Sustainable Approaches for Soil Gas Mitigation Systems

摘要

Soil gas mitigation is increasingly considered as a pre-emptive strategy at sites impacted with volatile contaminants, radon or naturally-occurring biogenic gases (e.g., methane). For new building construction, there is a range of possible mitigation solutions, which often involve modification of the building envelope in contact with soil to incorporate soil gas venting and/or barrier systems. New approaches are emerging; however, there is limited knowledge and guidance on the design and effectiveness of such measures. This paper focuses on sustainable approaches for soil gas mitigation where aerated sub-floors are used in conjunction with semi-passive (wind turbine) or low-power fans. Aerated sub-floors are much more permeable than gravel layers, and thus are more efficient in term of venting characteristics. Scoping calculations based on civil engineering principles are presented for evaluating the influence of wind forces and the stack effect on venting performance and show that even small frictional losses significantly reduce airflow rates. Because the frictional losses in aerated sub-floors are small, there is significant benefit in use of such systems. To illustrate the effect of venting on vapour intrusion, results are presented for a modified version of the Johnson and Ettinger model that uses semi-analytical mass balance calculations to quantify the reduction in the vapour attenuation factor achieved by venting of a sub-building layer. The air flow calculations, frictional loss calculations, and modified Johnson and Ettinger model represent an improved framework for quantitative analysis of sustainable mitigation options. The paper concludes describing three case study sites where soil gas mitigation systems utilizing aerated sub-floors were installed where volatile organic compounds (e.g., chlorinated solvents or petroleum hydrocarbons) were of potential concern. Building sizes varied from 18,800 to 95,600 sq ft. Performance testing was completed at each site for the purpose of determining the number and size of fans required to provide effective ventilation and to confirm that subslab depressurization was sufficient to overcome variations in building depressurization resulting from changes in weather and building operation. The performance testing indicated that the aerated sub-floors at all three sites could be ventilated using low-power radon fans. For wind turbines, the model calculations and limited testing indicated limited venting efficiency, although the efficiency may be improved through the use of aerated sub-floors and the adoption of design practices to minimize pipes frictional losses.