Formatted Title
A Local Empirical Vapor Intrusion Attenuation Factor Database
Background/Objectives
There is a well-established precedent for using empirical attenuation factors for considering vapor intrusion risk. There are several existing collections of empirical attenuation factors with one of the most well-known being the U.S. Environment Protection Agency’s Vapour Intrusion Database (US EPA 2012) which includes 2,929 paired measurements from 913 buildings at 41 sites and across 15 US states. This database includes 235 exterior soil vapor to indoor air records and 1,582 subslab to indoor air records, and identifies a recommended soil vapor to indoor air attenuation factor of 0.03.
The South Australian Environment Protection Authority (SA EPA) administers a program that investigates and manages “orphaned” site contamination where the original polluter is unable to be regulated under the relevant legislation. Since 2012, SA EPA has undertaken vapor intrusion testing at over 60 residential dwellings across the city of Adelaide, including measuring concentrations of chlorinated hydrocarbons in indoor air, crawlspace air, soil vapor and groundwater. While the dataset is relatively modest in size, it crucially provides empirical measurements relevant to the metropolitan Adelaide and interrogation of this dataset can reveal factors relevant to the assessment of vapor intrusion in similar settings.
Approach/Activities
The vast majority of collated data was collected under direction of the SA EPA and is supported by detailed technical reports presenting the scope of work, methodology, field and laboratory results and data quality evaluation. Similar to the U.S. EPA (2012), the available data were screened before addition to the database to ensure it met minimum requirements, including hazardous volatile contaminants of low biodegradation (predominantly trichloroethene with some tetrachloroethene), excluding records with very low subsurface concentrations, and inclusion of records with supporting data quality information. To date, the database includes 295 indoor air samples from 50 buildings, along with 131 crawlspace air samples, 23 subslab vapor samples, 71 soil vapor samples and 37 groundwater samples. There is a total of 677 paired measurements including 350 exterior soil vapor to indoor air records and 33 subslab to indoor air records.
The data are held in a Microsoft® PowerBI® model with customized dashboards allowing filtering, interrogation and visualisation of the dataset. Filters can be applied within the PowerBI® dashboards to produce a narrower data subset for further interrogation. Filters include specifying the contaminant(s), spatial and temporal windows, and minimum and maximum concentrations. While the database includes a variety of subsurface media records, a key attenuation factor for the purpose of assessing risk from vapor intrusion is that between soil vapor and indoor air.
Results/Lessons Learned
The dashboards allow users to see an overall summary and map of the available data, as well as summaries of the filtered dataset including key statistics such as number of records, the 95th percentile, maximum and minimum attenuation factor, as well as plots showing the distribution of the dataset.
After applying relevant filters, there were a total of 94 paired records considered representative of typical conditions and which produced a 95th percentile attenuation factor of 0.0019. This value is approximately an order of magnitude lower than the attenuation factor identified in U.S. EPA (2012) (also 95th percentile) for shallow soil vapor of 0.03.
In addition, the PowerBI® model allows users to consider the reliability of various attenuation factors. Users can choose from a selection of adopted attenuation factors and indoor air risk screening levels to see the proportion of records where the correct risk outcome is identified.
There have been significant efforts in the past to establish national empirical attenuation factor databases to identify suitable attenuation factors to assess vapor intrusion risk. However, with the increase in sampling capability, higher quality data and regulator led databases, there is potential to look at regional and local records to identify empirical attenuation factors more relevant to local conditions. The SA EPA will continue to grow the database with the potential for it to further inform the assessment and regulation of vapor intrusion in South Australia.