Evaluation of site-specific risk assessment for contaminated lands.

This report provides the results of a two-phased study conducted for the Canada Mortgage and Housing Corporation (CMHC) to examine the practices and variability amongst practitioners of contaminated sites risk assessment in Canada. Phase I consisted of a survey of practitioners in the private and regulatory sectors. The intent of the private sector survey was to characterize the capabilities and experience of private firms engaged in human health risk assessment across Canada. Insight gained from the private sector survey was used to assist in the selection of participants for the round robin study which comprised Phase II. The intent of the regulatory survey was to gain insight on a regional basis with regards to regulator experience and acceptance of human health risk assessment of contaminated sites. As part of Phase II, nine Canadian practitioners from various regions in Canada with varying levels of expertise performed a screening level risk assessment of a hypothetical case study. The purpose of the Phase II study was to assess the degree of variability in risk estimates among participants, and analyze the sources of variability and uncertainty. For Phase II, a hypothetical case study was designed and circulated to nine participants. The hypothetical case study consisted of a residential housing development proposed on former industrial lands and in this respect is reflective of a "brownfields development. The results from Phase I of the study indicate that Canadian risk assessment practitioners have broad expertise in relevant disciplines such as toxicology, biology, environmental engineering, chemistry, and hydrology. Many of the firms supplement their in-house capabilities with external consultants. The results of the regulatory survey indicate that governmental agencies support the use of human health risk assessment in the management of contaminated sites. However, at present the approach is generally based on informal policy and not formally regulated. The results of Phase II of the study indicate that Canadian firms vary considerably in their approach to performing human health risk assessments at contaminated sites. This results in risk estimates that ranged over several orders of magnitude for various chemical exposure pathways. The high variability in risk estimates was due to a combination of factors including differences in the assumed chemical toxicity, receptor characteristics, and differences in model type and assumptions used to predict vapour and dust concentrations in air. Since the magnitude of risk estimates typically varied over several orders of magnitude for a particular chemical exposure pathway, it is likely that in some cases real world business and/or risk management decisions may be erroneously influenced by screening risk assessment practices. Depending on the results of a screening level risk assessment, risk assessors may incorrectly conclude that chemicals present at a site are not a human health concern when in fact the health risks are significant or conclude that chemicals present at a site are a human health concern when in fact the health risks are minimal. Direct application of these findings to a definitive risk assessment is discouraged • because the latter situation is more likely to allow assessors greater time and effort to improve realism in risk estimates. However, it would seem prudent to provide guidance to practitioners and risk manager on how to apply risk assessment assumptions to encourage greater continuity in risk assessment and risk management.