3.1 Water quality parameters
The pH of the water bodies in Xi'an was 6.91–8.91, which was slightly alkaline overall (Table 3). The average values of DO, EC, and ORP in each water period followed the descending order of dry period > normal period > wet period. Due to the main influencing factors of DO content in water bodies, such as water temperature and oxygen partial pressure in the air, the water temperature was low in the dry period, while was high in the wet period. Additionally, during the wet period, the metabolic activity of phytoplankton and microorganisms was vigorous, consuming more dissolved oxygen, resulting in a lower DO content. The EC was 144–2050 µS·cm− 1 with significant spatio-temporal differences. In the dry period, the water body had poor fluidity and low flow rate, which lead to high conductivity value. During the wet period, there was more precipitation, leading to an increase in water volume and water level, resulting in a decrease in conductivity. Different water temperatures during different water periods could lead to differences in acidity, gas solubility, and biological activity, thereby affecting the ORP of the water body, resulting in the highest ORP value in the dry period. The mean values of COD and IMn in water bodies were the highest in the wet period, while the mean values of NH3-N and TP were the highest in dry period.
Table 3
| | Dry period | | | Normal period | | | Wet period | |
Maximum value | minimum value | mean value | standard deviation | Maximum value | minimum value | mean value | standard deviation | Maximum value | minimum value | mean value | standard deviation |
pH | 8.78 | 6.91 | - | - | 8.91 | 7.59 | - | - | 8.91 | 7.01 | - | - |
DO (mg·L− 1) | 13.50 | 7.60 | 11.04 | 1.44 | 16.70 | 6.60 | 8.74 | 2.17 | 10.00 | 4.20 | 7.62 | 1.50 |
EC (µS·cm− 1) | 2050 | 417 | 878.61 | 379.12 | 1850 | 260 | 668.29 | 370.78 | 2000 | 144 | 524.50 | 443.84 |
ORP (mV) | 360.00 | 184.00 | 273.07 | 32.48 | 261.00 | 208.00 | 234.07 | 14.97 | 242.00 | 166.00 | 206.11 | 19.49 |
COD | 20 | 4 | 12 | 4 | 20 | 5 | 10 | 4 | 20 | 6 | 13 | 3 |
IMn | 5.3 | 1.0 | 3.4 | 1.3 | 5.8 | 1.9 | 3.2 | 1.0 | 8.2 | 2.1 | 4.6 | 1.4 |
NH3-N | 1.71 | 0.07 | 0.79 | 0.44 | 1.67 | 0.06 | 0.50 | 0.42 | 1.03 | 0.04 | 0.34 | 0.31 |
TP | 0.29 | 0.01 | 0.12 | 0.08 | 1.67 | 0.01 | 0.09 | 0.05 | 0.19 | 0.01 | 0.09 | 0.05 |
3.2 DOM concentration
The DOM concentration was shown in Fig. 2. The DOC concentration during dry, normal, and wet periods were 2.88–9.74 mg·L− 1, 3.99–9.17 mg·L− 1, and 1.69–7.28 mg·L− 1, respectively, with the average values of 5.74 ± 1.68 mg·L− 1, 6.48 ± 1.41 mg·L− 1, and 4.96 ± 1.13 mg·L− 1, respectively. The order of average concentration from high to low was: normal period, dry period, and wet period (Fig. 2(a)). Rainfall flushing during wet period increased terrestrial inputs of DOM, but at the same time, higher water body flows resulted in lower DOC concentration. Moreover, during wet period, the water temperature was higher, and the vigorous metabolic activity of phytoplankton and microorganisms also caused fluctuations in DOC concentration in water body. In the dry period, the water flows were low, the exogenous input was reduced, and the temperature was low. The metabolic activities of plants and microorganisms were weak, resulting in a higher concentration of DOC than wet period and a lower concentration than normal period.
From a spatial perspective, the DOC contents in ZH2, LinH, HCH, and CYQ were relatively high, with the maximum values exceeding 8 mg·L− 1 (Fig. 2(b)). The Zaohe River and Caoyunqu Channel are components of the major flood discharge systems in the urban area of Xi'an, the inflow of surface runoff and the tail water from the sewage treatment plants along the route led to high DOC content in the water bodies. The DOC contents in the Linhe River, Jinghe River, Xingfuqu Channel and Jingyugou ditch varied significantly in different water periods.
During dry, normal, and wet period, the UV254 values of DOM were 0.024–0.110 cm− 1, 0.02–0.105 cm− 1 and 0.063–0.234 cm− 1, with the mean values of 0.063 ± 0.026 cm− 1, 0.061 ± 0.020 cm− 1 and 0.137 ± 0.043 cm− 1, respectively (Fig. 2(c)); the α355 values of DOM were 1.38–6.91 m− 1, 0.23–6.45 m− 1 and 2.76–14.51 m− 1, with the mean values of 2.93 ± 1.48 cm− 1, 2.41 ± 1.43 cm− 1 and 7.69 ± 2.62 cm− 1, respectively (Fig. 2(d)). The above indicated that the relative content of aromatic compounds containing C = C and C = O and CDOM were higher in the water bodies in wet period, followed by dry period. This might be due to the high rainfall during wet period, resulting in an increase in surface runoff. Aromatic compounds containing C = C and C = O and CDOM in the soil were washed into the water bodies by rainwater. Yang et al. (2013) found that the values of α350 in the north Jiulong River increased rapidly after rainfall. Shin et al. (2016) noted that after summer precipitation, the DOM in the five major rivers in South Korea was clearly characterized by terrestrial sources, further suggesting that rainfall flushed DOM from soils on riverbanks into nearby water bodies.
During the dry period, the values of UV254 and α355 were relatively higher in the lower reaches of the Bahe and Chanhe Rivers, Zaohe River, Linhe River, Huchenghe River, and Caoyunqu Channel. The values of UV254 and α355 in the lower reaches of the Bahe and Chanhe Rivers (BH3 and CH3) were significantly higher than those in the middle and upper reaches of the rivers (BH1 and BH2, CH1 and CH2), mainly due to the discharge of tail water from the 5th and 11th sewage treatment plants, and the 3rd sewage treatment plant into the downstream of the Bahe River and Chanhe River, which have led to the increase of the content of organic matter. In the Weihe and Fenghe Rivers, the values of UV254 and α355 decreased along the direction of water flow, probably due to the self-purification effect of the water bodies. During the normal period, the values of UV254 and α355 in the Zaohe River, Linhe River and Caoyunqu Channel were significantly higher. In the Weihe River, the values of UV254 gradually increased along the direction of water flow, mainly because the fact that organic pollutants (tail water of Lintong New District sewage plant and Luyuan municipal sewage plant) along the Weihe River converged through the sewage outlets. During the wet period, the values of UV254 and α355 in the middle reaches of the Bahe River (CH2) were the largest, and the differences in the values of UV254 and α355 of each water body were obvious.
3.3 Fluorescence components of DOM
It was identified that the DOM contained three fluorescent components (Fig. 3) in each period, including at least two types of humus like substances. During the dry and normal period, the DOM contained fulvic acid-like (C1), humus-like (C2) and protein-like (tryptophan-like) (C3). There were peak D and E (Ex/Em=240 (315) nm/390 nm (dry period); Ex/Em =245(325)nm/400 nm (normal period)) were found in the fluorescence spectrum of C1, with the representing substances belonging to ultraviolet and visible light fulvic acids, respectively. There were peak A and C (Ex/Em=260 (360) nm/442 nm (dry period); Ex/Em=265(360)nm/476 nm (normal period)) in the fluorescence spectrum of C2, which represented substances belonging to UV and visible humic substances, respectively. In the fluorescence spectrum of C3, there were peaks T1 and T2 (Ex/Em=235 (280) nm/326 nm(dry period); Ex/Em=240(295)nm/340 nm (normal period)), representing substances belonging to low excitation region tryptophan-like acid and high excitation region tryptophan-like acid, respectively.
During the wet period, the DOM contains fulvic acid-like(C1), humus-like (C2) and humus(C3, peak N, Ex/Em =280 nm/372 nm) generated by phytoplankton. There were peak D and E (Ex/Em=260(315) nm/434 nm) in the fluorescence spectrum of C1, representing substances belonging to ultraviolet and visible light fulvic acids, respectively. The presence of peaks H1 and H2 (Ex/Em=275 (365) nm/496 nm)in the fluorescence spectrum of C2 represented substances that were attributed to UVC humic-like acids and UVA humic-like acids, respectively. Meng et al. (2019) also identified similar components in parallel factor analysis of DOM in lnland Small Watersheds in Northwest China.
The mean values of the total contributions of C1, C2 and C3 in the DOM of all the water bodies during the dry period were 36.23 ± 5.76%, 37.90 ± 5.18% and 25.88 ± 6.78%, respectively, and the mean values of the total contributions of all humus-like substances (C1 + C2) were over 74%, indicating that the DOM in the dry period was humus-like dominated. The main reasons were vegetation along the rivers and canals, dead leaves and soil humus entering the water body with the runoff of ice melt and snow melt, and the water body accepting the tail water of sewage (wastewater) treatment. During the normal period, the mean values of the total contributions of C1, C2 and C3 in the DOM were 32.25 ± 8.70%、20.08 ± 6.13% and 47.67 ± 14.48%, respectively, the average fluorescence intensities of humic-like substances (C1 + C2) and protein-like substances (C3) were relatively close, and the fluorescence intensity of high excitation wavelength tryptophan-like substances in DOM was higher than that of other fluorescent components. High excitation wavelength tryptophan-like substances is closely related to the aromatic amino acids in DOM, it is mainly produced by microorganisms and phytoplankton, and might also be derived from the discharge of tail water, such as domestic sewage and industrial wastewater (Liu et al., 2020; He et al., 2018). The mean values of the total contributions of C1, C2 and C3 in the DOM during the wet period were 54.56 ± 2.39%、26.55 ± 4.19%, and 18.89 ± 6.37%, respectively, and the fluorescence components were dominated by humus-like substances. The composition of the fluorescence component of DOM in the wet period differed significantly from that in the dry and normal period. A obvious seasonal variation in the composition of DOM in the Mediterranean Sea during different periods was also found by Brogi et al. (2020).
3.4 PCA analyses
Principal component analysis (PCA) was applied to analyze the fluorescence peak intensities (ID、IE and IT), DOM concentrations (DOC, UV254 and α355) and major water quality indicators in the water body (Fig. 4). The first two PCA axes (PC1 and PC2) accounted for 42.9% and 21.8%, respectively with their cumulative variance contribution rate of 64.7%. COD, IMn, IE, ID, IT, UV254 and α355 were positively correlated on PC1 and had higher loadings. ORP, EC, NH3-N, TP and COD were positively correlated on PC2 and had higher loadings. The above indicated that PC1 was related to the relative content of DOM, and PC2 was related to the water quality and nutrient pollution of the water body. Most of the samples in the wet period were distributed on the positive side of PC1, and most of the samples in the dry period were distributed on the positive side of PC2, which indicated that the water quality and DOM composition in water bodies of Xi'an in different water periods had obvious temporal heterogeneity.
3.5 Correlation analysis
COD, IMn, DOC, UV254, α355, ID and IE in three water periods showed a significant positive correlation between each other (r = 0.594–0.975, P < 0.01) (Fig. 5), indicating that UV254, α355, Peak D fluorescence intensity, and Peak E fluorescence intensity could indirectly indicate the relative content of organic matter and DOM in the water bodies of Xi'an, and UV fulvic acid-like and visible light fulvic acid-like had similar sources. IT showed a significant positive correlations (r = 0.444–0.987, P < 0.01 or P < 0.05) with IMn, TP, DOC, UV254, α355, ID, and IE, respectively, indicating that the total fluorescence intensity of DOM reflected the relative content of organic matter, DOM, and total phosphorus. Kuang et al. (2022) also pointed out that the fluorescence intensity value could reflect the relative content of DOM. TP was significantly correlated with ID, IE and IT in three water periods (r = 0.468–0.694, P < 0.01 or P < 0.05), respectively, indicating that the fluorescence intensity of UV fulvic acid-like, visible light fulvic acid-like, and the total fluorescence intensity could indirectly reflect the content of TP in the water bodies, which might be due to the fact that the intensities of the fluorescence components were related to the migration and transformation of phosphorus in the aqueous environment (Yan et al., 2021; Sun et al., 2020).
3.6 Characteristics of DOM source
During the dry, normal and wet periods, the FI values ranged from 1.68 to 2.54, 1.57 to 2.14, and 1.50 to 2.00, with the mean values of 2.00 ± 0.16, 1.84 ± 0.16, and 1.67 ± 0.13, respectively; the HIX values ranged from 1.55 to 5.19, 0.58 to 13.03 and 3.31 to 78.26, with the mean values of 2.93 ± 0.85, 3.20 ± 2.18 and 20.30 ± 18.88; the BIX values ranged from 0.56 to 1.27, 0.81 to 3.98 and 0.58 to 1.26, with the mean values of 1.02 ± 0.14, 1.47 ± 0.60 and 0.80 ± 0.18; the mean values of β:α were 0.93 ± 0.13, 1.31 ± 0.46 and 0.75 ± 0.14, respectively (Fig. 6). In the dry period the mean value of FI was greater than 1.9, indicating that the source of DOM was mainly dominated by endogenous sources (biogenic sources), which might be mainly due to the low precipitation and low flow of the water bodies in the dry period, resulting in a reduction of exogenous inputs. In addition, previous studies have found that the biological substances generated during sewage discharge (tail water from sewage treatment plants) could increase the endogenous DOM content in the receiving water bodies(Wang et al., 2019; Yu et al., 2015). The average HIX values during the dry and wet seasons were less than 4, and the average BIX values were greater than 1, indicating that the DOM in the water bodies during these two periods was mainly autogenous and had weak humus characteristics, DOM was mainly produced by biological or bacterial activities. Zhang et al (2022) found that the proportion of local sources of DOM in water bodies in the dry period was higher than that in the wet season, and the degree of humification was lower, probably because of the long hydraulic retention time and the accumulation of nitrogen and phosphorus nutrients in the dry season (Kellerman et al., 2014). During the wet season, the average HIX value was greater than 6 and the average BIX value was less than 1, indicating that the DOM had strong humic characteristics, stronger aromaticity, a greater contribution of terrestrial inputs and was mainly characterized by a moderately neophytic or stronger autochthonous origin. He et al (2022) pointed out that terrigenous humus-like substances flowed into the river during rainfall, and in this case, the proportion of external sources was greater. The largest mean value of β:α was found in normal period, indicating that water bodies had the highest proportion of nascent DOM in normal period.
During three periods, the HIX value of the Zaohe River increased along the water flow direction, and the FI value in the downstream of the Zaohe River (ZH3) was higher than that in the upstream (ZH1) during the normal and wet period, indicating that the degree of DOM humification continued to increase along the water flow direction, and the endogenous proportion of DOM in the downstream of the Zaohe River during the normal and wet period was higher than that in the upstream. The HIX value of the Juehe River decreased while the value of BIX increased along the water flow direction. The BIX value (0.56) of Heihe River was the lowest in the dry period, and the autochthonous source contribution was not significant. The BIX value of the Heihe River was 0.81 during normal period, suggesting a moderately strong autochthonous origin. During the dry and normal period, the β:α value of the Heihe River was the lowest, with the least biological activity and the lowest percentage of newly produced DOM, probably because the Heihe River is a potable water source district, with less extracellular releases and exudates of bacteria and algae in the water, and lower endogenous DOM compared to other water bodies.
3.7 Differences in molecular properties of DOM
The mean values of UV253/UV203 in the dry, normal and wet period were 0.024 ± 0.012, 0.025 ± 0.009 and 0.069 ± 0.028, respectively; the mean values of E2/E3 were 6.2 ± 1.1, 9.9 ± 4.8 and 5.1 ± 0.6, respectively; the mean values of E3/E4 were 5.0 ± 1.1, 15.8 ± 7.8 and 4.7 ± 0.7, respectively (Fig. 7). The degree of benzene ring substitution in DOM followed the order of wet period > normal period > dry period, and the order of relative molecular weight was as follows: wet period > dry period > normal period. This indicated that the degree of benzene ring substitution and molecular weight of DOM in water bodies in Xi'an were relatively high during the wet period, while the degree of benzene ring substitution of DOM was relatively weak during the dry period. This is likely attributed to the development of humus in the soil around the water bodies in the wet period, and the input of aromatic organic matter from terrestrial sources (Praise et al., 2018). During the three periods, the mean values of E3/E4 in DOM were in the following order: normal period > dry period > wet period, and all of them were greater than 3.5, indicating that humus in DOM was mainly fulvic acid.
During the dry period, the values of UV253/UV203 and E3/E4 in the Heihe River were the lowest, and the values of E3/E4 was less than 3.5, indicating that DOM in the Heihe River had the least degree of benzene ring substitution, and contained humus mainly dominated by humic acid. The E2/E3 values of DOMin the upstream water bodies of the Bahe River, Chanhe River and Fenghe River were lower than those in the downstream, indicating that the molecular weight of DOM in the upstream water bodies of these rivers were higher than those in the downstream water bodies. During the normal period, the UV253/UV203 values of the Fenghe River decreased along the direction of water flow, while the E2/E3 values of the Bahe River, Chanhe River and Fenghe River decreased. This indicated that the degree of benzene ring substitution of the DOM in the Fenghe River decreased along the water flow direction, and the molecular weight of the DOM in the Bahe River, Chanhe River, and Fenghe River increased. During the wet period, the UV253/UV203 value of DOMin the upstream of the Juehe River was lower than that in the downstream, indicating that the degree of benzene ring substitution in DOM in the upstream of the Juehe River was weaker than that in the downstream. There was no significant difference in E2/E3 values among different water bodies. The E3/E4 values in the Huchenghe River and Meipohu Lake were less than 3.5, and the humus in DOM was mainly humic acid. The reasons for the changes in the molecular weight of DOM in water bodies during different water periods were the comprehensive effects of organic matter brought in by rainwater erosion of soil, self purification of water bodies, and the discharge of tail water containing a large amount of humus and protein macromolecules.