[Quantification of Nitrate Sources to Groundwater in Karst Trough-valley Areas Based on Dual Stable Isotopes of N-NO and O-NO and the IsoSource Model].

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 and N-NO and O-NO 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:① The NO concentration in groundwater ranged from 19.31 mg·L to 37.01 mg·L(mean of 28.21 mg·L) in the Longfeng karst trough-valley, and from 2.15 mg·L to 27.69 mg·L(mean of 10.31 mg·L) in the Longche karst trough-valley. The groundwater NO concentration exhibited an obvious seasonal variation in both valleys. ② The N-NO and O-NO 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 higher N-NO and O-NO 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 of N-NO and O-NO in groundwater in the Longfeng karst trough-valley suggest that groundwater NO was mainly sourced from agricultural N fertilizer, while the higher values of N-NO and O-NO in groundwater in the Longche karst trough-valley indicate that groundwater NO was primarily sourced from domestic sewage. Moreover, such N-NO and O-NO values in groundwater indicate that nitrification was the primary process for nitrogen conversion in both valleys. Meanwhile, significant seasonal differences in groundwater N-NO and O-NO were observed in both valleys; the N-NO and O-NO 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 the N-NO and O-NO 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 NO sources to groundwater in both valleys. During the rainy season, the groundwater NO concentration in the Longfeng karst trough-valley was mainly due to the nitrification of NH in precipitation and fertilizer as well as organic nitrogen in soil, whereas during the dry season, the groundwater NO concentration primarily originated from domestic sewage. In contrast, the groundwater NO concentration in the Longche karst trough-valley primarily originated from domestic sewage in both seasons. ③ The results of the IsoSource model indicated that the nitrification of NH from fertilizer and rainwater was the primary NO 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 NO from rainwater and fertilizer (<10%). During the rainy season, the groundwater NO concentration was mainly due to the nitrification of NH from fertilizer and rainwater (52.25% of the total) in Longfeng trough-valley, while groundwater NO concentration primarily originated from domestic sewage during the dry season (41% of the total). In contrast, the groundwater NO concentration was mainly from domestic sewage (36.17%) in Longche karst trough-valley, and was followed by the nitrification of NH from fertilizer and rainwater (23.5%), soil organic nitrogen (22.5%), and NO from rainwater and fertilizer (<10%). The groundwater NO 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.