Lugu Lake water quality index results
According to the Environmental Quality Standards for Surface Water (GB3838-2002), the water quality category of each monitoring section can be obtained by comparing the standard with each measured index. Tables 3, 4, and 5 exhibit the full results of the water quality index values of Lugu Lake in 2019, 2020, and 2021, respectively. And the statistical results of water quality categories of Lugu Lake in 2019, 2020 and 2021 are shown in Fig. 2.
From the data in the above three tables, it can be concluded that:
Comparing the average values of the water quality indicators of Lugu Lake with the standards in Environmental Quality Standards for Surface Water (GB3838-2002), it is found that: in 2019, 2020 and 2021, the water quality monitoring indicators of the three monitoring sections of Lugu Lake’s Changdao Bay, Lake center and Zhaojia Bay all reached the Category I standard.
According to the data comparison for three consecutive years, it is found that the water quality of Lugu Lake is good, and the changes of various monitoring indicators are not large, which meets the requirements of surface water functional zoning. According to certain assessments, the Lugu Lake Nature Reserve’s carrying capacity has reached a relatively saturated capacity in terms of space and tourism amenities, however the Lugu Lake Nature Reserve’s water supply and sewage treatment capacity can still be enhanced21. As a result of the three years of data, the pollution generated by the Lugu Lake’s surrounding environment can be discharged to the standard after treatment, allowing the lake’s water quality to remain good and reasonably steady.
Water quality evaluation and discussion
Evaluation of water quality by single factor pollution index method
According to the formula (1), the data of the four monitoring indicators in the Lugu Lake are calculated, and the single factor pollution index of each monitoring indicator in the Lugu Lake is shown in Table 6.
It can be seen from Table 6 that the single factor index method is based on the three types of water quality standards in the Environmental Quality Standards for Surface Water. In the 3 years of 2019, 2020 and 2021, the permanganate index, biochemical oxygen demand (BOD5), ammonia nitrogen, petroleum (petroleum ether extraction), volatile phenol, mercury, lead, cadmium, anionic surfactant, chromium, fluoride, total phosphorus, cyanide The water quality evaluation of sulfide, arsenic, CODcr, copper, zinc and selenium are all Category I standards.
Compared with the water quality standards in the Environmental Quality Standards for Surface Water, the single factor pollution evaluation method can more simply and intuitively reflect the pollution status of the water quality of Lugu Lake. The water quality of Lugu Lake is Category I standard, which is pollution-free.
Evaluation of water quality by Nemerow pollution index method
According to formula (2), the data of the four monitoring indicators in the Lugu Lake are calculated, and the Nemerow pollution index of each monitoring indicator in the Lugu Lake is shown in Figs. 3, 4 and 5.
The lower the Nemerow pollution index value, the higher the water body’s quality. It can be seen from the evaluation results of the Nemerow index method that the water quality pollution index of Lugu Lake is between 0.22 and 0.34 in the past three years. The water quality of the three monitoring sections of Changdao Bay, Lake Center, and Zhaojia Bay did not change significantly, and the water quality of Lake Center has always been better than other monitoring sections. Throughout the three years, all of Lugu Lake’s monitoring sections received a Category I water quality evaluation, indicating that the lake’s water quality was exceptional. From the data in Fig. 5, it can be seen that the overall water quality of Lugu Lake has improved in the past three years, and it has always been classified as Category I water quality, which is clean.
Compared with the evaluation results of the water quality pollution status of each monitoring section of Lugu Lake by the single factor pollution index method, the Nemerow index method comprehensively considers the more seriously polluted indicators, which reflects the degree of water pollution, and also takes the average value of the single factor index22, making up for the shortcomings of the single factor index method, so the Nemerow index method is more comprehensive than the single factor index method for water quality evaluation.
Analysis of temporal and spatial variation characteristics of water quality
According to formulas (3) and (4), the temporal and spatial variation characteristics of water quality in each monitoring section of the Lugu Lake are calculated. The calculation results of the temporal and spatial variation rates are shown in Tables 7 and 8.
Three monitoring sections of Changdao Bay, Lake center and Zhaojia Bay were selected to analyze the temporal and spatial changes of water quality. According to Table 7, from 2019 to 2020, there was no significant change in the water quality of the lake center and the whole, and the water quality of Changdao Bay and Zhaojia Bay deteriorated slightly, but the change was not obvious. In 2019, the water quality of Changdao Bay was slightly worse than the rest of the monitoring sections, but the overall water quality was good. In 2020, the water quality of the monitoring sections of Changdao Bay and Zhaojia Bay was poor, and the water quality in Lake center was the best. According to Table 8, the water quality in the center of Lugu Lake in 2021 was lower than that in 2020, but the change was not obvious. The overall water quality is good.
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