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
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摘要
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,δ~(15) N( NO_3~-),δ~(18) O( NO_3~-),δ~(18) O( H_2 O),andδ~(13) C( DIC) isotope data were used to investigate the NO_3~-source and its environmental effects. The results showed: ① The hydrochemistry of the two underground rivers is of the type HCO_3-Ca. The NO_3~-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 δ~(15) N( NO_3~-) value ranged from~(-3). 14‰ to 12. 67‰,with an average value of 7. 45‰. The δ~(18) O( NO_3~-) 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 NO_3~-sources throughout the year. In addition,rainfall,fertilizer,and soil nitrogen were the NO_3~-sources during the rainy season. There are no significant differences between the NO_3~-sources of the two underground rivers,and nitrification is the main nitrogen conversion process. ③ The molar ratio of( Ca~(2+)+ Mg~(2+))/HCO_3-varied from 0. 65 to0. 82. That of the Fenghuang River was 0. 75 and that of the Longju River was 0. 70. The δ~(13) C( DIC) value ranged from~(-1)_2. 46‰ to-9. 20‰,with a mean of-11. 10‰ in the Longju River and~(-1)0. 72‰ in the Fenghuang River. These values indicated that the HNO_3 derived from the nitrification of NH_4~+was involved in the weathering of carbonate rocks. ④ HNO_3 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.
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,δ~(15) N( NO_3~-),δ~(18) O( NO_3~-),δ~(18) O( H_2 O),andδ~(13) C( DIC) isotope data were used to investigate the NO_3~-source and its environmental effects. The results showed: ① The hydrochemistry of the two underground rivers is of the type HCO_3-Ca. The NO_3~-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 δ~(15) N( NO_3~-) value ranged from~(-3). 14‰ to 12. 67‰,with an average value of 7. 45‰. The δ~(18) O( NO_3~-) 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 NO_3~-sources throughout the year. In addition,rainfall,fertilizer,and soil nitrogen were the NO_3~-sources during the rainy season. There are no significant differences between the NO_3~-sources of the two underground rivers,and nitrification is the main nitrogen conversion process. ③ The molar ratio of( Ca~(2+)+ Mg~(2+))/HCO_3-varied from 0. 65 to0. 82. That of the Fenghuang River was 0. 75 and that of the Longju River was 0. 70. The δ~(13) C( DIC) value ranged from~(-1)_2. 46‰ to-9. 20‰,with a mean of-11. 10‰ in the Longju River and~(-1)0. 72‰ in the Fenghuang River. These values indicated that the HNO_3 derived from the nitrification of NH_4~+was involved in the weathering of carbonate rocks. ④ HNO_3 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.
引文
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[4] Musgrove M, Opsahl S P, Mahler B J, et al. Source,variability,and transformation of nitrate in a regional karst aquifer:Edwards aquifer,central Texas[J]. Science of the Total Environment,2016,568:457-469.
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[9] Shi Y L,Cui S H,Ju X T,et al. Impacts of reactive nitrogen on climate change in China[J]. Scientific Reports,2015,5:8118.
[10] Zhao Y H,Zhang L,Chen Y F,et al. Atmospheric nitrogen deposition to China:a model analysis on nitrogen budget and critical load exceedance[J]. Atmospheric Environment,2017,153:32-40.
[11] Vet R,Artz R S,Carou S,et al. A global assessment of precipitation chemistry and deposition of sulfur,nitrogen,sea salt, base cations, organic acids, acidity and pH, and phosphorus[J]. Atmospheric Environment,2014,93:3-100.
[12] Waldner P,Marchetto A,Thimonier A,et al. Detection of temporal trends in atmospheric deposition of inorganic nitrogen and sulphate to forests in Europe[J]. Atmospheric Environment,2014,95:363-374.
[13] van Grinsven H J M,Bouwman L,Cassman K G,et al. Losses of ammonia and nitrate from agriculture and their effect on nitrogen recovery in the European Union and the United States between 1900 and 2050[J]. Journal of Environmental Quality,2015,44(2):356-367.
[14] Xu W,Luo X S,Pan Y P,et al. Quantifying atmospheric nitrogen deposition through a nationwide monitoring network across China[J]. Atmospheric Chemistry and Physics,2015,15(21):12345-12360.
[15]国家统计局.中国统计年鉴[M].北京:中国统计出版社,1980-2017.
[16] Zhu Z L, Chen D L. Nitrogen fertilizer use in Chinacontributions to food production,impacts on the environment and best management strategies[J]. Nutrient Cycling in Agroecosystems,2002,63(2-3):117-127.
[17] Ju X T,Xing G X,Chen X P,et al. Reducing environmental risk by improving N management in intensive Chinese agricultural systems[J]. Proceedings of the National Academy of Sciences of the United States of America,2009,106(9):3041-3046.
[18] Kendall C. Tracing nitrogen sources and cycling in catchments,chapter 16[A]. In:Kendall C,Mc Donnell J J(Eds.). Isotope Tracers in Catchment Hydrology[M]. Amsterdam:Elsevier Science,1998. 519-576.
[19] Nestler A,Berglund M,Accoe F,et al. Isotopes for improved management of nitrate pollution in aqueous resources:review of surface water field studies[J]. Environmental Science and Pollution Research,2011,18(4):519-533.
[20] Heaton T H E. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere:a review[J]. Chemical Geology:Isotope Geoscience Section,1986,59:87-102.
[21] Finlay J C,Sterner R W,Kumar S. Isotopic evidence for in-lake production of accumulating nitrate in Lake Superior[J].Ecological Applications,2007,17(8):2323-2332.
[22] Xue D M,Botte J,de Baets B,et al. Present limitations and future prospects of stable isotope methods for nitrate source identification in surface-and groundwater[J]. Water Research,2009,43(5):1159-1170.
[23] Hoering T C,Ford H T. The isotope effect in the fixation of nitrogen by azotobacter[J]. Journal of American Chemical Society,1960,82(2):376-378.
[24] Gilliam R W,Cherry J A. Field evidence of denitrification in shallow groundwater flow systems[J]. Water Pollution Research,1978,13(1):53-71.
[25] Bottcher J, Strebel O, Voerkelius S, et al. Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer[J]. Journal of Hydrology,1990,114(3-4):413-424.
[26] Rivett M O,Buss S R,Morgan P,et al. Nitrate attenuation in groundwater:a review of biogeochemical controlling processes[J]. Water Research,2008,42(16):4215-4232.
[27] Kaown D,Koh D C,Mayer B,et al. Identification of nitrate and sulfate sources in groundwater using dual stable isotope approaches for an agricultural area with different land use(Chuncheon,mid-eastern Korea)[J]. Agriculture,Ecosystems&Environment,2009,132(3-4):223-231.
[28] Osaka K,Ohte N,Koba K,et al. Hydrological influences on spatiotemporal variations ofδ15N andδ18O of nitrate in a forested headwater catchment in central Japan:denitrification plays a critical role in groundwater[J]. Journal of Geophysical Research, 2010, 115(G2):G02021, doi:10. 1029/2009JG000977.
[29] Kim H,Kaown D,Mayer B,et al. Identifying the sources of nitrate contamination of groundwater in an agricultural area(Haean basin,Korea)using isotope and microbial community analyses[J]. Science of the Total Environment,2015,533:566-575.
[30] Yue F J,Li S L,Liu C Q,et al. Sources and transport of nitrate constrained by the isotopic technique in a karst catchment:an example from Southwest China[J]. Hydrological Processes,2015,29(8):1883-1893.
[31] Semhi K,Suchet P A,Clauer N,et al. Impact of nitrogen fertilizers on the natural weathering-erosion processes and fluvial transport in the Garonne basin[J]. Applied Geochemistry,2000,15(6):865-878.
[32] Perrin A S, Probst A, Probst J L. Impact of nitrogenous fertilizers on carbonate dissolution in small agricultural catchments:implications for weathering CO2uptake at regional and global scales[J]. Geochimica et Cosmochimica Acta,2008,72(13):3105-3123.
[33]刘长礼,张云,宋超,等.施用农肥对岩溶溶蚀作用的影响及其生态环境意义[J].中国地质,2009,36(6):1395-1404.Liu C L,Zhang Y,Song C,et al. The effect of farm manure on the dissolution of carbonate rocks and its eco-environmental impact[J]. Geology in China,2009,36(6):1395-1404.
[34]张兴波,蒋勇军,邱述兰,等.农业活动对岩溶作用碳汇的影响:以重庆青木关地下河流域为例[J].地球科学进展,2012,27(4):466-476.Zhang X B,Jiang Y J,Qiu S L,et al. Agricultural activities and carbon cycling in Karst areas in Southwest China:dissolving carbonate rocks and CO2sink[J]. Advances in Earth Science,2012,27(4):466-476.
[35]胡刘婵,蒋勇军,曾思博,等.岩溶关键带C-N耦合循环与碳酸盐岩风化——以重庆雪玉洞观测站为例[J].第四纪研究,2017,37(6):1251-1261.Hu L C,Jiang Y J,Zeng S B. et al. C-N coupling cycles and carbonate weathering in karst critical zone in Chongqing Xueyu Cave observing station[J]. Quaternary Science,2017,37(6):1251-1261.
[36] Wigley T M L. WATSPEC:a computer program for determining the equilibrium speciation of aqueous solutions[M]. London:British Geomorphological Research Group by Geo Abstracts ltd,1977. 48.
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