Proof-of-Concept Study of an Aerobic Vapor Migration Barrier Beneath a Building at a Petroleum Hydrocarbon-Impacted Site

Abstract

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.

Three-dimensional vapor intrusion modeling approach that combines wind and stack effects on indoor, atmospheric, and subsurface domains

“…4, 12 The findings showed that wind flow is important in influencing (an aerobically biodegradable) contaminant and oxygen concentrations in the soil under a building. The results showed low concentration of contaminant and high concentration of oxygen at the windward side of the building, while a high concentration of contaminant and depleted oxygen was reported on the leeward side of the building.…”

“…The results showed low concentration of contaminant and high concentration of oxygen at the windward side of the building, while a high concentration of contaminant and depleted oxygen was reported on the leeward side of the building. 4, 12 The study concluded that the oxygen delivered to the soil by wind on the windward side enhanced aerobic biodegradation of the contaminant. Luo 12 developed a numerical model that modified the 3-D VI model previously developed by Abreu and Johnson 2 to account for the influence of wind and stack effects; however the modified model did not consider the effect of wind and stack effects on AER when calculating the indoor air concentration.…”

“…Wind flow not only influences AER and air pressure inside a building, but also influences the ground surface pressure adjacent to the building, which consequently alters the soil gas flow and subsurface soil gas concentrations under the building. 12, 14, 15 Previous radon research has highlighted the importance of wind and stack effects for radon intrusion. For instance, Riley et al 14, 15 used the mean ground-surface pressure coefficients obtained from a wind tunnel experiment on a single family structure to investigate the effect of wind speed and direction on radon concentration in soil and indoor air.…”

“…Luo 12 developed a numerical model that modified the 3-D VI model previously developed by Abreu and Johnson 2 to account for the influence of wind and stack effects; however the modified model did not consider the effect of wind and stack effects on AER when calculating the indoor air concentration. 12 …”

“…4, 12, 14, 15, 18, 19 Reichman et al 22 noted that existing VI models require users to input generic building pressures and AERs even though indoor air science research provides tools to determine AERs and building pressures based on building characteristics, occupant behavior and weather conditions.…”