Oxygen transport from the atmosphere to soil gas beneath a slab-on-grade foundation overlying petroleum-impacted soil

Modeling and field study results suggest that, in the case of a building overlying an aerobically biodegradable vapor source (i.e., petroleum-impacted soil), the significance of vapor intrusion into the building depends on the source vapor concentration, the relative position of the vapor source and building, and the rate of O₂ transport from the atmosphere to the soil gas beneath the building. This work quantified the latter at a house having about a 250 m² slab-on-grade foundation footprint. It was constructed on 1.5 m of clean fill overlying a petroleum hydrocarbon-impacted soil layer undergoing methanogenesis. Soil gas O₂ and CH₄ profiles adjacent to and beneath the foundation were measured and then the soil gas beneath the slab was rapidly displaced with N₂. The natural replenishment of O₂ was monitored for 90 days using in situ O ₂ sensors, and the responses with time were similar, independent of location. The O₂ replenishment rate was about 2500 g-O₂/d immediately after the N₂ flood and then it declined to 200-500 g-O₂/d over 30 days. Weather events affected the O₂ replenishment rate; an increase occurred during a strong wind period (>3 m/s), and a decrease occurred during a heavy rainfall event. The spatial and temporal patterns in the O₂ sensor responses and quantified O ₂ replenishment rates could not be accounted for by simple mechanistic hypotheses involving lateral diffusion or advection through the bulk soil, and instead the data suggest rapid replenishment immediately below the foundation followed by downward diffusion.