Time variation of CODcr and permanganate index in Qinghai Lake from 2010 to 2020
In order to make clear of the background values of the Qinghai Lake watershed, the monitoring water quality indexes were collected and analyzed to conclude the characterizations of the water quality variations. According to the historical monitoring data of water quality( Fig. 3-1), the CODcr concentration in Qinghai Lake was between 18.11 and 38.00 mg/L from 2010 to 2020, with an average of 27.10 mg/L, which was at the surface class IV level. Except for the large fluctuation of CODCr concentration in 2010 and 2011, the fluctuation of CODCr concentration in Qinghai lake water in other years is relatively small. From the mean value of CODCr at each historical monitoring point, the mean value of CODCr at Sha Dao point is relatively the highest, 43.84 mg/L.
From 2010 to 2020, the annual mean value of CODcr of Qinghai lake water body first decreased and then increased, and fluctuated greatly, fluctuating between class III ~ V levels of surface water. Among them, the average value of CODCr in 2011 is relatively the highest, close to the upper limit of class V water quality standard for surface water, and the annual average value of CODCr in 2012 is relatively the lowest; In 2020, CODcr will rise to class IV water level, with an average of 36.02 mg/L.
The on-site monitoring in May, September and October 2020 shows that( Fig. 3-2) the annual CODcr of Qinghai lake water body ranges from 31 to 48 mg/L, with an average of 39.21 mg/L. The CODcr of all point water bodies exceeds the class IV standard of surface water (threshold≤30 mg/L), and the CODcr of some point water bodies exceeds the class V standard of surface water (threshold≤40 mg/L). From the perspective of monthly variation, the range of CODCr in Qinghai lake water body is 31~45 mg/L, 32~47 mg/L and 31~48 mg/L respectively, and the average values are 40.13mg/L, 39.43mg/L and 38.09mg/L. on the whole, the difference of CODCr in Qinghai lake water body in different months is small, and the CODcr concentration is at a high level. Qinghai Lake is a large salt water lake in Western China. The population in the basin is rare. Its CODcr is mainly affected by rivers entering the lake, grassland non-point sources and surrounding human activities.
According to the historical monitoring data( Fig. 3-3), the permanganate index concentration of Qinghai lake water from 2010 to 2020 is between 2.14 and 5.53 mg/L, with an average of 3.89 mg/L, which is at the surface class II level. Except for the large fluctuation of permanganate index concentration in water body in 2012, the fluctuation of permanganate index concentration in Qinghai lake water body in other years is relatively small. From the mean value of permanganate index at each historical monitoring point, the mean value of permanganate index at Shadao point is relatively the highest, which is 5.39 mg/L.
From 2010 to 2015, the annual mean value of permanganate index of Qinghai lake water body fluctuated as a whole, but all met the class III level of surface water. From 2016 to 2020, the annual average value of permanganate index of Qinghai lake water body showed a downward trend and was basically at the class II level of surface water. Among them, the permanganate index remained at the level of 2.14 ~ 2.35 mg/L from 2018 to 2020, slightly higher than the class I water quality limit. In recent years, the average value of permanganate index in 2011 is relatively the highest, which is 5.53 mg/L, close to the upper limit of class III water quality standard for surface water, and the average annual value of permanganate index in 2019 is relatively the lowest.
Time variation of CODcr and permanganate index of rivers entering Qinghai Lake from 2010 to 2020
From 2010 to 2020, CODcr of main rivers entering Qinghai Lake Basin was 7.50~13.94 mg/L, with an average of 10.99 mg/L; The CODcr of rivers entering the lake is lower than that of lakes as a whole( Fig. 4-1).
Except for the DaoTang River, the mean value of CODcr of major rivers is relatively similar from 2010 to 2020, and the interannual variation trend is relatively consistent. The average CODcr of the eight main rivers entering Qinghai Lake from 2010 to 2020 shows the law of Ganzi River < Buha River < jilmeng River < hargai River < Shaliu River < Quanji River < HeMa River < DaoTang river.
From 2010 to 2020, the permanganate index of main rivers entering the lake in Qinghai Lake Basin was 1.44~2.98 mg/L, with an average of 2.18 mg/L( Fig. 4-2); The permanganate index of rivers entering the lake is lower than that of lakes as a whole.
Except for Daotang River and Heima River, the mean value of permanganate index of main rivers is relatively similar from 2010 to 2020, and the interannual variation trend is relatively consistent. The average value of permanganate index of the eight main rivers entering Qinghai Lake from 2010 to 2020 shows the law of Buha River < Shaliu River < hargai River < jilmeng River < Ganzi River < Quanji River < Heima River < DaoTang river. Except Shaliu River and Ganzi River, the law of other rivers is consistent with CODcr.
Spatial distribution of CODcr and permanganate index in Qinghai Lake
There are obvious differences in the spatial distribution of CODCr concentration in Qinghai Lake( Fig. 4-3). In May, CODCr in the East is higher than that in the west, and that in the center of the lake is higher than that in the coast; In September and October, CODcr of water body showed two high areas, namely, near Jiangxi ditch wharf and Sand Island, Jiangxi ditch wharf and Qinghai Lake fishing ground wharf. From the perspective of spatial distribution characteristics, the higher values in the three months are mainly concentrated in Jiangxi ditch wharf and Qinghai Lake fishery wharf. Jiangxi ditch wharf and Qinghai Lake fishery wharf are located in the south of Qinghai Province, and the planting industry along the south coast is mainly. Therefore, the input of external sources leads to the higher concentration of CODCr in the water body in the south of Qinghai Lake than in other regions.
There are obvious differences in the spatial distribution of permanganate index concentration in Qinghai Lake( Fig. 4-4). In May, the permanganate index concentration range is 1.80 ~ 4.50 mg/L, with an average of 2.21 mg/L, in September, the permanganate index concentration range is 1.90~14.50 mg/L, with an average of 2.83 mg/L, and in October, the permanganate index concentration range is 1.80~4.50 mg/L, with an average of 2.29 mg/L. In May and September, the permanganate index in the East was higher than that in the West; In October, the permanganate index of water body showed three high areas, namely the center of the lake, the East and the west, and the shore area close to the center of the lake. From the perspective of spatial distribution characteristics, except for the center of the lake in October, the high-value areas appear in the areas close to the shore. Therefore, the external input may be the reason for the higher concentration of permanganate index in the water body of Qinghai Lake than in other areas.
Optical Properties Of Dom In Qinghai Lake
Colored soluble organic matter (CDOM) and fluorescent soluble organic matter (FDOM) are two important indicators of optical properties in DOM. CDOM is a component of DOM that can absorb ultraviolet and photosynthetic effective radiation; FDOM is a DOM component that can release fluorescent photons after absorbing light radiation. Therefore, CDOM and FDOM are commonly used to characterize the optical characteristics of DOM.
(1) Seasonal and spatial distribution of CDOM
The concentration of CDOM is characterized by its absorption coefficient a254 at 254 nm. The field monitoring data in May, September and October 2020 show that ( Fig. 5-1)the value of a254 in Qinghai lake water body is between 8.80~13.29 M-1, with obvious seasonal differences. Among them, a254 in water in May is between 8.80 ~ 9.83 M-1, with an average value of 9.17 M-1, which is significantly lower than that in other seasons (P<0.01); In September, a254 in the water body was between 8.73~10.64 M-1, with an average value of 9.48 M-1; In October, a254 in the water body is between 9.14~13.29M-1, with an average value of 9.81 M-1, showing the law of October>September>May. The concentration of CDOM in Qinghai Lake not only has obvious seasonal differences, but also shows different spatial differences. Among them, the highest value of CDOM at each sampling point in the water body in May, September and October appeared near erlangjian in the south of the lake. In addition, the value of CDOM in the water body on the West Bank of Qinghai Lake in May was also high.
(2) Fluorescence spectrum and fluorescence component characteristics of FDOM
The three-dimensional fluorescence spectrum matrix data of DOM in Qinghai Lake were analyzed by PARAFAC model, and four DOM fluorescence components with single maximum emission wavelength were analyzed. The maximum excitation / emission wavelength distribution of the four fluorescent components and the three-dimensional fluorescence spectra of the principal components are shown in the figure below( Fig. 5-2).
The excitation wavelength of component C1 is at 242 nm and the maximum emission wavelength is near 422 nm, which reflects the land-based high molecular weight humic acid, mainly from the degradation of higher plants or soil leaching; Component C2 has two obvious excitation wavelengths at 220 nm and 272 nm, and the maximum emission wavelength is 422 nm, which reflects the low molecular weight fulvic acid like fluorescence peak, which mainly comes from the fluorescence peak formed by biodegradable organic compounds; Component C3 has two obvious excitation wavelengths at 232 and 286 nm, and the maximum emission wavelength is 338 nm. It belongs to the binding peak of protein like and fulvic acid like, which has a red shift compared with the conventional tryptophan like peak; Component C4 has an obvious excitation wavelength at 270 nm and the maximum emission wavelength is 478 nm, reflecting the fluorescence peak formed by polymer humic acid.
Overall, the proportion of each component in the total fluorescence intensity of FDOM in May and September is C2>C1>C4>C3, and the proportion of each component in the total fluorescence intensity of FDOM in October is C3>C2>C1>C4, which has certain seasonal differences( Fig. 5-3).
In the three sampling surveys of Qinghai Lake in May, September and October(Fig. 5-4), the average total fluorescence intensity of FDOM in the water body of Qinghai Lake was 22.20, 24.17 and 42.80 R.U., showing a significant seasonal difference law of October>September>May. The spatial distribution of the total fluorescence intensity of FDOM in water is also different in different seasons. The spatial difference in October is more obvious than that in May and September. However, in the three surveys, the highest value of the total fluorescence intensity of FDOM in water appears near erlangjian in the south of Qinghai Province.
The mean values of C1 components in the overlying water of Qinghai Lake in May, September and October are 5.85, 6.86 and 7.23 R.U. (Fig. 5-5), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest value appears near erlangjian in the south of Qinghai and the lowest value appears in the center of the lake.
The mean values of C2 components in the water body of Qinghai Lake in May, September and October were 6.93, 6.96 and 9.93 R.U. (Fig. 5-6), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values in May and October appear in the south of Qinghai, and the highest values in September appear in the southwest of the lake.
The mean values of C3 components in the water body of Qinghai Lake in May, September and October were 4.59, 4.96 and 20.28 R.U. (Fig. 5-7), showing the law of October>September>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values appear near erlangjian in the south of Qinghai and Hunan.
The mean values of C4 components in the water body of Qinghai Lake in May, September and October were 4.82, 5.39 and 5.36 R.U. (Fig.5-8) respectively, showing a different law from other components, that is, September>October>May. The spatial distribution characteristics are similar to the total amount, and the spatial difference is obvious. The highest values appear near erlangjian in the south of Qinghai and Hunan.
Figure 5-8 spatial distribution characteristics of C4 fluorescence intensity in Qinghai lake water in different seasons
Bioavailability Of Dom In Qinghai Lake
Both COD and BOD5 are used to quantitatively reflect the degree of organic pollution in water. Previous studies have shown that when BOD5 / CODcr≥30%, it is biodegradable sewage; If BOD5 / CODcr<30%, it is difficult to biodegrade sewage. In this study(Fig. 6), the BOD5 / CODcr value of Qinghai Lake is between 1.90%~5.29%, with an average of 3.36%, which is obviously low, indicating that the bioavailability of organic matter in Qinghai Lake is poor and difficult to decompose. This non decomposable organic matter will continue to accumulate into the sediment over time and migrate between water and sediment under the disturbance of wind, waves and fish, which also reflects the reason for the high COD of Qinghai Lake from another aspect.