Formatted Title
Challenges of Vapor Mitigation in Residential Settings
Background/Objectives
The South Australian Government has funded the installation of vapor mitigation systems in 20 properties in the metropolitan suburbs of Beverley and Thebarton. The properties were affected by vapor intrusion from legacy chlorinated hydrocarbon groundwater plumes that had migrated away from the original source areas. Testing at the properties by the South Australian Environment Protection Authority (SA EPA) confirmed that indoor air concentrations of trichloroethene (TCE) were present at unsafe levels and action was needed to reduce concentrations to prevent long-term health impacts. The cost to retrofit and commission an active vapour mitigation system ranges between $40,000 AUD and $100,000 AUD per property depending on the building construction. Each system is designed to be actively run, fitted with a simple alarm to show it is operational and must meet noise standards.
Typically, these sub-floor systems comprise a network of extraction pipes beneath the floor fitted to an extraction fan which vents externally at roof level. From the time a property owner is first offered testing until the system is handed over there will have been hundreds of interactions with the EPA. Families experience disruption inside their home and may be asked to relocate to other parts of the house or offsite during the installation process. Issues that have arisen include alleged damage from moving furniture, dust generated during installation, dissatisfaction with floor finish and unacceptable noise levels of the system post installation. Issues retro-fitting a system also arise because access is restricted by dwarf walls and very shallow crawlspaces (sometimes as shallow as 200 mm).
Approach/Activities
The SA EPA aims to meet timeframes for action outlined in the Orphaned Site Contamination Management Framework (OSCM, 2022). When opportunities arise, the SA EPA looks to alternative approaches to improve the experience for the homeowner while mitigating the vapor intrusion risk. Alternatives must be benchmarked against the known costs and timeframes realized for the traditional approach to retrofitting active sub-floor systems to existing residential properties.
Such an opportunity arose in 2022 when high soil moisture conditions and lower groundwater TCE concentrations caused by three consecutive years of above average rainfall resulted in a reduction in predicted vapor intrusion risk to residential properties. These circumstances allowed time for a Remediation Options Assessment (ROA). If a viable alternative approach was identified it would negate the need to install traditional sub-floor systems at up to three individual properties in the Woodville North EPA assessment area where indoor air concentrations of TCE were measured above safe levels.
Results/Lessons Learned
The ROA comprised an initial screening of a broad range of 19 different remediation technologies against four assessment criteria addressing effectiveness, logistical practicability, cost and timing. A total of four potentially viable technologies were taken forward for more detailed evaluation.
Ultimately, the ROA identified sub-floor ventilation and/or depressurisation as the preferred remedial option to mitigate vapor intrusion caused by the shallow groundwater plume at Woodville North.
Logistical challenges identified as restrictions in relation to opportunities for alternative remedial approaches for the Woodville North vapor intrusion risk may not be present at other orphaned site contamination areas across South Australia. For example, areas with a deeper water table may allow for unsaturated zone depressurisation and both enhanced biodegradation with bioaugmentation and treatment of the groundwater plume using in situ chemical reduction utilising zero valent iron would be possible if the source zone wasn’t directly below residential properties, in that injection points and gases produced during the fermentation/reduction process wouldn’t have a negative impact on the properties.
The SA EPA is still investigating innovative approaches to mitigate vapor intrusion in residential settings.