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Metode til risikovurdering af gasproducerende lossepladser
During the last ten years in Denmark focus has been put on the risk landfill gas
constitutes to houses and other man made constructions situated on, or nearby, landfills.
Experiences have shown that it is very difficult to make a qualified risk assessment
concerning gas explosion due to leakage of landfill gas to the surroundings. In Denmark
risk assessments is an important and highly integrated part of dealing with soil and
ground water contamination. A procedure for risk assessment has been developed for heavy
metals, oil, chlorinated solvents etc. The risk related to the traditional soil and
groundwater contamination is frequently caused due to long-term exposure. Therefore, risk
assessment is often based on average conditions. The risk related to landfill gas is acute
and, therefore the risk has to be estimated from worst-case situations and not average
conditions.
Risk assessment concerning landfill gas differs from risk assessments made in relation
to soil and ground water contamination. Gas migration and gas escape from the ground into
houses is governed by mechanisms not yet known in detail. Therefore, it is very difficult
to give a description of what can be considered as worst-case.
Among authorities and consulting engineers there has been a need and a desire for a
concept for risk assessment in relation to landfill gas. Therefore, in 1995 the Danish
Environmental Protection Agency (Danish EPA) decided to develop a concept for risk
assessment of landfill gas. Preliminary studies were made and on this background the
Danish EPA under "Teknologiprogrammet" (The Technology Program) started the
development of a concept for risk assessment of landfill gas. Primarily the purpose is to
develop a tool for risk assessment in relation to gas explosions in buildings, houses
nearby, or at closed down landfills. This project was started in June 1999 and completed
with this report.
Data collection
In the beginning of the study, information was collected concerning Danish and foreign
landfill gas explosions and experience of gas migration from landfills in Denmark. All
Danish counties including the capital of Copenhagen were asked for information in relation
to experiences and strategies with landfill gas and furthermore about accidents as a
consequence of landfill gas. The data collection showed that there is only information
about a few accidents caused by landfill gas explosions, and that information about each
accident is limited.
Information from 70 international corporate business partners and environmental
authorities showed that accidents still happen, especially in connection with current on
going landfills. With respect to the amount of information on the accidents, the data
showed the same limitations as in Denmark.
Based on the collected information, the accidents are divided into major and minor
accidents. Major accidents include accidents such as explosions where people are
permanently injured or, where there is significant material damage. Minor accidents
normally take place in connection with maintenance for examples of landfill installations.
Minor accidents are those at which people are not permanently injured. The collected data
have not clarified the causes for the accidents.
The data collected showed that there is relatively little experience with risk
assessments in relation to landfill gas-induced accidents. Information has not been
received either from national or international corporate business partners and
environmental authorities about concepts for risk assessments.
Basic knowledge concerning landfill gas
In the available literature the knowledge about production and migration of landfill
gas is dispersed. Therefore, we have summarised the basic knowledge relevant to risk
assessment. This report gives a basic introduction to factors of importance to risk
assessment with respect to landfill gas. In appendix 1 a detailed description of the
different factors is given. The physical-chemical characteristics of landfill gas and
conditions concerning gas production and gas migration are described.
We have not come across information in the available literature concerning mechanisms
governing the escape of landfill gas into houses. Migration of radon into houses has been
fully investigated in connection with radon research in Denmark. We have used the results
from this research in our project to evaluate the escape of landfill gas into houses.
Concept for risk assessment
As mentioned earlier, it is not possible to employ the risk assessment methods normally
used for soil and ground water contamination for landfill gas. The fundamental idea for
this project, therefore, has been to use a barrier diagram method as approach for the risk
assessment. Instead of employing the concept for risk assessment normally used in the
chemical industry, off-shore and larger traffic installation, a barrier diagram method has
been used. It visualises in a well-arranged way even very complicated risk assessments.
It turns out that there are several identical factors that cause a gas explosion when
going through the conditions that influence gas production, gas migration and gas escape
from the ground. This is all described in the concept of risk assessment where you
systematically go through and analyse the sequence of events, evaluating each event and
those parameters (barriers) which can stop the event before the accident (gas explosion)
happens.
Barriers able to reduce, or prevent, the sequences of events are described, and for
each barrier points are suggested. Barriers can be physical arrangements for example
remedial actions, but can also be reduction of gas escape from the ground into the houses;
reduction of gas concentration in the house, or a combination of the two types.
Finally, the risk the event causes, has to be evaluated. It is recommended to use
classes of consequences in the evaluation of the safety level for a specific landfill. The
recommended requirement for the safety level has to be met for each building/house
assessed.
The barrier diagram method and the point score system has been given, along with the
safety level is describes thoroughly in appendix 3. The method has been verified, by
applying it to a number of landfills.
Investigations
In order to go through the risk assessment, it is necessary to collect the appropriate
information to ensure that it is possible to evaluate whether or not the landfill gas
poses a risk to humans or the environment.
Like for other soil and ground water investigations, the investigation of a landfill is
split into 3 phases. In the first phase information to be used in the evaluation of the
gas production and gas transport from the landfill has to be collected. In the second
phase, a preliminary investigation at the site has to be conducted to find out whether or
not gas production and gas migration are taking place.
Finally, the necessary supplementary investigations are carried out to evaluate the gas
migration and whether or not gas escapes from the ground to the houses/buildings on, or
nearby, the landfill. A detailed description of each element of the investigation is given
in appendix 2.
Conclusions
A concept for risk assessment of landfill gas in relation to both existing and closed
down landfills has been designed. The concept makes it possible for the authorities to
conduct landfill gas risk assessments on a uniform basis and thereby to function as a tool
for selecting where to aim efforts for investigations and remediation.
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