Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

Questioning the Johnson & Ettinger Model: Diffusive and Advective Flux Measurements in a Building

Background/Objectives:

Over the years, indoor air quality has become a major health concern, given that we spend an average of 80% of our time indoors and that, to avoid wasting energy, buildings are increasingly insulated from the outside world. As a result, vapors from pollutants in underground environments, such as soil or groundwater, can penetrate into buildings and degrade indoor air quality. The Johnson-Ettinger model is widely used to assess the impact of contaminated vapors on residential air quality. Typical use of this model is based on a series of estimated data with few site-specific measurements [1]. Several studies have investigated the impact of uncertainties in the model's input parameters on the reliability of the resulting results [1, 2]. Other studies have shown the importance of using appropriate input parameters for the J&E model [3]. Therefore, the main objective of this study is to carry out in situ measurements and compare them with the results obtained with the Johnson & Ettinger model in order to provide further insight into the limitations of this model.

Approach/Activities:

The measurements were carried out in a house located in a housing estate that was formerly a factory (textile cleaning company). The groundwater on this site is contaminated with trichloroethene (TCE). The main objective of the measurements was to determine in situ the diffusive and advective fluxes of TCE present in the house. Diffusive fluxes were measured using a flow chamber. This method is used to assess the transfer fluxes of volatile compounds at the soil/atmosphere interface. A specific method has been set up to measure advective flows, which will be explained during the presentation. Curiously, it seems that in situ measurements of advective flux at a specific point have not yet been carried out under real conditions. Fluxes were measured with and without ventilation of the house.

Results/Lessons Learned:

The measurements were used to generate a map of the diffusive and advective TCE fluxes emitted from the ground towards the house. The map shows high diffusive fluxes and very high spatial heterogeneity, even at a distance of a few meters. In terms of advective VOC fluxes, it appears that two cracks close to the poles account for the majority of these fluxes. The measurements confirmed that diffusive flows are independent of the pressure differential, whereas advective fluxes were very strongly influenced by this pressure differential. At the building scale, the mass balance is validated because the variation in concentration in the building can be predicted by the measured fluxes and the air exchange rate. However, we show that the diffusion coefficient through the slab is particularly high, outside the range of conventionally accepted values. In this example, the diffusive flux is clearly dominating. These observations do not correlate with those obtained with the Johnson & Ettinger (J&E) model. For example, in our case, soil permeability does not play a role, contrary to what is claimed in the sensitivity studies of the J&E model. In addition, the heterogeneity of the soil and the slab leads to highly variable diffusive fluxes in space, and calculating the J&E model from concentrations at depth leads to significant errors. It would therefore seem more efficient to carry out the calculations using flow chamber data, eliminating the uncertainty about diffusion in the soil. We therefore propose a simplified formulation of the J&E model which includes the measurements presented.

References

1. Weaver, J. W., & Tillman, F. D. (n.d.). Uncertainty and the Johnson-Ettinger Model for Vapor Intrusion Calculations. 2. Johnson, P. C. (2002). Identification of Critical Parameters for the Johnson and Ettinger (1991) Vapour Intrusion Model. API Soil and Groundwater Research Bulletin, 1–29. 3. Hers, I., Zapf-Gilje, R., Johnson, P., & Li, L. (2003). Evaluation of the Johnson and Ettinger Model for Prediction of Indoor Air Quality. Ground Water Monitoring and Remediation, 23, 119–133. https://doi.org/10.1111/j.1745-6592.2003.tb00678.x

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