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地热资源作为一种集水、矿和热为一体的清洁能源,还兼具医疗和旅游价值,是极具开发潜力的一种新型能源. 在经过补给后进入深部的地热水其赋存环境、运移规律和反应过程相当复杂,同时受到热力、水力以及化学等多种因素的影响;地热水复杂的形成过程也导致了难以对地热系统进行全面系统的了解,而开展地热系统成因模式、水岩相互作用过程研究对推进地热系统的了解有极大帮助[1]. 海南岛的地热资源呈现出以中低温为主、出露点多、储量大的特点,开发利用潜力巨大,具有代表性的琼海官新地热田、儋州兰洋温泉、保亭七仙岭温泉都得到了不同程度的开发利用. 随着地热资源的大面积开发,地热水高氟的特点而导致的氟污染问题受到越来越多的关注[2].
氟作为自然界中常见的元素,主要来源于深部地壳和地幔,以吸附态氟离子、固体氟化物以及氟矿石颗粒等赋存形态广泛分布于岩石圈、水圈、土壤圈中[3]. 氟是人体生长发育过程中必不可少的微量元素,在人体微量元素含量的第三位,摄入适量的氟能够预防龋齿和骨质疏松,但是人体对氟的最佳摄入量的范围十分小[4]. 当氟浓度的摄入量过大时,会导致矿化过度或异位矿化的病理现象,即地方性氟中毒. 地方性氟中毒广泛分布于中国各个地区,患病人数超过1亿,为降低患病人数国家政府投入了大量的人力物力,但效果并不是很理想. 过量的氟对土壤中各类微生物和植物的生长会造成一定程度的危害,同时也会造成人体肾上腺素功能紊乱[5].
地下热水的含氟量通常高于同地区的地表水和地下水,含量通常介于3—10 mg·L−1,远高于中国饮用水标准中规定的1 mg·L−1. 在地热资源当前大规模应用的情况下,高氟含量尾水的排放对于水、土壤环境中氟含量的影响将成为一个不容忽视的问题. 20世纪以来,有关于水环境中氟的研究日益增多,吕晓立[6]对新疆塔城盆地的含氟地下水的分布特征和富集规律进行了分析,李成城[7]总结了运城盆地的高氟地下水的成因机制,但地下热水中氟的研究却比较少. 琼北地区温泉出露点多,地热水中氟含量高,在开发过程中对环境的潜在威胁大. 针对这一问题,本文通过对琼北地区的地质资料、水化学数据、同位素数据进行分析,探究地热水的富集规律和分布特征. 探究内容可以为研究区防治地热水污染提供参考,同时对地热资源合理开发具有重要的指导意义.
琼北地区地热水中氟的富集规律
Study on enrichment of fluorine in geothermal water in Qiongbei area
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摘要: 以海南岛的琼北地区地热水为研究对象,对研究区的地热水、地下水和地表水水样进行测试分析,运用piper图、δ2H和δ18O图、钠碱指数等方法探究琼北地区高氟地热水的富集规律及分布特征. 结果表明,地热水氟浓度均超标,平均值高达14.25 mg·L−1;而地下水和地表水的氟含量未超标,变化范围为0.02—0.44 mg·L−1. 沿海地区到内陆地区地热水氟含量呈现逐渐增大的趋势,内陆地区平均含量高达21.69 mg·L−1. 地热水水化学类型以HCO3−Na为主,温度、pH和地层中的含氟矿物类型对地热水氟的富集具有重要的影响. 地下热水补给来源为现代大气降水,并且存在轻微的“氧漂移”现象,受到一定程度的蒸发浓缩的影响. 随着文石、方解石的沉淀反应、络合反应形成的络合物以及Na+、K+置换出Ca2+、Mg2+的离子交换反应,地热水中氟浓度不断增大.Abstract: The distribution and enrichment characteristics of high-fluorine geothermal water in Qiongbei area have been studied in this paper by a series of geochemical tools including Piper diagram, hydrogen and oxygen isotope and sodium alkali index feature. The results indicate that geothermal water samples are enriched in fluorine with an average value of 14.25 mg·L−1. However, the fluorine concentration in groundwater and surface water samples varying between 0.02 mg·L−1 and 0.44 mg·L−1 were less than the standard value. A gradual increase in fluorine concentration from coastal areas to inland areas has been observed in our study area, with the value of fluorine concentration up to 21.69 mg·L−1. The hydrochemistry type of geothermal water is mainly HCO3−Na, and the enrichment of fluorine is significantly affected by temperature, pH and the type of fluorine-containing minerals. The hydrogen and oxygen isotope compositions indicate that geothermal water is mainly originated from the meteoric water and influenced by evaporation concentration to some extent. The precipitation reaction of aragonite and calcite, the complex formed by the complex reaction and the ion exchange reaction of replacing Ca2+ and Mg2+ with Na+ and K+ result in the continuous increase of fluorine concentration in the geothermal water.
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Key words:
- fluorine /
- geothermal water /
- enrichment /
- Qiongbei area
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表 1 研究区水样水化学组分特征
Table 1. Water chemical composition characteristics of water samples in the study area
采样点
Sample类型
TypepH T/
℃TDS/
(mg·L−1)K+/
(mg·L−1)Na+/
(mg·L−1)Ca2+/
(mg·L−1)Mg2+/
(mg·L−1)Cl−/
(mg·L−1)SO42−/
(mg·L−1)HCO3−/
(mg·L−1)F−/
(mg·L−1)δ2H/
‰δ18O/
‰HH01 高氟地热水 8.10 35.1 746 5.1 778.1 4.8 1.5 310.3 38.6 1122.3 6.32 −49.4 −6.53 HH02 高氟地热水 7.99 43.8 1430 6.0 681.3 4.2 2.2 383.4 30.1 1106.0 7.36 −43.6 −6.48 HH03 高氟地热水 8.47 38.7 1306 4.9 653.8 3.0 1.1 290.6 19.2 1058.7 8.29 −42.7 −6.37 HH04 高氟地热水 7.59 70.1 3096 63.3 960.0 293.4 0.1 1957.3 42.6 71.9 5.30 −46.9 −6.18 HH05 超高氟地热水 8.92 47.3 266 3.1 125.5 4.2 0.1 19.1 94.5 83.7 22.00 −52.8 −7.38 HH06 超高氟地热水 8.18 83.5 281 5.8 114.8 10.8 1.1 21.5 78.0 125.5 20.40 −54.8 −6.98 HH07 超高氟地热水 9.07 40.1 442 2.6 110.5 2.4 0.4 8.7 63.6 77.3 23.10 −65.5 −8.92 HH08 超高氟地热水 8.19 55.4 131 3.0 112.0 4.8 0.7 19.7 49.6 131.0 21.25 −55.3 −7.41 HL01 地下水 6.89 26.1 221 1.0 9.2 17.9 10.9 8.1 5.0 102.8 0.02 −44.9 −6.7 HL02 地下水 6.93 26.3 668 3.6 4.0 4.2 0.4 3.0 4.2 32.7 0.02 −23.5 −3.42 HL03 地下水 7.85 26.5 3490 3.5 9.5 9.6 3.3 13.0 10.3 36.4 0.42 −36.5 −5.1 H01 地表水 7.59 28.7 421 2.7 6.9 10.8 2.2 5.4 5.7 46.4 0.44 −39.2 −5.45 H02 地表水 7.25 27.6 380 0.1 4.7 9.6 1.8 3.2 4.5 40.0 0.26 −38.8 −5.51 H03 地表水 6.85 27.5 329 2.6 7.2 8.4 2.0 7.2 5.3 38.2 0.32 −34.4 −4.97 H04 海水 7.15 27.1 1001 14.1 488.8 19.7 57.3 857.8 131.0 10.9 0.02 − − -
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