Risk assessment can be undertaken at three distinct
levels of detail.
At each tier, the five key tasks:
- Receptor Characterisation,
- Exposure Assessment,
- Toxicity Assessment, and
- Risk Characterisation
are undertaken to
provide information and data in order to make a risk management decision or to decide
whether it is necessary to proceed to the next level of detail.
Broadly, the degree
of detail and quality of the data at each level can be described as:
This tiered approach provides a systematic way of determining what level of
investigation is appropriate for the site of concern, minimising unnecessary investigations, and allowing more efficient use
of resources. It has been selected as the most appropriate model for risk
assessment in New Zealand.
USEPA risk assessment methods do not explicitly provide a tiered approach
but leave the
decision to risk assessors. However, in New
Zealand we believe that few risk assessments require the level of detail described in
some US models.
The Canadian and Australian models do provide for a tiered approach and, to allow for the
range of site sizes and complexity commonly found in New Zealand, a tiered model is
considered the most appropriate method of working through a risk assessment.
The tiered model, however, does not preclude the requirement for iterations
within each tier where necessary.
Iteration is an 'unprescribed re-evaluation of information that may occur at any time
during a risk assessment
done in response to an identified need, new information or
questions raised during an assessment. As such iteration is a normal characteristic of
risk assessments but is not a formal planned step' (USEPA 1998).
An example of an
iteration may be returning to sample uncontaminated land surrounding the site in order to
answer questions about background concentrations of contaminants.
While using a tiered RA model may reduce the need for iteration, it is not intended to
eliminate it entirely, as iteration is an essential method of identifying and obtaining
the information required.
It is important to note that by proceeding from Tier 1 to
Tiers 2 and 3, there is a
decreasing degree of conservatism and an increasing degree of certainty that the values
reached approximate the true values.
Note: Regardless of the level of detail described in each section, you
should choose the level of detail and degree of emphasis at each level of RA appropriate
to the characteristics, scale, and significance of your site of concern.
Tier 1 Risk Assessment
The Tier 1 RA is intended to be a qualitative screening process. At this
the collation of information either through literature review or site
investigation should be preliminary. The purpose of the Tier 1 RA is
to determine two main points:
- whether there is a potentially complete pathway between the
contaminant of concern and potential receptors, and
- whether contaminant concentrations exceed benchmark or guideline
values for relevant receptors or media of concern.
If the pathways are assessed as incomplete, or if
contaminant concentrations do not exceed benchmark criteria for
ecological or human health values, the RA process may be suspended without proceeding
to Tier 2 assessment. A risk management decision may be made, based
on the Risk Characterisation, to:
- do nothing further,
- perform an
- undertake a Tier 2 RA, or
- undertake remedial work.
A Tier 1 RA assesses contaminants of concern against
published assessment criteria (guideline values or benchmark criteria).
These criteria have been developed to help the
assessor undertake risk assessments and incorporate in one step many of
the factors involved in receptor identification, and toxicity and
In developing these criteria, specific receptors have
been identified, certain contaminant pathways have been assessed, and
specific contaminants of concern are identified. These criteria can be
either quite specific (e.g. maintenance worker exposure to benzene
through inhalation). In other cases they can be general. For example,
water quality guidelines for the protection of aquatic organisms provide
protection to sensitive aquatic species, regardless of whether these
receptors may actually be present in the receiving waters.
We recommend that the assessor is familiar with the
assumptions that are used to develop guideline values that are being
used at a Tier 1 assessment. These are usually described in detail in
the text accompanying the guideline values.
Tier 2 Risk Assessment
The Tier 2 RA is expected to be the most detailed level of
undertaken for most sites in New Zealand. Tier 2 RA may involve
more detailed site investigations if required, but is primarily intended to
involve a more intensive literature search to modify the assumptions of
the benchmark criteria used in Tier 1.
The intention of this stage is to undertake a preliminary
customisation of criteria for each contaminant specific to the
pathways, receptors, media, and environmental conditions found at the
site, and to establish modified assessment criteria.
If the contaminant concentrations do not exceed the modified
criteria, the RA process may be suspended without proceeding to Tier 3, and a ecological risk management decision made.
Tier 3 Risk Assessment
Few NZ sites may be large or complex enough to justify the
level of detail and investigation required for a Tier 3 RA. A Tier
3 RA will involve the development of complex models supported by further
intensive site investigations of the contaminants of concern, pathways,
and receptors characteristics.
The intention of this tier is to further customise the modified
criteria values calculated in Tier 2 to achieve a more accurate
representation of the risk posed by the contaminants to specific
The risk management decision may involve remediation or further
iterations and will be made based on the Tier 3 Risk Characterisation of the site.
assessment is a Tier 1 activity.
Advanced risk assessment is a Tier
2 & 3 activity.