Risk assessment

Numerical soil and water standards are most often used to determine when substance concentrations have been remediated to acceptable levels. At some sites, removing substances is not possible or practical.

In these cases, the substances may be managed on-site to ensure they do not pose a hazard to human or environmental health.

Using the risk-based approach allows a responsible person to estimate risks associated with leaving substances in place. This information is used to design appropriate risk management solutions to eliminate the risks or reduce them to safe levels.

Risk assessment is a tool that can be used to evaluate and predict the severity of existing and potential future effects from substances at a site.

It can only be used within a site-specific context, which means that every risk assessment is unique to the site for which it was prepared. 

Every risk assessment provides the following types of information:

• Documentation of the substances at a site, their location and the extent of any contamination occurring on and off-site
• Estimation of the size and likelihood of risks and hazards to humans and organisms at and near the site
• Documentation and evaluation of the effectiveness of measures proposed to manage contamination in place

The fundamental goal of risk assessment is to estimate levels of risk and hazard to human and environmental health.

The mere presence of a substance or contaminant at a site does not necessarily constitute a risk.

For a risk to exist, the following 3 basic conditions must be met:

• Substances must be present
• These substances must have the potential to cause toxic or other adverse effects (that is, the substances must be hazardous)
• Pathways must exist by which living organisms (receptors) may be exposed to the substances

Risk assessment considers all the exposure pathways for existing and potential future receptors on and off-site. A receptor could be any living organism including microbes, birds, humans, fish, wildlife and more.

An exposure pathway is the potential route a substance may take to come in contact with a receptor while the receptor is present at the site and doing activities, such as:

• Breathing
• Eating
• Drinking
• Working

Substances in environmental media such as soil, air, food, surface water, sediment, or groundwater may come into contact with receptors through different routes.

For example, substances in soil may be ingested, inhaled, or absorbed through the skin, or it may accumulate in food, such as crops or livestock grown on-site.

Risk assessment uses mathematical models to predict the dose (that is, the amount of a substance a receptor receives through any specific exposure pathway). These predicted doses can be added and compared with a dose of the substance that's considered safe.

If the safe dose is not exceeded, it's assumed that there's little risk that exposure to the substance will harm the health of receptors.

The hazard and risk calculated for a site can be expressed in mathematical terms as either a hazard quotient and/or a risk estimate:

• Hazard quotients are calculated for substances that do not cause cancer
  • A hazard quotient is the dose of a substance received from a site (the estimated daily intake) divided by the safe dose for the substance (the reference dose)
• Risk estimates are derived for cancer-causing substances
  • In the Contaminated Sites Regulation (CSR), risks are expressed as the probability of cancer occurring in an individual from exposure to a substance

Hazard and risk estimates calculated for a site can be compared with risk-based standards. If the risk estimates exceed the risk-based standards, the site may need to be managed to reduce estimated impacts to levels equal to, or less than, the standards.

If a site uses risk management solutions to address the hazard and risk, the site is called a risk-managed site. These types of sites require special monitoring and inspection to ensure the remedial solutions used are maintained and effective.

Field studies, such as habitat assessments and biological surveys, as well as laboratory studies provide the foundation for assessing ecological risks.

Information collected from these studies may be used in a detailed ecological risk assessment report to assess the following:

• The potential impacts arising from substances before and after site remediation or redevelopment
• The procedures, including monitoring requirements, designed to reduce significant ecological impacts identified from contaminants remaining in place