Nitrate source apportionment in groundwater using Bayesian isotope mixing model based on nitrogen isotope fractionation.

Accurate identification of nitrate (NO3-) sources is critical to address the issue of groundwater pollution. The nitrogen (N) isotopic enrichment factor (ɛp/s) is an important parameter to explain the N cycle and determine the proportional contribution of NO3- sources. Considering the isotopic fractionation effects in N transformation processes, this study quantitatively analyzed the NO3- sources in groundwater using stable isotopes (δ15N-NO3- and δ18O-NO3-) and the Bayesian isotope mixing model (SIAR). For the first time, the ɛp/s values (0.0‰, -8.7‰, -8.7‰, and 14.7‰) of atmospheric deposition (AD), soil nitrogen (SN), chemical fertilizers (CF), and manure and sewage (M&S) were calculated to determine the NO3- source apportionment in groundwater. It was proved that the isotopic fractionation effect could produce a more accurate NO3- source apportionment. We also found that the NO3- source contributions were closely related to the cropping system. In the vegetable cultivation area, CF (54.32%) and SN (37.75%) were the dominant NO3- source, while in the grain cultivation area, NO3- pollution was largely influenced by SN (33.67%), CF (33.27%), and M&S (30.16%). According to this study, the isotope fractionation is strongly recommended for NO3- source apportionment in groundwater system.