Where potential contamination has been identified on a site, a risk assessment should be undertaken to determine if the contamination would actually affect human health or the environment. It may only be suitable for the contaminated material to remain on site if it can be demonstrated that human health or the environment will not be affected.
A conceptual site model (CSM) should be developed for all levels of risk assessment. A CSM is a description of potential sources, pathways and receptors at a given site and is often shown using a diagram. As the risk assessment’s level of complexity increases, the CSM’s level of detail should also increase to reflect the knowledge of the site.
This is the simplest form of risk assessment and involves comparing concentrations of substances found in soil or water against pre-determined guidelines. These are generally based on a wide range of assumptions and conditions. If concentrations of substances are below the accepted criteria, then further assessment of risk is not necessary. If substances exceed the criteria, then a simple risk assessment may be required or where a criteria is not available a Detailed Risk Assessment may be required.
A simple modification to published criteria may be required to ensure that the criteria is relevant to a site-specific situation. This will often involve examining the underlying assumptions of the criteria and where appropriate, making simple adjustments to them. Simple risk assessments may also include basic modelling, such as dispersion calculations. If substances fail these adjusted criteria, a detailed risk assessment may be required.
A greater understanding of a site can be achieved by characterising the nature of substances on a site and the pathways where exposure to these substances may occur. This may involve specialised fate and transport modelling, or a detailed toxicity assessment of the specific chemicals. Site-specific criteria may be developed through these methods, however detailed justification of the development and use of any derived criteria must be provided to the EPA when being relied upon in an assessment.
It is generally expected that Tier 1 and Tier 2 assessments should adequately define elements of risk at the majority of Tasmanian sites, and Tier 3 assessments will only be required at more complex sites. Risk assessments involving petroleum hydrocarbons must consider Application of the CRC CARE HSLs - Interim Guidance Note.