Theme: 09. Vapor Intrusion

Topic Area: 9c. Vapor Intrusion Preferential Pathways

Formatted Title:

Identification of New Preferential Pathways for Vapor Intrusion of Chlorinated Solvents Found by Combining VaporSafe^TM and Detection Dogs

Background/Objectives:

One of Stockholm’s largest dry cleaners operated at the Site from the 1940s until the end of the 1960s after which the buildings were demolished, and the area was redeveloped. The Site is particularly challenging to investigate as there is both an expressway on a viaduct and a tramline crossing the area. There are also adjacent apartment buildings with radon gas mitigation systems together with various utility lines for sewage, storm water, electricity and telephone etc.

Vapor intrusion of chlorinated aliphatic hydrocarbons (CAH) to the adjacent apartment buildings was identified in the initial investigation in 2017 as the primary risk driver for the Site. The current project, that will be completed this year, aims for a full risk assessment of the contamination together with a remedial alternative evaluation process. The investigations have been carried out dynamically in stages. The results of less expensive and less intrusive screening techniques (i.e., soil gas, indoor air, tree cores and air and water in sewer and stormwater pipes) have been used to direct more expensive ground penetrating investigations with drilling rigs (MIP, soil cores and groundwater wells), while the conceptual site model (CSM) was refined after each stage.

The results of these standard investigations were puzzling in regard to the mechanism of vapor intrusion into the adjacent apartment buildings, were elevated concentrations of perchloroethylene (PCE) and trichloroethylene (TCE) were detected. A contaminated area, which acts as the source, was identified several meters below clay sediments and the potentiometric surface of the aquifer at the site. However, compared to what was observed in the indoor air in the apartment buildings, located approximately 30 meters east of the source area, the soil gas concentrations within the source area were relatively low, as well as the CAH concentrations close to and under the apartment buildings. Furthermore, the vapor concentrations of CAH in the sewer and storm water pipes were around one order of magnitude higher than in the surrounding soil gas, indicating that pipes could be a preferential pathway.

The source’s inaccessibility makes it very difficult to remediate, it is therefore of utter importance to correctly assess how vapor enters the apartment buildings so that the remediation correctly addresses these preferential pathways.

Approach/Activities:

To identify the cause of the elevated concentrations of CAH in the indoor air, a combination of new techniques was used. To discover possible vapor intrusion entry points the newly certified Swedish detection dogs were used. The dogs are trained on detecting PCE and TCE and mark when they smell these compounds. In Sweden and Denmark detection dogs have been used successfully for several years and have proven effective in identifying vapor intrusion entry points into buildings. Based on previous results and the markings of the dogs a Vaporsafe^TM investigation was performed for the first time in Sweden. It uses a highly sensitive portable gas chromatograph (GC) which can analyze air samples from several different places continuously whilst also collecting pressure and weather data. In total approximately 120 discrete air and soil gas samples were analyzed with the GC, to both narrow down the locations for continuous monitoring and refine the CSM. By continuous monitoring it was also possible to investigate the effect of the radon gas mitigation system in real time.

Results/Lessons Learned:

By combining Vaporsafe^TMand detection dogs it was possible to dynamically perform an extensive mapping of the buildings and discover unexpected new pathways for vapor intrusion, which had previously not been considered when using more conventional methods. By continuous monitoring, fluctuations in indoor air concentrations could be seen and evaluated together with other data and identify vapor entry points. The project shows the importance of working with a CSM and gradually updating it and not to rely on “common knowledge” of where the highest concentrations in different medias will be found. It also shows how the use of these new techniques can lead to a better CSM, which allows for a more accurate human risk assessment and leads to remedial actions/mitigations that can address the problem.

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Inger Johansson