Technical Guidance 02: Protocols and techniques for characterising sites with subsurface petroleum hydrocarbons – a review

About this document

Characterisation and monitoring of petroleum-impacted sites can be costly. Poor characterisation can lead to uncertain mapping of the mass and distribution of petroleum hydrocarbons in subsurface environments. This in turn can lead to poor decisions which may compromise human or environmental health or can increase costs where remediation is prolonged, misapplied or not well targeted due to lack of appropriate data.

Characterisation and monitoring of petroleum-impacted sites can be costly. Poor characterisation can lead to uncertain mapping of the mass and distribution of petroleum hydrocarbons in subsurface environments. This in turn can lead to poor decisions which may compromise human or environmental health or can increase costs where remediation is prolonged, misapplied or not well targeted due to lack of appropriate data.

From discussions between regulators, oil companies, consultants, CSIRO, CRC CARE and other researchers a scope of work related to site characterisation was developed. CRC CARE agreed to a review stage project proposal developed by CSIRO. The review stage is a first step in a larger project to document techniques and protocols that are used nationally and internationally – to bring greater unity and improved approaches to contaminated site assessment for petroleum impacts in soil and groundwater environments of importance to Australia.

This report:

1. documents typical properties of hydrocarbon compounds and fuel/oil behaviours
2. provides an overview of traditional staged and accelerated site characterisation strategies including a discussion of the Triad approach
3. summarises available guidance, protocol and standards documentation from State government agencies and the NEPC, from industry, and from overseas (United States, United Kingdom, Europe, New Zealand)
4. documents some standard and not so standard sampling and investigation techniques, and
5. provides summary comment.

Important issues, protocols and technologies related to petroleum hydrocarbons in soil and groundwater environments are highlighted.

The review provides an introduction to the multiphase behaviour of petroleum hydrocarbons and the need to address gas, liquid and solid phases (air, water and soil) as well as the released non-aqueous phase liquid (NAPL) in site assessments – and to address both the vadose zone and groundwater. Fuels and oils are highly complex mixtures that change their composition over time. The intensity of investigative effort required in each phase is somewhat dictated by environmental and jurisdictional drivers, but also by typical properties of some petroleum hydrocarbons, which govern the likely distribution of the hydrocarbon product and individual compounds in the subsurface. A limited discussion of fuel additives is also given.

The importance of having site characterisation goals in a risk-based framework is emphasised – what is the overall goal and what is to be achieved at a site, what is the compliance point, and what phase is dominant? To assist with this, it is important to have a well developed conceptual site model, along with a targeted program of activity to improve the three-dimensional spatial and temporal understanding of petroleum contamination distributions in the subsurface. Such a program would be developed out of a full conceptualisation of the site model – inclusive of hydrogeology and soil strata, primary fuel/compound types, the presence of additives, compliance boundaries, principal transport mechanisms in each phase, the proximity of receptors, etc. The intensity of effort can be refined using data quality objectives, where ‘data quality’ may more generally refer to data that adequately represents the site conditions. It is concluded that an investigation program needs to be purposeful, adequate and representative to achieve its aims (i.e. on PAR).

The traditional staged approach to site characterisation (Phase 1 and Phase 2 environmental site assessments – ESAs) is compared with accelerated site characterisation (ASC) and in particular the so-called ‘Triad’ approach, which links systematic planning and dynamic work strategies with on-line and on-site measurement techniques to enable site characterisation in a (hoped for) single mobilisation. No implementation of ASC techniques is apparent in Australia, despite savings estimates of up to 40–50% for some applications in the United States of America.

Information on some of the standard and more novel site characterisation technologies is reviewed, including direct push technologies, geophysics, on-site analytical techniques, NAPL characterisation, capillary fringe and vadose zones, combined technologies, directional and alternate drilling, flux estimation techniques, passive and on-line sampling/monitoring devices, and reactive transport modelling. Further development, validation and combination of technologies is required if rapid and cost-effective site characterisation is to have greater adoption in Australia.

Summary observations from the review include:

1. A risk-based approach should be taken as the starting point for site characterisation, since this forces consideration of the regulatory regime, exposure pathways, and focuses effort on establishing a robust conceptual model – with field investigations that target the improvement and modification of this model.
2. A clear information objective is important – embodying data quality objectives and achieving representative sampling of a site.
3. Important to any site characterisation is the derivation of a site conceptual model that integrates what is already known about a site, and identifies both what still needs to be discovered, and how that information should be used.
4. Choices of site investigation approaches should be made on the basis of improvement of the site conceptual model. The site conceptual model also serves as the basis for risk assessment and remediation. The information needed for risk assessment and remediation planning is not necessarily the same.
5. Site characterisation plans need to be flexible and adaptable to allow feedback on possibly real-time results. This kind of dynamic approach to site characterisation needs to be considered as a part of overall site investigation strategy, before site characterisation mobilisation. It also impinges on the reliance on conventional sample to laboratory systems, as laboratory data can take longer to be generated. Dynamic approaches may also need to encompass remediation planning.
6. If representivity is to be emphasised, perhaps the analytical precision of conventional laboratory (off-site) analyses could be traded off against greater data intensity on-site, especially when comparing sources of error from sampling, and sources of uncertainty from site heterogeneity and variability.
7. For groundwater investigations, guidance documents generally promote a minimum of three or four boreholes. In practice, on average a greater number of sampling boreholes is used. Greater use could be made of statistical sampling theory, statistical tests of significance, and geostatistical analysis to handle aquifer and geochemical heterogeneity.
8. Site investigation data needs to be assessed over time, as well as in space, so that trends in contaminant behaviour can be assessed. The depth dimension is often neglected in air, water and soil phases. Whilst depth sampling of soils/sediments (whether in the vadose zone or groundwater) is mentioned in several guidance documents – it has not been formalised. Depth discrete sampling of groundwater is mostly ignored.
9. A variety of site characterisation techniques are available for on-site analyses, from on-site sensors to geophysical techniques. Validation and improvement is required to avoid false negative and positive outcomes, and to build reliability and confidence into rapid assessment practices.
10. Development of new tools for on-site use in site characterisation is increasing – but if ASC or Triad is to be pursued, further development of new or novel sensor and site characterisation techniques should be pursued – especially those that would allow rapid on-site decision making and achieve reliable outcomes at reduced cost. Combining ‘continuously available’ data with models on-line could also be pursued. This is near-readily available for groundwater flow, however, where geochemistry and reactive transport is important on-line real-time linkages between data collection and models is problematic due to the computational intensity of such codes.
11. The use of ‘direct push’ tools as opposed to conventional drilling, and a variety of attachments and sensors which can be used with these direct push tools to collect site investigation information is desirable. This allows more real-time information gathering and implementation of dynamic site characterisation strategies. Innovation in this area should be encouraged.
12. In Australia, implementation of Triad would require upskilling of all elements of site characterisation (planning, on-site interpretation including sophisticated modelling, and technology availability and deployment) in consulting agencies and/or industry, and integration of these skills with the regulatory approvals process – possibly on-site.
13. Harmonisation of the regulation of site characterisation through NEPM or other mechanisms may assist the practice of site characterisation, but reduced flexibility needs to be avoided to allow for continued innovation.
14. Greater guidance on the use and application of numerical models to integrate site characterisation data could be useful. The Murray-Darling Basin Commission issued guidelines for water resource groundwater flow modelling in 2000. Prommer et al. (2003) provided an initial basis for petroleum plumes in groundwater. There are no Australian guidelines for modelling multiphase hydrocarbon behaviour.
15. There is a need to continue to facilitate knowledge transfer across regulators, contaminated site owners, service providers and the academic community.
16. The scalability of site investigation plans and technologies should be assessed – from UST and service station scale, to depot and terminal scale and perhaps refinery or multiple source complex mega-site scale.
17. US documentation (ASTM, API, etc.) includes management as part of guidance in many instances, whilst the NEPM does not. Interestingly, the NSW EPA Service Station guidelines did include remediation options as part of the guidance. A perceived difficulty in embedding management options in such guidance is the lack of stability in the range of technologies on offer.
18. Research and innovation is needed to quantify and optimise the value of gathering additional data so as to minimise uncertainty during assessment – to provide more precise definition of risks and costs for clean-up applications. For example, whilst the multiphase behaviour of petroleum hydrocarbons is recognised, the value of extra data in any one phase in reducing uncertainties in risk assessment or the selection of remedial options is not well defined.

A number of challenges remain – none the least of these is maintaining flexibility to adopt new ideas and technologies and, where needed, change the regulatory and industry norm to allow alternative assessments, whilst maintaining protection of human health and the environment.