Federal Contaminated Site Risk Assessment In Canada Part I: Guidence on Human Health Preliminary Quantitive Risk Assessment (PQRA)

2.4 Problem Formulation

It is essential that a brief but thorough problem formulation be provided. Specifically, report subsections will likely include but not necessarily be limited to:

• screening and identification of contaminants of potential concern (COPCs);
• identification and description of potential receptors;
• identification of operable exposure pathways;
• a brief summary paragraph describing the COPCs, critical receptor(s), and exposure pathways; and
• presentation of the Problem Formulation Checklist (see section 2.4.4, Table 2).

2.4.1 Screening and Identification of Contaminants of Potential Concern

For soil-borne contaminants, COPCs should be identified (screened) employing CCME Environmental Quality Guidelines for protection of human health, where possible. Where CCME human health guidelines are not available, human health-based provincial guidelines may be used, provided those for non-carcinogens are derived on the basis of 20% of the toxicological reference value (TRV). The CCME applies 20% of the tolerable daily intake (TDI; also termed a reference dose (RfD) or acceptable daily intake (ADI)) when setting guidelines for soil and other media. Where no Canadian jurisdiction has established a human health-based environmental quality guideline for a particular contaminant, the U.S. Environmental Protection Agency's preliminary remediation goals (PRGs) (U.S. Environmental Protection Agency [U.S. EPA], 2002) may be used, again adjusting those for non-carcinogens to reflect 20% of the U.S. EPA RfD.

In the event that a contaminant has no corresponding health-based soil quality guideline, the contaminant should be included as a COPC for further risk assessment, unless the measured concentrations are consistent with natural or background concentrations (see below).

For contaminants in groundwater, the Health Canada Guidelines for Canadian Drinking Water Quality (http://www.hc-sc.gc.ca/hecs-sesc/water/index.htm) should be used for screening of COPCs if the groundwater is potable. If it is non-potable, available provincial guidelines should be reviewed and employed as appropriate in the professional judgement of the risk assessor.

Before a site is considered contaminated, on-site concentrations of contaminants, particularly natural elements, should also be compared to data from local or regional surveys of background soil quality and groundwater quality (and surface water quality if relevant) in uncontaminated areas, if data are available. If it is found that concentrations of contaminants of potential concern at the site are representative of background levels, then the site may not be contaminated despite the fact that generic guidelines are exceeded. A further discussion of background levels is presented in Appendix A.

Various sampling procedures will have been applied to the site to collect samples of contaminated environmental media that could include soil, indoor dust, drinking water, indoor or ambient air, vegetation and/or other biota. A variety of methods could have been used to select sampling locations, including random, systematic (grid), or targeted (at known or suspected "hot spots" or in locations of frequent/continuous receptor occupation), etc. The soil sampling conducted at contaminated sites during typical environmental site assessments (ESAs) is usually targeted at zones of known or suspected contamination. As a result, the sampling is not random, and areas with elevated concentrations will typically be subject to more frequent sampling than are areas without contamination. Therefore, the maximum concentration determined from such targeted sampling will in all probability exceed the true average, on-site soil concentration of contaminants.

Depending on the quantity and quality of available data for a given site, and on professional judgment, a variety of possible statistics may be used to represent the on-site contaminant concentration in appropriate media (air, water, soil, etc.) for screening purposes. The statistic could be the maximum concentration, the arithmetic average, the 95% upper confidence limit (UCL) of the mean, or the 90^th or 95^th percentile value of the available data, etc.

In most cases, it is anticipated that the maximum measured concentration of a contaminant will be used to characterize its concentration at the site under investigation. However, where in the opinion of the risk assessor the data are sufficiently rigorous, the arithmetic average concentration should be used for screening purposes. In any case, a brief justification for the statistic selected should be provided (for example, only 20 samples were collected and, therefore, the maximum concentration was most appropriate).