Water quality assessment of Lugu Lake based on Nemerow pollution index method

In this paper, three monitoring sections were set up in Lugu Lake, and water samples were collected in 2019, 2020, and 2021 for the determination of physical and chemical properties such as permanganate index, chemical oxygen demand, biochemical oxygen demand (BOD5) and so on. By using the single factor pollution index method and the Nemerow pollution index method, the water quality of three monitoring sections and the whole Lugu Lake was assessed, and the temporal and spatial changes of water quality were analyzed. The findings demonstrate that Lugu Lake's overall water quality is excellent, and that it has not altered significantly in three years.The results of evaluating the water quality by the single factor pollution index method show that, in the past three years, the water quality of the three monitoring sections and the whole of Lugu Lake is Category I, which belongs to no pollution, and the measured indicators all meet the water quality standard of Category I. 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 and the water quality evaluation of Changdao Bay, Lake center, Zhaojia Bay and the whole are Category I standards in 2019, 2020 and 2021. In terms of time changes, the water quality of Lugu Lake has remained stable between 2019 and 2021, and the water quality has been good. From the perspective of spatial changes, in 2019 and 2020, the water quality in Lake center is better than the monitoring sections of Changdao Bay and Zhaojia Bay.


Scientific Reports
| (2022) 12:13613 | https://doi.org/10.1038/s41598-022-17874-w www.nature.com/scientificreports/ study of Lugu Lake primarily focuses on the ecological environment and changes in dissolved oxygen, but the study of other various monitoring indicators such as permanganate index and a variety of heavy metal ions is less monitored, so the single factor pollution index method and the Nemero pollution index method are used to study a number of indicators in order to provide new information. Therefore, this paper uses the water quality monitoring data of Lugu Lake from 2019 to 2021, adopts the single factor evaluation method and the Nemerow pollution index method to evaluate and analyze the water quality changes of Lugu Lake in three years, and discusses the impact of human activities on the water quality of Lugu Lake. In order to provide a scientific basis for the environmental protection of Lugu Lake and the sustainable development of water resources.

Materials and method
1Research areas and data. Lugu Lake (27°36'-27°47' N, 100°43'-100°54' E) is located in the hinterland of Hengduan Mountains at the junction of Ninglang County in Yunnan Province and Yanyuan County in Sichuan Province. It belongs to the Yalong River system. The elevation of the lake is 2690.8 m, the watershed area is 247.6 km 2 , the average water depth of the lake is 40.3 m, and the maximum water depth is 105.3 m 13 . The lake is 9.4 km long and 5.2 km wide 14 , with a catchment area of 187 km 2 and a supply coefficient of 3.54. The lake's only outlet, Caohai, is located on the east coast. The water exchange cycle of the lake is 18.5 years, and it is a semi-closed lake 15 . The water quality of Lugu Lake was assessed using the annual average values of monitoring indicators in 2019 and 2020, as well as the average values of testing indicators from January to June 2021, for the Changdao Bay, Lake center, Zhaojia Bay, and the whole. The sample map of Lugu Lake is shown in Fig. 1.

Research methods.
Collection and determination of water samples. In this study, water samples were collected in 2019, 2020, and the first half of 2021 in accordance with the standards of the national surface water environmental quality monitoring network for collection, measurement and separation (on-site monitoring technical guidelines).Permanganate Index, COD, BOD 5 , TP, Ammonia Nitrogen, Mercury, Lead, Cadmium, Chromium (hexavalent), Arsenic, Copper, Selenium, Fluoride, Cyanide, Sulfide , petroleum (petroleum ether extraction) and volatile phenol of water samples was carried out in the laboratory according to the national standard method.
Water quality evaluation method. According to the research purpose, the single factor pollution index method and the Nemerow pollution index method were selected to comprehensively evaluate the water quality of the Lugu Lake in three years.
(1) Single factor pollution index method The comprehensive water quality category of the single factor index method is determined by comparing the water quality monitoring results with the relevant classification standards, that is, the category of the worst single   16 . Through single factor evaluation, the main pollutants in the water and their hazard levels can be determined. The pollution index of the specific pollutant is calculated by dividing the actual measured value of the pollutant and the evaluation standard, and the evaluation result is expressed by the pollution index. The formula of the single factor pollution index method is: In the formula, Pi represents the pollution index of single water quality index i; Ci is the measured value of pollutant content (mg/L); S i is the standard value of environmental quality (mg/L), which is the Category III water standard in the Environmental Quality Standards for Surface Water (GB3838-2002) 17 .
The environmental quality standard evaluation grading standard of the single factor pollution index method is shown in Table 1.
(2) Nemerow pollution index method The Nemerow index is a weighted multi-factor environmental quality index that takes into account extreme values or outstanding maximum values 18 . The calculation method of the comprehensive pollution index is as follows: In the formula, P N is the comprehensive pollution index of the sampling point; P imax is the maximum value of the single-item pollution index of the pollutants at the sampling point; P 1 = 1 n n i=1 P i is the average value of the single-factor index.
The grading standard for environmental quality evaluation by the Nemerow pollution index method 19 is shown in Table 2.
(3) Spatial and temporal variation characteristics of water quality in Lugu Lake.
The degree of water quality change with time and space is judged according to the time change rate T and the space change rate S of water quality, respectively. The calculation formula of T and S are as follows 20 ,  www.nature.com/scientificreports/ In the formula, G t1 is the Nemerow index at the start time of the comparison time. G t2 is the Nemerow index of the sampling point at the termination time; G S1 and G S2 are the Nemerow index in the sampling points of the start section and the end section in the comparison space, respectively. Data processing. Excel was used for data processing, and Origin 2021 was used for graphing.

Results and discussion
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: (1) 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.
(2) 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 enhanced 21 . 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 (BOD 5 ), 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.  www.nature.com/scientificreports/ 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.   Table 5. Statistical results of various indicators in Lugu Lake in 2021.

Monitoring indicators
The whole Lake center 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   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 index 22 , 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.

Average value Measured category Average value Measured category
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  www.nature.com/scientificreports/ 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.

Conclusion
The overall water quality of Lugu Lake is good, and the water quality has not changed much in three years. In 2019, 2020 and 2021, it will meet the Category I water quality standard in the Environmental Quality Standards for Surface Water.
The results of evaluating the water quality by the single factor pollution index method show that, in the past three years, the water quality of the three monitoring sections and the whole of Lugu Lake is Category I, which belongs to no pollution, and the measured indicators all meet the water quality standard of Category I. It can be seen from the evaluation results of the Nemerow index method that the water quality pollution index of Lugu   www.nature.com/scientificreports/ Lake is between 0.22 and 0.34 in the past three years and the water quality evaluation of Changdao Bay, Lake center, Zhaojia Bay and the whole are Category I standards in 2019, 2020 and 2021. From the perspective of time changes, the water quality of Lugu Lake has not changed significantly from 2019 to 2021, and the water quality has been good. From the perspective of spatial changes, in 2019 and 2020, the water quality in Lake center is better than the monitoring sections of Changdao Bay and Zhaojia Bay. www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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