Coupling stable isotopes and water chemistry to assess the role of hydrological and biogeochemical processes on riverine nitrogen sources

WATER RESEARCH

卷:150

Article

Hu, Minpeng^1,2  Liu, Yanmei¹  Zhang, Yufu¹  Dahlgren, Randy A.⁴  Chen, Dingjiang^1,3 

[1] Zhejiang Univ, Coll Environm & Resource Sci, Hangzhou 310058, Zhejiang, Peoples R China

[2] Zhejiang Univ, Zhejiang Prov Key Lab Subtrop Soil & Plant Nutr, Hangzhou 310058, Zhejiang, Peoples R China

[3] Zhejiang Univ, Minist Educ, Key Lab Environm Remediat & Ecol Hlth, Hangzhou 310058, Zhejiang, Peoples R China

[4] Univ Calif Davis, Dept Land Air & Water Resources, Davis, CA 95616 USA

关键词: Nitrogen dynamics;    Dual stable isotopes;    Source identification;    Legacy effect;   

DOI  :  10.1016/j.watres.2018.11.082

来源: Elsevier

【 摘 要 】

Accurate source identification is critical for optimizing water pollution control strategies. Although the dual stable isotope (N-15-NO3-/O-18-NO3-) approach has been widely applied for differentiating riverine nitrogen (N) sources, the relatively short-term (<1 yr) N-15-NO3-/O-18-NO3- records typically used in previous studies often hinders rigorous assessment due to high temporal variability associated with watershed N dynamics. Estimated contributions of legacy N sources in soils and groundwater to riverine N export by modeling approaches in many previous studies also lack validation from complementary information, such as multiple stable isotopes. This study integrated three years of multiple stable isotope (N-15-NO3-/O-18-NO3- and H-2-H2O/O-18-H2O) and hydrochemistry measurements for river water, groundwater and rainfall to elucidate N dynamics and sources in the Yongan watershed (2474 km(2)) of eastern China. Nonpoint source N pollution dominated and displayed considerable seasonal and spatial variability in N forms and concentrations. Information from delta N-15-NO3- and delta O-18-NO3- indicated that riverine N dynamics were regulated by contributing sources, nitrification and denitrification, as well as hydrological processes. For the three examined catchments and entire watershed, slow subsurface and groundwater flows accounted for >75% of river discharge and were likely the major hydrological pathways for N delivery to the river. Riverine NO3- sources varied with dominant land use (p<0.001), with the highest contributions of groundwater (60%), wastewater (35%), and soil (50%) occurring in agricultural, residential and forest catchments, respectively. For the entire watershed, groundwater (similar to 50%) and soil N (>30%) were the dominant riverine NO3- sources, implying considerable potential for N pollution legacy effects. Results were consistent with observed nitrous oxide dynamics and N sources identified in previous modeling studies. As the first attempt to apply multiple isotope tracers for exploring and quantifying N transformation and transport pathways, this study provides an integrated approach for verifying and understanding the N pollution legacy effects observed in many watersheds worldwide. This study highlights that river N pollution control in many watersheds requires particular attention to groundwater restoration and soil N management in addition to N input control strategies. (C) 2018 Elsevier Ltd. All rights reserved.

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