Combining multi-isotope technology, hydrochemical information, and MixSIAR model to identify and quantify nitrate sources of groundwater and surface water in a multi-land use region

Abstract

Accurate identification of nitrate (NO₃^-) sources is the premise of non-point source pollution control in watersheds. The multiple isotope techniques (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ²H-H₂O, δ¹⁸O-H₂O), combined with hydrochemistry characteristics, land use information, and Bayesian stable isotope mixing model (MixSIAR), were used to identify the sources and contributions of NO₃^- in the agricultural watershed of the upper Zihe River, China. A total of 43 groundwater (GW) and 7 surface water (SFW) samples were collected. The results showed that NO₃^- concentrations of 30.23% GW samples exceeded the WHO maximum permissible limit level, whereas SFW samples did not exceed the standard. The NO₃^- content of GW varied significantly among different land uses. The averaged GW NO₃^- content in livestock farms (LF) was the highest, followed by vegetable plots (VP), kiwifruit orchards (KF), croplands (CL), and woodlands (WL). Nitrification was the main transformation process of nitrogen, while denitrification was not significant. Hydrochemical analysis results combined with NO isotopes biplot showed that manure and sewage (M&S), NH₄⁺ fertilizers (NHF), and soil organic nitrogen (SON) were the mixed sources of NO₃^-. The MixSIAR model summarized that M&S was the main NO₃^- contributor for the entire watershed, SFW, and GW. For contribution rates of sources in GW of different land use patterns, the main contributor in KF was M&S (contributing 59.00% on average), while M&S (46.70%) and SON (33.50%) contributed significantly to NO₃^- in CL. Combined with the traceability results and the situation that land use patterns are changing from CL to KF in this area, improving fertilization patterns and increasing manure use efficiency are necessary to reduce NO₃^- input. These research results will serve as a theoretical foundation for controlling NO₃^- pollution in the watershed and adjusting agricultural planting structures.

Keywords: Agricultural watershed; MixSIAR; Multiple stable isotopes; Nitrate; Trace.