The comprehensive evaluation factors of agricultural geological environment in Zhejiang Province can be selected according to the types of main pollutants in the region and the requirements of evaluation purposes. The participating factors of soil environmental quality assessment mainly include the following indicators:
1) toxic heavy metal elements, nonmetallic elements or compounds, such as mercury, cadmium, lead, arsenic, copper, chromium, nickel, zinc, fluorine and cyanide;
2) Organochlorine, organophosphorus, organosulfur pesticides, detergents, phenols, oils, Escherichia coli and other organic poisons and pathogenic bacteria;
3) pH, total nitrogen, total phosphorus, etc.
When selecting the evaluation factors, it is considered that Hg, Cd, Pb, As, Cu and Cr are stipulated in People's Republic of China (PRC) Soil Environmental Quality Standard (GB15618-1995) only according to the land use function, farming methods (paddy field and dry farming), pH value and cation exchange capacity.
(B) the establishment of evaluation criteria
In the evaluation of soil environmental quality in China, the soil background value is often used as the initial value of the evaluation standard, and the soil background value plus 2 or 3 times standard deviation is used as the evaluation grading standard.
1995 promulgated the People's Republic of China (PRC) Soil Environmental Quality Standard (GB15618-1995). According to the application function and protection goal of soil, the soil environmental quality is divided into three categories and three grades at the same time. The first grade soil is implemented to protect the natural ecology of the region. The second-class soil implements the second-class standard to ensure agricultural production and protect human health; Three standards are implemented for the three types of soil to ensure agricultural and forestry production and normal plant growth. The promulgation and implementation of this standard provides a new basis for the classification system of soil pollution in China.
The assessment standard of AGEIS soil environmental quality is mainly GB15618-1995 (Table 7-2). At the same time, in order to meet the needs of various users and facilitate communication, AGEIS also provides Dutch national soil environmental quality standards (Table 7-3) for users to choose flexibly according to the actual situation.
Table 7-2 Standard value of soil environmental quality in China (GB15618-1995) Unit: mg/kg.
Note: Heavy metals (Cr is mainly trivalent) and As are calculated according to the amount of elements, which are suitable for soils with cation exchange capacity > 5cmol/kg. If ≤5cmol/kg, the standard value is half of the value in the table. BHC is the total amount of four isomers and DDT is the total amount of four derivatives; The soil environmental quality standard of paddy-upland rotation field is As for paddy field and Cr for dry land.
Table 7-3 Dutch National Soil Environmental Quality Standard Unit: mg/kg
Note: ① Mercury and organic mercury; ② Methylmercury. Target value-below this value, the impact on the environment can be ignored; Intervention value-above this value, the function of soil on human body, plants or animals will be seriously or sharply reduced, which may lead to serious pollution incidents and is "potentially dangerous"; Serious pollution.
(c) Selection of evaluation methods
The models that can be used for the investigation and evaluation of agricultural geological environment in Zhejiang Province include index method, mathematical statistics method, fuzzy mathematics method, expert evaluation method, grey system method, genetic algorithm and artificial neural network method. When different models are used to evaluate the same target object, the results are often different, even significantly different, that is, the evaluation results are false. Therefore, it is particularly important to choose an appropriate evaluation model in the evaluation (Hou et al., 2003). Soil environmental quality evaluation can adopt "one-vote veto" method and index method.
The comprehensive evaluation of soil environmental quality draws lessons from the existing environmental pollution evaluation methods, and carries out multi-index comprehensive evaluation on the basis of single factor evaluation results, so as to understand the overall pollution degree of soil. Because the crustal abundance of various elements is obviously different, and the background values of elements in different regions are different, the pollution index method can be used to compare the pollution degree of various elements in different regions.
(1) single element pollution index (p)
Taking the grid As the basic evaluation unit, the content data of Hg, Cd, Pb, As, Cu, Cr, Ni, Zn and other elements in the surface soil samples of 1 /4km2 obtained from regional geochemical survey, and the residue of DDT and BHC in the deep soil samples of 1 /64km2 were analyzed. Compared with the soil environmental quality grading standards of corresponding elements or pesticide residues under different land use patterns (paddy field/dry land), soil pH value and cation exchange capacity, the quality grades of each element and pesticide residue at each measuring point were determined, and then a single-index environmental quality grading map was made in the form of color block diagram and isoline diagram, and the sampling points, areas and percentage of the total area of each index belonging to each environmental quality grade were counted.
The pollution index calculated with reference to geochemical background values can truly reflect the pollution degree of elements. Geochemical background refers to the normal variation range of element content, which should be calculated by using the upper limit of regional geochemical background or geochemical sub-region background threshold, generally expressed by the average value of the region (sub-region) after excluding abnormal data plus 2 times standard deviation.
Pij = cij/SJ (j = 1, 2, …, m, assuming that * * * has m soil samples)
Where: Pij is the pollution index of soil element I at point J; Cij is the concentration of soil element I measured at point j; Si is the evaluation standard of element I, that is, the upper limit of regional (sub-regional) geochemical background threshold.
(2) Comprehensive pollution index (P)
Comprehensive pollution index refers to the organic synthesis of single element pollution index reflecting the degree of soil pollution. The superposition of single factor evaluation results refers to taking the lowest environmental quality grade in each measuring point as its comprehensive environmental quality grade on the basis of the single factor evaluation results of each measuring point and following the principle of "from bad to excellent", and then making a comprehensive environmental quality grade map in the form of color block map and isoline map, and counting the sampling points, areas and percentages of each environmental quality grade to the total area.
There are mainly the following mathematical calculation models.
1) simple weighted average method. That is, the arithmetic average of the single index of N pollution in soil at a certain measuring point, and the formula is
Design and Implementation of Agricultural Geological Environment Geographic Information System in Zhejiang Province
Among them, Pj is the comprehensive pollution index of N element in soil at point J; Pij is the pollution index of soil element I at point J.
2) Nemuro pollution comprehensive index method. This is a comprehensive method considering extreme value, which not only considers the role of a single element, but also highlights the importance of the most polluted element, namely
Design and Implementation of Agricultural Geological Environment Geographic Information System in Zhejiang Province
or
Design and Implementation of Agricultural Geological Environment Geographic Information System in Zhejiang Province
Among them, Pj is the comprehensive pollution index of N element in soil at point J; Ij。 Aver is the average value of N pollution index in soil at point J; Ij。 Max is the maximum pollution index of N element in soil at point J..
3) Weighted average comprehensive index method. The weighted average comprehensive index model is widely used in various evaluations and is a classic comprehensive evaluation model. It is the sum of all the indicators that reflect the evaluation results. The outstanding advantages of this model are: considering the difference of different elements' contribution to pollution, its evaluation model is simple and easy to calculate. However, the evaluation results are simple accumulation, which may cover up the jumping characteristics of some factors, thus making the evaluation results biased. In addition, there is no particularly effective method to determine the weight of evaluation factors, which is greatly influenced by human factors. Mathematical expression of the model:
Pj =∑WiIij
Among them, Pj is the comprehensive pollution index of N element in soil at point J; Wi is the weight value of element I (0 < wi < 1, and the total weight of n elements is equal to1); Iij is the pollution index of soil element I at point J.
The "one-vote veto" method is relatively simple. As long as the numerical value of the analysis project is compared with the corresponding standard value, as long as one item exceeds the standard, the evaluation unit will be rated as unqualified. Of course, in the specific evaluation process, the model base of the above single factor evaluation and multi-index comprehensive evaluation can be established. In practical application, users can choose different evaluation models according to their own needs.
(d) Establish an index system.
For the single-element evaluation index, the national soil environmental quality standard value can be directly adopted. Using its index value, the pollution grade map of a single element is generated.
Multi-index comprehensive evaluation should use the weight determined by analytic hierarchy process and calculate the comprehensive pollution index by combining the pollution index of a single element. Using its index value, a hierarchical coloring map is generated.
Because soil is always affected by water, air and human factors with different characteristics (Wang Xiaodan et al., 2006; Li Ruimin et al., 2005), the contents of various pollutants are different. According to the application function and protection goal of soil and referring to the research results at home and abroad, the soil environmental quality is divided into five grades.
The first level (background area): all pollutants in the soil are within the background level, and the soil is not polluted, which has no adverse effect on the growth of planted agricultural products and does not cause pollutant accumulation. The soil environment of green food agricultural base, export-oriented agricultural base and drinking water protection area should be within this grade range. During the period of 1987, the State Environmental Protection Administration organized the "National Soil Background Value Survey". Because the soil background value investigation avoided the polluted samples when sampling, the abnormal values were eliminated by statistical method in the determination results, and the soil background samples basically represented the unpolluted soil.
The second level (safety zone): all kinds of pollutants in the soil have accumulated to a certain extent, and after planting crops, there is no adverse effect on their growth. Individual elements accumulate slightly in some crops, and their pollution residues will not exceed the standard. The content of pollutants is roughly twice or less than the soil background value. Suitable for general field agricultural production, there is more farmland soil at this level.
Grade III (warning zone): The soil is obviously polluted, and the content and concentration of pollutants have doubled or even doubled. Most crops will not be harmed by growth, and pollutants will exceed the standard, but it is easy to cause obvious pollutant accumulation or even exceed the standard for highly enriched plants.
Grade 4 (moderately polluted area): land that exceeds the warning zone of Grade 3 and is not suitable for agricultural production.
Grade 5 (heavily polluted area): The pollution of soil crops is quite serious, which should be highly valued by relevant departments.
The above five levels are continuous, and the transition from one level to the adjacent level is not absolute.