Emissions of VOCs and particulate matter during site disturbances, such as excavation, may be several orders of magnitude greater than the emission levels of an undisturbed site. The potential air emissions from the undisturbed and disturbed site must be understood before developing a site mitigation strategy. EPA has developed a systematic approach, called an Air Pathway Analysis (APA), for determining what air contaminants are present and at what level these compounds may be released into the atmosphere. The APA method is outlined in a four-volume series (Air Superfund National Technical Guidance Study Series, EPA, 1989).
Emissions from treatment systems (e.g., SVE or incinerators, etc.) may be approximated by using soil contaminant concentrations and flow or throughput rate.
If the use of thermal combustion or certain oxidation systems is planned for off-gas treatment of extracted vapors, then an adequate supply of air/oxygen must be provided in order to operate these efficiently.
Information regarding the concentration and permeability/percent flow at discrete vertical intervals is extremely useful in optimized recovery from the regions of highest contaminant mass/removal potential. In other words, if 90% of the contaminant mass is being extracted from only 5% of the vertical interval, then off-gas treatment is biased by the large contribution of uncontaminated soil gas. Thus, changes in screened intervals, flow rates, mass transfer rates, and residual contaminant composition over time can dramatically affect off-gas treatment and should be evaluated.