Soil vapour intrusion of subsurface volatile organic compounds (VOCs) into indoor air of buildings is a significant potential concern at existing sites where chemical releases occur, or at new buildings at Brownfield sites with residual chemical impacts. While soil vapour intrusion mitigation systems are increasingly being implemented, there are limited published data on mitigation performance for VOCs particularly for industrial or commercial buildings or high density residential buildings with below ground parking garages. Data gaps include the effectiveness of passive and active venting systems and reduction in vapour intrusion that can be achieved relative to unmitigated buildings. Because of lack of knowledge and standardization, design practices and post-mitigation monitoring requirements vary widely and are, in some cases, over-conservative. To address these gaps, a comprehensive empirical review of data on the performance of active and passive venting systems and a study using the Modified Johnson and Ettinger Model was completed. The empirical data indicate performance of passive venting systems are variable in terms of venting air flow rates and pressures. The results of modelling for passive venting indicate a wide range of predicted reduction factors, defined as the vapour attenuation factor for a baseline unmitigated building divided by the attenuation factor for the mitigated case. Because of the potential for depressurized buildings and/or reverse vent stack effect, for passive venting systems a continuous leak free barrier that reduces the potential for soil gas diffusion and advection is essential. The performance of active venting systems can be more readily controlled and quantified based on design principles as supported by the results of modelling, which indicated higher reduction factors than for passive venting systems. For both passive and active venting systems, improved efficiency in venting can be achieved through aerated subfloors. A monitoring framework that is robust but efficient and sustainable is presented that incorporates the concept of a concentration exceedance factor and the type of mitigation system
