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。

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.