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2 Background to the Risk Screening System

The Risk Screening System (RSS) is based on a risk equation made up of the hazard, the exposure pathway and the receptor. The presence of all three components means there is some level of risk, while the absence or near absence of any of the components means there is no or minimal risk.

The hazard and pathway components of the risk equation are in turn defined by a variety of parameters that are considered to be the most important in determining the degree to which the hazard exists or a pathway to a receptor is completed. The equation is:

risk = hazard x pathway x receptor


hazard = toxicity x quantity x mobility

pathway = containment x pathway barrier 1 x pathway barrier 2 x ...

(the likelihood of there being a complete pathway being defined by various barriers in the pathway)


receptor = a single value between 0 and 1 defining the sensitivity or vulnerability of the receptor, whether people or an ecological environment.

A low value assigned to a parameter indicates a limitation to the overall risk (ie, a small hazard, or a large barrier to contact or transport along a pathway, or a low sensitivity receptor), while a high value suggests a high potential for risk. The combination of several high, low or intermediate values then gives a measure of the overall risk. This is a similar, but simplified, conceptual framework to the 1993 Rapid Hazard Assessment System (RHAS). The fundamental assumptions behind the design and use of the RSS are as follows.

The assessment is carried out for the hazard, pathways and receptors existing at the time of the assessment. The hazard (hazardous substances in the environment) at the time of the assessment must include consideration of historical uses and impacts. The risk ranking will be for the time of the assessment.

The three most common exposure pathways are:

  • surface water migration
  • groundwater migration
  • direct contact (including ingestion, dermal contact and inhalation).

Each of these exposure mechanisms has a similar set of parameters to represent, and affect, the three parts of the risk equation:

  • the contaminant source (the hazard)
  • the receptors
  • the transport pathways and exposure mechanisms (the pathway) between the source and receptors.

Only those parameters considered to be most important to the risk have been included. Parameters that require excessive efforts to evaluate relative to their contribution to a site's risk ranking have been excluded.

The ranking system is multiplicative. A low score in any of the risk components (hazard, pathway or receptor) reflects a lack of hazard or pathway and therefore a low risk associated with the site. For example, hydrocarbon-affected groundwater does not present a risk if there is no viable pathway between the groundwater and a receptor (perhaps because the groundwater is not used, is sufficiently deep that inhalation is unlikely and does not seep to the surface).

All parameters have been assigned a maximum input value of 1 (indicating a high contribution to the hazard, or a minimal barrier to contact or transport). Minimum values are set relative to their estimated impact on the overall site score (as outlined below). No parameter can be assigned a zero score (which would imply that either there is no hazard, the pathway is incomplete or there is no receptor), in recognition that all sites on the HAIL will have some risk, even if low.

The parameter input value ranges have been set according to their relative importance (weighting) in contributing to a site's risk. For example, the parameter values for depth (to the contaminant in the direct contact pathway) range from 0.5 to 1, whereas those for toxicity range from 0.2 to 1.

The RSS is not intended to be used like a recipe book by a non-specialist. You will need to have a knowledge of contaminated site issues and hazardous substances. If you have to deal with substances that are not commonly encountered, there are many databases available on the Internet that will allow you to make assessments of relative toxicity or mobility (see Additional Information).

The idea is that the information required to assign parameter values in the RSS should be easily available - through maps, regional council databases, phone calls, site visits, and the like. The RSS should not require detailed site investigation information. In any case, the ranking is too coarse to greatly benefit from such detail, although it may boost the confidence placed on the final ranking. If a more detailed assessment is required, consider using the RHAS (Ministry for the Environment, 1993).

The site ranking is taken as the worst-case risk ranking of the three pathways, because a site is considered a high risk even if only one pathway poses a significant risk. The three exposure pathways are presented separately only for the purposes of the user's convenience, and the overall site ranking will not be diluted or masked by low risk rankings for the other exposure pathways.

The site score prioritises sites into one of three risk categories - high, medium or low. The method does not allow for, and is not intended to produce, fine distinctions between sites, hence the 'screening system' of the title. Sites within a particular risk category are considered equal, and any distinction between these sites must be determined separately using other factors, such as prioritising the investigation of certain types of site use as a matter of policy.

The numerical cut-off between the three categories is based on:

  • a high-risk site having no more than two medium parameters (or in some cases one low) with the remainder high (overall score &#8805 0.4)
  • a low-risk site having no more than three medium parameters, with the remainder being low (overall score &#8804 0.02)
  • a medium risk site falling between the other two (> 0.02 < 0.4).

The special case mode allows finer distinctions to be made between similar sites. This mode holds the hazard parameters constant (by bypassing these parameters, in effect assigning them a value of 1 in the multiplicative system), resulting in fewer parameters being multiplied together and therefore a greater apparent variation between numerical scores. However, scores obtained by using the system in the special case mode cannot be compared with scores obtained from inputting all the parameter values.