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.