Figure 8 Uncertainty in the proportional contribution of four potential nitrate pollution sources (a) surface water in November 2021, (b) groundwater in November 2021, (c) surface water in April 2022, (d) groundwater in April 2022
Sensitivity analysis is a crucial part of nitrogen source apportionment studies, which involves assessing the sensitivity of the results to changes in the input values of the potential source end-members. In this study, four potential nitrate sources were analyzed using the SIAR model, with their δ15N and δ18O mean values being changed by -50% to +50% at 10% intervals. The contributions of different nitrate sources were then calculated using the model. The results, shown in Figure 9, demonstrate that surface water and groundwater are most sensitive to fecal and domestic sewage for nitrogen isotopes. When the mean δ15N value of fecal and domestic sewage was changed from -50% to +50%, the contribution of nitrate to surface water in November 2021 decreased from 0.682 to 0.229. Sensitivity analysis results for δ18O show that changing the oxygen isotope composition of AD had a relatively large impact on the results of the isotopic mixing model. The importance of accurately measuring the δ15N and δ18O characteristics of potential nitrate sources, especially fecal and domestic sewage, is highlighted by the sensitivity analysis.