Distribution characteristics of iodine in karst groundwater in Xintian county, Hunan Province and the analysis on the causes of high iodine
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摘要: 在湖南新田县部分岩溶区发现高碘地下水,威胁着周边居民的饮水安全,查明该区域地下水中碘的分布特征及其控制因素具有重要意义。采集新田县66组泉水样和45组井水样,采用水化学图解法、主成分分析法和GIS技术,分析了泉水和井水的水化学特征,查明了地下水中碘的空间分布特征,剖析了碘富集的主要控制因素。研究发现泉水与井水中碘含量分别为2.7~92.8 μg·L−1和4.15~3 861 μg·L−1,其中,53.3%井水样品碘含量超过《水源性高碘地区和高碘病区的划定》(GB 19380-2016)标准中的界定值100 μg·L−1。受沉积环境、pH、Eh和地下水径流条件影响,高碘地下水主要沿着一条NE−SW向的河谷分布,从峰林谷地地区到地势低洼的河谷平原地带,地下水碘含量整体随着径流条件变差呈现逐渐增加的趋势。海相沉积所形成的富碘富有机质地层是高碘地下水形成的地质基础,发生有机质降解和竞争吸附的弱碱性偏还原环境是导致碘被释放到地下水中的主要因素;此外,水流滞缓的封闭地下水环境也是控制高碘地下水形成的重要因素。Abstract:
Iodine is one of the essential trace elements for human body, which maintains the growth and normal metabolism of the organism. Iodine deficiency or excess will have different degrees of impact on human health. A large field of strontium-rich mineral water was discovered in Xintian county, Hunan Province. However, the iodine content in some strontium-rich groundwater is abnormal, threatening the drinking water safety of local residents. Therefore, it is important to find out the distribution characteristics of iodine in groundwater and the controlling factors of the formation of high iodine groundwater so as to implement the project for the safety of drinking water and to prevent endemic iodine diseases in the study area. In a hydrogeological survey, 66 groups of spring water samples and 45 groups of well water samples were collected in Xintian county to analyze their hydrochemical characteristics, identify the spatial distribution characteristics of iodine in groundwater and analyze the main factors controlling iodine content in groundwater by means of hydrochemical graphical method, principal component analysis and GIS technology. The results showed that the iodine concentration in spring water and well water ranged from 2.7 to 92.8 μg·L−1 and 4.15 to 3,861 μg·L−1, respectively, with the respective median value of 5.4 μg·L−1 and 168 μg·L−1. It can be seen that all the groundwater with high iodine was well water, and 53.3% of well water samples had iodine concentration exceeding the permitted national standard of 100 μg·L−1 (GB 19380-2016). In contrast, the iodine content in spring water was relatively low, and the overall iodine-deficient groundwater was predominant. High iodine groundwater was mainly distributed along a river valley in the northeast-southwest direction. The iodine content in groundwater showed a gradual increase from the peak-forest valley to the plain area of low-lying river valley, and the hydrochemical type also changed from water with single HCO3-Ca type to water with complex HCO3-Na, HCO3-Na·Ca type, etc. The marl stratum formed by marine sedimentation of Shetianqiao Formation is rich in iodine and organic matter, which provides good geological conditions for the enrichment of iodine in groundwater. The microbial degradation of organic matter occurs in the closed and partially reducible groundwater environment of the marl aquifer. CO2 generated by organic matter decomposition exists in groundwater mainly in the form of ${\rm{HCO}}_3^{-}$, which will compete with I- for adsorption. Meanwhile, the process of organic matter decomposition to form ${\rm{HCO}}_3^{-}$ is accompanied by the release and migration of iodine deposited on it, making the iodine content in the groundwater increases with the increase of ${\rm{HCO}}_3^{-}$ content. At the same time, the alkaline karst water environment can increase the electronegativity of organic matter surface to reduce the adsorption of iodine ions by organic matter. OH− in groundwater also competes with I− for adsorption, which accelerates the release and migration of adsorbed iodine into groundwater, and hence increases the iodine content. The distribution area of high iodine groundwater is located in a groundwater discharge area. It is also an area distributed with strata rich in iodine and organic matter. In this area, the sluggish groundwater flow due to flat topography, together with a relatively closed groundwater environment, further facilitates the enrichment of iodine. The strata rich in iodine and organic matter formed by marine sediments, closed and partial reduction chemical environment with karst water of weak alkalinity and sluggish groundwater runoff are the main factors controlling the formation of high iodine groundwater in the study area. -
Key words:
- karst groundwater /
- iodine /
- spatial distribution /
- principal component analysis /
- controlling factors
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表 1 研究区地下水样品水化学分析统计表
Table 1. Hydrochemical analysis of groundwater samples in the study area
参数 单位 最小值 最大值 平均值 中间值 标准差 变异系数 TDS mg·L−1 136.91 732.81 334.20 322.52 118.05 0.35 总硬度 mg·L−1 65.56 612.20 271.70 254.13 88.49 0.33 Eh mV −96.2 123.9 47.8 64.5 50.13 1.05 pH 6.74 8.91 7.25 7.23 0.29 0.04 K+ mg·L−1 0.06 24.00 2.11 1.13 3.44 1.63 Na+ mg·L−1 0.28 219.72 15.61 2.29 34.76 2.23 Ca2+ mg·L−1 14.85 159.42 89.99 85.23 28.09 0.31 Mg2+ mg·L−1 0.94 59.05 11.40 4.94 13.39 1.17 Cl− mg·L−1 1.30 93.32 11.69 6.36 14.95 1.28 ${\rm{SO}}_4^{2-}$ mg·L−1 4.89 236.34 25.48 16.17 27.02 1.06 ${\rm{HCO}}_3^{-}$ mg·L−1 132.50 630.30 316.55 297.19 97.99 0.31 ${\rm{NO}}_3^{-}$ mg·L−1 1.78 91.12 11.76 5.51 17.02 1.45 NO$_2^{−}$ mg·L−1 <0.002 8.650 0.290 <0.002 1.31 4.58 F− mg·L−1 0.04 3.50 0.35 0.16 0.55 1.58 TFe mg·L−1 <0.003 1.260 0.140 0.061 0.22 1.62 I− ug·L−1 2.7 3 861.0 116.7 12.5 401.83 3.44 CODMn mg·L−1 <0.50 2.29 0.53 0.54 0.53 1.00 游离CO2 mg·L−1 1.06 12.67 4.73 4.51 2.51 0.53 Sr2+ ug·L−1 2.9 8 465.0 1 002.2 195.0 1 811.83 1.81 表 2 新田岩溶地下水旋转因子载荷矩阵
Table 2. Rotation factor loading matrix of Xintian karst groundwater
因子 泉水 井水 F1 F2 F3 F1 F2 F3 F4 TDS 0.648 0.729 0.180 0.348 0.855 0.261 0.237 TH 0.444 0.857 0.085 −0.647 0.697 0.228 0.010 pH −0.062 -0.723 −0.119 0.764 −0.092 0.102 −0.047 K+ 0.821 0.011 0.179 −0.025 0.136 −0.001 0.887 Na+ 0.689 0.282 0.450 0.950 0.139 0.084 0.026 Ca2+ 0.291 0.901 0.130 -0.739 0.527 −0.364 0.075 Mg2+ 0.545 0.116 −0.100 −0.004 0.381 0.880 −0.091 Cl− 0.842 0.171 0.194 −0.071 0.747 0.022 0.117 ${\rm{SO}}_4^{2-}$ 0.740 0.277 0.435 −0.039 0.545 0.335 0.373 ${\rm{HCO}}_3^{-}$ 0.071 0.960 0.023 0.534 0.606 0.321 −0.260 ${\rm{NO}}_3^{-}$ 0.904 −0.010 −0.031 −0.121 0.114 −0.170 0.882 F− 0.380 −0.017 0.809 0.844 −0.015 0.169 −0.034 I− −0.043 0.466 0.789 0.829 0.191 −0.116 −0.135 Sr2+ −0.263 0.695 0.409 0.217 0.064 0.908 −0.077 特征值 6.749 2.813 1.292 4.675 3.603 1.892 1.156 贡献率/% 48.2 20.1 9.2 33.4 25.7 13.5 8.3 累计方差贡献率/% 48.2 68.3 77.5 33.4 59.1 72.6 80.9 -
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