Technical Guidance 03: Natural attenuation – a scoping review

‘Natural attenuation’ refers to naturally occurring physical, chemical and biological processes, or any combination of those processes, that reduce the concentration or mass of polluting substances in groundwater. It has been recognised that natural attenuation processes can form the basis of a viable remediation strategy for contaminated sites. Monitored natural attenuation has the potential to reduce contaminant clean-up costs while avoiding exposures to humans and sensitive environmental receptors. Whilst the benefit of using monitored natural attenuation on a site can be significant compared with active remediation, there are also risks associated with this strategy that need to be managed.

CRC CARE commissioned this review of natural attenuation processes as part of its priority scopes of work related to the environmental fate of petroleum hydrocarbons. The goal of the scoping report was to review international, national and industry information about the natural attenuation of petroleum hydrocarbons with a focus on Australian studies. This project seeks to identify scientific knowledge gaps which are relevant to the application of monitored natural attenuation in Australia.

The following are the major observations arising from this review:

1. A risk-based approach using lines of evidence is widely used as a framework for monitored natural attenuation (MNA) guidance.
2. Six years ago, MNA was recognised as a well-established remediation approach for only a few types of contaminants, but importantly these included the petroleum compounds benzene, toluene, ethylbenzene and the xylene isomers (BTEX), and some oxygenated hydrocarbons.  For the other classes of petroleum hydrocarbons and oxygenated hydrocarbons, there was either not a complete understanding of biodegradation processes or its confidence of success in an MNA-based remediation strategy. Since then greater knowledge has been gained particularly about methyl tertiary-butyl ether (MTBE) and polycyclic aromatic hydrocarbons (PAH).
3. Biodegradation is recognised as the key natural attenuation mechanism for fuel hydrocarbons. To support the application of MNA, extensive compilations and reviews of BTEX biodegradation rates for laboratory and field sites are available, primarily based on studies from the United States.
4. Australian data that might help define the scope of impact arising from the use of fuel additives (MTBE, ethanol and biodiesel) are scarce. The Australian experience of groundwater contamination from MTBE is likely to differ from the United States, as MTBE is potentially only present in fuels imported from overseas.
5. Approaches for implementing MNA at sites contaminated with PAH are poorly developed compared with BTEX or chlorinated solvent sites. Knowledge about PAH degradation in the field and sorption/desorption processes is reasonably limited.
6. There have been relatively few studies that have addressed the natural weathering of residual and mobile non-aqueous phase liquid (NAPL) fuels compared with studies into the processes affecting dissolved phase contaminants resulting from NAPL. This knowledge gap will be important where MNA is linked as a remedial strategy with NAPL clean-up.
7. Groundwater recharge has recently been recognised as an important attenuation mechanism near the contaminant source. High-resolution multi-level sampling and scenario modeling are increasing our understanding in this area.
8. Whether the assimilative capacity of an aquifer can be sustained over the multi-year life of a MNA remedial strategy is an area of knowledge that is poorly understood. One area of uncertainty relates to the longevity of cycling of redox-active elements (Fe, S) between the dissolved and mineral phases. Another is the prediction of changes in electron acceptors in response to changes in environmental conditions (e.g. recharge).
9. Further knowledge about the expected value of particular biogeochemical indicator data to effectively discriminate contaminant degradation mechanisms at a fuel-impacted site is needed. This knowledge is needed to cost-effectively set up monitoring regimes capable of validating attenuation processes long-term.
10. Published literature on natural attenuation processes at Australian sites for petroleum hydrocarbons is patchy and lacks a degree of specificity about site location, degradation rate and contaminant data, which therefore limits the usefulness of the literature for contributing to a generalised understanding of contaminant behaviour across those specific subsurface environments.
11. In Australia, there have only been two published studies on petroleum hydrocarbon plume characteristics at multiple MNA sites. There is not enough data to conclude whether the generalised behaviour of petroleum hydrocarbon plumes under Australian conditions is similar to that reported from overseas. Therefore the general applicability of overseas data (e.g. degradation rates) to Australian sites is a knowledge gap.
12. Published Australian natural attenuation literature has focused on shallow sand aquifer systems. These include the Quaternary aquifer around Perth and the Botany aquifer near Sydney. Studies on these sites could provide the fundamental data for scenario modelling at those locations. Generally, there were few papers which described contaminant behaviour in fractured rock and clay sites.