Technical Guidance 12: Biodegradation of petroleum hydrocarbon vapours

In this report we review the role of biodegradation in reducing petroleum hydrocarbon vapour intrusion into slab-on-ground buildings for application at a Tier 1 or human health screening level. This work comprises part of the efforts through CSIRO and the Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE) to provide technical input to the current review of the Australian National Environment Protection (Assessment of Site Contamination) Measure (NEPM).

Vapours emanate from subsurface spills and leaks of petroleum fuels such as gasoline, diesel and other petroleum products. Historic or recent events may lead to contamination, and investigation of vapour risks may be triggered by previous site investigations, knowledge of site history or use, or where land redevelopment is occurring. Vapours naturally attenuate or decrease in concentration as they move from the subsurface through the soil towards the ground surface and potentially into buildings. Here we consider the additional attenuation potential due to aerobic biodegradation of petroleum hydrocarbon vapours, and the consequent additional reduction in human health exposure that may be applicable at a Tier I or screening level. The report relates to the evaluation of long-term chronic low-level indoor air concentrations of petroleum hydrocarbon chemical constituents, rather than short-term concerns such as high-level explosive hazards or acute toxic concentrations.

Included in this report are: (i) published information on the biodegradation of petroleum hydrocarbon vapours; (ii) additional exposure reduction factors proposed by researchers and suggested by jurisdictions attributable to biodegradation of petroleum hydrocarbon vapours; (iii) data on petroleum hydrocarbon vapours and oxygen concentrations from four states of Australia along with an assessment of generalised trends; (iv) recommendations as to reduction factors that might be applied when oxygen is present in the subsurface and aerobic biodegradation of petroleum vapours is occurring; and (v) guidance on the application of such reduction factors.

The studies and modelling suggest that where oxygen was present petroleum vapours degraded rapidly. Where oxygen was absent little biodegradation was observed.  Additional attenuation due to biodegradation of petroleum hydrocarbon vapours have been reported to vary from 1 (i.e., no change) to many orders of magnitude, depending on the depth to the source of vapours, the concentration resident in the source zone, and the potential for oxygen ingress. Australian and overseas data from studies in and beneath buildings show that significant biodegradation occurs, especially for modest sized buildings.

Four recommendations are made concerning:

1. the need for measurement and confirmation of the presence of oxygen in the subsurface
2. an exclusion/inclusion criterion related to depths to vapour sources
3. an exclusion/inclusion criterion related to the scale of the building foundations to which the recommendations apply, and
4. the magnitude of the additional exposure reduction that is applicable at a Tier 1 or screening level due to biodegradation of petroleum hydrocarbon vapours.