Diffusive and advective flux measurements of trichloroethene from soil into a building: a case study

In order to validate some assumptions and calculations of Johnson and Ettinger's model, a mapping of measured VOC fluxes in a heavily contaminated building was undertaken. To this end, both advective and diffusive flux measurements were carried out under real conditions. Diffusive fluxes were measured with flux chambers recording the initial concentration rise during the first minutes. The results showed high diffusive fluxes and considerable spatial heterogeneity, even at a distance of a few meters. A novel method, described in this article, was used to measure advective fluxes. It appears that the majority of these fluxes originate from two cracks close to poles. Measurements confirmed that diffusive fluxes are independent of the indoor-outdoor pressure differential, whereas advective ones are strongly influenced by the pressure differential. At the building scale, the mass balance is validated, as the variation in concentration within the building can be predicted by the measured fluxes and air exchange rate. However, we show that the order of magnitude of the diffusion coefficient through the slab is particularly high, outside the range of conventionally accepted values. In this building, diffusive fluxes, independent on pressure differences, are clearly dominant while in Johnson and Ettinger's model, the fluxes through the slab depend on the crack surface and vary with pressure.