Study Of The Building Pressure Cycling Method For Evaluating Vapor Intrusion

Vapor intrusion(VI)refers to the migration of volatile organic compounds(VOCs)from contaminant sources into the building of interest,mainly through the joint effects of diffusion,advection,and biodegradation.The collection of indoor air samples is always decisive in assessing the risk of VI and determining the level of groundwater remediation.However,human activities and function of ventilation equipment would change the indoor-outdoor pressure difference,and accordingly,the soil gas entry rate and the indoor air concentration.As a result,in recent VI investigations,the building pressure cycling(BPC)method has been applied to help minimize ambiguity caused by temporal variability of indoor air samples that are important to risk assessments.It can also help investigators to identify the sources of VOCs and preferential pathway.In this study,we use a three-dimensional numerical model to examine the dynamic migration of VOCs after the application of BPC.The role of BPC depressurization was examined numerically in the scenario of the chlorinated vapor intrusion.First,we validated the numerical model with field measurements.Then the verified model is used to investigate the effects of site-specific features in determining the performance of BPC operation.At last,we summarize past field applications of BPC to examine the simulated results.Our study indicates that the BPC induced indoor depressurization can increase the building loading rate in the first 2-3 hours,which would then drop to 2-3 times of that with natural conditions in most cases of groundwater contamination.In some cases involving a strong source,e.g.,a vapor source above the capillary fringe or a groundwater source with sandy soil above the groundwater level,the normalized building loading rates can be maintained as high as 4-9 without decrease after the first 2-3 hours.Significantly higher increase in building loading rate may indicate a potential presence of a preferential pathway between the groundwater contamination and concerned building.The role of BPC application was examined numerically in enhancing subsurface oxygen-limited biodegradation.The numerical model was first validated with field observations and then used to simulate short-term and long-term BPC applications in petroleum vapor intrusion.The results indicated that during the 48-hour application of the BPC test,the responses of normalized building loading rates at petroleum sites are slightly lower than those at sites involving chlorinated solvents,regardless of the source depth,source concentration,and reaction rate constant.The role of long-term BPC pressurization was examined numerically in generating a subslab aerobic barrier through indoor pressurization at a petroleum site.The numerical model was first validated with field observations and then used to simulate BPC applications in petroleum vapor intrusion scenarios.The results indicated that,with a long-term BPC operation,a subslab aerobic barrier could be generated with an adequate air injection rate(10 L/min in this study)caused by indoor pressurization.In the simulated scenario,the induced sub-foundation aerobic conditions were comparable to those in a field study with a subsurface air delivery system.The effects on hydrocarbon soil gas concentration profiles can last for weeks even after the test.Moreover,our investigations show the performances of the BPC application are virtually independent of hydrocarbon’s reaction rate constant.This study provides an alternative approach to mitigate petroleum vapor intrusion at a low cost and nearly free of engineering work.