Proof-of-concept study of an aerobic vapor migration barrier beneath a building at a petroleum hydrocarbon-impacted site.

A proof-of-concept study was conducted to evaluate an alternative to traditional extraction-based subslab vapor mitigation systems at sites with petroleum hydrocarbon and/or methane vapor impact concerns. The system utilizes the slow delivery of air beneath a foundation to attenuate vapor migration to the building via aerobic biodegradation. The study was conducted at a site having elevated hydrocarbon plus methane and depleted O(2) vapor concentrations (160 mg/L and <1% v/v, respectively) beneath a building having a 195 m(2) footprint and a basement extending 1.5 m below ground surface (BGS). Nonaqueous phase liquid (NAPL)-impacted soils, first encountered at about 7.6 to 9.1 m BGS, were the source of hydrocarbon and methane vapors, with the latter being generated by anaerobic methanagenesis of the former. O(2) concentrations beneath and around the building were monitored prior to and during air injection through a horizontal well installed about 1.5 m beneath the foundation. The air injection rate was increased from 1 to 5 to 10 L/min, with each held steady until the O(2) distribution stabilized (46-60 d). The 10 L/min flow rate achieved >5% v/v soil gas O(2) concentrations beneath the foundation and spanning a 1.5 m vertical interval. It was within 3× of the pretest stoichiometric requirement estimate of 3.8 L/min. This resulted in reductions in subslab hydrocarbon plus methane concentrations from 80 to <0.01 mg/L and benzene, toluene, ethylbenzene, and xylenes (BTEX) reductions to below detection limits (0.5-0.74 ppb(v)). This air injection rate is <1% of flows for typical extraction-based mitigation systems.