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1 Introduction

The purpose of this guideline is to provide a nationally consistent system for ranking and prioritising contaminated sites for further investigation. The intention is that sites are consistently ranked, regardless of location and who is carrying out the assessment.

The previous site assessment system was the Rapid Hazard Assessment System (RHAS) (Ministry for the Environment, 1993), based on the Canadian Classification System for Contaminated Sites (CCME, 1992). This revised Risk Screening System (RSS) is a simplification of the original, which proved to be unnecessarily complicated for rapid screening. However, the RSS is not intended to completely replace the original RHAS. There may still be situations where a more rigorous hazard assessment is desirable, in which case the RHAS could be suitable.

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 considered to be the most important in determining the degree to which the hazard exists, or in defining whether a pathway to a receptor is complete.

The system operates by assigning values to these parameters to reflect estimates of such things as the toxicity and quantity of the hazard, the degree to which there are barriers to a pathway being complete, and the sensitivity or vulnerability of the receptor. In applying the RSS the user works through a series of analyses. Each analysis is specific to a particular contaminant that may exist in the ground from some period in the site's history, whether from the current use or from one or more past uses. The worst case is then reported as the current site risk.

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-specific parameters are bypassed.

The usual operation is the standard mode, in which a ranking of high, medium or low risk is returned. This allows comparison between similar or dissimilar sites, as defined on the Hazardous Activities and Industries List (HAIL) (Ministry for the Environment, 2004). The HAIL defines industries or activities that have a greater probability of causing land contamination because of the hazardous substances associated with the activity or industry.

The standard mode is not intended to - and in fact cannot - provide fine distinctions between sites of a similar type or risk, hence the name 'screening system'. Many sites that appear to be different will in fact fall into the same risk category. These sites should be considered to have an equal risk, 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 RHAS can also be used to differentiate between such sites. The standard mode is the only way to compare sites in different HAIL categories, or sites of the same HAIL category but with what appears to be a different (typically a different extent) hazard.

The special case mode can be used where distinctions are sought between sites in which the nature of the hazards is similar, such as similar-sized sites with the same contaminants. Typically these will be sites with the same HAIL category, but may also be in different HAIL categories provided the hazardous substances are common and the perceived quantity or extent of contamination is similar (eg, a small timber treatment site and a large animal dip site). This is because the special case mode bypasses the hazard-specific parameters of toxicity, quantity/extent and mobility (in effect holding them constant) in order to gauge the differences caused by site-specific factors relating to the likelihood of the contaminant coming into contact with, or being transported to, a receptor. The resultant special case score cannot and must not be compared with that obtained using the full calculation, but may enable prioritisation within a group of like sites.

This document provides the conceptual background to the RSS development and serves as a guide for its use. The screening is performed on an RSS template, either on paper or in a Microsoft Excel spreadsheet. The Excel spreadsheet is included on the attached CD-ROM, and is also available from the Ministry for the Environment's web site,