里湖地下河N、S来源及其水-岩作用过程

发布时间：2018-08-12 17:24

【摘要】：地下水是世界上主要的淡水资源之一,在岩溶地区,特别是在我国西南岩溶区,由于地下水储量相对丰富,水质优良,成为良好的供水水源。然而,近年来,岩溶地下水遭受污染的报道却只增不减,其中由硝酸盐、硫酸盐超标引起的地下水污染日益严重并引起世界各国的普遍关注,治理由硝酸盐、硫酸盐超标引起的地下水水质恶化已刻不容缓,为此,厘清城市污染物质来源,从源头上减少硫酸盐、硝酸盐输入显得尤为重要。我国西南岩溶区具有地上、地下双层结构,“三水”转换迅速,硝酸盐、硫酸盐污染发生具有随机性,机理过程的复杂性,排散的途径及排放的不确定性,并且还具有远距离运移的特点,要追踪来源十分困难,运用单一的指标判别其污染源往往缺乏针对性和科学性,需要结合多种同位素分别示踪污染物质来源。同时,人为排放的硫酸和硝酸进入地下河系统后参与到水-岩作用过程,引起水-岩作用强度时空差异,改变了地下水的物质组成及其地球化学循环,干扰了由碳酸溶蚀碳酸盐岩产生的碳汇效应。本文以里湖地下河流域为研究区,将硫同位素、氮氧同位素与传统水化学方法相结合,分析地下水物质组成及其影响因素,揭示里湖地下河硫酸盐、硝酸盐来源,运用水化学方法估算硫酸和硝酸参与下的水-岩作用强度。通过数据分析,得出以下结果:里湖地下河流域水化学类型为HCO_3~-Ca型,地下水地球化学组成受控于碳酸盐岩的自然风化溶解,在人为输入、降水稀释和河流自净作用的综合影响下,各离子浓度在旱季大于雨季,空间上从上游到下游整体呈现波动下降的趋势。地下河δ~(34)S-SO_4~(2-)值在-4.12‰~-0.93‰间,通过与潜在硫源的δ~(34)S比对,推断地下河SO_4~(2-)受到城镇污水、化肥和酸雨的影响,δ~(34)S值在农耕季略大于旱季,且波动更大,可能是地下水中混入了相对更高δ~(34)S-SO_4~(2-)值的化肥;δ~(15)N-HCO_3~-值在-0.05‰~18.15‰之间,平均为8.20‰,δ~(18)O-HCO_3~-在-6.71‰~77.04‰之间,平均为12.58‰,结合硝酸盐氮氧同位素组成分析,地下河HCO_3~-来源具有明显的季节差异,2015年1月其来源复杂,2014年5月和10月地下水HCO_3~-主要受到合成化肥、人畜粪便和污水及土壤氮的不同程度的混合,2015年7月主要来源为合成化肥,并受到大气降水高δ~(18)O-HCO_3~-的影响。地下水δ~(13)C-DIC分布在-10.74‰~-4.34‰之间,平均为-7.36‰,碳同位素组成同时受到自然的碳酸溶蚀和人为输入的硫酸、硝酸溶蚀的影响。与桂林、南山、乌江等地相比较为偏正,指示可能受到硫酸、硝酸溶蚀碳酸盐岩产生的碳同位素偏正的DIC的影响。地下水[Ca~(2+)+Mg~(2+)]/[HCO_3~-]当量比值在1.12~1.56之间,[SO_4~(2-)+HCO_3~-]/[HCO_3~-]比值在0.11~0.50之间,也揭示硫酸、硝酸参与了水-岩作用。根据水化学方法计算得出碳酸溶蚀碳酸盐岩的比例介于28.04%~78.55%之间,平均为50.01%,受水-岩作用时间影响,旱季溶蚀比例大于雨季,空间上溶解能力最强为下游的小龙洞处;硫酸和硝酸溶蚀碳酸盐岩的比例介于21.45%~71.96%之间,平均为49.71%,在农业活动的影响下,其溶蚀能力最强出现在2014年5月,受人类活动强度和水体自净作用的综合影响,其溶蚀强度总体上表现为中游农业活动区域上游城镇和工业排污区域下游人为活动较少干扰区域。
[Abstract]:Groundwater is one of the main freshwater resources in the world. In karst areas, especially in the karst areas of Southwest China, groundwater has become a good source of water supply because of its relatively abundant reserves and excellent water quality. However, in recent years, reports of pollution of karst groundwater have only increased, including groundwater pollution caused by excessive nitrate and sulfate. It is urgent to control the deterioration of groundwater quality caused by excessive nitrate and sulfate. Therefore, it is very important to clarify the source of urban pollutants and reduce sulfate from the source. Rapid transformation, nitrate and sulfate pollution occur randomly, the mechanism process is complex, the way of dispersal and emission is uncertain, and also has the characteristics of long-distance migration, it is very difficult to trace the source, using a single index to identify its pollution source is often lack of pertinence and scientific, need to combine a variety of isotopes separately. At the same time, anthropogenic discharge of sulfuric acid and nitric acid into the underground river system participates in the process of water-rock interaction, resulting in spatio-temporal differences in the intensity of water-rock interaction, changing the composition of groundwater and its geochemical cycle, interfering with the carbon sink effect produced by carbonate dissolution carbonate rocks. In the study area, sulfur isotopes, nitrogen and oxygen isotopes were combined with traditional hydrochemical methods to analyze the composition of groundwater and its influencing factors, reveal the sources of sulfate and nitrate in the underground rivers of Lihu Lake, and estimate the intensity of water-rock interaction under the participation of sulfuric acid and nitric acid by hydrochemical methods. The hydrochemical type of the basin is HCO_3~-Ca. The geochemical composition of groundwater is controlled by the natural weathering and dissolution of carbonate rocks. Under the combined influence of artificial input, precipitation dilution and river self-purification, the ion concentrations in the dry season are higher than those in the rainy season. The spatial fluctuation of the groundwater is decreasing from upstream to downstream. It is inferred that SO_4~ (2-) in underground rivers is affected by urban sewage, chemical fertilizer and acid rain by the ratio of - 4.12 ~ - 0.93, and the value of 34 S is slightly higher in agricultural season than that in dry season, and the fluctuation is larger. It is possible that the groundwater is mixed with chemical fertilizers with relatively higher value of (34) S-SO_4 (2-); and the value of (15) N-HCO_3 82 Between, the average is 8.20, between - 6.71 ~77.04, the average is 12.58. Combined with the analysis of nitrate nitrogen and oxygen isotope composition, the source of HCO_3 - in underground rivers has obvious seasonal differences. The sources of HCO_3 - in January 2015 are complex. In May and October 2014, the groundwater HCO_3 - is mainly subject to synthetic fertilizers, human and animal manure, sewage and soil. In July 2015, the main source of nitrogen mixing was synthetic fertilizer and was affected by the high precipitation of (18)O-HCO_3~-. The groundwater (13)C-DIC was distributed between - 10.74 - 4.34, with an average value of - 7.36. The carbon isotope composition was affected by both natural carbonation and human input of sulfuric acid and nitric acid. In Nanshan, Wujiang and other places, the ratio of Ca~ (2+) + Mg~ (2+) / [HCO_3~-] equivalent in groundwater is between 1.12 and 1.56, [SO_4~ (2-) + HCO_3~-] / [HCO_3~-] ratio is between 0.11 and 0.50, indicating that sulfuric acid and nitric acid are involved in water-rock interaction. According to hydrochemical calculation, the proportion of carbonate dissolution carbonate rocks ranges from 28.04% to 78.55%, with an average of 50.01%. Affected by the time of water-rock interaction, the ratio of dissolution in dry season is larger than that in rainy season, and the spatial dissolution capacity is the strongest in the Xiaolong cave downstream; the ratio of sulfuric acid and nitric acid dissolution carbonate rocks ranges from 21.45% to 71.96%, with an average of 49.71%. Under the influence of agricultural activities, the strongest dissolution capacity appeared in May 2014, which was affected by the intensity of human activities and the self-purification of water. The dissolution intensity was generally shown in the upper reaches of agricultural activities and the lower reaches of industrial sewage discharge areas with less disturbance of human activities.
【学位授予单位】：西南大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：X523;P641.3

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