As defined in the National Environment Protection (Assessment of Site Contamination) Measure 1999 (the NEPM), the purpose of contaminated site assessment is to determine whether site contamination poses an actual or potential risk to human health and the environment, either on or off the site, of sufficient magnitude to warrant remediation [or management] to the current or proposed land use.
The assessments should:
- Be undertaken in accordance with the NEPM; and
- Be undertaken by professionals who have the relevant qualifications, competencies and experience for the site contamination issues relevant under investigation (refer to the Engaging a Contaminated Site Assessment Consultant page for further information).
It is recommended that a contaminated site assessment be undertaken in a staged approach based on the level of information required. Each stage of site assessment should include the relevant level of Risk Assessments, which ensures a comprehensive knowledge of the source, pathway and receptors in each case.
Stage 1 – Site History Reviews and Preliminary Site Investigations
The first stage of contaminated site assessment involves establishing a site history of potentially contaminating activities (PCAs). This Site History Review involves the collation of information, from various sources, to determine if the site is or may have hosted or is likely to have been impacted by a potentially contaminating activity. Where a site history review clearly demonstrates that site activities have been non-contaminating this information can be used to justify not progressing with further assessment.
However, where there is an indication that the land may potentially be contaminated, as a result of current or past PCAs, a Preliminary Site Investigation (PSI) should be conducted by a suitably qualified and experienced consultant. A PSI usually includes a desktop study to identify the site characteristics (site location, site layout, building construction, geological setting, historical land uses and activities at the site and on adjacent sites) and a site inspection (including interviews with site representatives).
The PSI should be sufficient to determine contaminants of concern and identify areas of potential contamination, including all areas of potential contamination sources. The PSI may also include initial sampling to provide a preliminary assessment of site contamination and need for further investigation.
A starting point of a Site History Review (and a PSI) is consideration of the Potentially Contaminating Activities List and whether the land in question has hosted or currently hosts or is adjacent to a site that has hosted or currently hosts a PCA.
The risk to underground services from contamination should be fully assessed, including the possibility of ingress by way of seepage or permeation. This is of particular importance where subsurface plastic pipes deliver potable water through or near hydrocarbon or solvent contaminated land.
A Property Information Request may also form part of a Site History Review.
The information collected as part of the PSI should result in the development of an initial conceptual site model (CSM) - a representation of site-related information regarding potential or known contamination sources and receptors, and the potential exposure pathways between those sources and receptors.
Refer to the Site History Reviews and Developing a Conceptual Site Model page or the NEPM Schedule B2 for more information.
Where a PSI demonstrates that the land is not contaminated or the potential for risk to human health and the environment is limited based on current or intended future land use, there may be no need for further investigation. However, where contaminating activities are suspected or known to have occurred, or if the site history is incomplete or where contamination requires full delineation to determine the risk as sampling indicates levels above guidelines, it may be necessary to undertake a Detailed Site Investigation (DSI).
Stage 2 - Detailed Site Investigations
A Detailed Site Investigation (DSI) involves the taking of samples in the field from air, soil, groundwater or other water sources to confirm the presence or absence of contamination identified or suspected in the PSI.
The DSI should involve the development of Data Quality Objectives (DQOs) based on an initial CSM and be undertaken in accordance with a Sampling & Analysis Quality Plan (SAQP). The SAQP sets the sampling program and data quality objectives as well as the Quality Assurance and Quality Control methodologies to be employed to manage the field work stage of the assessment.
A number of sampling events may be required to delineate the contamination and determine the risks to human health and the environment. Any additional sampling events or changes to the sampling methodology should be reflected by amending the SAQP.
Note that in accordance with the NEPM (Schedule B6) 'The assessment process for groundwater contamination differs from that of land contamination in that there is greater emphasis on suitability for current and realistic future uses, compared with the emphasis on current and intended uses for soil assessment'.
The NEPM Field Checklist, which is part of the NEPM Toolbox provides details of the parameters that should be collected in the field based on the objectives of the investigation and the contaminants of concern.
The DSI should be comprehensive enough to identify the nature of the contamination and delineate its lateral and vertical extent to a sufficient degree that an appropriate level of human health and environmental risk assessment may be undertaken and, if necessary, to provide the basis for the development of an appropriate remediation or management strategy.
Where management is proposed for residual contamination and not remediation, the consultant should justify the approach and ensure the management measures are appropriate to the current and/or future use or development of the site.
If site contamination is found to pose an actual or potential risk to human health and/or the environment, either on or off the site and therefore the site may not be suitable for its current or proposed use, remediation and/or management should be considered.
The method of remediation or management should take into consideration the hierarchy of options for site clean-up and management as defined under Principle 16 of the NEPM, and may vary depending on:
- the type and extent of contamination;
- where the contamination is (soil, sediments or ground/surface water); and
- the risk that the material poses.
Where remediation involves asbestos the WA Heath Department publications including the Guidelines for the Assessment, Remediation and Management of Asbestos Contaminated Sites in Western Australia (2009) should be considered.
Complex remediation should be supported by the development and implementation of a Remediation Action Plan (RAP). The key components of a RAP are:
- Identification of the key stakeholders and responsibilities.
- Development of remediation goals and clean-up acceptance criteria.
- Assessment of the remediation options and determination of the preferred remediation option.
- Documentation of the remediation methodology including any regulatory permit/licensing requirements.
- Development of an Environmental Management Plan.
- Defining the validation program to demonstrate the successful completion of the remediation, including monitoring.
Stage 4 - Validation
Following remediation, it must be proven that remediation has met the remediation goals and remnant contamination does not present an unacceptable risk to human health or the environment based on the current or proposed land use, through appropriate sampling of soil, groundwater and/or vapour. Validation of groundwater requires ongoing groundwater monitoring over a pre-determined period of time.
Where monitoring is necessary to demonstrate that remediation is occurring or has been successful monitoring should take into account a range of factors that may effect results, including but not limited to seasonal changes in groundwater levels, artifacts from sampling and changes in conditions due to remediation.
A contingency plan is often required (i.e. a change in remedial method or further remediation) if the chemical levels identified during monitoring exceed a pre-determined trigger level.