(Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences)
Urban agglomeration not only promotes China's industrialization and economic development, but also causes many water environment and its derivative problems. The formation and development of urban agglomeration is inevitably accompanied by the increase of urban density, the development of industrial agglomeration and the adjustment of industrial structure. The development of these factors will inevitably lead to excessive consumption of water resources, ecological destruction, water environmental pollution and other problems, and the water environmental carrying capacity will gradually increase. The carrying capacity of water environment varies greatly in a region. The present situation of surface water and groundwater environment in plain areas and mountainous areas is different, and the external carrying capacity is also different. These two aspects determine the industrial choice, urban layout and development direction in the land planning of the whole urban agglomeration, and affect the stability of the national food supply. Therefore, it is the basic work of urban agglomeration land planning to find out the present situation of water environment in this area and evaluate the water environment carrying capacity.
First, the research status at home and abroad
Water environmental carrying capacity (WECC) is a natural combination of the concept of carrying capacity and the field of water environment. The special research on water environmental carrying capacity abroad is still in its infancy, and a unified and mature method has not yet been established. The related research abroad often regards the water environment as an isolated system, and does not study the water environment carrying capacity from the perspective of various factors' interrelation and mutual restriction. There are relatively few reports on water environmental carrying capacity for regional geography, climate and economic development. At present, the theoretical and practical research of water environmental carrying capacity in China is still in the exploratory stage, and there is no systematic report on the connotation, characteristics, change relationship and quantitative characterization of water environmental carrying capacity. 1997, Tang Jianwu and others put forward the concept of water environmental carrying capacity, and applied the study of water environmental carrying capacity to water pollution carrying capacity, water environmental capacity, water environmental value and benefit accounting. However, the scientific definition, research theory and method of water environmental carrying capacity have not yet reached a * * * understanding in academic circles. The water environmental carrying capacity defined by Wang Shucheng, Minister of Water Resources, refers to the regional environmental quality of a water area, that is, if the water area system is ecologically healthy and has self-purification ability, its carrying capacity will be strong and it is a well-functioning water area system. Cui Shubin believes that the water environmental carrying capacity is usually "water environmental capacity", "water environmental pollution capacity" or "water environmental allowable pollution load".
To sum up, the evaluation of water environmental carrying capacity at home and abroad is often based on surface water, considering its pollution capacity (environmental capacity) and regulation function, while ignoring the environmental carrying capacity of groundwater. The objective fact is that there is a close relationship between groundwater and surface water, and the recharge relationship is inseparable and a whole; At the same time, in addition to the self-purification ability of surface water, the groundwater system structure, especially the soil layer and vadose zone, has the ability to resist and purify pollutants. In addition to the above-mentioned groundwater purification capacity, it actually increases the capacity of the whole water environment. Therefore, the water environmental carrying capacity should include groundwater and surface water environmental carrying capacity. Therefore, the water environment assessment work is to take surface water and groundwater as a whole and evaluate the water environment carrying capacity from the characteristics and natural laws of urban agglomerations. Therefore, through the study of water environmental carrying capacity of urban agglomerations, the threshold of water environmental carrying capacity of economic and social development is further clarified, which provides technical support for scientific and rational planning of economic and social structure adjustment planning of urban agglomerations to adapt to water environmental carrying capacity, and for the overall layout and full implementation of national land planning.
Second, the objectives and tasks of water environmental carrying capacity evaluation
(1) target
Guided by the system theory and based on the water cycle theory, this paper analyzes the surface water quality, calculates the water environmental capacity and predicts the water quality in the planning level year from the perspective of the land planning demand of urban agglomerations. According to the characteristics of hydrogeological conditions, evaluate the anti-pollution performance of groundwater environment; Combined with the evaluation results of groundwater renewable capacity, based on maintaining the safety of rapid development of urban agglomeration, the regional water environment carrying capacity of urban agglomeration is evaluated. Suggestions on land planning based on water environment carrying capacity are put forward, which provides technical support for exploring the friendly corresponding relationship between benign evolution and rapid development of water environment in urban agglomerations and land planning.
(2) Task
1. Verification of surface water environmental capacity
The spatial emission characteristics of surface water environmental pollutants are analyzed, the local surface water environmental capacity is calculated, and the total amount control evaluation is carried out according to the water environmental capacity.
2. Evaluation of anti-pollution performance of groundwater environment
The evaluation factors that can represent the anti-pollution ability of groundwater environment are selected, and the anti-pollution ability of groundwater environment is evaluated by DRASTIC model.
3. Evaluation of water environmental carrying capacity
Taking surface water and groundwater as a whole, selecting appropriate evaluation factors, establishing corresponding evaluation system and weight system, and evaluating water environmental carrying capacity.
4. Put forward suggestions on land planning based on water environmental carrying capacity.
According to the needs of urban agglomeration land planning and the evaluation results of water environmental carrying capacity, some suggestions on land planning based on water environmental carrying capacity are put forward.
(3) Technical methods
On the basis of collecting and sorting out the basic data, this study will identify the main water environment problems currently faced according to the water environment characteristics of urban agglomerations, analyze the pollution sources of surface water and evaluate the compliance of functional areas, calculate the environmental capacity of surface water, and conduct total amount control assessment based on the environmental capacity of surface water; The hydrogeological conditions in the study area are analyzed, and the DRASTIC model is selected to evaluate the anti-pollution performance of groundwater environment. Combined with the evaluation results of groundwater renewal capacity, the index system, calculation model and evaluation method of water resources carrying capacity are constructed, and the water environment carrying capacity of urban agglomeration areas is calculated. Finally, suggestions on land and resources planning to improve the water environment carrying capacity of urban agglomerations are put forward (Figure 1).
Figure 1 Technology Roadmap
Thirdly, the evaluation of water environmental carrying capacity needs in-depth study.
(A) Water environmental pollution sensitivity assessment
The sensitivity of water environmental pollution refers to the capacity of regional ecosystem to water pollutants under natural precipitation, that is, the possibility of water environmental pollution under normal precipitation. It mainly depends on the size of regional precipitation and the dilution ability of surface runoff formed by precipitation to pollutants. According to the depth of precipitation runoff, the present situation of water environmental quality and the intensity of pollutant discharge in urban agglomerations, the following index system is selected for zoning (table 1), and the sensitivity of river water environmental pollution is divided into five grades: general zone, mild sensitivity, moderate sensitivity, high sensitivity and extreme sensitivity (table 1).
Table 1 Water environmental pollution sensitivity index and classification standard
(2) Assessment of water environmental capacity
Water environmental capacity is a management and control goal based on the full scientific study of hydrological characteristics, sewage discharge mode and pollutant migration and transformation law of the basin, combined with environmental management requirements. The water environmental capacity not only reflects the natural attributes (hydrological characteristics) of the basin, but also reflects the human demand for the environment (water quality objectives). The water environmental capacity will change with the constant change of water resources and the continuous improvement of people's environmental demand.
The calculation of water environmental capacity in urban agglomeration areas is mainly based on the water quality objectives and existing parameters formulated in the water environmental function zoning, and compared with the current situation of river basin water quality, with COD and ammonia nitrogen as the main total control indicators to evaluate the water environmental capacity.
(3) Evaluation of groundwater environmental pollution resistance.
Groundwater environment is an important part of human environment and an important branch of water environment, which plays an important role. Toxic and harmful elements or pollutants discharged into the external environment by human beings in daily life and production are recycled into the groundwater environment, which leads to the deterioration of the groundwater environment. The deteriorating groundwater environment in turn adversely affects human production and life, restricts social and economic development, and may even endanger human health in serious cases. Compared with surface water pollution, groundwater pollution is more hidden and irreversible. Compared with the surface water environment, the groundwater environment not only has the self-purification function of surface water, but also has the barrier effect on pollutants entering the water body, which reflects the self-protection ability of the groundwater environment. The evaluation of groundwater environmental anti-pollution ability is to select evaluation factors that can represent groundwater environmental anti-pollution ability from the perspective of effectively protecting groundwater resources from pollution, establish corresponding scoring system and weight system, and divide groundwater environmental anti-pollution ability grades.
There are vector analysis, comprehensive index method, analytic hierarchy process, grey correlation method and aerodynamic method which are suitable for evaluating the anti-pollution ability of groundwater environment. Considering that the influencing factors and evaluation index system of groundwater environmental anti-pollution ability are the same as groundwater vulnerability, the DRASTIC method promoted by EPA in the United States is widely used in groundwater vulnerability evaluation. This method is also used to evaluate the anti-pollution ability of groundwater environment.
The DRASTIC method mainly considers the following seven parameters: groundwater depth, aquifer net recharge, aquifer lithology, soil type, topography, vadose zone (seepage zone) influence and aquifer permeability coefficient. These seven indicators correspond to the main factors that affect the groundwater environment, so it is appropriate to use the seven parameters of DRASTIC as the evaluation index system of this evaluation. See Table 2, Table 3 and Table 4 for the grades of each index and their corresponding standard characteristic values.
Table 2 Index level and its corresponding standard eigenvalue table
Table 3 Index level and its corresponding standard eigenvalue table
Table 4 Index Level and Its Corresponding Standard Eigenvalue Table
Four. Evaluation of water environmental carrying capacity
evaluation methodology
There are many methods to evaluate the water environmental carrying capacity. Analytic Hierarchy Process (AHP) is chosen this time. AHP is a statistical method, which is widely used to determine the weight of each index in multi-index evaluation. The main steps are as follows:
1. Establish the hierarchical structure of the problem
Firstly, according to the understanding and preliminary analysis of the problem, the complex problem is decomposed into various components called factors according to specific objectives, criteria and constraints, and the attributes of these factors are arranged in different layers. Factors at the same level dominate some factors at the lower level, and at the same time, they are dominated by factors at the higher level, forming a top-down hierarchical structure. The simplest hierarchy is divided into three layers. The top layer generally has only one factor, which is the goal of the system, and is called the target layer; The middle level is the criterion, and the criterion for measuring whether the goal is achieved is arranged; The bottom layer is the indicator layer, which indicates the specific indicators selected, as shown in Figure 2.
Fig. 2 Schematic diagram of analytic hierarchy process structure
Evaluation of water environmental carrying capacity is a complex decision-making system, which can be divided into three levels according to the basic principle of analytic hierarchy process:
(1) Target layer: Take the water environmental carrying capacity as the working target.
(2) Criteria layer: The factors that affect the water environmental carrying capacity are listed as the main aspects as the basic criteria for evaluation, such as surface water environmental capacity, groundwater environmental anti-pollution ability, water environmental renewal ability, shallow groundwater exploitation sustainability, etc.
(3) Indicator layer: According to the evaluation criteria, the above influencing factors are further subdivided into several specific evaluation indicators.
2. Build a judgment matrix
Compare the relative importance of each element in the same level to each criterion in the previous level, and construct a pairwise comparative judgment positive matrix A:
A=(aij)n n
Among them, aiji (I, j = 1, 2, ..., n) is the relative importance value of each factor, and the judgment matrix of each factor at the same level is established by L ~ 9 scale method, and its value significance is shown in Table 5.
Table 5 Judgment Matrix Symbols of Analytic Hierarchy Process and Their Significance
3. Weight determination and conformity test
According to the constructed judgment matrix, the index weight is calculated (that is, the maximum eigenvector of the judgment matrix is solved).
(1) Calculate the product of n elements in each row of the matrix:
Earth Science, Water and Cities
(2) Calculate the root of n times.
(3) normalizing the vectors, that is, forming a matrix of n vectors obtained by square roots of n times, and normalizing the vectors;
Earth Science, Water and Cities
Get: w = (w 1，w2，…，Wn)，
Where: w is the approximate value of the obtained feature vector and the weight of each index.
(4) Calculate the eigenvalue Amax of the matrix:
Earth Science, Water and Cities
Where: [BWτ]i is the i-th element of the vector BWτ.
(5) Because of the complexity of objective things and the one-sidedness of subjective knowledge, the judgment matrix is not necessarily a consistency matrix, but when it deviates too much from consistency, it will lead to some self-contradictory problems. Therefore, after the results are obtained, a random consistency test is needed, and the test formula is:
CI =(λmax-n)/(n- 1)；
CR=CI/RI
Where: CI is the consistency index;
Amax is the largest characteristic root;
N is matrix dispersion;
RI is the average random consistency index;
CR is a random consistency ratio. Only when Cr < 0. 10, the judgment matrix has satisfactory consistency and the weight obtained is considered reasonable.
(B) the establishment of evaluation index system and determine the weight
1. Evaluation index system
According to the factors that affect the evaluation of water environmental carrying capacity, surface water environmental capacity intensity, groundwater environmental anti-pollution ability, groundwater renewable ability and groundwater function are determined as the first-class factors; The four primary factors include several secondary factors, and the evaluation factor system is shown in Figure 3.
Fig. 3 Structure diagram of water environmental carrying capacity evaluation factor system
2. Quantification of evaluation factors
For the factors that can directly get the specific value through measurement, directly select the value for fuzzy evaluation and give the grade quota; for the factors that can't directly get the specific value, the expert scoring method is used for grade division (10 scale). The measured values are directly used for the environmental capacity intensity of surface water, and the anti-environmental pollution ability, renewable ability and function of groundwater are evaluated by experts.
3. Weight determination
According to the constructed judgment matrix, the index weight is calculated by AHP model (that is, the maximum eigenvector of the judgment matrix is solved, Table 6).
Table 6 Evaluation Factors and Their Weights of Water Environmental Carrying Capacity
sequential
Consistency ratio of judgment matrix: 0.0056.
(III) Zoning standard of bearing capacity
Using Arc GIS platform, the four vector diagrams of surface water environmental capacity intensity, groundwater environmental anti-pollution ability, groundwater renewal and groundwater function are converted into grid diagrams, which are adopted by using grid functions.
Water environmental carrying capacity = RS WS+RC WC+RR WR+RF WF.
Where: r is the grade characteristic value (measured value);
W is the weight.
According to the standards shown in Table 7, the comprehensive evaluation results are divided into strong water environment bearing capacity, strong water environment bearing capacity, general water environment bearing capacity, weak water environment bearing capacity and weak water environment bearing capacity.
Table 7 Zoning Criteria for Water Environmental Carrying Capacity Evaluation
Application of verb (abbreviation of verb) to evaluation results of water environmental carrying capacity
The research on water environmental carrying capacity of urban agglomeration aims to provide technical support for the overall layout and comprehensive implementation of land planning in urban agglomeration areas, and provide scientific basis for formulating economic and social structure adjustment planning based on water environmental carrying capacity in urban agglomeration areas.
(1) Areas with strong water environment carrying capacity
The area with the strongest water environment carrying capacity, but if it is located in a mountainous area, the terrain slope is large, which is not conducive to industrial layout and human settlement, and the area is sensitive to the ecological environment, it is suitable to become a soil and water conservation area; If the slope is small, it is suitable for the layout of heavily polluted industries, with large surface water environmental capacity and relatively strong anti-pollution ability of groundwater environment.
(2) Areas with strong water environment carrying capacity
The mountainous area with small slope is suitable for industrial and mining enterprises with strong pollution, with large surface water environmental capacity, slightly higher anti-pollution ability of groundwater environment and strong groundwater renewal ability; Low mountain area has large water environmental capacity, relatively good anti-pollution performance of groundwater environment and strong water supply capacity, which is suitable for arranging industrial and mining enterprises. However, due to the relatively developed general geological disasters in this area, it should be prevented.
(3) Water environmental carrying capacity of general area
These areas are generally located in the piedmont zone, suitable for small polluting industrial enterprises, human habitation and agricultural irrigation, with large surface water environmental capacity, slightly higher anti-pollution ability of groundwater environment, slightly less pollution and general groundwater renewal ability.
(d) Areas with weak water environment carrying capacity
Plain areas are generally suitable for human habitation and agricultural irrigation. Because of its small water environment capacity, the anti-pollution performance of groundwater environment is slightly lower, it is slightly easy to be polluted, and the groundwater renewal ability is average.
(5) Areas with weak water environment carrying capacity
Valley areas should control the scale of population development, limit industrial layout, be suitable for agricultural irrigation, control the use of pesticides, and prevent and control non-point source pollution of water environment.