Formatted Title
Prioritizing Vapor Intrusion Risk across Entire Residential Suburbs
Background/Objectives
Aquifer conditions in metropolitan Adelaide are conducive to the migration of legacy chlorinated groundwater plumes over large distances. Over decades these plumes have moved away from the industrial source areas towards residential suburbs and can pose significant vapor intrusion risks to the public.
For cases where the regulation of the party liable for the contamination is not possible, the South Australian Environment Protection Authority (SA EPA) administers a program that provides timely investigation and management of this “orphaned” contamination that can pose a significant risk to public health. For sites impacted by chlorinated hydrocarbons, it is often the uncontrolled vapor inhalation risk via the vapor intrusion pathway that is difficult to characterize and subsequently prioritize given the available resources. Nevertheless, it is important to have a consistent and clear process for prioritizing the assessment and mitigation of vapor intrusion risk at residential properties. Since 2016 the SA EPA has established 10 orphaned site contamination assessment areas and funded indoor air testing at over 60 properties.
Approach/Activities
In 2014, the SA EPA, in conjunction with state government health department (SA Health), developed a framework for response to indoor air concentrations of trichloroethene (TCE), a common risk driver for vapor intrusion assessment. The framework includes five risk ranges based on the concentration of TCE in indoor air relevant to the agreed safe level in a residential building (<2 µg/m3). The response ranges include the lowest range as safe with no further action, through to the significant health risk posed by high concentrations prompting accelerated intervention.
The indoor air level response ranges are used by the SA EPA to assess vapor intrusion in two parts. Firstly, as part of predicting potential risk, estimated indoor air concentrations derived from numeric models are compared against the response ranges. The properties within the assessment area are identified as falling within one of the response ranges allowing prioritization of funds and resources to administer property-specific assessment, where warranted. Secondly, property level assessment is conducted to acquire measured indoor air concentrations that are compared with the response ranges to determine if vapor mitigation may be necessary. This prioritization has been documented in a detailed framework available to the public on the SA EPA website.
Results/Lessons Learned
Residential properties predicted within the highest risk response range will be assigned highest priority, including prompt testing of indoor air. In some cases, the SA EPA has adopted use of a portable gas chromatograph (GC) to provide real-time indoor and sub-floor air measurement. In cases where indoor air is measured within the highest risk response range, SA EPA will assist the owner in applying interim mitigation measures such as increased building ventilation and air purification.
For residential properties predicted within the low or moderate risk response ranges, but still above the long-term safe level, property vapor intrusion assessment will include testing of indoor air with samplers deployed for longer durations (7-14 days) to provide assessment of time-weighted average indoor air concentrations. This also includes measurement of sub-floor air and soil vapor to provide multiple lines of evidence, along with seasonal sampling to account for variations in building use.
In cases where residential properties have measured indoor air concentrations above safe levels, the SA EPA has funded and managed the installation of retrofitted vapor mitigation systems. Once successfully commissioned, the mitigation system becomes the responsibility of the current and any future owners under an enduring statutory agreement. To date 20 vapor mitigation systems have been installed at residential properties in metropolitan Adelaide.
Developing an agreed, concise and clear prioritization process for considering vapor intrusion risks is critical for an efficient assessment campaign and enables proportionate use of finite resources to address the greatest risks first. The SA EPA has gained significant knowledge in developing the prioritization framework for TCE and has begun developing similar frameworks for other key contaminants.