Impacts of Changes of Indoor Air Pressure and Air Exchange Rate in Vapor Intrusion Scenarios

Abstract

There has, in recent years, been increasing interest in understanding the transport processes of relevance in vapor intrusion of volatile organic compounds (VOCs) into buildings on contaminated sites. These studies have included fate and transport modeling. Most such models have simplified the prediction of indoor air contaminant vapor concentrations by employing a steady state assumption, which often results in difficulties in reconciling these results with field measurements. This paper focuses on two major factors that may be subject to significant transients in vapor intrusion situations, including the indoor air pressure and the air exchange rate in the subject building. A three-dimensional finite element model was employed with consideration of daily and seasonal variations in these factors. From the results, the variations of indoor air pressure and air exchange rate are seen to contribute to significant variations in indoor air contaminant vapor concentrations. Depending upon the assumptions regarding the variations in these parameters, the results are only sometimes consistent with the reports of several orders of magnitude in indoor air concentration variations from field studies. The results point to the need to examine more carefully the interplay of these factors in order to quantitatively understand the variations in potential indoor air exposures.

Keywords: Vapor intrusion; indoor air exchange rate; indoor air pressure; modeling.

Figure 1

Normalized indoor air contaminant vapor concentration as a function of calculated soil gas flow rate in (positive) or out (negative, shaded area) of the building through the simulated crack

Figure 2

Transient vapor mass entry rate through the crack into the building, normalized by the groundwater vapor concentration, as a function of time. (a) indoor pressure varied between − 5 Pa to + 5 Pa, with soil permeability 10⁻¹² m². (b) indoor pressure varied between − 10 Pa to + 10 Pa, with soil permeability 10⁻¹² m². s-s stands for steady state

Figure 3

Scenario (c): Transient vapor mass entry rate through the crack into the building, normalized by the groundwater vapor concentration, as a function of time, with indoor pressure varied between −10 Pa to +10 Pa, with soil permeability 10⁻¹¹ m²

Figure 4

Calculated transient soil gas entry rate into (positive) or out of (negative) the building through crack, for scenario (a) of Figure 2

Figure 5

Simulated transient indoor air contaminant vapor concentration with varying building air exchange rate (A_E) during (a) fall season, and (b) summer

Figure 6

Fall season: simulated temporal indoor air contaminant vapor concentration with varying building air exchange rate (A_E) or the indoor gauge pressure (±5 Pa), or both. A_E is assumed to have the seasonal mean as in fall. In scenario (a) A_E increase at t = 0. In scenario (b) A_E decrease at t = 0. In both scenarios, J_ck varied as in that in Figure 2(a), or Equation (12)

Figure 7

Summer season: simulated temporal indoor air contaminant vapor concentration with varying building air exchange rate (A_E) or the indoor gauge pressure (±5 Pa), or both. A_E is assumed to have the seasonal mean as in fall. In scenario (a) A_E increase at t = 0. In scenario (b) A_E decrease at t = 0. In both scenarios, J_ck varied as in that in Figure 2(a), or Equation (12)

Figure 8

Upper panel: the calculated TCE concentrations in indoor air for hypothetical spring and summer scenarios. Lower panel: the calculated TCE concentrations (in boxes) compared to the field data from Holton et al. (2013). The green boxes represent spring, and orange boxes represent summer condition.