Figure 3 Composition of hydrogen and oxygen isotopes in surface
water and groundwater in the Huashan watershed
3.3 Qualitative Analysis of Nitrate
Sources
A high
NO3-/Cl- molar ratio
with low Cl- in groundwater is supposed to be affected
by intensive agricultural activities (Liu et al. 2006), while low
NO3-/Cl- molar ratio
with high Cl- in groundwater is typical considered to
be influenced by manure and municipal sewage (Yue et al. 2017). The
NO3-/Cl- molar ratio
versus Cl- concentration (μmol/L) dual-logarithmic
plot is widely utilized in numerous studies to identify the origin of
groundwater NO3- concentration (Torres
Martínez et al., 2021; Li et al., 2022). As shown in Figure 4, soil
organic nitrogen is an important source of nitrate in the study area.
Groundwater in April 2022 was influenced by sewage. There was some
overlap between agricultural inputs and soil organic nitrogen,
indicating that NO3- in the study area
was also influenced by agricultural inputs. This conclusion is
consistent with the results shown in Figure 5.
Figure 4 Relationships
between NO3-/Cl-molar ratio and Cl- molar concentration in Huashan
watershed
Dual isotope approach
(δ15N-NO3- and
δ18O-NO3-) was
generally applied to identify nitrate source by comparing the isotopic
signals between the water samples and the possible nitrogen source
(Nestler et al. 2011). As shown in fig.5, the observed isotopic signal
of surface water samples in November 2021 were mainly located in the
expected ranges of M&S and SN, and the isotopic values of groundwater
mainly fall in the NF and SN intervals, indicating M&S and SN sources
of nitrate for surface water and the NF and SN sources of nitrate for
groundwater in winter. The
δ15N-NO3-and δ18O-NO3- values
of surface water in April 2022 (spring) mainly fell in the NF and SN
intervals, while groundwater fell mainly in M&S, SN and NF. Therefore,
nitrate in spring surface water comes mainly from NF and SN, while
groundwater comes from M&S, SN and NF, which is consistent with the
analysis results of water chemistry methods. The seasonal difference in
nitrate sources between November 2021 (winter) and April 2022 (spring)
mainly due to the application of chemical fertilizers (ammonia-nitrogen
fertilizers) during planting period in spring. Half a month before
sampling, the nitrogen fertilizers were washed into the river by a heavy
rainfall (107 mm) which led to the NF source of nitrate in the river in
spring. Furthermore, some samples fall outside the endmember mixing
frame, as shown in the bottom-left corner of the figure 5, which may be
due to isotope fractionation. It is known that fractionation increases
with decreasing temperature (Mariotti et al., 1981; Yun et al., 2011).
During the sampling period, the temperature was relatively low with an
average of 3℃, which could have led to the preferential utilization of14N and 16O during the conversion of
Ammonium fertilizer to nitrate, resulting in lower
δ15N and δ18O values in the nitrate
infiltrating into groundwater, consistent with the findings of Yu et al.
(2020).
Figure 5 δ15N-NO3-
versus δ18O-NO3- diagram. Typical ranges of δ15N-NO3- and δ18O-NO3-
values for different nitrate sources were taken from Xue et al. (2009).
The isotopic signals and the contents of nitrate were general applied to
identify denitrification processes in water as the ratios of
fractionation factors (Δδ15N/Δδ18O)
usually vary from 1.3 to 2.1 (Xue et al., 2012). As shown in Figure 6b,
δ15N-NO3- is
positively correlated with
δ18O-NO3- with a
slope of 0.67 for groundwater
samples in winter, confirmed the existence of denitrification. No
significant correlation was observed between
δ15N-NO3- and
Ln[N-NO3-] for groundwater samples
in winter may be because the mixed source of groundwater and its
dilution effect on NO3- (Fig 6a). No
relation was observed between
δ15N-NO3- and
δ18O-NO3- for
groundwater samples in spring and
surface water samples in winter and in spring, suggesting no
denitrification existed. The DO content of surface water samples and
part of groundwater samples in spring generally higher than 6 mg/L also
suggested the same viewpoint, as denitrification generally occurred in
an anaerobic environment with DO content lower than 4 mg/L (Shang et
al., 2020). Water samples with lower dissolved oxygen concentrations
were predominantly observed during winter and located in proximity to
the outlet of the watershed for groundwater. High nitrogen isotopes and
low DO concentrations are typically observed at the watershed outlet,
thus denitrification in groundwater is mainly observed in the outlet
area during winter (Li et al. 2019). The relationship between DO and pH,
as shown in Figure 6c, reflects the same conclusion. In addition,
related studies have shown that 2/3 of the oxygen atoms bound during
nitrification come from H2O and 1/3 from
O2, and the nitrate
δ18O-NO3- produced
by nitrification ranges from -10‰ to 10‰ (Kendall et al, 2007). The
δ15N-NO3- of surface
waters in the Huashan watershed lie in this interval range, indicating
that denitrification is non-existent in surface waters (Fig 6b). The
water samples of groundwater in both phases, 30% of which had
δ15N-NO3- beyond
this range, and these points were mainly distributed downstream of the
watershed, indicating the presence of denitrification in groundwater
downstream of the watershed. Overall, denitrification was not detected
in the winter and spring surface water or in the spring groundwater of
the Huashan watershed. Weak denitrification was found in the winter
groundwater, mainly located near the outlet of the watershed.