.21 14.10 0.42 HCO3- (mg/L) 536 1 495 728 164 0.23 115 713 528 192 0.36 As (μg/L) 60.98 5 812 4 476.74 929 2.08 2.90 1 884 254 559 2.20 Fe2+ (mg/L) 0.21 38.50 8.91 7.74 0.87 0.71 22.30 8.45 6.54 0.77 S2‒ (μg/L) 0.00 58.00 5.54 10.03 1.81 1.00 12.00 3.07 3.68 1.20 As(III) (μg/L) 50.37 5 332 375 857 2.28 16.50 1 450 213 449 2.11 As(V) (μg/L) 1.75 282 23.46 56.78 2.42 0.93 346 29.90 113 3.77 MMA(μg/L) 3.38 350 26.79 56.36 2.10 0.00 54.70 9.52 16.84 1.77 DMA(μg/L) 2.78 94.59 9.89 15.13 1.53 0.00 37.33 7.36 11.15 1.52 ∑MeAs(μg/L) 6.16 444 36.69 71.23 1.94 0.00 92.04 14.77 26.92 1.82 
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表 2 研究区地下水中3个荧光组分特征与已有文献报道的对比
Table 2. Descriptions of the three-component PARAFAC model of Ex/Em wavelengths data and their comparison with previous identified components
组分 Ex/Em.max(nm) 荧光类型 文献报道 C1 255(340)/466 陆源类腐殖质；分子量大；与高分子量的芳香族有关 C2:350/454 (Lambert et al., 2017)
C2: < 260/448-480 (Fellman et al., 2010)
C3:250(330)/456 (Yang et al., 2020a, 2020b)C2 < 220/418 陆源类富里酸，分子量较低 C3:240(305)/425 (Osburn et al., 2017)
C2: < 240/404 (Walker et al., 2009)
C4: < 240/405 (Lambert et al., 2016)C3 240(320)/398 微生物源类腐殖质；中等分子量大小 C2:325(< 260)/385 (Yamashita et al., 2010)
C1:240(310)/405 (Chen et al., 2017)
C3:315/410 (Lambert et al., 2017)
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