Keywords: water environment; Quality; Evaluation criteria; evaluation methodology
1 water environment quality assessment
Water environmental quality evaluation is a process of quantitatively describing the water environmental quality (or converting quantity and quality into comments) through certain mathematical methods and other means. The evaluation of water environmental quality must be based on the monitoring data, and the statistics (characteristic values) and various representative values of the environment can be obtained through mathematical statistics, and then the environmental quality can be evaluated according to the evaluation methods and grading standards of water environmental quality.
2 the role and classification of water environmental quality assessment
Water environmental quality assessment is one of the important means of environmental management. Through the evaluation of water environmental quality, we can understand the past, present and future development trend and its changing law of environmental quality, and formulate comprehensive prevention measures and programs; We can understand and master the main pollution factors and sources that affect the environmental quality in this area, so as to formulate pollution source control schemes and comprehensive prevention and control plans to improve environmental quality; Can provide scientific basis for the formulation of national or local environmental standards and regulations; The environmental quality can be predicted in advance, and the environmental impact report and prevention plan of new construction, reconstruction, expansion, tapping potential, innovation and transformation can be compiled, which can provide scientific basis for site selection, design and production layout. It can also be used to summarize the environmental protection work in the region, identify the effect of prevention and control measures, write annual environmental quality reports, compare the environmental quality between different regions, exchange information, conduct national environmental quality statistics, and promote the development of environmental protection research and technology and whether to sacrifice the water environment.
According to different classification methods, water environmental quality assessment can be roughly divided into the following categories: 1) According to time, it can be divided into retrospective assessment, current assessment and predictive assessment; 2) According to the geographical type, it can be divided into cities, regions or river basins, scenic spots, etc. 3) According to the specialty of using environment, it can be divided into drinking water, irrigation water and fishery water.
3 water environment quality evaluation method
3. 1 Brief introduction of evaluation method
There are a variety of environmental quality assessment methods at home and abroad, but at present there is no unified evaluation method standard for environmental protection workers in China. There are many methods to evaluate the quality of water environment, such as Brown water quality index, Pratt water quality index, Ross water quality index, Nemerow water quality index, comprehensive pollution index, fuzzy mathematics method and map superposition method. The last method is widely used in China at present, which is simple and practical. All the above evaluation methods must first determine the representative value of a single index of the section, and most of them take the average value as the representative value. Nemerow water quality index method not only considers the average value, but also considers the influence of the final value on the evaluation results, but there are many specific problems that are difficult to solve in the evaluation work and are rarely used. Other methods also involve the weight of standards or evaluation indicators and are rarely used.
3.2 Basic steps of water environment quality assessment
Evaluation Purpose According to the function of water environmental quality evaluation, the evaluation purpose is defined.
Select the evaluation scope of water environment quality evaluation of countries, river basins, water systems, cities, lakes and reservoirs;
Selection of evaluation data The current evaluation generally selects the data of the last year or month as the current data, and the retrospective evaluation selects the data from the basic year to the current year, and the time period must be representative. For example, in the recent retrospective evaluation of Huaihe River Basin, most of the data from 1994 to 2000 were selected, because the large-scale pollution control in this basin started from 1994;
The selection of evaluation items generally includes natural indicators, organic pollution indicators and toxic pollution indicators;
The selection of evaluation criteria can choose different water quality evaluation criteria according to the evaluation purpose;
Selection of evaluation methods For watershed evaluation, it is generally necessary to carry out single evaluation, classified evaluation (natural, organic and toxic) and comprehensive evaluation in different water periods. Map superposition method is often used for classification evaluation and comprehensive evaluation. Generally, water quality index (WQI) should be calculated for water quality evaluation of water supply sources, and the evaluation results should be published to the public regularly.
The evaluation results describe the evaluation results, find out the main pollution factors, main pollution areas, pollution trends and changing rules, and analyze the pollution causes.
The purpose of water environmental quality evaluation is to understand the past, present and future development trend and its changing law of environmental quality through evaluation, and to master the main pollution factors and main pollution sources that affect the environmental quality in this area, so as to formulate pollution source control schemes and comprehensive prevention plans and improve environmental quality. Therefore, according to the above steps, the evaluation should be purposeful, flexible and changeable, and give full play to the subjective initiative of individuals. Status assessment can be monthly assessment, quarterly assessment and annual assessment. Retrospective evaluation is generally a retrospective analysis of water quality in recent years.
Monthly water quality assessment should describe the overall water quality of this month with charts. Comparing the water quality of last month with that of the same period last year, there should be a clear conclusion whether the water quality has improved or deteriorated. Find out the main pollution factors and rivers or sections with serious water pollution this month. The quarterly status assessment is basically similar to the monthly water quality status assessment, but the representative value of the time period must be determined first.
To evaluate the present situation of annual water quality, it is necessary to describe the overall water quality situation this year with charts. Compared with the water quality of last year or reference year, it is necessary to have a clear conclusion whether the water quality has improved or deteriorated. In time, because the river water quality is obviously affected by seasons, it can also be compared and analyzed in different periods, quarters or months; In space, it can be evaluated according to different regions, such as rivers and water systems. River evaluation should also analyze the changes of water quality along the way, and it is best to highlight the seriously polluted places with the concentration hydrograph of main pollution indicators. When analyzing the contribution of tributaries to the main stream pollution, it can be solved by ranking the pollution load ratio with equal standard, but it may be unrepresentative due to accidental results. It is suggested to calculate the transport capacity of a pollutant by combining the flow data. If Lutaizi Station, the main stream of Huaihe River, is taken as the downstream control point, its upstream includes Huaibin, He Ying, the first tributary, Honghe, Pi River and Shihe River, the second largest tributary. It is generally believed that Heying River and Wohe River have great influence on the pollution of Huaihe River. However, after calculating the annual cod-Mn transport in the estuaries of the tributaries, it is found that the Honghe and Huaibin of Huaihe River contribute the most to the pollution of Lutaizi, accounting for about 30% ~ 70% of the total amount of Lutaizi, and the variation law of pollutant transport in the second section is similar to Lutaizi, on the contrary, He Ying and Guo River have little influence. This is mainly because the Honghe and Huaibin of Huaihe River are flat rivers, while the Heying River and the Wohe River have many sluices. If the latter is a smooth river, the Huaihe River will be submerged. The above examples can illustrate many problems, but they also have some limitations, that is, they do not consider the degradation factors of pollutants and need complete and reliable flow series data.
4 Problems existing in water environmental quality assessment
Recently, the relevant departments issued the Environmental Quality Standard for Surface Water (GHZB1-1999). Although some contents involve water quality evaluation, they are still not too detailed. Article 5. 1 of the standard stipulates that "the evaluation of surface water environmental quality should select a single index and evaluate the compliance rate item by item", which mainly highlights the evaluation of single compliance rate; Article 5.2 stipulates that "for water bodies with obvious characteristics in wet season, normal season and dry season, the compliance rate should be evaluated by time periods, and the data used should not be instantaneous monitoring values and annual average values, and there should be no less than two data in each time period", which emphasizes the assessment of compliance rate and denies the role of average monitoring values in each time period and the whole year. For example, the water quality of Hongshizui section of Shihe River in Huaihe River system has always been good, with Grade II in most months and Grade III in a few months. However, the water quality objectives in the Huaihe River Basin Water Pollution Prevention and Control Plan and the Ninth Five-Year Plan are Class IV, and the water quality compliance rate is definitely 100%. If people want to know what kind of water this section is in all water periods and all year round, there is no quantitative concept. This problem can be solved if there is a representative value of the cross section. The water quality evaluation of the basin is a regional evaluation, and its content will inevitably involve the overall water quality of the basin. When there is no representative value of each section, the information of the overall compliance rate of the basin obtained by evaluating the compliance rate according to the number of monitoring stations will be distorted, because this result is greatly influenced by the monitoring frequency of each station to a certain extent, and the weight is inadvertently set for each station. For example, the monitoring frequency of one station is 3 times/year, and most of them are super-V water bodies, while the monitoring frequency of another station is 12 times/year, and most of them are Class III water bodies. When the percentage of annual water quality categories in this area is counted according to the number of monitoring stations, the percentage of good water quality will be too large to reflect the real situation.
According to the principle of statistics, in order to describe a sample sequence completely, it is necessary to understand the relationship between the possible values of random variables and the probability. The evaluation of qualified rate, over-standard rate and detection rate is actually equivalent to probability evaluation, but in practical work, it is necessary to describe not only the relationship between probability and the corresponding values of random variables, but also the overall generalization of the distribution of random variables, that is, the digital characteristics of random variables, and determine the water quality category of a section in the evaluation period according to the eigenvalues. When dealing with practical problems, we still need a limited number of experiments or observations to solve them. There are usually two kinds of digital characteristics: one is to reflect the concentration degree of random variables, that is, the average value, and its digital characteristic values are average value, median value, mode and geometric average value; One is to reflect the dispersion degree of random variables, and the numerical eigenvalues include mean deviation, variance and mean square deviation.
The average value of environmental statistics refers to the average statistical index of environmental phenomena in a homogeneous population at a certain time, place and condition. Its remarkable feature is that a numerical value is used to represent the overall level of environmental phenomena studied, and the differences of variables in the overall are abstracted. In fact, it is a process from personality to sex. Using the average, we can compare the overall level of similar phenomena in different regions, help people understand the overall situation of the studied environmental objects, and study the development and changes of the overall level of something in different periods, so as to observe and analyze its development process and trend. Only in this way can we reveal the law of interdependence among various environmental phenomena.
Mode is a random variable with the highest frequency in the sample sequence, which is very representative. However, it is generally difficult to find patterns in short-term water quality data, so it is impossible to get real patterns in practical work. Geometric average is mainly used to study the average growth rate or average ratio, and is often used for factors with large amplitude changes. Therefore, the mode and geometric mean are generally not regarded as the representative values of water quality evaluation.
The positional relationship of arithmetic mean, median and mode If the abscissa represents the variable value and the ordinate represents the corresponding frequency, if the curve distribution is symmetrical, the three are combined into one. If the curve distribution is positive or negative (see figure 1 and figure 2), then the arithmetic mean is most affected by the final value and far from the mode; However, the median is influenced by the number of terms and is closer to the mode.
Dispersion reflects the honey degree of variables distributed on both sides of the average, and the small dispersion indicates that the difference between the variable value and the average is small, and the average is stable and representative. On the contrary, a large deviation means that the values of variables are quite different from the average, which means that the average is unstable and unrepresentative.
Evaluation accuracy refers to the difference between the obtained evaluation results and the actual environmental quality. The accuracy of evaluation representative values determines the accuracy of evaluation results. Assuming that the average value is the representative value, according to the standard deviation formula of the average value, it is not difficult to see that the greater the sampling density (that is, the greater the n), the smaller the standard deviation of the average value, the more reliable and accurate the average value is, and the same evaluation accuracy has been improved accordingly.
From the above analysis, it can be seen that the water environmental quality evaluation must count the representative values of sections, which is the basis of water quality evaluation. It depends on the specific situation whether it is appropriate to take the average or the median. When evaluating water quality, calculating the standard deviation of the average value first and then judging the representativeness of the average value will add more workload to the evaluation work. Generally speaking, the frequency of the median is close to the frequency of the mode, and the median is very representative. When the monitoring frequency is once a month, the average or median value of the water period can be taken as the representative value of the section; When the monitoring frequency is once every two months, the median value can be taken as the representative value of the water period section; Monitoring once every three or four months, the average value and median value are not representative. Obviously, it is debatable to emphasize that the annual average value is not representative or that the annual average value cannot reflect the extreme value when evaluating water quality.