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

Outdoor Air Restriction

• 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.

Gaseous and Particulate Substances

• 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).

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Vapor Pressure Ranges for Gaseous and/or Particulate Hazardous Substances

16.2Likelihood of Release

Observed Release by Direct Observation

• 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: or
  • 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.

Observed Release by Chemical Analysis

• 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.

Special Considerations for Air Sampling

• 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

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Potential to Release

• 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

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Gas Containment

• The gas containment value is assigned from HRS Table 6-3.
• These tables require that specific observations are made during the site visit.
• Critical Source Features
  • depth of uncontaminated soil cover
  • extent of vegetation
  • releases of biogas (e.g., methane)
  • active fires
  • condition of containers

Gas Source Type

• 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.

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Gas Migration Potential

• Obtain the gas migration potential value for each gaseous substance found in the source from 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.

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Gas Potential to Release

• 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.

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Particulate Containment

• 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

Particulate Source Type

• 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.

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Particulate Migration Potential

• 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.

Particulate Potential to Release

• 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.

Overall Potential to Release

• The overall potential to release value is the higher of the gas and particulate potential to release values.

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