Research on the Evaluation of Water Environment Carrying Capacity in Urban Agglomeration Planning

Zhang Fawang Chen Li Cheng Yanpei Dong Hua

(Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences)

Urban clusters, while promoting the industrialization and economic development of our country, also give rise to many water environment and its derivative problems. The process of formation and development of urban agglomerations is inevitably accompanied by phenomena such as increase in town density, development of industrial agglomeration, and industrial restructuring. The development of these elements will inevitably cause excessive consumption of water resources, ecological damage, water pollution and other problems, water environment carrying capacity is gradually increasing. There is a big difference in the carrying capacity of water environment in a region, and the surface water and groundwater environment status of plain area and mountainous area are different, and their ability to bear the outside is also different. These two aspects determine the choice of industries, town layout and development direction in the territorial planning of the entire urban agglomeration, and affect the stability of the national food supply. Therefore, mapping the current status of the water environment in the urban agglomeration area and evaluating the carrying capacity of the water environment in the area is the basic work of the urban agglomeration territorial planning.

I. 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, and the specialized research on water environmental carrying capacity in foreign countries is still in the initial stage, and has not yet established a unified and mature method. Overseas studies tend to treat the water environment as an isolated system, and do not study the carrying capacity of the water environment from the perspective of interconnection and mutual constraints of various elements, and there are relatively few reports on the study of the carrying capacity of the water environment in terms of regional geography, climate and economic development. Domestic theoretical and practical research on the carrying capacity of the water environment is still in the exploratory stage, the connotation of the carrying capacity of the water environment, the characteristics, the relationship between changes and quantitative characterization has not yet seen a more systematic report. 1997, Tang Jianwu et al. gave the concept of the carrying capacity of the water environment, and will be used in the study of the carrying capacity of water environment for the sewage capacity of watersheds, the capacity of the water environment, water environment value and benefit accounting and so on. However, the scientific definition of the carrying capacity of the water environment, research theory and methodology, the academic community has not yet reached **** knowledge. Water Resources Minister Wang Jucheng definition of the carrying capacity of the water environment refers to the regional environmental quality of a watershed, that is, the ecological health of the watershed system, there is a self-purification capacity, its carrying capacity is strong, is a well-functioning watershed system. Cui Shubin believes that the carrying capacity of the water environment is the usual sense of the "water environment capacity", "water environment (water body) of the polluting capacity" or "water environment allowable pollution load" and so on.

Based on the above, the evaluation of the carrying capacity of the water environment at home and abroad is often from the surface water, considering the size of its pollution-carrying capacity (environmental capacity) and regulation, while ignoring the carrying capacity of the groundwater environment. The objective fact is that groundwater and surface water are closely connected, and the recharge relationship is inseparable, which is a whole; at the same time, the groundwater body itself has the self-purification ability of surface water, and its system structure, especially the soil layer and air-packed belt, has the ability to resist and purify the pollutants. Combined with the above groundwater purification ability, it actually increases the capacity of the whole water environment. Therefore, the carrying capacity of water environment should include two aspects of groundwater and surface water environmental carrying capacity. Therefore, the evaluation of water environment is to take surface water and groundwater as a whole, and to evaluate the carrying capacity of water environment from the characteristics of urban agglomerations and laws of nature. Therefore, by carrying out research on the carrying capacity of the water environment in the urban agglomeration area, further clarifying the carrying capacity threshold of the water environment for economic and social development, and providing technical support for the scientific and rational planning of the urban agglomeration to adapt to the economic and social structural adjustment program of the carrying capacity of the water environment, and for the overall layout and full implementation of the territorial planning.

Second, the objectives and tasks of the evaluation of the carrying capacity of the water environment

(I) Objectives

Taking the system theory as the guidance, based on the theory of the water cycle, and from the perspective of the needs of the urban agglomeration of territorial planning, the analysis of the existing water quality of the surface water situation and the measurement of the water environment capacity, and the prediction of the water quality situation in the planning level year; based on the characteristics of the hydrogeological conditions, the evaluation of the groundwater environment to resist the pollution of performance; combined with the renewable capacity of groundwater, the evaluation of the groundwater environment to resist the pollution of the groundwater environment. Evaluate the pollution resistance of groundwater environment based on hydrogeological characteristics; evaluate the carrying capacity of water environment in the urban agglomeration area by combining the results of groundwater renewability evaluation and maintaining the safety of urban agglomeration's rapid development; put forward the suggestion of national land planning based on the carrying capacity of water environment, and provide the technical support for exploring the friendly and corresponding relationship between the benign evolution of water environment and urban agglomeration's rapid development, as well as the national land planning.

(II) Tasks

1. Approval of environmental capacity of surface water

Analyze the characteristics of spatial discharge of pollutants in the surface water environment, measure the environmental capacity of the local surface water bodies, and evaluate the total amount of control based on the environmental capacity of water.

2. Evaluation of anti-pollution performance of groundwater environment

Select evaluation factors that can characterize the anti-pollution capacity of groundwater environment, and adopt DRASTIC model to evaluate the anti-pollution performance of groundwater environment.

3. Evaluation of carrying capacity of water environment

Taking surface water and groundwater as a whole, selecting appropriate evaluation factors, establishing corresponding evaluation system and weighting system, and evaluating carrying capacity of water environment.

4. Propose the national planning based on water environment carrying capacity

From the needs of national planning of urban agglomerations, based on the evaluation results of water environment carrying capacity, propose the national planning based on water environment carrying capacity.

(III) Technical Approach

Based on collecting and organizing basic information, this study will identify the main water environment problems currently facing the urban agglomeration, analyze the sources of surface water pollution and evaluate the attainment of functional zones, calculate the environmental capacity of surface water, and calibrate the total amount of control based on the environmental capacity of surface water; analyze the hydrogeological conditions of the study area, select the DRASTIC model, and evaluate the environmental capacity of groundwater. DRASTIC model, evaluating the anti-pollution performance of groundwater environment; combining the results of groundwater renewal capacity evaluation, constructing the water resources carrying capacity index system, calculation model and evaluation method, calculating the water environment carrying capacity of the urban agglomeration area, and finally putting forward the suggestions of land and resources planning to improve the water environment carrying capacity of the urban agglomeration area (Figure 1).

Figure 1 Technical roadmap

Three, the evaluation of the water environment carrying capacity needs to be in-depth study of the problem

(a) water environment pollution sensitivity evaluation problem

Water environment pollution sensitivity refers to the regional ecosystem under the conditions of natural precipitation, the size of the capacity of the water pollutants, that is, occurs in the normal precipitation, the likelihood of water pollution occurs in the water environment, and the possibility of the size. Size. It mainly depends on the size of regional precipitation and precipitation formation of surface runoff on the dilution of pollutants. According to the depth of regional precipitation runoff in the urban agglomeration, combined with the current status of water environment quality and the intensity of pollutant discharges, the zoning selects the following indicator system (Table 1), and divides the sensitivity of water environment pollution of the river into five levels: general area, mildly sensitive, moderately sensitive, highly sensitive and extremely sensitive (Table 1).

Table 1: Indicators of water pollution sensitivity and grading standards

(II) Assessment of water environment capacity

Water environment capacity is based on the hydrological characteristics of the watershed, sewage disposal, pollutant migration and transformation of the law of the Basin based on adequate scientific research, combined with the needs of environmental management to determine the management and control objectives. The capacity of water environment reflects the natural attributes of the basin (hydrological characteristics), and at the same time reflects the needs of human beings for the environment (water quality objectives), the capacity of the water environment will continue to change with the continuous changes in water resources and people's environmental needs continue to improve and change.

The calculation of water environment capacity in urban agglomeration area is mainly based on the water quality objectives and existing parameters formulated by Water Environment Functional Zoning, comparing with the current status of water quality in the watershed, and evaluating the capacity of the water environment by taking COD and ammonia nitrogen quantity as the main target of total control.

(C) Evaluation of the pollution-resistant capacity of the groundwater environment

The groundwater environment is a kind of important part of the human environment, and it is an important branch of the composition of the water environment, occupying a pivotal role. The toxic and harmful elements or pollutants discharged into the external environment by human beings in their daily life and production process enter into the groundwater environment by the water cycle, resulting in the deterioration of the groundwater environment. The deteriorated groundwater environment in turn has a negative impact on human production and life, restricting social and economic development, and may even jeopardize human health in serious cases. Groundwater pollution is more insidious and irreversible than surface water pollution. Compared with the surface water environment, groundwater environment, in addition to the self-purification function of surface water bodies, its own composition of pollutants into the water body has a blocking effect, which reflects the self-protection ability of the groundwater environment. The evaluation of the pollution-resistant capacity of groundwater environment is to select the evaluation factors that can characterize the pollution-resistant capacity of groundwater environment from the perspective of effectively protecting groundwater resources from pollution, and establish the corresponding scoring system and weighting system to classify the pollution-resistant capacity of groundwater environment.

The methods applicable to the evaluation of groundwater environmental pollution resistance are vector analysis, comprehensive index method, hierarchical analysis, gray correlation method, aerodynamic method, etc. Considering that the factors affecting the groundwater environmental pollution resistance as well as the evaluation index system are the same as those affecting the vulnerability of groundwater and the evaluation index system, the groundwater vulnerability evaluation has been widely applied to the DRASTIC method, which is promoted by the U.S. EPA. DRASTIC method. This method is also adopted for the evaluation of the pollution resistance of groundwater environment.

The DRASTIC method mainly considers the following seven parameters: depth of groundwater, net recharge of aquifer, lithology of aquifer, soil type, topography, influence of air pocket (seepage zone) and permeability coefficient of aquifer. These seven indicators correspond to the main factors affecting the groundwater environment, so it is appropriate to take the seven parameters of DRASTIC as the evaluation index system for this evaluation. The level of each index and its corresponding standard eigenvalue are shown in Table 2, Table 3 and Table 4.

Table 2: Table of standard eigenvalues corresponding to the level of indicators

Table 3: Table of standard eigenvalues corresponding to the level of indicators

Table 4: Table of standard eigenvalues corresponding to the level of indicators

Fourth, evaluation of the carrying capacity of the aquatic environment

(A) Evaluation methods

There are many methods to evaluate the carrying capacity of the aquatic environment. There are many methods to evaluate the carrying capacity of the water environment, and this time, the hierarchical analysis method is chosen, which is a statistical method widely used in the determination of the weights of indicators in the multi-indicator evaluation problem. The main steps are as follows:

1. Establishment of the hierarchical structure of the problem

First of all, according to the understanding of the problem and the preliminary analysis, the complex problem is decomposed into the components known as factors according to the specific objectives, guidelines and constraints, and the attributes of these factors are arranged in different levels. Factors at the same level dominate certain factors at the next level, and at the same time it is dominated by factors at the previous level, forming a top-down hierarchy. The simplest recursive hierarchy is divided into 3 levels. The top level generally has only one factor, which is the goal of the system, and is called the goal level; the middle level is the criterion, in which the criteria for measuring whether the goal is achieved are arranged; and the bottom level is the indicator level, which indicates the specific indicators selected, etc., as shown in Figure 2.

Figure 2 Hierarchical analysis structure diagram

Water environment carrying capacity evaluation is a complex decision-making system, according to the basic principles of hierarchical analysis, can be divided into three levels:

(1) Goal layer: the water environment carrying capacity as the goal of the work.

(2) Guideline layer: the factors affecting the carrying capacity of the water environment are categorized into one main aspect and listed as the basic guidelines for evaluation, such as the capacity of the surface water environment, the pollution resistance of the groundwater environment, the capacity of the water environment to renew, and the sustainability of shallow groundwater exploitation.

(3) Indicator layer: according to the evaluation guidelines, the above influencing factors are further subdivided into a number of specific evaluation indicators.

2. Constructing judgment matrix

The relative importance of each element of the same level to each criterion of the previous level is compared two by two, and a positive two-by-two judgment matrix A is constructed:

A=(aij)n-n

In which: aij(i,j=1,2,......n) is the relative importance of the factor Value, using l to 9 scale method to establish the judgment matrix of the same level of each factor, the meaning of its value is shown in Table 5.

Table 5 Hierarchical analysis of the judgment matrix signs and their meanings

3. Determination of weights and consistency test

According to the constructed judgment matrix, the calculation of indicator weights (i.e., solving the maximum eigenvector of the judgment matrix).

(1) Calculate the product of n elements of each row of the matrix:

Earth Science-Water and Cities

(2) Calculate the nth root.

(3) Normalize the vectors: form a matrix of the n vectors obtained from the above nth square root, and normalize the vectors:

Earth Science-Water and Cities

To get: W = (W1,W2,..., Wn),

where: W is the approximation of the eigenvectors obtained, i.e. is the weight of each indicator.

(4) Calculate the eigenvalue Amax of the matrix:

Earth Science-Water and Cities

Eq: [BWτ]i is the ith element of the vector BWτ.

(5) Due to the complexity of objective things and the one-sidedness of subjective understanding, the judgment matrix constructed is not necessarily a consistency matrix, but when the deviation from consistency is too large, it will lead to some self-contradictory problems. Therefore, after obtaining the results, the stochastic consistency test is also needed, and the test formula is:

CI = (λmax-n)/(n-1);

CR=CI/RI

Where: CI is the consistency index;

Amax is the maximum characteristic root;

n is the matrix order dispersion;

RI is the average stochastic consistency index;

CR is the stochastic consistency ratio, only when CR < 0.10, the judgment matrix has satisfactory consistency, and the obtained weights are considered reasonable.

(II) Establishment of evaluation index system and determination of weights

1. Evaluation index system

Based on the evaluation of water environment carrying capacity influencing factors, it is determined that the strength of environmental capacity of surface water, the pollutant-resistant nature of groundwater environment, the renewable capacity of groundwater and the function of groundwater are the first-level factors; the four first-level factors contain a number of second-level factors, and the system of evaluation factors is shown in Figure 3.

The evaluation factor system is shown in Figure 3.

Figure 3: Structure of water environment carrying capacity evaluation factor system

2. Quantification of evaluation factors

For the factors that can be directly measured to obtain specific values, the values are directly selected for fuzzy judgment to give the rating quota, and for the factors that can not be directly obtained by the specific values, the expert scoring method is used for the rating (10-point system). The strength of surface water environmental capacity directly uses the measured value, and the groundwater environmental pollution resistance, groundwater renewability and groundwater function adopts the expert scoring method.

3. Weight Determination

AHP model is used to calculate the weights of the indicators according to the constructed judgment matrix (i.e., the maximum eigenvectors of the judgment matrix are solved, Table 6).

Table 6 Evaluation factors of water environment carrying capacity and their weights

Continued

Judgment matrix consistency ratio: 0.0056.

(C) Carrying capacity zoning criteria

Using Arc GIS platform, four vector maps, namely, surface water environment capacity strength, groundwater environment pollution resistance, groundwater renewability, and groundwater function, are converted into raster maps, borrowing the Arc GIS platform. into a raster map, borrowed the raster function, and adopted

Water environment carrying capacity = Rs-WS + RC-WC + RR-WR + RF-WF

Where: R is the rank characteristic value (measured value);

W is the weight.

The results of the comprehensive score will be divided into strong water environment carrying capacity, strong water environment carrying capacity, general water environment carrying capacity, weak water environment carrying capacity and weak water environment carrying capacity according to the criteria shown in Table 7.

Table 7: Evaluation of water environment carrying capacity zoning standards

V. Application of water environment carrying capacity evaluation results

The purpose of the study of water environment carrying capacity of urban agglomerations is to provide technical support for the overall layout and implementation of the territorial planning of the urban agglomerations, and to provide scientific evidence for the urban agglomerations to make economic and social structural adjustment programs based on the conditions of the carrying capacity of the water environment.

(A) strong water environment carrying capacity area

The strongest water environment carrying capacity area, but if located in the mountainous areas, terrain slope, is not conducive to industrial layout and human habitation, and the area of ecological sensitivity, suitable for soil and water conservation area; if the slope is small, it is suitable for the layout of the strong pollution of the industry, the surface water environment has a large capacity, and the groundwater environment is relatively anti-pollution ability. Stronger.

(2) Water Environment Carrying Capacity Zone

Mountainous areas and small slopes, it is suitable for the layout of strong pollution of industrial and mining industries, the surface water environment has a large capacity, the groundwater environment is slightly higher anti-pollution, groundwater renewal capacity; low mountainous areas because of its water environment has a large capacity, the groundwater environment is relatively good anti-pollution performance, the water resources supply capacity, suitable for the layout of industrial and mining enterprises, but due to the general development of geologic hazards relative development conditions, it is appropriate to layout of industrial and mining enterprises. General geologic disaster relative development status, should be prevented.

(C) water environment carrying capacity of the general area

Generally located in the mountainous area, these areas are suitable for the layout of small pollution industrial enterprises, suitable for human habitation, suitable for agricultural irrigation, the surface water environment has a large capacity, the groundwater environment is slightly higher anti-pollution performance, slightly more difficult to contaminate, the groundwater updating capacity in general.

(4) Weak Water Environment Carrying Capacity Area

The plain area is generally suitable for human habitation and agricultural irrigation, because of its small water environment capacity, slightly lower pollution-resistant performance of groundwater environment, slightly easier to pollute, and general renewability of groundwater.

(E) weak carrying capacity of the water environment

The river valley area should control the scale of population development, limit the layout of industry, suitable for irrigated agriculture, and control the use of pesticides, prevention and control of surface pollution of the water environment to enhance.