What creates odour during the remediation?
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.
What do the odours smell like?
Various odours were emitted from the contaminated soil during the project excavations but most smelled like petrol, oil, tar or mothballs. The human nose is very sensitive so people living adjacent to the site occasionally smelled odour.
It is important to note that odour from VOCs can be noticeable when VOC concentrations are still below safe levels. The presence or level of unpleasant odour is therefore not necessarily a good indicator of a chemical’s harmfulness. Monitoring to detect VOCs in the air and their measurement provides the best indicator of health risk. Extensive monitoring was undertaken during the project, which is discussed further on the Volatile Organic Compounds page.
Controlling odour: the ECE and ECS
Odorous 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 odour in the air include:
- 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.
What is the difference between odour monitoring and VOC monitoring?
During odour monitoring an environmental officer uses a portable piece of equipment, known as a Nasal Ranger, to determine the ambient air dilution rate of any odour detected. This dilution rate indicates the intensity of the odour, which is used in combination with the trained officer’s characterisation of the odour, and the prevailing meteorological conditions, to identify its precise source. In contrast, VOC monitoring measures the level of one or more chemicals that cause odour through the use of equipment only.
Where and how often was odour monitoring conducted?
Odour monitoring was conducted every working day at a number of locations on and around the project site.
Odour Response Levels
A number of project goals and limits, known as Response Levels (RLs), were established at or below the maximum allowable limits for odour. Measurements of odour collected on and around the project site were compared to these RLs. A Nasal Ranger instrument was used to measure the dilution-to-threshold (D/T) ratio of an odour, which is represented as an odour unit. The RLs were:
- 1-2 odour units at the site boundaries as the project goal.
- 2-4 odour units at the site boundaries as the project limit or maximum RL.
- These project criteria RLs were developed using: Technical Framework: Assessment and management of odour from stationary sources (NSW EPA, 2006).
Odour monitoring results
Monthly summaries of the environmental monitoring results obtained during the project, including odour level results, are provided under the Monitoring Results section. See here