Abstract
In order to analyze and compare the characteristics and applicability of different water quality evaluation methods applied to lake water quality evaluation, four monitoring sections were set up in Sancha Lake in 2019, 2020, and 2021, and 20 water quality parameters were selected. The single factor index method, the comprehensive pollution index method, the Nemerow pollution index method, and the improved Nemerow pollution index method were used to comprehensively evaluate water quality. The research results showed that the single factor index evaluation method is simple to operate and can quickly determine the water quality category by identifying the worst single water quality indicator. The comprehensive pollution index method and the Nemerow pollution index method determine the degree of water pollution based on the numerical values representing the overall pollution level of the representative water body. The evaluation results showed that except for the evaluation results of the single factor evaluation method with categories II and III, the results of other evaluation methods were all category I, indicating that the water quality was good.
HIGHLIGHT
The single factor index method, the comprehensive pollution index method, the Nemerow pollution index method, and the improved Nemerow pollution index method were used to comprehensively evaluate water quality.
INTRODUCTION
Water resources are important materials for human survival and precious resources that cannot be replaced in industrial and agricultural production and economic development (Wang et al. 2016). Lakes and reservoirs are important freshwater resource reservoirs on Earth and important strategic resources in China. As an important part of the terrestrial hydrosphere and a participant in the natural water cycle process, lakes and reservoirs have multiple functions such as flood regulation, water quality purification, and climate regulation (Liu et al. 2022). They are also an important guarantee for the ecological environment and play an important role in maintaining regional ecological security (Cichoń 2017). Currently, water quality evaluation of lakes and reservoirs is a fundamental work for water environment management (Saeed & Hashmi 2014). Only by using reasonable water quality evaluation methods can the current water quality status of the water body be accurately reflected, providing a scientific basis for water pollution control and water environment management, and further promoting the rational utilization of the resources of lakes and reservoirs.
At present, there are more methods used at home and abroad to evaluate the water quality condition of rivers and lakes and reservoirs, commonly used are the single factor index evaluation method based on grading criteria (Huang 2020; Liu et al. 2020; Peng et al. 2020), the comprehensive pollution index method (Chen et al. 2017; You et al. 2021), and the Nemerow pollution index method (Yin et al. 2023), and also such as principal component analysis based on multivariate statistical analysis (Liu et al. 2022), gray correlation method (Du et al. 2022), and analysis of variance (Xia et al. 2022), and artificial neural network method (Zhang et al. 2022) and support vector machine (Zhou et al. 2021) based on data mining and pattern recognition between data. In recent years, researchers have used different water quality evaluation methods to evaluate the water quality of rivers and lakes. Yin used the single factor index method, the principal component analysis, the Nemerow pollution index method, and the Shannon-Weaver diversity index method to conduct a comprehensive evaluation of Dongting Lake water quality (Yin et al. 2023); Cui applied the single factor index method, the comprehensive pollution index method, the pollutant sharing rate, the flux analysis, and other methods to evaluate the water quality of Hongze Lake CODMn, NH3-N, TP, and TN. The water quality evaluation and pollutant change trend analysis were carried out for four indexes (Cui et al. 2021); Shan used the comprehensive water quality identification index method to analyze and evaluate the water quality of the inlet and outlet of Shifosi Reservoir from 2009 to 2015 (Shen 2019); Liu selected the fuzzy comprehensive evaluation method for the comprehensive evaluation of the water quality of Sancha Lake, and found that the main pollutants in Sancha Lake are TN and TP, and in many years, they are more than the standard of Class III water quality requirements (Liu 2012).
Sancha Lake is located in Sancha Town, Janyang City, Chengdu City, Sichuan Province, which is entrusted to the management of Chengdu High-tech Zone as a whole, called High-tech East Zone, 60 km from Tianfu Square in the north and 32 km from Janyang City in the northeast, and is the core area of ‘Two Lakes and One Mountain’, the new five major tourist areas in Sichuan Province. There are numerous large and small weirs and ditches and creeks used for agricultural irrigation within the scope of Sancha Lake; Sancha Lake is the production and living drinking water source of Sancha Township, with abundant water resources. Sancha Lake's basic water quality belongs to a class of water quality, local areas have two or three types of water quality pollution situations, at present, many pollution prevention and control and environmental management measures to make the pollution of the Sancha Lake watershed tends to decline mode, and has achieved obvious results. However, water eutrophication with nitrogen and phosphorus pollution is the prominent cause (Li et al. 2019). In terms of the water environment, the water quality of Sancha Lake has not yet reached the good water quality required by the tourism landscape water system in some areas, and the water body of the east shore lake shows a mild eutrophic state. Dealing with water quality problems and major pollutants and establishing a good water ecological environment is the key to the sustainable operation of Sancha Lake. The previous studies on the Sancha Lake mainly focused on water body eutrophication and sediment migration change patterns (Jia et al. 2015), while there were fewer studies on the evaluation of various monitoring indicators, so the single factor pollution index method, the integrated pollution index method, and the Nemerow pollution index method were used to study a number of indicators in order to obtain a more scientific and reasonable basis for the management of the Sancha Lake.
RESEARCH AREAS AND METHODS
Research areas and data
(a) Relative location of monitoring points. (b) Location of sampling points in the 1# water intake, 2# water intake, and lake tail. (c) Location of sampling points in the lake center.
(a) Relative location of monitoring points. (b) Location of sampling points in the 1# water intake, 2# water intake, and lake tail. (c) Location of sampling points in the lake center.
Sampling method and determination of water samples
In this study, water samples were collected in 2019, 2020, and the first half of 2021 according to the Technical Specifications Requirements for Monitoring of Surface Water and Waste Water (HJ/T91-2002). The sampling frequency is once a month, 12 times a year, and the water quality index data is averaged. This specification is suitable for water quality monitoring of rivers, lakes, reservoirs, and other water bodies. According to the methods specified in Table 4 of the Environmental Quality Standards for Surface Water (GB3838-2002), the 20 basic water quality parameters included in the standards were determined in the laboratory. In addition, for the determination of COD, potassium dichromate (K2Cr2O7) was chosen as the oxidizing agent for the determination, as this method is easy to operate, highly accurate, and suitable for all types of water bodies.
Water quality evaluation method
Single factor pollution index method
In the formula, Pi represents the single pollution index of indicator i; Ci is the measured concentration (mg/L), Coi is the standard concentration (mg/L), which is the Category III water standard in the Environmental Quality Standards for Surface Water (GB3838-2002).
Comprehensive pollution index method
Nemerow pollution index method
In the formula, PN represents the Nemerow pollution index; Pimax is the maximum value of Pi; and Piave is the average value of Pi.
Improved Nemerow pollution index method
In the formula, ri is the correlation ratio, Si is the standard value of each pollution factor, Smax is the maximum standard value of the ith pollution factor, and ωi is the weight of the ith pollution factor.
In the formula, represents the Nemerow pollution index; Pimax is the maximum value of Pi; and
is the mean value with weights.
Data processing
Excel was used for data processing, and Origin 2021 was used for graphing.
RESULTS
Determination of the water quality classification standard
According to the regulations of the Chengdu Ecological Environment Bureau, the specified category of this sampling section is the Class III standard of the Environmental Quality Standards for Surface Water (GB3838-2002). According to the formula of the evaluation method, the classification standard is calculated based on the Class III water. The grading standards of three water quality evaluation methods are shown in Tables 1–3.
Water quality category determination based on the comprehensive pollution index method
Assessment of pollution . | Water quality category . | Pc . |
---|---|---|
No pollution | Ⅰ | ≤ 0.484 |
Slight pollution | Ⅱ | (0.484, 0.743] |
Moderate pollution | Ⅲ | (0.743, 1.000] |
Heavy pollution | Ⅳ | (1.000, 2.417] |
Serious pollution | Ⅴ | >2.417 |
Assessment of pollution . | Water quality category . | Pc . |
---|---|---|
No pollution | Ⅰ | ≤ 0.484 |
Slight pollution | Ⅱ | (0.484, 0.743] |
Moderate pollution | Ⅲ | (0.743, 1.000] |
Heavy pollution | Ⅳ | (1.000, 2.417] |
Serious pollution | Ⅴ | >2.417 |
Water quality category determination based on the Nemerow pollution index method
Assessment of pollution . | Water quality category . | PN . |
---|---|---|
No pollution | Ⅰ | ≤ 0.786 |
Slight pollution | Ⅱ | (0.786, 0.881] |
Moderate pollution | Ⅲ | (0.881, 1.00] |
Heavy pollution | Ⅳ | (1.00, 7.275] |
Serious pollution | Ⅴ | >7.275 |
Assessment of pollution . | Water quality category . | PN . |
---|---|---|
No pollution | Ⅰ | ≤ 0.786 |
Slight pollution | Ⅱ | (0.786, 0.881] |
Moderate pollution | Ⅲ | (0.881, 1.00] |
Heavy pollution | Ⅳ | (1.00, 7.275] |
Serious pollution | Ⅴ | >7.275 |
Water quality category determination based on the improved Nemerow pollution index method
Assessment of pollution . | Water quality category . | P′N . |
---|---|---|
No pollution | Ⅰ | ≤ 0.783 |
Slight pollution | Ⅱ | (0.783, 0.792] |
Moderate pollution | Ⅲ | (0.792, 1.00] |
Heavy pollution | Ⅳ | (1.00, 8.817] |
Serious pollution | Ⅴ | >8.817 |
Assessment of pollution . | Water quality category . | P′N . |
---|---|---|
No pollution | Ⅰ | ≤ 0.783 |
Slight pollution | Ⅱ | (0.783, 0.792] |
Moderate pollution | Ⅲ | (0.792, 1.00] |
Heavy pollution | Ⅳ | (1.00, 8.817] |
Serious pollution | Ⅴ | >8.817 |
Single factor index method evaluation results
Monitoring section . | 2019 . | 2020 . | 2021 . | |||
---|---|---|---|---|---|---|
Water quality category . | Main pollutants . | Water quality category . | Main pollutants . | Water quality category . | Main pollutants . | |
1# water intake | Ⅲ | TP | Ⅰ | CODCr | ||
2# water intake | Ⅰ | CODCr | Ⅰ | CODCr | ||
Lake tail | Ⅰ | CODCr | Ⅲ | TP | ||
Lake center | Ⅱ | DO | Ⅲ | TP | Ⅲ | TP |
The whole | Ⅰ | CODCr | Ⅲ | TP | Ⅲ | TP |
Monitoring section . | 2019 . | 2020 . | 2021 . | |||
---|---|---|---|---|---|---|
Water quality category . | Main pollutants . | Water quality category . | Main pollutants . | Water quality category . | Main pollutants . | |
1# water intake | Ⅲ | TP | Ⅰ | CODCr | ||
2# water intake | Ⅰ | CODCr | Ⅰ | CODCr | ||
Lake tail | Ⅰ | CODCr | Ⅲ | TP | ||
Lake center | Ⅱ | DO | Ⅲ | TP | Ⅲ | TP |
The whole | Ⅰ | CODCr | Ⅲ | TP | Ⅲ | TP |
Analysis of the water quality change of Sancha Lake based on the single factor pollution index method
The single factor pollution index method is used to evaluate the water quality of each sampling point of Sancha Lake, and the single factor index calculation results of each index for each year are shown in Supplementary Table S4 according to formula (1). It can be seen that the single factor index of each index is less than 1, and the overall water quality basically meets the Class III standard of the Environmental Quality Standards for Surface Water (GB3838-2002) stipulated by the Chengdu Ecological Environment Bureau.
Water quality category measured by the single factor pollution index method of Sancha Lake in 2019, 2020, and 2021.
Water quality category measured by the single factor pollution index method of Sancha Lake in 2019, 2020, and 2021.
Analysis of the water quality change of Sancha Lake based on the comprehensive pollution index method
Comparison of the comprehensive pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
Comparison of the comprehensive pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
Analysis of the water quality change of Sancha Lake based on the Nemerow pollution index method
Comparison of the Nemerow pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
Comparison of the Nemerow pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
Analysis of the water quality change of Sancha Lake based on the improved Nemerow pollution index method
Comparison of the improved Nemerow pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
Comparison of the improved Nemerow pollution index of various monitoring sections of Sancha Lake in 2019, 2020, and 2021.
DISCUSSION
Comprehensive comparison of evaluation results
(a) Comparison of the single factor pollution index method and three comprehensive evaluation methods in 2019. (b) Comparison of the single factor pollution index method and three comprehensive evaluation methods in 2019.
(a) Comparison of the single factor pollution index method and three comprehensive evaluation methods in 2019. (b) Comparison of the single factor pollution index method and three comprehensive evaluation methods in 2019.
Comprehensive comparison of evaluation methods
Based on the characteristics of the four evaluation methods, a comprehensive comparative analysis is conducted on the calculation formulas, scientific rationality, and evaluation effects of the four evaluation methods in the water quality evaluation of Sancha Lake. The functions of the four evaluation methods are shown in Supplementary Table S5. The single factor evaluation method is convenient to calculate and intuitive to evaluate. The comprehensive pollution index method is relatively easy to calculate and converts the water quality pollution caused by each indicator into a water quality index, while it can be used for comparison of water quality of similar water bodies, but it does not consider the weight of each indicator. The calculation method of weighted average considers each indicator as the same weight. The Nemerow index method considers the influence of extreme values and average values on water quality, and the evaluation results are more comprehensive. The improved Nemerow index method is more cumbersome to calculate and requires the specific weight of each indicator to be calculated.
CONCLUSION
Overall, the water quality of Sancha Lake is good and meets the requirements of water functional zoning. From 2019 to 2021, the monitoring indicators of the monitoring sections met the water quality standards of Class III or above in the ‘Environmental Quality Standards for Surface Water’. According to the results of the single factor index evaluation, some monitoring sections of Sancha Lake had water quality of Class II and Class III. Due to regional characteristics, the main pollutants were concentrated in TP, CODCr, and DO. The comprehensive pollution index method, the Nemerow pollution index method, and the improved Nemerow pollution index method showed consistency in the evaluation results of Sancha Lake's water quality. The evaluation results showed that from 2019 to 2021, all monitoring sections were Class I water quality, indicating no pollution.
Through the comparison of the application of different methods, the advantages and disadvantages of each method have been demonstrated. In actual water quality evaluation management work, when a certain indicator exceeds the standard severely, especially when the indicator with biological toxicity exceeds the standard, it is recommended to use the single factor evaluation method in order to quickly determine the excessive indicator. When the categories of various indicators in a lake are similar, in order to reflect the comprehensive impact of various indicators on water quality, or when the categories of various indicators differ greatly, in order to compare the water quality of lakes belonging to the same category, it is recommended to use the comprehensive pollution index method, the Nemerow pollution index method, and the improved Nemerow pollution index method.
Overall, compared with the other three comprehensive evaluation methods, the single factor evaluation method is more rigorous and conservative, and its results are more pessimistic. The latter considers factors more comprehensively and has a wider range of applications. Therefore, in the future, when evaluating the water quality of lakes, the data should be fully analyzed, and appropriate evaluation methods should be chosen according to the evaluation objectives. The single factor evaluation method can be used in combination with other methods to make the water quality evaluation results more scientifically reasonable.
ACKNOWLEDGEMENT
This work was supported by the Sichuan Science and Technology Program (2021YFS0284).
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.