基于CFCs及多同位素示踪技术的城乡交错带地下水硝酸盐污染源解析及迁移转化规律研究

Development of urban-rural ecotone is the result of rapid urbanization. Characteristics of groundwater nitrate pollution in urban-rural ecotone can be summarized as having multi-pollution sources, complex chemical compositions and complicated migration and transformation processes. Traditional hydrochemistry and nitrogen isotope method shows limitations when being applied to trace nitrate pollution in urban-rural ecotone. The limitations were summarized as (1) nitrogen isotope characteristics for typical sources of pollution show wide range of δ15N and overlap with each other, it makes multi-sources identification difficult without referring other isotope and hydrochemical information; (2) underlying surface change due to human activities disturb the natural groundwater recharge process, resulting subsequent changing of groundwater residence time and hydrodynamic conditions. Longer groundwater residence time combined with redox conditions changing show significant influence on the transport of nitrate in groundwater. It is necessary to illustrate groundwater flow system and evaluate groundwater residence time to analyze nitrate transport processes; (3) it is hard to discriminate two typical denitrification processes: Heterotrophic Denitrification and Autotrophic Denitrification. And these two processes are key factors when evaluating the natural attenuation ability for groundwater. So in this study, an investigation based on CFCs (CFC-12, CFC-11, CFC-113) age-dating technique and multi-isotope tracing method combined with traditional hydrochemical tools is conducted to deal with the nitrate pollution problems in urban-rural ecotone. In this study, it is emphasized that the groundwater flow system and element migration processes are two key factors when study groundwater nitrate pollution. So first, groundwater apparent age is calculated based on CFCs to trace groundwater hydrodynamic conditions, and it gives a direct estimation of nitrate transport rate in the study area. Then based on multi-isotopes (δD-H2O and δ18O-H2O, δ15N-NO3- and δ18O- NO3-, δ34S-SO42- and δ18O-SO42- and δ13C-DIC), redox reactions among C, N, S and isotope fraction principles, both the identification of multi-pollution sources and understanding of complicated migration and transformation processes of nitrate in urban-rural ecotone are investigated. Multi-sources are identified by a statistical multiple end-members mixing model, and uncertainty analysis is also conducted. At last, two typical denitrification processes: Heterotrophic Denitrification and Autotrophic Denitrification are also discriminated by synthesizing hydrochemistry and isotope information. This study may contribute to the water resource and environment management in urban-rural ecotone.

城乡交错带地下水硝酸盐污染具有多来源、复杂组分及复杂迁移转化过程等特点。这些特点导致传统水化学结合氮同位素示踪在应用中存在局限性，主要表现为：多来源识别困难且无法定量评估污染源贡献比；无法有效揭示城乡交错带地区下垫面条件变化导致的地下水滞留时间及水动力条件的变化情况，不利于硝酸盐迁移过程考察；无法有效区分地下水中异养及自养两类反硝化过程。针对以上问题，本研究以CFCs地下水测龄及多同位素示踪（δD-H2O和δ18O-H2O、δ15N-NO3-和δ18O-NO3-、δ34S-SO42-和δ18O-SO42-及δ13C-DIC）为主要技术手段，地下水动力学及同位素分馏原理为理论基础，重点解决城乡交错带地下水硝酸盐多来源解析和地下水异养及自养反硝化能力评价两大问题，建立研究多来源、复杂组分、复杂迁移转化过程的硝酸盐污染问题的来源识别和地下水反硝化能力评价的综合体系。

作为人类活动剧烈影响区，土地利用类型以及物质流等的剧烈变动使得城乡交错带地下水环境严重退化，并伴随产生了严重的硝酸盐污染，探索识别城乡交错带地下水硝酸盐污染来源以及迁移转化规律意义重大。本项目以珠江三角洲典型农业区、城乡交错带及城镇化地区为研究对象，利用水化学以及多同位素示踪技术探索交错带硝酸盐污染来源及迁移转化特征，并对地下水中硝化及反硝化作用产生的N2O在地下水溶解性气体N2O中的占比进行评估。结果如下：.（1）地下水硝酸盐污染具有有点的普遍性，在农业区、城乡交错带及城镇化地区均有检出，其中农业区硝酸盐浓度分布范围为4.73-20 mg/L，交错带及城镇化地区范围为0.36-63.28 mg/L。.（2）基于多同位素示踪结果发现：针对浅层地下水，典型农业区地下水硝酸盐污染主要以农村化粪池渗漏、生活污水排放及化肥施用为主；城乡交错带及城镇化地区浅层地下水硝酸盐污染主要来自于城中村粪肥及生活污水的排放。针对深层地下水，硝酸盐污染并不明显，三氮主要组成为氨氮，南沙番禺地区深层地下水的铵含量极高且主要来源于天然有机质矿化作用。.（3）深层地下水的碳氮硫循环耦合。深层地下水无机碳的主要来源为碳酸岩溶解和有机质矿化作用，硫酸盐的主要来源为海水入侵。δ13C-HCO3-，δ34S-SO42-及δ18O-SO42-表明产甲烷及硫酸还原作用的存在，且硫酸还原及CO2还原是深层地下水氨释放的关键，C-N-S形态在深层地下水中同时受到还原作用控制且相互影响；δ13C-HCO3-，δ34S-SO42-及δ18O-SO42-同位素整体变化微弱，同样无法有效指示异样反硝化作用。.（4）δ18O-N2O及SP值则表明该区浅层地下水发反硝化作用生成N2O的占比约为27.74％；硝化作用生成N2O的占比的范围约为72.26％。发生硝化或者反硝化作用后，生成的产物N2O被还原成N2的比例约为52.31％。整体上可以看出，浅层地下水中主要发生的反应为硝化作用，反硝化作用相对较为微弱。