Identification of nitrate sources of groundwater and rivers in complex urban environments based on isotopic and hydro-chemical evidence

Ruidong Chen; Qihang Hu; Wanqi Shen; Jiaxun Guo; Long Yang; Qiqi Yuan; Xiaoming Lu; Lachun Wang

doi:10.1016/j.scitotenv.2023.162026

Identification of nitrate sources of groundwater and rivers in complex urban environments based on isotopic and hydro-chemical evidence

Sci Total Environ. 2023 May 1:871:162026. doi: 10.1016/j.scitotenv.2023.162026. Epub 2023 Feb 7.

Authors

Ruidong Chen¹, Qihang Hu¹, Wanqi Shen¹, Jiaxun Guo², Long Yang¹, Qiqi Yuan¹, Xiaoming Lu³, Lachun Wang⁴

Affiliations

¹ School of Geography and Ocean Science, Nanjing University, Nanjing, Jiangsu province 210023, China.
² Center for Water and Ecology, State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China.
³ Jiangsu Province Hydrology and Water Resources Investigation Bureau, Nanjing, Jiangsu province 210029, China.
⁴ School of Geography and Ocean Science, Nanjing University, Nanjing, Jiangsu province 210023, China. Electronic address: wang6312@nju.edu.cn.

PMID: 36754334
DOI: 10.1016/j.scitotenv.2023.162026

Abstract

Groundwater and rivers in Chinese cities suffer from severe nitrate pollution. The accurate identification of nitrate sources throughout aquatic systems is key to the water nitrate pollution management. This study investigated nitrogen components of groundwater for twelve years and analyzed the sources of nitrate in the aquatic system based on dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) in the city of Nanjing, a core city of the Yangtze River Delta region, China. Our results showed that the ratio of nitrate to the sum of ammonia and nitrate in groundwater show an increasing trend during 2010-2021. The nitrate concentration was positively correlated with the proportion of cultivated land and negatively correlated with the proportion of forest land in the buffer zone. The relationship between Cl^- and NO₃^-/ Cl^- showed that agriculture and sewage sources increased during 2010-2015, sewage sources increased during 2016-2018, agriculture sources increased during 2019-2021. Manure and sewage were the primary sources of groundwater nitrate (72 %). There was no significant difference between the developed land (78 %), cultivated land (69 %), and aquaculture area (72 %). This indicates that dense population and intensive aquaculture in the suburbs have a significant impact on nitrate pollution. The contributions of manure and sewage to the fluvial nitrate sources in the lower reaches of the Qinhuai River Basin were 61 %. The non-point sources, including groundwater N (39 %) and soil N (35 %), were 74 % over the upper reaches. This study highlights the necessity of developing different N pollution management strategies for different parts of highly urbanized watersheds and considers groundwater restoration and soil nitrogen management as momentous, long-term tasks.

Keywords: Aquatic systems; Dual nitrate isotopes; Non-point sources; Spatiotemporal distribution; Yangtze River Delta.