Formatted Title
Advances in Existing Building Methane Mitigation
Background/Objectives
Over the past three decades, we have learned much about mitigating vapor intrusion in existing buildings. The standard of care has included understanding the porosity of the soil below the slab, defining the entry pathways and convective driving forces, and designing an active soil depressurization system (ASDS) that will maintain an effective pressure differential at the outer extent of the vacuum field during adverse pressure conditions. For chlorinated solvent sites, the risk of fire or explosion is rarely considered. Historically, ASDS airflow has been viewed as a byproduct of achieving the depressurization objective, with little attention given to understanding or quantifying the source of the airflow. These considerations are more important for sites with subsurface methane, such as low-lying coastal developments and buildings constructed over organic fill material.
Approach/Activities
Methane (CH4) is a highly flammable, odorless, and colorless gas that can accumulate below building slabs and in enclosed spaces, posing risks of explosion, fire, and asphyxiation. Accessing the sub-slab for sampling and pressure field extension testing poses unique risks that must be defined and managed. Designing a system requires not just creating a negative pressure below the slab but also identifying sub-slab airflow sources and quantifying distance and directional airflow. A controlled or regulated fresh air inlet system may be part of the design so that conveyed soil gases are continuously below the lower explosive limit (LEL). Designs may need to include explosion-proof blowers and accommodation for sufficient dilution air volumes to ventilate the sub-slab and maintain concentrations below the LEL throughout the mitigation system. Along with motor response actions initiated by soil gas or indoor air methane concentrations, additional sensor-based logic actions that control motor speed and sub-slab ventilation air volume through inlet pipes can be integrated to maintain safety and system efficiency.
Results/Lessons Learned
The presentation will walk through diagnostics, design, implementation, and remote management with an emphasis on methane-specific differentiators. Excerpts from three case studies will share lessons learned, such as procedures to improve safety during the pressure field extension testing, how sub-slab structural components and variability in fill material can influence the relationship between vacuum field extension and airflow. System logic, response actions, and alarm notification hierarchy will be discussed. The findings show that sensor-controlled systems can provide improved performance and reduced energy consumption over traditional methane systems where motors are on-off actuated by methane concentrations in riser pipes.