This guideline describes the background to and use of the Risk Screening System (RSS). The system aims to provide a nationally consistent means of ranking sites that are, or are suspected of being, contaminated, using readily available information, regardless of location and who is carrying out the assessment. The purpose of ranking a site is usually so that it may be prioritised for further investigation.
The RSS is a simplified version of the Ministry for the Environment's Rapid Hazard Assessment System, published in 1993. The 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 assigned values to reflect the degree to which the hazard exists or a pathway to a receptor is complete, and the sensitivity of the receptor. The hazard parameters include toxicity and quantity of a hazardous substance, and the pathway is defined by parameters such as depth to the hazard or distance to a receptor, as estimates of the completeness of the pathway.
Three pathways are independently analysed: surface water exposure, groundwater exposure and direct contact with soil. A site's risk ranking - reported as low, medium or high - is the pathway giving the worst case. In applying the RSS the user works through a series of analyses for each site, evaluating the risk posed by different substances that could exist in the environment at the time of the evaluation, either from historical activities or the current site use.
To enable rapid analyses the RSS is performed on the RSS template, either by hand or in a spreadsheet. The electronic version has the advantages of built-in guidance and error checking. The electronic RSS template is available on the attached CD-ROM or on the Ministry for the Environment's website, www.mfe.govt.nz.
The system may be operated in two modes:
- the standard mode, in which values must be assigned to all parameters
- the special case mode, a reduced version in which the hazard parameters may be bypassed.
The usual operation is the standard mode, which allows comparison and prioritisation between similar or dissimilar sites as defined on the Hazardous Activities and Industries List (HAIL). The standard mode is not intended to, and indeed cannot, provide fine distinctions between sites of similar type or risk, which is why only a coarse low, medium or high ranking is given.
The special case mode can be used where distinctions are sought between sites in which the nature of the hazard is similar. Typically this will involve sites with the same HAIL category, but may also involve different HAIL categories provided the hazardous substances are common and the perceived quantity is similar. The resultant special case score cannot be compared with the standard mode ranking, but may enable prioritisation within a group of like sites.