Hydrocarbon and oxygen transport in the vicinity of a building overlying a NAPL source zone

Within the context of developing guidelines for assessing petroleum-impacted sites for vapor intrusion pathway significance, the concepts of "exclusion criteria" and an "exclusion distance" are often introduced. The concept is that one uses site criteria (i.e. depth to source, soil setting, etc.) to exclude the need to conduct further pathway assessment, rather than sample chemical concentrations in soil gas or indoor air. This is appealing, particularly for petroleum-impacted sites, because modeling results and field studies suggest that there will be some settings where vapor transport is significantly attenuated by aerobic biodegradation. Three-dimensional modeling results have been used by some to propose depth and source concentration combinations that can be assumed to have de minimis risks. The potential weakness of this approach is that base-case simplified geologies are often used and it is assumed that the base-case inputs are inherently conservative. Case studies are needed to validate modeling results and that is the intent of this work. Soil gas concentrations were monitored in the vicinity of a one-story 2100 ft² building having a basement extending 5 ft below ground surface (BGS). Non aqueous-phase liquid (NAPL) impacted soils are first encountered at about 9.2 m (30 ft) BGS to 10.6 m (35 ft) BGS. The data show relatively uniform and elevated (60-160 mg/L) hydrocarbon vapor concentrations and depleted O₂ beneath and around the building foundation, which is not anticipated by simplistic scenario modeling results. Detailed soil respiration and air permeability test results suggest two possible reasons for the observed behavior, a) significant background O₂ uptake in surface soils or b) physically limited O₂ transport from the atmosphere. Soil O₂ uptake rates at four locations at the depth of 0-4.0 ft BGS around building foundation ranged from 2-25 mg-O ₂/kg-soil/day. There also was a silt/clay zone between 2-5 ft BGS and 7-8 ft BGS with an air permeability of less than 10^-14 m ². The results from this study showed that simplistic generic scenario modeling results should be used with caution, and that factors reducing O₂ transport from the atmosphere to the subsurface can significantly affect the vapor distribution at petroleum hydrocarbon sites.