Risk Characterisation can be defined as 'the process of estimating the magnitude
and probability of effects' (Environment Canada 1994). The objective of Risk
Characterisation is to collate and summarise the information obtained during the previous
tasks. You are essentially attempting to answer the questions
contaminant? What pathway? What receptor? What exposure? and What effect?'
It combines the results of the Receptor Characterisation,
Toxicity Assessment and Exposure
Assessment to determine the probability that a risk may exist, and if so what
the magnitude of that risk is.
The purpose of Risk
Characterisation is to provide land managers with sufficiently clear information to
allow risk management decisions to be made. If the risks are not adequately defined, it
may be necessary to proceed with iterations of whichever aspects are lacking or proceed to
the next tier of RA.
A variety of methods can be used to estimate and characterise the
risk. Characterisation of risk can be extremely difficult to achieve, particularly at larger
scales of ecological receptors and sites.
|'Natural ecosystem variation can make it very difficult to observe
(detect) stressor-related perturbations.
For example, natural fluctuations in
marine fish populations are often large, with intra- and interannual variability in
population levels covering several orders of magnitude. Furthermore, cyclic events of
various periods (e.g. bird migration, tides) are very important in natural systems and may
mask or delay stressor-related effects.
Predicting the effects of anthropogenic stressors against this background of variation
can be very difficult.' (USEPA 1998).
Risk Characterisation needs to take into account the limitations of the
data collected, and the assumptions and uncertainties inherent in the data and
Having characterised the risk, and anticipating the necessity of making a risk
management decision, it may also be appropriate to consider the performance in terms of
speed and effectiveness of remediation options with regard to the recovery of the affected
It is important to remember that risk characterisation should also take
into account the effects of remedial actions on ecological receptors as the adverse effect
of mitigation may negate the benefits of removing the contaminant.
Tier 1 – Risk characterisation tasks
A Tier 1 Risk Characterisation involves comparing the
on-site contaminant concentrations with the generic guideline values for the various media (e.g. soil,
aquatic) and deciding on the likelihood of an adverse effect being generated.
Essentially several simple questions are asked:
In making this assessment you should also consider the
quality, the limitations of the data that have been collected, and the
appropriateness and assumptions of the guideline values chosen. These criteria are developed to provide very
conservative values with margins of safety incorporated. However, if the biota
present in/near the site were not considered in the derivation of the criteria,
then further investigation may be required.
You will also need to critically assess how well the
problem has been identified or characterised.