基于双同位素(δ^15N-NO^--δ^18O-NO3^-)和IsoSource模型的岩溶槽谷区地下水硝酸盐来源的定量示踪

Quantification of Nitrate Sources to Groundwater in Karst Trough-valley Areas Based on Dual Stable Isotopes of δ^15N-NO3^- and δ^18O-NO3^- and the IsoSource Model

由于岩溶水文系统的脆弱性,岩溶地下水的NO-3污染成为全球普遍且严峻的环境问题,为保证居民的饮水安全,准确识别地下水中NO-3污染来源并量化各来源的贡献具有重要意义.选择重庆市近郊受城市化和农业活动影响显著的中梁山北部的龙凤和龙车两个岩溶槽谷地下河系统为研究对象,于2017年2月~2018年2月采集地下河水水样,分析其水化学和(δ^15N-NO^--δ^18O-NO3^-),并利用IsoSource模型定量评估地下水中NO-3的来源.结果表明:①龙凤和龙车槽谷地下水NO^-3浓度变化范围为19.31~37.01 mg·L^-1和2.15~27.69 mg·L^-1,平均值分别为28.21 mg·L^-1和10.31 mg·L^-1,季节变化明显;②龙凤和龙车槽谷地下水δ^15N-NO3^-和δ^18O-NO3^-分别变化于3.29‰~11.03‰、0.88‰~7.51‰和5.25‰~11.40‰、2.90‰~19.94‰,平均值分别为6.74‰、3.18‰和7.95‰、11.18‰,龙凤槽谷较低的δ^15N-NO3^-和δ^18O-NO3^-值暗示其地下水NO3^-主要来源于农业N肥,而龙车槽谷较高的δ^15N-NO3^-和δ^18O-NO3^-值意味着其地下水NO3^-主要来源于生活污水,也表明硝化过程是本区地下水N的主要转化过程;同时,两槽谷地下水δ^15N-NO3^-和δ^18O-NO3^-存在明显的季节差异,龙凤槽谷旱季和雨季地下水δ^15N-NO3^-和δ^18O-NO3^-的平均值分别为8.83‰、2.79‰和4.64‰、3.58‰,龙车槽谷旱季和雨季地下水δ^15N-NO3^-和δ^18O-NO3^-的平均值分别为9.79‰、14.56‰和5.12‰、7.8‰,表明两槽谷地下水NO-3来源存在显著的季节差异,龙凤槽谷雨季地下水NO3^-主要来源于降水和化肥中NH+4的硝化作用、土壤有机氮,而旱季主要来源于人畜粪便及污水,龙车槽谷旱、雨季地下水NO3^-都主要来源于人畜粪便及污水;③IsoSource模型解析结果表明,龙凤槽谷地下水NO3^-污染以降水和化肥中的NH+4来源贡献最大(44.63%),其次为人畜粪便及污水(29.5%)和土壤氮矿化(22.38%),大气沉降和化肥贡献率较低,不足10%.其中,雨季主要来源为降水和化肥中的NH+4(52.25%),旱季则是人畜粪便及污水(41%);龙车槽谷NO3^-污染以人畜粪便及污水来源最大(36.17%),其次为降水和化肥中的NH4^+硝化(23.5%)和土壤氮矿化(22.5%),大气沉降和化肥贡献率皆低于10%,旱、雨季人畜粪便及污水来源的贡献率都较大,分别为47%和25%.

Due to the vulnerability of karst hydrological systems,nitrate pollution in karst groundwater has become a global common and serious environmental problem.In order to ensure drinking water safety,it is very important to accurately identify groundwater nitrate sources.The groundwater hydrochemistry andδ^15N-NO3^- andδ^18O-NO3^- isotopes were analyzed in samples taken from a suburb of Chongqing:the Longfeng karst trough-valley,which is mainly affected by agricultural activities,and the Longche karst trough-valley,which is primarily affected by urbanization.The IsoSource model was then used to quantify the groundwater nitrate sources.The results showed that:①The NO3^- concentration in groundwater ranged from 19.31 mg·L^-1 to 37.01 mg·L^-1(mean of 28.21 mg·L^-1)in the Longfeng karst trough-valley,and from 2.15 mg·L^-1 to 27.69 mg·L^-1(mean of 10.31 mg·L^-1)in the Longche karst trough-valley.The groundwater NO3^- concentration exhibited an obvious seasonal variation in both valleys.②Theδ^15N-NO3^- andδ^18O-NO3^- isotopes in groundwater in the Longfeng karst trough-valley ranged from 3.29‰to 11.03‰(mean of 6.74‰)and 0.88‰to 7.51‰(mean of 3.18‰),respectively.In contrast,groundwater in the Longche karst trough-valley presented higherδ^15N-NO3^- andδ^18O-NO3^- values that ranged from 5.25‰to 11.40‰(mean of 7.95‰)and 2.90‰to 19.94‰(mean of 11.18‰),respectively.The lower values ofδ^15N-NO3^- andδ^18O-NO3^- in groundwater in the Longfeng karst trough-valley suggest that groundwater NO3- was mainly sourced from agricultural N fertilizer,while the higher values ofδ^15N-NO3^- andδ^18O-NO3^- in groundwater in the Longche karst trough-valley indicate that groundwater NO3- was primarily sourced from domestic sewage.Moreover,suchδ^15N-NO3^- andδ^18O-NO3^- values in groundwater indicate that nitrification was the primary process for nitrogen conversion in both valleys.Meanwhile,significant seasonal differences in groundwaterδ^15N-NO3^- andδ^18O-NO3^- were observed in both valleys;theδ^15N-NO3^- andδ^18O-NO3^- values were higher during the dry season(means of 8.83‰and 2.79‰,respectively)than during the rainy season(means of 4.64‰and 3.58‰,respectively)in the Longfeng karst trough-valley,whereas theδ^15N-NO3^- andδ^18O-NO3^- values were lower during the dry season(means of 9.79‰and 14.56‰,respectively)than during the rainy season(means of 5.12‰and 7.8‰,respectively)in the Longche trough-valley.This suggests that there were differences in the seasonal NO3^- sources to groundwater in both valleys.During the rainy season,the groundwater NO-3 concentration in the Longfeng karst trough-valley was mainly due to the nitrification of NH+4 in precipitation and fertilizer as well as organic nitrogen in soil,whereas during the dry season,the groundwater NO3^- concentration primarily originated from domestic sewage.In contrast,the groundwater NO3^- concentration in the Longche karst trough-valley primarily originated from domestic sewage in both seasons.③The results of the IsoSource model indicated that the nitrification of NH+4 from fertilizer and rainwater was the primary NO3^- source to groundwater(44.63%of the total)in the Longfeng trough valley,and was followed by domestic sewage(29.5%),soil organic nitrogen(22.38%),and NO3^- from rainwater and fertilizer(<10%).During the rainy season,the groundwater NO3^- concentration was mainly due to the nitrification of NH+4 from fertilizer and rainwater(52.25%of the total)in Longfeng trough-valley,while groundwater NO3^- concentration primarily originated from domestic sewage during the dry season(41%of the total).In contrast,the groundwater NO3^- concentration was mainly from domestic sewage(36.17%)in Longche karst trough-valley,and was followed by the nitrification of NH+4 from fertilizer and rainwater(23.5%),soil organic nitrogen(22.5%),and NO3^- from rainwater and fertilizer(<10%).The groundwater NO3^- concentration in the Longche karst trough-valley primarily originated from domestic sewage in both seasons,and accounted for 47%and 25%during the rainy season and dry season,respectively.