Volatile Organic Compounds
What are volatile organic compounds?
The most odorous chemicals that contaminated the former Macdonaldtown Gasworks site belong to a group known as volatile organic compounds (VOCs). When soil that contains VOCs is excavated, these liquid compounds ‘volatilise’, or become vapour, causing odour to be emitted to the atmosphere. Hours after excavation activities finish, volatilisation may still occur, creating odour outside regular working hours. Volatilisation may also be caused by humidity or rapid temperature change, creating odour in the early morning or evening. The nature of VOCs makes odour in the air difficult to control.
Controlling VOCs: the ECE and ECS
VOC emissions were controlled during the main soil excavations by undertaking them inside an Environmental Control Enclosure (ECE) and by filtering all air extracted from the enclosure through an Emission Control System (ECS). Other controls that were used to minimise VOC levels in the air included:
- Reducing the pace of excavation;
- Minimising the excavation area through the use of covers; and
- Working in a different excavation area for a period of time.
Where and how often was VOC monitoring conducted?
A direct reading instrument called an AreaRAE continuously monitored total VOC levels in the air at the western boundary of the project site. In addition, a piece of portable equipment known as a photo-ionisation detector, or PID, was used to monitor VOCs at various locations on and around the site every working day during remediation works.
VOC Response Levels
A number of project goals and limits, known as Response Levels (RLs), were established at or below the maximum allowable limits for total VOCs. Measurements of total VOCs collected on the site outside the ECE and at site boundaries through AreaRAE or PID monitoring were compared to these RLs. The RLs were:
- A daily average concentration of 3ppm (parts per million) as the project goal.
- A daily average concentration of 5ppm as the project limit or maximum RL.
It is important to note that odour from VOCs may still be noticeable when VOC concentrations are below RLs. This means that the presence of an odour does not mean RLs are being exceeded.
VOC monitoring results
Monthly summaries of the environmental monitoring results obtained during the project, including VOC results, are provided under the Monitoring Results section. See here