Occurrence and Formation of High Iodine Groundwater Inoxbows of the Middle Reach of the Yangtze River
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摘要: 高碘地下水(碘浓度大于100 μg/L)广泛分布于我国沿海地区和干旱内陆盆地,威胁近千万人口的饮水安全,但目前对湿润区河湖平原地下水中碘的分布与成因机制的认识还十分薄弱.通过采集长江中游故道区75组浅层地下水样品和7组地表水样品进行了水化学分析,查明了地下水中碘的空间分布特征,并运用主成分分析识别了碘富集的水环境要素和水文地球化学过程,并结合4个不同位置的钻孔岩性特征探讨了沉积环境演化对地下水中碘富集的控制作用.研究发现地下水中碘的浓度范围为0.78~1 590 μg/L,其中25%样品超过我国水源性高碘地区水碘含量判定值(100 μg/L).高碘地下水主要赋存于长江河曲凹岸和粘土充填的牛轭湖区的浅层承压含水层.长江中游故道埋藏的丰富有机质形成的强还原环境有利于碘从沉积物释放至地下水中,微生物介导下沉积物有机质降解和铁的氢氧化物还原性溶解是区内高碘地下水形成的主要水文地球化学过程.牛轭湖区后期填充的低渗透率粘粒填塞体和河曲凹岸沉积的厚层粘土层创造了利于碘富集的封闭且水流滞缓的地下水环境.Abstract: High iodine groundwater (iodine concentration greater than 100 μg/L) is widely distributed in coastal areas and arid inland basins in China, threatening the drinking water safety of nearly ten million people, but the understanding of the distribution andformationof iodine in groundwater in humid areas is still very weak. In this study, 75 shallow groundwater samples and 7 surface water samples were collected fromoxbowsof the middle reach of the Yangtze River for hydrogeochemical analysis, the spatial distribution characteristics of iodine in groundwater were identified, the water environmental factors and hydrogeochemical process of iodine enrichment were identified by principal component analysis, and the control effect of sedimentary environment evolution on iodine enrichment was discussed through the lithological analysis of four boreholes at different locations. The concentration of iodine in groundwater ranged from 0.78 μg/L to 1 590 μg/L, and 25% of the samples exceed the determination value of water iodine content in water source areas with high iodine content in China (100 μg/L). The high iodine groundwater mainly occurs in the shallow confined aquifer, distributed in the concave bank of the Yangtze River and the clayfilled oxbow lake. The strong reducing environment rich in organic matter buried under the deposition in oxbows of the middle reach of Yangtze River is conducive to the release of iodine from the sediments to groundwater. Microbially degradation of organic matter and reductive dissolution of iron hydroxide in aquifer sediments are the main hydrogeochemical processes controlling the formation of high iodine groundwater in the study area. The low-permeability clay packing in the later period of the oxbow lake and the thick clay layer deposited on the concave bank of the river meander created a closed and stagnant groundwater environment for iodine enrichment.
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图 5 研究区地下水碘含量与pH(a)、Eh(b)、NO3-(c)的关系图;(d)碘的pH-Eh图(据Fuge and Johnson, 2015绘制)
Fig. 5. Relationship between groundwater iodine concentration and pH (a), Eh (b), NO3-(c); (d)Eh/pH diagram for iodine (revised from Fuge and Johnson, 2015).
表 1 研究区水样水化学指标统计
Table 1. Statistics of water sample chemical indicators in the study area
参数 地表水 浅层潜水 浅层承压水 最小值 最大值 平均值 中位数 变异系数 最小值 最大值 平均值 中位数 变异系数 最小值 最大值 平均值 中位数 变异系数 pH 7.57 9.26 8.22 8.16 0.06 5.98 7.29 6.76 6.78 0.06 6.69 7.57 7.14 7.18 0.03 Eh(mV) 43.6 217 138 135 0.41 -43.80 252 90.10 114 0.97 -203 11.7 -90.6 -93.2 -0.5 DO(mg/L) 6.72 11.2 8.48 8.07 0.18 0.86 5.32 2.89 2.51 0.48 < 0.01 6.6 2.21 1.76 0.71 Na(mg/L) 4.66 15.9 10.8 9.92 0.41 4.63 46.9 15.7 13.3 0.63 6.16 96.1 18.6 14.6 0.75 Mg(mg/L) 6.74 17.4 10.8 9.9 0.34 4.23 63.6 19.3 8.37 0.89 4.39 128 35 34.8 0.5 K(mg/L) 1.8 6.02 3.59 3.27 0.41 0.39 41.3 7.47 3.94 1.50 0.44 111 5.71 3.11 2.63 Ca(mg/L) 23 43.8 35.5 37.8 0.22 14.1 194 72.1 37.3 0.87 32.4 273 137 142 0.32 Cl-(mg/L) 11.3 64.1 22.5 16 0.83 3 84.6 23.7 18.8 0.91 < 0.01 57.8 12 5.20 1.04 NO3-(mg/L) 0 5.71 1.95 0.54 1.23 < 0.01 132 22.6 4.05 1.60 < 0.01 33.6 1.78 0.45 3.34 SO42-(mg/L) 4.05 36.5 20 18.5 0.5 < 0.01 100 24.9 17.9 1.02 < 0.01 52.9 8 < 0.01 1.74 HCO3-(mg/L) 102 192 154 157 0.21 32 874 317 176 0.89 157 1 750 681 698 0.36 TDS(mg/L) 120 229 182 193 0.24 78 872 344 186 0.76 183 1 320 558 573 0.33 NH4⁃N(mg/L) 0.10 0.64 0.31 0.28 0.62 0.01 3.2 0.46 0.11 1.87 0.04 71 4.50 1.55 2.45 Fe2+(mg/L) - - - - - < 0.01 13.8 1.32 0.01 2.65 0.12 15.4 4.05 2.03 1.07 Fe(mg/L) < 0.01 0.03 0.01 0.01 1 < 0.01 13.6 0.87 0.10 3.24 0.09 18.3 4.36 1.79 1.2 I(μg/L) 1.3 10.8 5.14 4.54 0.71 0.78 36.8 8.24 4.55 1.09 2.44 1 590 142 35.7 2.11 DOC(mg/L)* - - - 7.83 - 1.21 7.33 3.32 2.98 0.63 2.58 21.9 6.64 5.29 0.71 注:-.表示数据未获取;*.仅29组水样测试DOC含量,其中地表水样品1组,浅层潜水样品8组,浅层承压水20组. 表 2 研究区最大方差法旋转主成分荷载
Table 2. Principal component loadings with varimax rotationin the study area
变量 PC1 PC2 PC3 TDS 0.979 0.118 0.106 Ca 0.951 0.209 -0.031 Mg 0.948 0.135 -0.065 HCO3- 0.968 0.151 -0.146 I- 0.123 0.742 -0.055 Fe 0.233 0.667 -0.159 Mo 0.117 0.812 -0.050 Cl 0.058 -0.184 0.881 NO3- -0.175 0.078 0.814 SO42- 0.055 -0.442 0.581 特征值 4.255 2.135 1.264 贡献率(%) 42.550 21.349 12.638 累计贡献率(%) 42.550 63.899 76.537 注:加粗表示显著荷载. -
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