Importance of isotope fractionation in SIAR model for quantifying NO3- sources in groundwater of China.

Quantifying NO3- sources by the combination of dual nitrate isotopes (δ15N-NO3- and δ18O-NO3-) with Stable Isotope Analysis in R (SIAR) models is crucial for mitigating NO3- pollution in groundwater. However, isotope fractionation effects during denitrification lead to significant uncertainties when quantifying groundwater NO3- sources using the SIAR model. In this study, hydrochemical data, water isotopes (δD-H2O and δ18O-H2O), and dual nitrate isotopes of groundwater at the West Lake watershed, East China were measured to estimate the isotope fractionation effect of denitrification in groundwater and assess its impact on quantifying NO3- source contributions using the SIAR model. The significant spatial (εN: -6.9‰ and εO: -3.1‰ in G1; εN: -15.1‰ and εO: -10.0‰ in G2) and temporal (εN: -17.0‰ and εO: -4.1‰ in spring; εN: -4.9‰ and εO: -2.5‰ in summer; εN: -7.2‰ and εO: -6.0‰ in autumn) variations in isotope fractionation effects of denitrification in groundwater at the West Lake watershed were observed. By incorporating these respective isotope fractionation enrichment factors into the SIAR model, more accurate NO3- source apportionments for G1 and G2 were obtained, confirming that the isotope fractionation effect of denitrification is an important parameter for quantifying NO3- sources in groundwater using the SIAR model. Furthermore, the national δ15N-NO3- and δ18O-NO3- observations of groundwater were compiled and the SIAR model integrated with isotope fractionation effect of denitrification were used to quantify NO3- sources in groundwater of China. It was found that regional differences in human activities directly influenced spatial variations of δ15N-NO3- and δ18O-NO3- values. The SIAR model outputs on a national scale revealed that sewage/manure (22.9-42.1%) and chemical fertilizers (23.0-42.7%) were the main NO3- sources in groundwater of China, attributable to large populations and extensive agricultural cultivation areas. These results provide direct evidence for formulating suitable policies and measures to control and reduce groundwater NO3- in China.