| 新疆阿克苏典型山前洪积扇内高氟地下水的化学特征及氟富集机制 |
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| 引用本文： | 潘欢迎,邹常健,毕俊擘,刘运德,黄丽文.新疆阿克苏典型山前洪积扇内高氟地下水的化学特征及氟富集机制[J].地质科技通报,2021,40(3):194-203. |
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| 作者姓名： | 潘欢迎 邹常健 毕俊擘 刘运德 黄丽文 |
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| 基金项目： | 国家重点研发项目课题2017YFC0406105中国地质调查局"新疆阿拉尔-图木舒克-昆玉综合地质调查"DD20179605 |
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| 摘 要： | 在内陆干旱区，作为重要饮用水源的地下水常面临氟含量超标问题。查明内陆干旱区高氟地下水的分布规律，了解氟在地下水中的富集过程及其影响因素，既可丰富高氟地下水的研究体系，也是保证内陆干旱区饮水安全的重要基础。以新疆阿克苏地区典型山前洪积扇——依格齐艾肯河-喀拉玉尔滚河河间地带为研究区，基于水文地球化学调查结果，刻画了高氟地下水的分布区；结合氟离子含量与特征性水化学指标间的关系，揭示了高氟地下水的成因机制。结果表明:①地下水中氟含量的变化范围为0.8~6.1 mg/L，83%的水样氟含量超过《生活饮用水卫生标准》（GB 5749-2006）规定的上限（1.0 mg/L）；②总体上，氟含量沿地下水流动路径逐渐增大，低氟地下水（ρ（F-）≤1.0 mg/L）分布在国道314以北的补给区，高氟地下水（ρ（F-）>1.0 mg/L）分布在国道314以南的径流区和排泄区；③高氟地下水的水化学类型以Cl·HCO3-Na型为主，而低氟地下水则以Cl·SO4-Na型为主，高氟地下水相比于低氟地下水优势阴离子偏向于HCO3-；④地下水的pH值范围为7.9~8.9（均值为8.4），表明其处于弱碱环境中。地下水中ρ（F-）与pH值呈正相关，此外构成浅层含水层的上更新统沉积物中含有黑云母、氟磷灰石等矿物，其表面存在一定数量的可交换F-，这表明水中OH-与矿物表面F-间的阴离子交换可能对氟的富集有一定贡献；⑤地下水的F-含量与Ca2+含量呈负相关，即高氟地下水中ρ（Ca2+）小于低氟地下水。考虑到氟化钙（CaF2）是自然界中的主要含氟矿物，也是地下水中氟的主要来源，ρ（F-）与ρ（Ca2+）间的这种负相关指示着高氟地下水中可能存在去Ca2+、Mg2+作用，如阳离子交替吸附或碳酸盐岩沉淀等。研究区地下水样中ρ（F-）与ρ（Mg2+）间也呈负相关关系，且和ρ（F-）与ρ（Ca2+）间的关系高度相似，也佐证了高氟地下水中去Ca2+、Mg2+作用的存在；⑥绝大部分地下水样品都位于氯碱性指数图的负值区域，且ρ（F-）与CAI-1和CAI-2均呈较好负相关，CAI-1和CAI-2都随ρ（F-）的增大而减小，这表明高氟地下水中存在Ca2+、Mg2+与Na+间更强的交换作用，对氟富集起着重要作用。地下水中ρ（F-）与SAR间呈较好正相关关系，且高氟地下水样的SAR均值（5.71）远大于低氟地下水SAR均值（1.67），这也进一步证明高氟地下水中的Ca2+、Mg2+与含水介质的Na+间存在强烈的交替作用，对氟的富集起着重要作用；⑦所有地下水样中的萤石均处于未饱和状态，且萤石的饱和指数（SI）与F-含量间呈现较好的正相关，这表明地下水对含氟矿物（主要是萤石）的持续溶解应是导致研究区地下水中氟富集的主要原因。与之相反，研究区所有地下水样中的方解石均处于过饱和状态（SI>0）。这表明CaCO3的沉淀可能促进了CaF2的溶解，导致地下水中氟离子质量浓度增高；⑧研究区低氟地下水的δ18O值介于-11.20‰~-10.67‰间，平均值为-10.94‰，而高氟地下水的δ18O值介于-11.65‰~-11.21‰间，平均值为-11.49‰，即低氟地下水较高氟地下水富集δ18O。此外，F-质量浓度较低（ρ（F-）≤3.0 mg/L）的地下水样中δ18O值与F-质量浓度呈负相关，即低氟地下水具有更正的δ18O值；F-质量浓度较高（ρ（F-）≥4.8 mg/L）的地下水样中δ18O值与F-质量浓度的相关性不显著，随F-质量浓度的增高，δ18O值基本维持不变。以上表明蒸发浓缩作用对地下水中氟的富集贡献较小；⑨研究区地下水中ρ（F-）/ρ（Cl-）比值与ρ（F-）间呈现正相关，即ρ（F-）/ρ（Cl-）比值随ρ（F-）增高呈增大趋势，这也说明地下水中氟富集的主要原因是含氟矿物的溶解，而不是蒸发浓缩作用。此外，Gibbs图也提供了证据:研究区地下水样基本处于水岩作用主导区域，表明地下水化学特征（包括氟的富集）主要受水岩作用控制，蒸发浓缩影响很小。总之，地下水中氟的富集主要由溶解作用引起，OH-与矿物表面F-间的交换也有贡献，但蒸发浓缩作用影响微弱。含氟矿物持续溶解的驱动机制是阳离子交替吸附（地下水中Ca2+与岩土颗粒表面Na+之间）及方解石沉淀所引起的地下水中Ca2+的衰减。
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| 关 键 词： | 高氟地下水 富集机制 萤石溶解 离子交换 方解石沉淀 |
| 收稿时间： | 2020-09-28 |
Hydrochemical characteristics and fluoride enrichment mechanisms of high-fluoride groundwater in a typical piedmont proluvial fan in Aksu area,Xinjiang, China |
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| Abstract: | In inland arid regions, high fluoride concentrations are frequently reported in groundwater which is an important source of drinking water. Investigating its distribution, enrichment and controlling factors could provide insights for better understanding the geochemistry of high-fluoride groundwater, and are critical foundation to ensure the safety of water supply in inland arid regions. To delineate the distribution of high-fluoride groundwater, a comprehensive hydrogeochemical investigation has been conducted in a typical piedmont proluvial fan in Aksu area, Xinjiang, China. The relationship between fluoride concentration and various geochemical parameters has been analyzed for identifying the controlling processes of groundwater fluoride enrichment in this region. The results are: ①Fluoride concentrations in groundwater range from 0.8 to 6.1 mg/L, and 83% of the samples exceed the maximum limit of 1.0 mg/L set by the sanitary standards for drinking water (GB 5749-2006). ②The fluoride concentration was found increasing along groundwater flow paths, and low-fluoride groundwater (ρ(F-)≤1.0 mg/L) mainly distributed in recharge areas by the north of National Highway 314 while high-fluoride groundwater (ρ(F-)>1.0 mg/L) mainly occurred in runoff and discharge areas by the south of National Highway 314. ③The groundwater with high and low fluoride concentration is classified as Cl·HCO3-Na type and Cl·SO4-Na type, respectively, indicating the dominant anion of high-fluoride groundwater is biased towards HCO-3. ④The pH range of groundwater is 7.98.9 with an average value of 8.4, demonstrating a weakly alkaline environment. The groundwater F- content is positively correlated with the pH value. Anion exchange between OH- in groundwater and F- on mineral surface might contribute to the enrichment of fluoride, since there is a certain amount of exchangeable F- in black mica, fluorapatite and other minerals in surrounding Upper Pleistocene sediments. ⑤The F- concentration is negatively correlated with the Ca2+ concentration in groundwater. Considering that calcium fluoride (CaF2) is the main fluorine-containing mineral in the nature and the major source of fluoride in groundwater, the negative correlation between ρ(F-) and ρ(Ca2+) indicates the removal of Ca2+ and Mg2+ in high-fluoride groundwater via cation exchange, adsorption and/or carbonate precipitation. The F- content and Mg2+ concentration of groundwater are also in negative correlation and are highly similar to that between ρ(F-) and ρ(Ca2+), which also evidences the removal of Ca2+ and Mg2+. ⑥The chlor-alkalinity index (CAI) are negative for the majority of groundwater samples, and both CAI-1 and CAI-2 are negatively related with ρ(F-), indicating the exchange between Ca2+, Mg2+ and Na+ in high-fluoride groundwater. There is a great positive correlation between F- content and SAR value, and the average SAR value of high-fluoride groundwater (5.71) is much higher than that of low-fluoride groundwater (1.67), which further proves a strong alternation of Ca2+ and Mg2+ with Na+ in the aquifer and this exchange process plays a significant role in fluoride enrichment. ⑦All groundwater samples are undersaturated with respect to fluorite. An obvious positive correlation between the saturation index (SI) of fluorite and the F- content demonstrates that the continuous dissolution of fluorine-containing minerals (mainly fluorite) is the primary contribution to the accumulation of fluoride in groundwater in the study area. In contrast, the calcite is supersaturated in all groundwater samples, which suggests that the precipitation of calcite may promote the dissolution of CaF2, leading to the increase of F- content in groundwater. ⑧ The δ18O value is between -11.20‰ and -10.67‰ with an average value of -10.94‰ in low-fluoride groundwater, and between -11.65‰ and -11.21‰ with an average value of -11.49‰ in high-fluoride groundwater. Low-fluoride groundwater is more enriched with δ18O than high-fluoride groundwater. Furthermore, the groundwater δ18O value is negatively correlated with F- content when ρ(F-)≤3.0 mg/L, while it remains the same when ρ(F-)≥4.8 mg/L. The above pattern suggests that evaporation contributes little to the enrichment of fluoride in groundwater. ⑨The ρ(F-)/ρ(Cl-) ratio in groundwater is positively correlated with ρ(F-), which also evidences that the dissolution of fluorine-containing minerals leads to the enrichment of fluoride in groundwater, rather than the evaporation. In addition, Gibbs diagram shows that all groundwater chemistry are controlled by water-rock interaction, with little influence of evaporation. In conclusion, our results indicates that high fluoride concentrations in groundwater are controlled by fluorite dissolution and anion exchange between OH- in groundwater and F- in minerals, whereas the influence of evaporation is negligible. And the fluorite dissolution is promoted by the Ca2+ depletion due to calcite precipitation and cation exchange between Na+ absorbed on mineral surface and Ca2+ in groundwater. |
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