How to Use Risk Assessment to Design a Landfill

Landfill design has historically been based on a selection of design features from a range of alternative methods and measures to achieve a set of target objectives.



The primary requirement these days is containment. Containment has been set as an objective which can be achieved either by using predefined and accepted prescribed methods, or the designer may seek to achieve same result by one of a range of techniques according to availability of materials, and cost factors.



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After a number of research studies were completed in very well controlled circumstances, by contractors observing good practice and closely monitored for their performance, it has been shown that large-scale engineering projects, say using HDPE plastic membranes or natural clay liners, still allow the passage of some flow. They leak!



It now has to be accepted that total perfection is rarely if ever achievable. It would furthermore be dangerous to assume that perfect construction without a single defect causing a small hole anywhere can be routinely achieved.



As a result designers and regulators have understood and come to accept the fact that all liners will allow some leakage. At another ‘materials technology’ level it is also a fact that even very low permeability materials like HDPE plastic membranes will pass a tiny flow through the material both water and gas, even when perfect.



Landfills are large, so a tiny and almost un-measurable flow through a small area becomes measureable when multiplied up by the huge areas of a modern landfill.



A probability distribution can be applied to each of the factors governing the rate of leakage for a given liner design, such that its probable leakage rate and consequent impact can be predicted by modelling.



The acceptability of a landfill design in the environment in which it will be placed should therefore be assessed by a process which quantifies the probable impact, or the probability distribution for an impact, of each source-term (typical concentration of a potentially damaging emission) on each relevant receptor, that is, the persons or features affected by it.



This process is known as risk assessment and should be used in an iterative way to produce an acceptable design.



By being used in an iterative way we simply mean that the designer or impact assessor starts by making a reasonable set of assumptions about a suitable landfill containment system. If at the first risk assessment stage it becomes clear that one or more of the impacts is unacceptable then the designer will repeat the assessment using a varied design having adjusted the design to reduce the excessive impact or impacts identified.



This process is repeated until an acceptable assessment has been achieved.



The process of environmental risk assessment will be governed by background levels set by the local regulatory authority, by statutory limits, and by policy.



These governing factors may, and probably will, change with time. The process by which risk assessment should lead to an acceptable quantified impact, and thereby after any necessary iteration, to an appropriate design, is outlined below.



· Identify all potential impacts.
· For each potential impact identify the source, the receptors or targets, and the pathway.
· Establish the sensitivity of the receptor, and the source term by geotechnical and other investigations.
· Derive a relationship between the landfill and the receptor using an agreed methodology and probability distribution for the source term and each element of the pathway
· Repeat this process to obtain a satisfactory design for the landfill consistent with an acceptable impact.

The process should be repeated iteratively for each potential impact until both an acceptable overall impact and an appropriate design for environmental protection is produced.



The risk assessment process should be applied to any quantifiable impact including, for example, leachate, landfill gas, traffic, noise and even public health risks one of which might be the potentially contaminative effects of seagulls, providing that the effects of mitigation can also be quantified.



Inherent in the use of probability based risk assessment is the possibility that improbable events might occur. In earthquake zones, for example, these will need consideration, while in river valley bottoms flooding might require consideration.




So, consideration should be given to possible responses to extreme events, and whether contingency measures need to be built-in from the start (for example, a leakage detection/interception system) or could be installed subsequently, in sufficient time, should the need arise (for example, a cut-off wall to hold back a leachate flow).




To allow for contingency measures which would be implemented as and when required, can be a practical option offering reduced initial costs, but may require pre-planning and in that there may be costs, such as in the acquisition and reservation of any necessary additional land.



For extensions of existing landfills, the risk assessment approach must take into account the effect of the existing waste and known levels of contamination which may be at present within acceptable limits but could be pushed over the edge by the additional pollutant loading from further landfill extension.



In some instances more stringent controls may be required, whereas in others it may no longer be appropriate to attempt to safeguard a damaged resource.



Steve Evans has written for the Wastersblog since 2006. It recently received nearly 50,000 hits in a month, and continues to grow. If you have any interest in free waste settlement ebook download-ready shouldn't you take a look?