|
Fig. 1.
Geographical location, land use and distribution of sampling points within the study area
Fig. 2.
Variations in the temperature, precipitation, snow depth, river runoff in the BRW and total nitrogen (TN) load at the confluence of the Buerhatong River and Hailan River in 2022
–
2023.
Fig. 3.
Distribution of
δ
18
O-H
2
O and
δ
D-H
2
O and the relationships between water samples and global and local meteoric water lines (a) (
Craig, 1961
). Spatial distributions of
δ
18
O-H
2
O of SW and GW (b). Diagram of the GW
–
SW exchange ratio; the green arrows indicate SW recharging GW, whereas the red arrows indicate GW recharging SW (c). Variations in shallow groundwater and river levels and normalized data during the wet season from 2006 to 2011 (d), and wells YJ-4 and YJ-2 were located 3.6 km upstream and 8.8 km downstream of the SW level monitoring station, with YJ-2 180 m from the riverbank and YJ-4 1,080 m away
Fig. 4.
Boxplots of nitrate in GW (a) and SW (c) and spatiotemporal distributions of nitrate in GW (b) and SW (d).
Fig. 5.
NO3
–/Cl
-
/
δ
18
O-H
2
O spatiotemporal changes in the BRW, and the distance is from the downstream outlet. GW/SW_main represents the main stream and GW/SW_minor represents the tributaries. Up, mid and down are upstream, midstream and downstream, respectively.
Fig. 6.
Relationship between the molar ratio of NO3
–/Cl
-
and Cl
-
in SW (a) and GW (b). The relationship between
δ
18
O-NO3
– and
δ
18
O-H
2
O (c) and the dashed lines represent the exchange of O
2
with the atmosphere (
δ
18
O-NO3
–=
δ
18
O-Air), the nitrification process (
δ
18
O-NO3
–=
2/3
δ
18
O-H
2
O
+
1/3
δ
18
O-Air) and the exchange with H
2
O (
δ
18
O-NO3
–=
δ
18
O-H
2
O), respectively. Dual isotope (
δ
15
N-NO3
– and
δ
18
O-NO3
–) distributions (d), and the different rectangles indicate potential nitrate sources.
Relationship between
δ
15
N
–
NO
3
δ
18
O
–
NO
3 -
and ln[NO3
–]; the slope reflects denitrification and the fractionation coefficient (e).
Fig. 7.
Mean proportional contribution (%) of each nitrate source in GW and SW (a) and the mean proportional contribution (mg/L) of each nitrate source in GW (b) and SW (c). SN: soil nitrogen, NF: NO3
– fertilizer, MS: manure and sewage, AF: NH4
+ fertilizer, AD: atmospheric deposition.
Fig. 8.
Spatiotemporal changes in the proportional contributions of nitrate sources.
Fig. 9.
Mantel test (a) and random forest (b) analysis of nitrate and related variables in groundwater.
Fig. 10.
A conceptual model of the GW
–
SW interaction and nitrate sources, migration and geochemical processes between the wet and dry seasons.
|
