Identifying sources and transformations of nitrate in different occurrence environments of carbonate rocks using a coupled isotopic approach (δ15N, δ18O, 87Sr/86Sr) in karst groundwater system, North China

Boyang Xu; Yun Lin; Yazun Wu; Yiyang Wang

doi:10.1016/j.scitotenv.2023.169300

Identifying sources and transformations of nitrate in different occurrence environments of carbonate rocks using a coupled isotopic approach (δ¹⁵N, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr) in karst groundwater system, North China

Sci Total Environ. 2024 Feb 20:912:169300. doi: 10.1016/j.scitotenv.2023.169300. Epub 2023 Dec 14.

Authors

Boyang Xu¹, Yun Lin², Yazun Wu³, Yiyang Wang¹

Affiliations

¹ School of Resources & Environment, Henan Polytechnic University, Jiaozuo 454003, China.
² School of Resources & Environment, Henan Polytechnic University, Jiaozuo 454003, China; Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo 454003, Henan Province, China. Electronic address: ylzz5211@sina.com.
³ School of Resources & Environment, Henan Polytechnic University, Jiaozuo 454003, China; Collaborative Innovation Center of Coalbed Methane and Shale Gas for Central Plains Economic Region, Jiaozuo 454003, Henan Province, China.

PMID: 38103615
DOI: 10.1016/j.scitotenv.2023.169300

Abstract

Karst water as the vital water supply source is an increasingly serious problem suffering from NO₃^- pollution. Identifying sources and transformations is the key to effectively controlling diffuse NO₃^- pollution. In this study, 25 karst groundwater samples were collected from the Xujiagou karst groundwater system in June 2023, and chemical variables and stable isotopes (δ¹⁵N, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr) were determined in different occurrence environments of carbonate rocks (exposed, covered, and buried carbonate rock areas). The results showed that the karst groundwater is dominated by nitrification. Human activities have affected the water quality of karst groundwater. The nitrate concentration ranged from 5.69 to 124.22 mg/L, and 4 % exceeds the quality indexes of class III water in China's standard for groundwater quality (20 mg/L as NO₃^--N). NH₄⁺ in fertilizer, manure and septic waste, and soil N were the main sources of nitrate pollution in the karst groundwater system. The distribution of NO₃^- sources is closely related to land-use types. Soil N (72.2 %) became the dominant nitrate source in the exposed area due to the small amount of urban land and the large distribution of forest and grassland. There were more cultivated land and large agricultural activities in the covered area, NH₄⁺ in fertilizer (59.1 %) contributes the most to NO₃^- sources. The buried area dominated by urban land, the influence of human activities (densely population and agricultural production activities) caused the highest concentration and coefficient of variation of nitrate in this area, and manure and septic waste (64.2 %) were the most to NO₃^- sources. This study can provide an important scientific basis for the protection of karst groundwater, and provide theoretical support for the treatment of karst groundwater pollution sources in the "monoclinic paraclinal" strata in northern China.

Keywords: (87)Sr/(86)Sr; Bayesian isotope mixing model; Nitrate source; Occurrence environments.