Source analysis of nitrate pollution in a typical tributary of the upper Yellow River using a combined approach of stable isotopes and DOM fluorescence characteristics

Abstract

Identifying the sources and characteristics of water pollution is essential for the protection and management of water environments. Nitrate (NO₃⁻) is a key pollutant affecting water quality in the Qingshui River, a typical semi-arid tributary in the upper Yellow River basin. This study aimed to investigate the spatiotemporal patterns and sources of NO₃⁻ in the basin using an integrated analysis of hydrochemistry, stable isotopes, and dissolved organic matter (DOM) fluorescence. Hydrochemical results revealed that the water quality exhibited distinct seasonal variations, influenced by the unique hydrological and climatic conditions of the upper Yellow River. The surface water showed elevated concentrations of NO₃⁻ with limited denitrification, and NO₃⁻ accumulation was driven by multiple sources. Based on the dual isotopes (δ¹⁵N and δ¹⁸O) and the MixSIAR model, it was quantitatively determined that allochthonous inputs—including chemical nitrogen fertilizer, soil nitrogen, domestic sewage, and manure—constituted the primary sources of NO₃⁻. DOM fluorescence analysis revealed active biological or microbial metabolic activities, while allochthonous DOM significantly contributed to NO₃⁻ contamination. Both the MixSIAR model and DOM fluorescence results confirmed that protein-like substances were mainly derived from domestic sewage, whereas humic-like substances originated from non-point source pollution in the study area. These findings demonstrate the feasibility of combining isotopic and DOM fluorescence approaches to trace NO₃⁻ sources in surface water. This integrated methodology can support the design of targeted zonal management strategies to protect the surface water environment and maintain sustainable socioeconomic systems in semi-arid regions.

Keywords

dissolved organic matter / hydrochemistry / nitrate / source identification and apportionment / stable isotopes

References

[1] Antonio torres-Martínez, J., Mahlknecht, J., Mora, A., Kaown, D., Koh, D. C., Mayer, B., & Tetzlaff, D. (2024). Unveiling nitrate origins in semiarid aquifers: A comparative analysis of Bayesian isotope mixing models using nitrate and boron isotopes and a Positive Matrix Factorization model. Journal of Hydrology, 639, 131622. https://doi.org/10.1016/j.jhydrol.2024.131622

[2] APHA. (2005). Standard methods for the examination of water and wastewater (21st edn). American Public Health Association/American Water Works Association/Water Environment Federation.

[3] Beusen, A. H. W., Bouwman, A. F., Van Beek, L. P. H., Mogollón, J. M., & Middelburg, J. J. (2016). Global riverine N and P transport to ocean increased during the 20th century despite increased retention along the aquatic continuum. Biogeosciences, 13(8), 2441-2451. https://doi.org/10.5194/bgd-12-20123-2015

[4] Birdwell, J. E., & Engel, A. S. (2010). Characterization of dissolved organic matter in cave and spring waters using UV-Vis absorbance and fluorescence spectroscopy. Organic Geochemistry, 41(3), 270-280. https://doi.org/10.1016/j.orggeochem.2009.11.002

[5] Cai, W., Xu, X., Du, X., et al. (2012). Parallel factor analysis with EEM on dissolved organic matter in Chongqing section of Jialing River. Research on Environmental Sciences25(3), 276-281. https://doi.org/10.5555/20123168373

[6] Cao, X., Yang, S., Wu, P., Liu, S., & Liao, J. (2021). Coupling stable isotopes to evaluate sources and transformations of nitrate in groundwater and inflowing rivers around the Caohai karst wetland, Southwest China. Environmental Science and Pollution Research, 28(33), 45826-45839. https://doi.org/10.1007/s11356-021-13827-4

[7] Chai, H., Xiang, Y., Chen, R., Shao, Z., Gu, L., Li, L., & He, Q. (2019). Enhanced simultaneous nitrification and denitrification in treating low carbon-to-nitrogen ratio wastewater: treatment performance and nitrogen removal pathway. Bioresource Technology, 280, 51-58. https://doi.org/10.1016/j.biortech.2019.02.022

[8] Chen, A., Du, Y., Wang, Z., Sun, X., Xu, R., Xiong, Y., Yang, L., Liu, J., & Gan, Y. (2024). Source identification of nitrate in groundwater of an agro-pastoral ecotone in a semi-arid zone, Northern China: Coupled evidences from MixSIAR model and DOM fluorescence. Applied Geochemistry, 175, 106197. https://doi.org/10.1016/j.apgeochem.2024.106197

[9] Chen, L., Wang, Y., Yang, N., Zhu, K., Yan, X., Bai, Z., Zhai, L., & Shen, Z. (2023). Improving crop-livestock integration in China using numerical experiments at catchment and regional scales. Agriculture, Ecosystems & Environment, 341, 108192. https://doi.org/10.1016/j.agee.2022.108192

[10] Chen, M., Kim, J. H., Nam, S. I., Niessen, F., Hong, W. L., Kang, M. H., & Hur, J. (2016). Production of fluorescent dissolved organic matter in Arctic Ocean sediments. Scientific Reports, 6(1), 39213. https://doi.org/10.1038/srep39213

[11] Chen, S., Du, Y., Das, P., Lamore, A. F., Dimova, N. T., Elliott, M., Broadbent, E. N., Roebuck., J. A., Jaffé, R., & Lu, Y. (2021). Agricultural land use changes stream dissolved organic matter via altering soil inputs to streams. Science of the Total Environment, 796, 148968. https://doi.org/10.1016/j.scitotenv2021.148968

[12] Derrien, M., Yang, L., & Hur, J. (2017). Lipid biomarkers and spectroscopic indices for identifying organic matter sources in aquatic environments: A review. Water Research, 112, 58-71. https://doi.org/10.1016/j.watres.2017.01.023

[13] Ding, K., Zhang, Y., Zhang, H., Yu, C., Li, X., Zhang, M., Zhang, Z., & Yang, Y. (2024). Tracing nitrate origins and transformation processes in groundwater of the Hohhot Basin's Piedmont strong runoff zone through dual isotopes and hydro-chemical analysis. Science of the Total Environment, 919, 170799. https://doi.org/10.1016/j.scitotenv.2024.170799

[14] Duan, P., Wei, M., Yao, L., & Li, M. (2022). Relationship between non-point source pollution and fluorescence fingerprint of Riverine dissolved organic matter is season dependent. Science of the Total Environment, 823, 153617. https://doi.org/10.1016/j.scitotenv.2022.153617

[15] Fellman, J. B., Hood, E., & Spencer, R. G. M. (2010). Fluorescence spectroscopy opens new windows into dissolved organic matter dynamics in freshwater ecosystems: a review. Limnology and Oceanography, 55(6), 2452-2462. https://doi.org/10.4319/lo.2010.55.6.2452

[16] Feng, L., Zhang, J., Fan, J., Wei, L., He, S., & Wu, H. (2022). Tracing dissolved organic matter in inflowing rivers of Nansi Lake as a storage reservoir: Implications for water-quality control. Chemosphere, 286, 131624. https://doi.org/10.1016/j.chemosphere.2021.131624

[17] Fu, P., Wu, F., Liu, C., Wang, F., Li, W., Yue, L., & Guo, Q. (2007). Fluorescence characterization of dissolved organic matter in an urban river and its complexation with Hg (II). Applied Geochemistry, 22(8), 1668-1679. https://doi.org/10.1016/j.apgeochem.2007.03.041

[18] Gao, Y., Tian, Y., Zhan, W., Li, L., Sun, H., Zhao, T., Zhang, H., Meng, Y., Li, Y., Liu, T., & Ding, J. (2023). Characterizing legacy nitrogen-induced time lags in riverine nitrogen reduction for the Songhuajiang River Basin: Source analysis, spatio-seasonal patterns, and impacts on future water quality improvement. Water Research, 242, 120292-120302. https://doi.org/10.1016/j.watres.2023.120292

[19] Guéguen, C., McLaughlin, F. A., Carmack, E. C., Itoh, M., Narita, H., & Nishino, S. (2012). The nature of colored dissolved organic matter in the Southern Canada Basin and East Siberian Sea. Deep Sea Research Part II: Topical Studies in Oceanography, 81-84, 102-113. https://doi.org/10.1016/j.drs2.2011.05.004

[20] Hansen, A. M., Kraus, T. E. C., Pellerin, B. A., Fleck, J. A., Downing, B. D., & Bergamaschi, B. A. (2016). Optical properties of dissolved organic matter (DOM): effects of biological and photolytic degradation. Limnology and Oceanography, 61(3), 1015-1032. https://doi.org/10.1002/lno.10270

[21] He, S., Li, P., Su, F., Wang, D., & Ren, X. (2022). Identification and apportionment of shallow groundwater nitrate pollution in Weining Plain, northwest China, using hydrochemical indices, nitrate stable isotopes, and the new Bayesian stable isotope mixing model (MixSIAR). Environmental Pollution, 298, 118852. https://doi.org/10.1016/j.envpol.2022.118852

[22] Hu, Y., Lu, Y., Edmonds, J., Liu, C., Zhang, Q., & Zheng, C. (2021). Irrigation alters source-composition characteristics of groundwater dissolved organic matter in a large arid river basin, northwestern China. Science of the Total Environment, 767, 144372. https://doi.org/10.1016/j.scitotenv.2020.144372

[23] Jin, Z., Zheng, Q., Zhu, C., Wang, Y., Cen, J., & Li, F. (2018). Contribution of nitrate sources in surface water in multiple land use areas by combining isotopes and a Bayesian isotope mixing model. Applied Geochemistry, 93, 10-19. https://doi.org/10.1016/j.apgeochem.2018.03.014

[24] Jonnalagadda, S. B., & Mhere, G. (2001). Water quality of the odzi river in the eastern highlands of Zimbabwe. Water Research, 35(10), 2371-2376. https://doi.org/10.1016/S0043-1354(00)00533-9

[25] Kang, X., Niu, Y., Yu, H., Gou, P., Hou, Q., Lu, X., & Wu, Y. (2022). Effect of rainfall-runoff process on sources and transformations of nitrate using a combined approach of dual isotopes, hydrochemical and Bayesian model in the Dagang River basin. Science of the Total Environment, 837, 155674. https://doi.org/10.1016/j.scitotenv.2022.155674

[26] Kritzberg, E. S., Cole, J. J., Pace, M. L., Granéli, W., & Bade, D. L. (2004). Autochthonous versus allochthonous carbon sources of bacteria: Results from whole-lake 13C addition experiments. Limnology and Oceanography, 49(2), 588-596. https://doi.org/10.4319/lo.2004.49.2.0588

[27] Lee, M. H., Lee, Y. K., Derrien, M., Choi, K., Shin, K. H., Jang, K. S., & Hur, J. (2019). Evaluating the contributions of different organic matter sources to urban river water during a storm event via optical indices and molecular composition. Water Research, 165, 115006. https://doi.org/10.1016/j.watres.2019.115006

[28] Lee, M. H., Osburn, C. L., Shin, K. H., & Hur, J. (2018). New insight into the applicability of spectroscopic indices for dissolved organic matter (DOM) source discrimination in aquatic systems affected by biogeochemical processes. Water Research, 147, 164-176. https://doi.org/10.1016/j.watres.2018.09.048

[29] Li, J., Zhu, D., Zhang, S., Yang, G., Zhao, Y., Zhou, C., Lin, Y., & Zou, S. (2022). Application of the hydrochemistry, stable isotopes and MixSIAR model to identify nitrate sources and transformations in surface water and groundwater of an intensive agricultural karst wetland in Guilin, China. Ecotoxicology and Environmental Safety, 231, 113205. https://doi.org/10.1016/j.ecoenv.2022.113205

[30] Ma, Z., Yang, Y., Lian, X., Jiang, Y., Xi, B., Peng, X., & Yan, K. (2016). Identification of nitrate sources in groundwater using a stable isotope and 3DEEM in a landfill in northeast China. Science of the Total Environment, 563-564, 593-599. https://doi.org/10.1016/j.scitotenv.2016.04.117

[31] Maqbool, T., Qin, Y., Ly, Q. V., Zhang, J., Li, C., Asif, M. B., & Zhang, Z. (2020). Exploring the relative changes in dissolved organic matter for assessing the water quality of full-scale drinking water treatment plants using a fluorescence ratio approach. Water Research, 183, 116125. https://doi.org/10.1016/j.watres.2020.116125

[32] Mcilvin, M. R., Altabet, M. A. (2005). Chemical conversion of nitrate and nitrite to nitrous oxide for nitrogen and oxygen isotopic analysis in freshwater and seawater. Analytical Chemistry, 77(17), 5589-5595. https://doi.org/10.1021/ac050528s

[33] Meng, F., Huang, G., Yang, X., Li, Z., Li, J., Cao, J., Wang, Z., & Sun, L. (2013). Identifying the sources and fate of anthropogenically impacted dissolved organic matter (DOM) in urbanized rivers. Water Research, 47(14), 5027-5039. https://doi.org/10.1016/j.watres.2013.05.043

[34] Mitchell, M. J., Piatek, K. B., Christopher, S., Mayer, B., Kendall, C., & Mchale, P. (2006). Solute sources in stream water during consecutive fall storms in a northern hardwood forest watershed: A combined hydrological, chemical and isotopic approach. Biogeochemistry, 78(2), 217-246. https://doi.org/10.1007/s10533-005-4277-1

[35] Mostofa, K. M. G., Jie, Y., Sakugawa, H., & Liu, C. Q. (2019). Equal treatment of different EEM data on PARAFAC modeling produces artifact fluorescent components that have misleading biogeochemical consequences. Environmental Science & Technology, 53(2), 561-563. https://doi.org/10.1021/acs.est.8b06647

[36] Murphy, K. R., Stedmon, C. A., Waite, T. D., & Ruiz, G. M. (2008). Distinguishing between terrestrial and autochthonous organic matter sources in marine environments using fluorescence spectroscopy. Marine Chemistry, 108(1-2), 40-58. https://doi.org/10.1016/j.marchem.2007.10.003

[37] Parnell, A. C., Inger, R., Bearhop, S., & Jackson, A. L. (2010). Source partitioning using stable isotopes: Coping with too much variation. PLoS One, 5(3), e9672. https://doi.org/10.1371/journal.pone.0009672

[38] Reinhardt, M., Müller, B., Gächter, R., & Wehrli, B. (2006). Nitrogen removal in a small constructed wetland: An isotope mass balance approach. Environmental Science & Technology, 40(10), 3313-3319. https://doi.org/10.1021/es052393d

[39] Sazawa, K., Tachi, M., Wakimoto, T., Kawakami, T., Hata, N., Taguchi, S., & Kuramitz, H. (2011). The evaluation for alterations of DOM components from upstream to downstream flow of rivers in Toyama (Japan) using three-dimensional excitation-emission matrix fluorescence spectroscopy. International journal of environmental research and public health, 8(5), 1655-1670. https://doi.org/10.3390/ijerph8051655

[40] Shammi, M., Pan, X., Mostofa, K. M. G., et al. (2017a). Seasonal variation and characteristic differences in the fluorescent components of extracellular polymeric substances from mixed biofilms in saline lake. Science Bulletin, 62, 764. https://doi.org/10.1016/j.scib.2017.04.016

[41] Shang, P., Lu, Y., Du, Y., Jaffé, R., Findlay, R. H., & Wynn, A. (2018). Climatic and watershed controls of dissolved organic matter variation in streams across a gradient of agricultural land use. Science of the Total Environment, 612, 1442-1453. https://doi.org/10.1016/j.scitotenv.2017.08.322

[42] Shang, X., Huang, H., Mei, K., Xia, F., Chen, Z., Yang, Y., Dahlgren, R. A., Zhang, M., & Ji, X. (2020). Riverine nitrate source apportionment using dual stable isotopes in a drinking water source watershed of southeast China. Science of the Total Environment, 724, 137975. https://doi.org/10.1016/j.scitotenv.2020.137975

[43] Shimotori, K., Watanabe, K., & Hama, T. (2012). Fluorescence characteristics of humic-like fluorescent dissolved organic matter produced by various taxa of marine bacteria. Aquatic Microbial Ecology, 65(3), 249-260. https://doi.org/10.3354/ame01552

[44] Singh, S., Dutta, S., & Inamdar, S. (2014). Land application of poultry manure and its influence on spectrofluorometric characteristics of dissolved organic matter. Agriculture, Ecosystems & Environment, 193, 25-36. https://doi.org/10.1016/j.agee.2014.04.019

[45] Stedmon, C. A., & Bro (2008). Characterizing dissolved organicmatter fuorescence with parallel factor analysis: A tutorial. Limnol Oceanogr-Meth6, 572-579. https://doi.org/10.4319/lom.2008.6.572

[46] Stedmon, C. A., Seredyńska-Sobecka, B., Boe-Hansen, R., Le Tallec, N., Waul, C. K., & Arvin, E. (2011). A potential approach for monitoring drinking water quality from groundwater systems using organic matter fluorescence as an early warning for contamination events. Water Research, 45(18), 6030-6038. https://doi.org/10.1016/j.watres.2011.08.066

[47] Stock, B. C., Jackson, A. L., Ward, E. J., Parnell, A. C., Phillips, D. L., & Semmens, B. X. (2018). Analyzing mixing systems using a new generation of Bayesian tracer mixing models. PeerJ, 6, e5096. https://doi.org/10.7717/peerj.5096

[48] Su, C., Jiang, J., Xie, X., Han, Z., Wang, M., Li, J., & Shi, H. (2023). Sources and cycling processes of nitrogen revealed by stable isotopes and hydrochemistry in a typical agricultural lake basin. Applied Geochemistry, 156, 105662. https://doi.org/10.1016/j.apgeochem.2023.105662

[49] Takić, L., Mladenović-Ranisavljević, I., Vuković, M., & Mladenović, I. (2012). Evaluation of the ecochemical status of the Danube in Serbia in terms of water quality parameters. The Scientific World Journal, 2012, 930737. https://doi.org/10.1100/2012/930737

[50] Torres-Martínez, J. A., Mora, A., Mahlknecht, J., Daesslé, L. W., Cervantes-Avilés, P. A., & Ledesma-Ruiz, R. (2021). Estimation of nitrate pollution sources and transformations in groundwater of an intensive livestock-agricultural area (Comarca Lagunera), combining major ions, stable isotopes and MixSIAR model. Environmental Pollution, 269(15), 115445. https://doi.org/10.1016/j.envpol.2020.115445

[51] Wang, S., Wang, W., Chen, J., Zhang, B., Zhao, L., & Jiang, X. (2020). Characteristics of dissolved organic matter and its role in lake eutrophication at the early stage of algal blooms-A case study of Lake Taihu, China. Water, 12(8), 2278. https://doi.org/10.3390/W12082278

[52] Wang, X., Cao, Y., Han, Y., Tang, H., Wang, R., Sun, X., Sun, Y. (2015). Determination of nitrogen and oxygen isotope ratio of nitrate in water with a chemical conversion method. Acta Pedol. Sin.52(3), 558-566. https://doi.org/10.11766/trxb201405080224

[53] Wang, Y., Peng, J., Cao, X., Xu, Y., Yu, H., Duan, G., & Qu, J. (2019). Isotopic and chemical evidence for nitrate sources and transformation processes in a plateau lake basin in southwest China. Science of the Total Environment, 711, 134856. https://doi.org/10.1016/j.scitotenv.2019.134856

[54] Wu, Q. Y., Zhou, T. H., Du, Y., Ye, B., Wang, W. L., & Hu, H. Y. (2020). Characterizing the molecular weight distribution of dissolved organic matter by measuring the contents of electron-donating moieties, UV absorbance, and fluorescence intensity. Environment International, 137, 105570. https://doi.org/10.1016/j.envint.2020.105570

[55] Xue, D., Botte, J., De Baets, B., Accoe, F., Nestler, A., Taylor, P., Van Cleemput, O., Berglund, M., & Boeckx, P. (2009). Present limitations and future prospects of stable isotope methods for nitrate source identification in surface- and groundwater. Water Research, 43(5), 1159-1170. https://doi.org/10.1016/j.watres.2008.12.048

[56] Yang, B., Wang, R., Xiao, H., Cao, Q., & Liu, T. (2018). Spatio-temporal variations of soil water content and salinity around individual Tamarix ramosissima in a semi-arid saline region of the upper Yellow River, northwest China. Journal of Arid Land, 10(1), 101-114. https://doi.org/10.1007/s40333-017-0072-9.

[57] Yang, X., Yuan, J., Yue, F.-J., et al. (2021). New insights into mechanisms of sunlight- and dark-mediated high-temperature can accelerate diurnal production-degradation transformation of lake fluorescent DOM. Science of the Total Environment, 760, 143377. https://doi.org/10.1016/j.scitotenv.2020.143377

[58] Yi, Y., Zhong, J., Bao, H., Mostofa, K. M. G., Xu, S., Xiao, H. Y., & Li, S. L. (2021). The impacts of reservoirs on the sources and transport of riverine organic carbon in the karst area: A multi-tracer study. Water Research, 194, 116933. https://doi.org/10.1016/j.watres.2021.116933

[59] Yue, F. J., Li, S. L., Liu, C. Q., Mostofa, K. M. G., Yoshida, N., Toyoda, S., Wang, S. L., Hattori, S., & Liu, X. L. (2018). Spatial variation of nitrogen cycling in a subtropical stratified impoundment in southwest China, elucidated by nitrous oxide isotopomer and nitrate isotopes. Inland Waters, 8(2), 186-195. https://doi.org/10.1080/20442041.2018.1457847

[60] Yue, F. J., Li, S. L., Liu, C. Q., Zhao, Z. Q., & Ding, H. (2017). Tracing nitrate sources with dual isotopes and long term monitoring of nitrogen species in the Yellow River, China. Scientific Reports, 7(1), 8537. https://doi.org/10.1038/s41598-017-08756-7

[61] Zark, M., & Dittmar, T. (2018). Universal molecular structures in natural dissolved organic matter. Nature Communications, 9(1), 3178. https://doi.org/10.1038/s41467-018-05665-9

[62] Zhang, Y., Liu, M., Qin, B., & Feng, S. (2009). Photochemical degradation of chromophoric-dissolved organic matter exposed to simulated UV-B and natural solar radiation. Hydrobiologia, 627(1), 159-168. https://doi.org/10.1007/s10750-009-9722-z

[63] Zhang, Y., Shi, P., Li, F., Wei, A., Song, J., & Ma, J. (2018). Quantification of nitrate sources and fates in rivers in an irrigated agricultural area using environmental isotopes and a Bayesian isotope mixing model. Chemosphere, 208, 493-501. https://doi.org/10.1016/j.chemosphere.2018.05.164

[64] Zhang, Y., Zhang, E., Yin, Y., van Dijk, M. A., Feng, L., Shi, Z., Liu, M., & Qina, B. (2010). Characteristics and sources of chromophoric dissolved organic matter in lakes of the Yungui Plateau, China, differing in trophic state and altitude. Limnology and Oceanography, 55(6), 2645-2659. https://doi.org/10.4319/lo.2010.55.6.2645

[65] Zhao, M. M., Wang, S., Chen, Y., Wu, J., Xue, L., & Fan, T. T. (2020). Pollution status of the Yellow River tributaries in middle and lower reaches. Science of the Total Environment, 722, 137861. https://doi.org/10.1016/j.scitotenv.2020.137861

[66] Zhao, P., Tang, X., Tang, J., & Wang, C. (2013). Assessing water quality of Three Gorges Reservoir, China, over a five-year period from 2006 to 2011. Water Resources Management, 27(13), 4545-4558. https://doi.org/10.1007/s11269-013-0425-x

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