Section 16: Air - Likelihood of Release

16.1 Observations

It is rarely beneficial to score the air pathway in areas of low population density.
Due to distance-weighting, the air pathway score is frequently very low in the absence of an observed release.
Determining an air pathway score is straightforward, particularly as an adjunct to scoring the soil exposure pathway.
Data from health and safety sampling can frequently be used to:
- support an observed release through direct observation
- provide evidence of biogas release

The air pathway addresses releases to the outside atmosphere only.
Indoor air contamination data can be used to support an outside observed release by chemical analysis.
Indoor air contamination cannot be scored using the air pathway except in terms of its potential to escape into the outside air.
Buildings containing indoor air contamination located on a site can be considered sources in certain circumstances. Consultation with EPA is necessary in such circumstances.
Interstitial air spaces in soil is not considered to be part of the outside atmosphere.

Distinguish between gaseous substances (vapor pressure greater than or equal to 10^-9 torr) and particulate substances (vapor pressure less than or equal to 10^-1 torr).

Highlight 10-2
Vapor Pressure Ranges for Gaseous and/or Particulate Hazardous Substances

An observed release by direct observation can be documented when one of the following occurs:
- Material that contains one or more hazardous substances has been seen entering the atmosphere directly.
- Demonstrated adverse effects may establish an observed release when evidence documents that a material containing hazardous substances has been released into the atmosphere.
- Documentation that the material contains hazardous substances is essential. Such documentation includes: sampling data (e.g., dust samples from piles) and documentary evidence (e.g., permits and compliance records).
- Observed releases by direct observation can usually be established near any permitted air emitting facility.

An observed release to the atmosphere (outdoor air) by chemical analysis is documented when both of the following occur:
- Analysis of air samples shows that the concentration of ambient hazardous substance(s) has increased significantly over background for the site.
- Some portion of the release is attributable to the site.
Establishing an observed release to air through chemical analysis is difficult due to the variability of the atmosphere. Specialized expertise is needed to design and execute air sampling programs.
Methane and similar "landfill gases" are not hazardous substances and cannot be used to establish an observed release through chemical analysis.

Determination of wind direction during sampling is essential
"Background" can be determined using an upwind or cross-wind sample.
Longer sampling durations increase likelihood of release detection but also increase likelihood of changes in wind direction.
All air samples must be comparable:
- taken duration the same, approximate time period
- taken at the same elevation relative to sources at the site
- collected using comparable equipment under comparable operating conditions (e.g., sampling duration)
- analyzed with comparable equipment
Air sampling is not restricted to the "bearing zone."
Soil gas samples cannot be used to demonstrate a significant increase above background level.
Soil gas samples can be used to support attribution.
In special circumstances, health and safety sampling can be used to establish an observed release through chemical analysis. Special attention must be given to sampling QA/QC in such cases.
EPA has prepared specific guidance to support air pathway analyses in the Superfund program: List of Guidances

In a single source, there may be some substances that are gaseous, some substances that are particulate, and some that are both.
The release of gaseous substances is governed by different processes than the release of particulate substances. Therefore, two separate evaluations are needed.
Separate calculations are performed for each source.
For purposes of potential to release evaluations, sources with similar characteristics can be aggregated to form a single "source."
- same containment factor value
- same source type
- significant overlap in hazardous substances
Combining sources is advantageous for small sources or for purposes of expediency
Factors
- Containment
- Source Type
- Migration Potential

The gas source type factor value is assigned from HRS Table 6-4.
Different gas and particulate source type factor values may be assigned to any particular source.
The minimum size requirement applies to source type rather than containment in the air pathway.
If a source does not meet the minimum size requirement (i.e., a hazardous waste quantity value of 0.5 or greater), assign that source a value of 0 for both source type factors.
Evidence of releases of biogas (e.g., methane) substantially increases the source type value for containers/tanks, landfills, and buried surface impoundments.

Obtain the gas migration potential value for each gaseous substance found in the source from the Superfund Chemical Data Matrix (SCDM).
EPA should be consulted whenever values are needed for substances not appearing in SCDM.
Take the average of the three highest values.
Assign a source gas migration potential to the source from HRS Table 6-7.

For each source, sum the gas source type factor value and gas migration potential factor value and multiply this sum by the gas containment factor value.
Select the highest product calculated for the sources evaluated and assign it as the gas potential to release value for the site.

The particulate containment value is assigned from HRS Table 6-9.
These tables require that specific observations are made during the site visit.
Critical Source Features
- depth of uncontaminated soil cover
- presence of liquids
- extent of vegetation
- condition of containers

Different source type factor values are assigned to gas and particulate potential from HRS Table 6-4.
The minimum size requirement applies to source type rather than containment in the air pathway.
If a source does not meet the minimum size requirement (i.e., a hazardous waste quantity value of 0.5 or greater), assign that source a value of 0 for source type.

This value is the same for all sources at the site and is based on the location of the site.

Particulate Migration Potential Factor Values, Figure 6-2

If the site is not located on the map or is too near a boundary on the map, calculate the Thornthwaite precipitation-effectiveness index using mean monthly precipitation and temperature data for the site.

For each source, sum the particulate source type factor value and particulate migration potential factor value and multiply this sum by the particulate containment factor value.
Select the highest product calculated for the sources evaluated and assign it as the particulate potential to release value for the site.