Geological environmental risk assessment is an integral part of environmental risk assessment. In 1970s, environmental risk assessment rose in several industrialized countries, especially in the United States. For more than 20 years, as far as the development of environmental risk assessment technology is concerned, it has generally gone through three periods: from the 1970s to the early 1980s, risk assessment was in its infancy, and its connotation was unclear, and only the method of toxicity identification was adopted; In the mid-1980s, the National Academy of Sciences (NAS) proposed that risk assessment consists of four parts: hazard identification, dose-effect relationship assessment, exposure assessment and risk characterization, so the basic framework of risk assessment has been formed. Since 1989, the scientific system of risk assessment has basically taken shape and is in the stage of continuous development and improvement. At present, foreign environmental risk assessment mainly includes human health risk assessment and environmental risk assessment. Relatively speaking, human health risk assessment methods are basically finalized, and environmental risk assessment is in the stage of summary and improvement. There have been sporadic studies on the geo-environmental risk assessment of landfill disposal abroad, but there is no research result on the geo-environmental risk assessment method of urban landfill disposal in China so far.
Second, the hazards and risks of geological environment
(A) the general performance of geological environmental risks
Geological environmental risk can generally be expressed as follows: Geological environmental risk is a concept composed of the possibility P of unfortunate geological environmental events and the damage they will cause after they occur. The possibility of unfortunate geological environment events is called "risk probability" P(x), also called risk degree, and the loss or harm caused by this event is called "risk consequence" D(x), and the risk can be characterized as:
R(x)=P(x) D(x)
Where x is a specific event or accident. Generally speaking, an actual environmental accident is composed of several independent events, so the risk R(x) of this environmental accident is:
Investigation and evaluation method of geological environment impact of municipal solid waste
or
Investigation and evaluation method of geological environment impact of municipal solid waste
Therefore, the task of geological environmental risk assessment is to find out its R(x) (that is, P(x), D(x)).
According to the above two formulas, the probability P(x) of risk accident and the possible harm or loss D(x) are calculated respectively, and then the risk R(x) is calculated, which is a classic basic method. This is the basic idea of geological environment risk assessment.
(2) Geological environment risk
Geological environment risk refers to the possibility of pollution or destruction of geological environment. If it can be expressed quantitatively, it refers to the risk probability P(x) in the above geological environment risk expression. Geological environmental risks include risk assessment. There are many researches on geological environment risk assessment at home and abroad, such as geological disaster risk assessment and groundwater pollution risk assessment. Relatively speaking, the work of geological environmental risk assessment was carried out late, with few results.
Three. Geological environment risk assessment of garbage disposal site
Geological environment risk assessment of garbage disposal site generally refers to the product of the accident probability of polluting geological environment and the loss caused by garbage dump. The geological environment here generally refers to soil and groundwater. In view of the slow movement of landfill leachate in soil, the influence range is generally small, and the losses caused are generally small. Therefore, the "geological environment risk assessment of garbage disposal site" here focuses on the risk assessment of groundwater pollution.
Four. Contents and steps of geological environment risk assessment
The contents and key steps of geological environment risk assessment are as follows:
(1) Hazard identification: judge what will happen: for the generalized geological environment risk identification, the possible accident of a landfill is that landfill gas escapes into stratum, soil and groundwater, polluting air, surface water and groundwater and spreading diseases; Landfill leachate seeps into stratum, soil and groundwater, polluting soil, groundwater and surface water. Hazard identification is to analyze these accidents item by item.
(2) Risk assessment: analyze and calculate the possibility of accidents: that is, analyze, calculate or evaluate the above possible probability of accidents.
(3) Consequence assessment: to assess what consequences will occur once an accident occurs, that is, to conduct qualitative analysis and quantitative assessment on the consequences (economic losses and health losses that may be caused by environmental pollution and disease spread). ) the above possible accidents.
(4) Risk inspection and analysis: point out the risks.
(5) Risk assessment: to judge the acceptability of risks, and make a comprehensive judgment according to the probability of accidents, their harmful consequences and the affordability of people or other recipients, so as to evaluate the acceptability of risks by people or other recipients.
(6) Risk reduction: put forward countermeasures or measures to avoid or reduce risks.
Five, commonly used environmental risk assessment methods
Because of the different nature and types of regional development, regional environmental protection objectives and standards, sensitive environmental protection objectives, and different risk factors, the evaluation and comprehensive evaluation methods of each risk factor are also different. Generally speaking, the current methods of regional environmental risk assessment are qualitative and semi-quantitative, and it is difficult to fully quantify them. Taken together, it can be summarized as follows:
(A) Optimization of probabilistic design scheme
This method is suitable for the comparison of several schemes. List the relative weight of the possible consequences of several schemes one by one, choose a scheme to implement according to the specific requirements and actual situation, and analyze the possible losses caused by the failure of this scheme.
(2) quotient method
Quotient method, also known as ratio method, is the most commonly used method for ecological risk assessment. It requires that a reference concentration index be established for the protected receptor first, and then compared with the estimated environmental concentration. The modified quotient method uses the risk index Hi to represent the risk quantity. When Hi≤ 1, the probability of environmental damage is low; 1 & lt； Hi< 10, the environment may be affected; When Hi≥ 10, the probability of environmental damage is high, and field assessment is needed.
(3) Extrapolation method
Extrapolation is the most commonly used method in health risk assessment. According to epidemiological or animal toxicology research data, the risk of toxic substances to organisms (or human bodies) is extrapolated at environmental level.
(D) Logical analysis method
AHP (Analytic Hierarchy Process), fault tree and other logical analysis methods are applied to regional environmental risk assessment. Through the analysis of accident source items, the risk "relative size" of each risk factor is obtained, that is, the "contribution" to regional comprehensive risk is measured.
(5) Statistical analysis method
Collect relevant historical data, and calculate the probability of similar accidents by statistical analysis, that is, "bring forth the old and bring forth the new", such as the calculation of meteorological conditions and the estimation of disease incidence.
(6) Formula evaluation method
Through the simulation analysis, deduction or experiment of the accident, the empirical formula is obtained, and the possible risk is calculated by using the formula. Through further experiments and observations, the formula is revised step by step. For example, the leakage of toxic gas, using the atmospheric diffusion model under similar conditions; The leakage of pollutants in water adopts water migration and diffusion model; Human health risks can also be calculated by exposure hazard formula.
(7) Fuzzy mathematics method
Regional environmental risk involves complicated causality, which is often difficult to be solved by accurate methods. There is no clear boundary between big and small risks. Fuzzy mathematics can express the intermediate transition of this difference and objectively describe the size of risk, and its research and application are gradually deepening.
(8) Graphic superposition method
The results of single-factor environmental risk assessment are sometimes represented by graphs, especially when the consequences of risk are difficult to be calculated by other methods, such as the leakage and diffusion of toxic and dangerous gases, the concentration contour map is generally drawn. In the comprehensive risk assessment, the distribution maps of various environmental risk factors are reasonably superimposed to obtain the relative risks of different functional areas in the whole study area.
(9) Event Tree Analysis (ETA)
Event tree analysis is an analysis method that starts from the initial cause events, divides them into stages according to the time sequence of accident development, and analyzes the subsequent events step by step. Each step considers two or more possible states of success and failure (possible and impossible), and finally expresses its possible consequences with a horizontal tree diagram. It can qualitatively and quantitatively reflect the dynamic change process of the whole accident and the occurrence probability of various States.
According to the different fault events selected as the primary cause events, the accident discharge top event can be regarded as the primary cause event of the event tree through simple pollution source analysis. ETA can analyze different event chains. The accident emission accident tree analysis determines that all kinds of events that can cause pollutants to be discharged into the environment, because of their different fault causes and different pollutant discharge forms, make the intensity of accident emission different, so they should be regarded as the primary cause of accidents in the source intensity event tree analysis. Using ETA, we can analyze the probability distribution spectrum of accident source intensity, its subsequent events and final results. ETA can also be used to analyze the process of environmental media causing safety risks to recipients after accidental discharge of pollution sources.
(X) Fault Tree Assessment Method
It has been introduced before, so I won't repeat it here.
It is worth noting that there are many environmental risk factors studied in this region, and each risk has its own characteristics. Appropriate methods should be selected for specific risk problems during evaluation.
(1 1) subjective probability and objective probability method
Risk analysis must obtain information about the probability distribution of state variables. There are generally two ways to obtain probability information: one is to make statistical calculation on the basis of a large number of experiments; Secondly, according to the classical definition of probability, the event set is decomposed into basic events and calculated by analytical method. Because the above two estimates are based on objective data. So it is called objective estimation of probability, and the probability obtained in this way is called objective probability.
In practical work, sometimes we can't get enough information to calculate the objective probability, but in risk decision analysis, we must estimate the probability. At this point, the decision-maker or analyst must make a subjective estimate of the probability of the event. This kind of probability, which is not based on a large number of historical data, has not been tested or accurately calculated, but mainly depends on personal subjective judgment, is called subjective probability. Generally speaking, the definition of subjective probability can be described as: according to the individual's subjective judgment on whether an event occurs and the possibility of it. Use a number between 0 and 1 to describe the possibility of an event, that is, subjective probability.
Like the objective probability distribution, the probability distribution of subjective probability has two types: discrete and continuous. For continuous distribution, it is common that the phase of normal distribution is uniform. Besides the analyst's subjective judgment, the subjective probability estimation can also be obtained by using the probability turntable method. The probability turntable is a disk with black and white sectors. There is a rotatable pointer in the center of the disc, which can rotate at will and is located in any sector in the turntable. The areas of sectors with different colors can be adjusted as required, as shown in figure 10- 1.
Figure 10- 1 schematic diagram of probability turntable
Judging from the above evaluation contents and methods, it is more difficult to identify, check and analyze the geological environmental risks of the garbage disposal site, and to evaluate and reduce the risks than to evaluate the hazards and consequences. In other words, the key to the environmental risk assessment of the disposal site lies in the analysis and evaluation of risk hazards and consequences, and in the accurate calculation and evaluation of parameters.
The following mainly describes three examples to illustrate the evaluation of groundwater pollution risk in garbage disposal sites.
In fact, the risk assessment of groundwater pollution in landfill can be divided into two aspects: "the leakage risk of landfill leachate" and "the risk that leachate may enter groundwater through the stratum after leakage". The second and third parts below discuss these two aspects with two examples respectively. The fourth section takes the groundwater pollution risk assessment of garbage disposal sites on both sides of Hutuo River in Shijiazhuang as an example, and comprehensively expounds the groundwater pollution risk assessment of garbage disposal sites.