岩溶槽谷区地下河硝酸盐来源及其环境效应:以重庆龙凤槽谷地下河系统为例

Sources of Nitrate in Groundwater and Its Environmental Effects in Karst Trough Valleys: A Case Study of an Underground River System in the Longfeng Trough Valley,Chongqing

以重庆典型岩溶槽谷龙凤槽谷地下河系统为研究对象,于2017年5月～2018年4月收集大气干、湿沉降和两条地下河(凤凰河、龙车河)水样,利用水化学、δ^(15)N(NO_3^-)、δ^(18)O(NO_3^-)、δ^(18)O(H_2O)和δ^(13)C(DIC)同位素等数据来探讨岩溶地下河水NO_3^-来源及其环境效应.结果表明:①两条地下河水化学类型均属于HCO_3-Ca型,NO_3^-浓度变化范围在17. 58～32. 58mg·L^(-1)之间,平均值为24. 02 mg·L^(-1),雨季略高于旱季,存在明显污染迹象;②两条地下河水δ^(15)N(NO_3^-)、δ^(18)O(NO_3^-)值变化于-3. 14‰～12. 67‰和-0. 77‰～12. 05‰之间,均值分别为7. 45‰和2. 90‰,表现为旱季偏正、雨季偏负的特点,且两条地下河水NO_3^-来源无明显差异,动物排泄物和生活污水是全年稳定来源,降雨、化肥和土壤氮是雨季地下河水NO_3^-的主要来源,硝化过程是地下河系统氮的主要转化过程;③两条地下河水(Ca^(2+)+Mg^(2+))/HCO_3-的量比介于0. 65～0. 82之间,凤凰河均值为0. 75,龙车河均值为0. 70,δ^(13)C(DIC)在-12. 46‰～-9. 20‰之间,凤凰河均值为-10. 72‰,龙车河均值为-11. 10‰,说明各个来源的HNO_3和NH_4^+硝化形成的HNO_3参与了碳酸盐岩的风化过程;④地下河水中8%的DIC来源于HNO_3溶蚀碳酸盐岩,凤凰河、龙车河分别为9%和7%.

Water samples from the two underground rivers (Fenghuang River and Longju River) and samples of the dry and wet deposition of atmospheric dissolved inorganic nitrogen were taken from the Longfeng karst trough valley located in the Zhongliang mountain in the suburbs of Chongqing from May 2017 to April 2018. Anions, cations,δ15 N(NO3-),δ18 O(NO3-),δ18 O(H2O), and δ13C(DIC) isotope data were used to investigate the NO3^- source and its environmental effects. The results showed:① The hydrochemistry of the two underground rivers is of the type HCO3^-Ca. The NO3^- concentration varied from 17.58 to 32.58 mg·L^-1, with an average of 24.02 mg·L^-1, and was slightly higher in rainy season than the dry season, revealing that the underground rivers were polluted.② The δ15 N(NO3-) value ranged from -3.14‰ to 12.67‰, with an average value of 7.45‰. The δ18 O(NO3-) value ranged from -0.77‰ to 12.05‰ with an average value of 2.90‰, and was higher in the dry season than the rainy season, indicating that animal excreta and domestic sewage were main NO3^- sources throughout the year. In addition, rainfall, fertilizer, and soil nitrogen were the NO3^- sources during the rainy season. There are no significant differences between the NO3- sources of the two underground rivers, and nitrification is the main nitrogen conversion process.③ The molar ratio of (Ca^2++Mg^2+)/HCO3- varied from 0.65 to 0.82. That of the Fenghuang River was 0.75 and that of the Longju River was 0.70. The δ13C(DIC) value ranged from -12.46‰ to -9.20‰, with a mean of -11.10‰ in the Longju River and -10.72‰ in the Fenghuang River. These values indicated that the HNO3 derived from the nitrification of NH4+ was involved in the weathering of carbonate rocks.④ HNO3 dissolved carbonate rocks and aggravated the chemical weathering of carbonate rock in the basin, contributing 8% of the DIC in groundwater, and 9% and 7% in Fenghuang River and Longju River, respectively.