Federal Contaminated Site Risk Assessment In Canada Part I: Guidence on Human Health Preliminary Quantitive Risk Assessment (PQRA)
1.3 Preliminary Quantitative Risk Assessment versus More Complex Site-Specific Risk Assessment
Preliminary quantitative risk assessments (PQRAs) and the more complex site-specific risk assessment (SSRA) are not independent but represent opposite ends of a continuum of complexity in risk assessment. The general characteristics of SSRA versus PQRA are outlined in Table 1. PQRA is not intended as a substitute for SSRA. A complex SSRA may be particularly appropriate in those situations where there is a large degree of variability across the site in terms of land use, contaminant types and concentrations, soil quality and other site characteristics, and receptors and their interaction with the site.
The increased detail and complexity of SSRA will generally reduce the degree of uncertainty associated with PQRA, resulting in the more accurate, precise, realistic, reliable, and defensible quantification of risks, as well as serving as a critical tool in the identification of complex remedial and risk management alternatives. When PQRA determines that, for maximal exposures, potentially unacceptable human health risks may exist, it may be appropriate to undertake a more detailed and complex SSRA prior to defining remedial or risk management options.
Guidance on conducting complex site-specific risk assessments is currently being formulated by Health Canada and will be published when work on it is completed.
Table 1: Specific Characteristics of Preliminary Quantitative Risk Assessments (PQRAs) vs Site-Specific Risk Assessments (SSRAs)
| |
Preliminary Quantitative Risk Assessment (PQRA) |
Tier 2/3 Site-Specific Risk Assessment (SSRA) |
| Environmental Media Sampled |
Generally, soil only; occasionally groundwater, if a concern |
Generally, will include soil, groundwater, vegetation, indoor air, outdoor air (volatiles and/or particulate), indoor dust, other environmental media as required |
| Quantity of Data |
Limited; generally restricted to data collected during ESA 2/3 for confirmation of contamination and very limited delineation of hot spots |
Extensive; Tier 2/3 SSRA generally includes a sampling plan designed to provide reliable and representative quantification of the contaminant(s) in each environmental medium/pathway |
| Statistic Used to Represent COPC Level(s) |
Generally, the maximum measured concentration |
Generally, the arithmetic average or the upper 95% confidence limit on the arithmetic average. |
| Use of Modelling |
Extensive, since COPC concentrations in all media but soil (and perhaps groundwater) are usually estimated with the use of models. |
Limited; generally direct data will be collected for all environmental media that are expected to be contaminated and/or contribute significantly to exposure. |
| Characterization of Site |
Limited to measurement of COPCs in soil (and perhaps groundwater) |
Extensive; physical (soil grain size, depth to groundwater, etc.) and chemical (pH, organic carbon content, buffering capacity, etc.) characterization of on-site soils and groundwater; precise measurement of distance from on-site structures (house, etc.) to contamination sources (hot spots); other characteristics as required |
| Characterization of Receptors |
Limited to standard, conservative assumptions available from published sources |
May be site-specific, particularly with respect to the nature and extent of land use as well as time-activity patterns (when and how the land is used by receptors); quantification of receptor characteristics tends toward greater precision and less conservatism |
| Risk Characterization |
For non-carcinogens, based on 20% of the tolerable daily intake since exposure from background sources (unrelated to the site) is not quantified
For carcinogens, based on 100% of the acceptable risk value of 1 x 10-5 since the incremental lifetime cancer risk (ILCR) is independent of background sources |
Based on 100% of the tolerable daily intake since exposure from background sources is quantified
For carcinogens, based on 100% of the acceptable risk value of 1 x 10-5 since the ILCR is independent of background sources |
1.4 Petroleum Hydrocarbons and Radiological Contaminants
The guidance presented below focuses exclusively on chemical contaminants other than petroleum hydrocarbon compounds (PHCs) or radiological contaminants. For PHCs, a Canada-Wide Standard (CWS) has been established and published by the Canadian Council of Ministers of the Environment (CCME) (2000, 2001), including spreadsheets to assist in the derivation of modified generic (Tier 2) soil quality guidelines incorporating limited site-specific data. Those methods should be employed where PHCs are encountered.
For sites presenting radiological risks, Health Canada should be consulted for advice on the most appropriate methods and approach to risk assessment for the type of contaminant and site in question.
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