(1) Geographical location
The study area belongs to the semi-arid continental monsoon climate zone in the north temperate zone, with hot summer and cold winter. The annual and interannual variation of atmospheric precipitation is very different. The average annual rainfall is 549.4 mm, and 70% ~ 80% of the annual rainfall is concentrated in June ~ August. The evaporation in this area is strong, and the evaporation is 900 ~ 1200mm.
Shijiazhuang section of Hutuo River has a wide valley and developed floodplain. Before 1959, the Hutuo River had water all the year round. After the Huangbizhuang Reservoir is impounded, the river runoff is regulated by the reservoir. Generally, when there is water in flood season, the riverbed will generally dry up, and it has dried up almost every year in recent years.
(2) Geological conditions
Generally speaking, the present situation of land use in the evaluation area is relatively simple, with cultivated land and green belt as the main parts. Although the geological and hydrogeological conditions are complex, they are mainly composed of clay, silt, sand and other lithology, and its complexity is mainly reflected in the large change of lithology and complex structure in a small area. Its simplicity mainly lies in the relatively simple sedimentary environment type of stratigraphic lithology. Except that the northwest boundary of the evaluation area is mountainous alluvial-diluvial deposit type and the stratigraphic deposit in the evaluation area is Hutuo River deposit, the research degree of stratigraphic and hydrogeological conditions is high.
Vadose zone stratum: The vadose zone stratum above the groundwater level is mainly composed of sand, clayey silt, loam and silt, and the cumulative maximum thickness of low permeability clayey soil layer is generally less than 20m (Figure 10-5).
Figure 10-5 Geological Profile of Evaluation Area
Groundwater aquifer: It is basically composed of fine sand, medium sand and gravel sand layers, with well-developed cross bedding and rich water-bearing conditions. It is one of the main water supply sources in Shijiazhuang (Figure 10-5).
Most of the evaluation area is located in the groundwater drop funnel area of Shijiazhuang City (Figure 10-6). After the pollutants enter the groundwater aquifer, they will spread to the center of the funnel and pollute the groundwater.
Figure 10-6 Groundwater Isogram of Shijiazhuang City
(3) the pollution protection ability of the study area
According to the research results of clay's protective performance against pollutants (Table 6-2), combined with the specific conditions of the evaluation area, the protective performance of vadose zone lithology in the evaluation area is evaluated and divided, and the division results are shown in Figure 10-7. As can be seen from the figure, the protection ability can be dIVided into four areas: area with good protection ability (i), area with good protection ability (II), area with general protection ability (III) and area with poor protection ability (iv). The details of each grade area are as follows:
1. Good protection ability
This is the area with the strongest pollution protection ability. The vadose zone is mainly composed of clay and loam, and its cumulative thickness is generally greater than 18m, which is mainly distributed in the front of the piedmont alluvial fan in the southwest of the river and the bend of the eastern river. The distribution range is shown in figure 10-7.
2. Good protection ability
In the area with the second best pollution protection ability, the formation lithology of vadose zone is mainly composed of clay, loam and clayey silt, with the cumulative thickness of clay and loam ranging from 9.5- 18m and clayey silt ranging from 16-30m. It is distributed on both banks far from the river (Figure 10-7).
3. The protection ability is average
It belongs to an area with general pollution protection ability. The vadose zone is composed of clay, loam and clayey silt. The cumulative thickness of clay and loam ranges from 5-5 to 9.5m, and that of clayey silt ranges from 8.4-16m. Distributed on both banks near the river (Figure 10-7).
4. Poor protection ability
It is an area with poor pollution protection ability. The vadose zone is clay and loam, with the cumulative thickness less than 5m, and the cumulative thickness of clayey silt is less than 8.4m, which is mainly distributed in the river zone (Figure 10-7).
Second, the hazard identification
In order to identify the hazards of geological environment risks in garbage disposal sites, it is necessary to analyze possible accidents one by one, such as landfill gas escaping into strata, soil and groundwater, polluting air, surface water and groundwater, spreading diseases and landfill leachate leaking into strata, soil and groundwater, and polluting soil, groundwater and surface water. However, because the risk of groundwater pollution is only evaluated for the disposal site here, it is obvious that there is only one geological environmental accident in which garbage pollutes groundwater.
Third, hazard assessment.
Hazard assessment is to analyze, calculate or evaluate the probability of groundwater pollution accidents in garbage disposal sites. As mentioned in the first section of this chapter, the risk assessment of groundwater pollution in landfill can actually be divided into two aspects: "the risk of landfill leachate leakage" and "the risk that leachate may enter groundwater through the stratum after leakage". At the same time, there are regular landfill sites (such as Chahe garbage disposal site) built according to standards and a considerable number of garbage dumps without formal design and construction in the evaluation area. The risk assessment of the former includes "the risk of leachate leakage" and "the risk of leachate leakage polluting groundwater", while the latter can only assess "the risk of leachate leakage polluting groundwater".
(1) Probability Assessment of Leakage Risk of Landfill Leachate
According to the Construction Standard of Sanitary Landfill Treatment Project of Urban Domestic Waste (theoretical method of standard formulation and natural resource value evaluation [57]) by the Ministry of Construction, the maximum economic loss caused by groundwater pollution in a garbage disposal site is 23.27 million yuan.
Verb (abbreviation of verb) risk assessment
(A) the risk of dumping garbage at will
As explained above, risk is equal to risk probability and loss caused by accident. If garbage is randomly stacked and disposed in Area I, Area II, Area III and Area IV of the evaluation area, the economic loss indicators of risk are respectively:
Area I: the economic loss caused by groundwater pollution is 0;
Area II: The economic loss caused by groundwater pollution is: 0.37×2327 = 86 1 10,000 yuan;
Zone III: The economic loss caused by groundwater pollution is: 0.63× 2327 =14.66 million yuan;
Area IV: The economic loss caused by groundwater pollution is:1× 2327 = 23.27 million yuan.
After groundwater is polluted, pollutants may enter the human food chain through drinking by people and animals or irrigating farmland with polluted groundwater, which will endanger human health. It is also possible to irrigate plants with polluted groundwater, which will affect plants or biological species and even biodiversity. These consequences are serious and difficult to assess.
The above assessment is only the direct consequence of groundwater pollution. In fact, the construction of garbage disposal sites in the evaluation area will also lead to the pollution of air, soil and surface water and worsen the ecological environment.
(b) Risks arising from the construction of sanitary landfills
In areas I, II, III and IV of the evaluation area, if it is used to construct sanitary landfill to treat garbage, the risk economic loss indicators are respectively:
Area I: the economic loss caused by groundwater pollution is 0;
Area II: The economic loss caused by groundwater pollution is: 0.039× 0.37× 2327 = 336,000 yuan;
Zone III: The economic loss caused by groundwater pollution is: 0.039× 0.63× 2327 = 572,000 yuan;
Zone IV: The economic loss caused by groundwater pollution is 0.039×/kloc-0 /× 2327 = 908,000 yuan.
Obviously, the risk of polluting groundwater by sanitary landfill is much lower than that by stacking garbage at will.
Evaluation areas II, III and IV belong to groundwater protection areas, flood discharge areas, farmers' concentrated areas, agricultural planting areas and ecological protection areas. From the point of view of groundwater pollution, the construction of garbage disposal sites in these areas (whether sanitary landfill or random stacking) will cause huge economic losses, especially in the river zone (Ⅳ area); It will also lead to the pollution of air, soil and surface water and worsen the ecological environment. The direct beneficiaries of this risk consequence are local residents, who are unacceptable.
In addition, the relevant laws and regulations do not allow the construction of garbage disposal sites in the second, third and fourth districts, so the institutions or enterprises that build garbage dumps cannot bear legal risks.
Countermeasures of reducing risk by intransitive verbs
At present, there are 1 landfill and 13 dump in the evaluation area. The geological environment risk of landfill is relatively small, so the completed landfill should be comprehensively monitored according to standards or specifications, and once problems are found, corresponding measures should be taken to remedy them in time. Garbage dumps piled at will should be moved to suitable places according to relevant specifications and technical standards, and sanitary landfill disposal should be carried out according to relevant specifications and standards.