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Charge Questions for Expert Panel

Charge Questions for Expert Panel. Modeling Vapor Attenuation Workshop Annual International Conference on Soils, Sediments, and Water October 19, 2004 Amherst, MA. 1: Models and Measurements. Does the VI Guidance use the proper balance of model predictions and observations?

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Charge Questions for Expert Panel

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  1. Charge Questions forExpert Panel Modeling Vapor Attenuation Workshop Annual International Conference on Soils, Sediments, and Water October 19, 2004 Amherst, MA

  2. 1: Models and Measurements • Does the VI Guidance use the proper balance of model predictions and observations? • Models (Predicting Exposure Pt. Conc. from GW or SG) • Semi-site-specific (Figure 3) • Site-specific • Measurements (GW or SG; indoor air)

  3. 2: VI Database • Is the VI database comprehensive enough to make decisions? If not, what is missing? Topics: • Data quality and extent • Resistant vs. degradable compounds • Precluding factors • Generic alphas

  4. 3: Figure 3 and Observed Alphas • In cases where observed alphas plot above or well below the line in Figure 3, are there reasons that could be used to improve the VI Guidance? Topics: • Values higher than Figure 3 attenuation factors • Values well below Figure 3 attenuation factors

  5. 4: Indoor Air Background • How do background VOC concentrations in indoor air affect the reliability of observed alpha datasets? • How should we screen for background influences? • If we could remove “background,” what would the measured alphas look like?

  6. 5: Factors Impacting Attenuation • What variables influence attenuation and how do they rank in terms of impacts on alphas? How should they be considered in the VI Guidance? Variables: • Chemical type (resistant, degradable) • Depth to source • Soil type • Weather conditions • Building (type, foundation, ventilation)

  7. 6: Precluding Factors • Are the VI Guidance Precluding Factors appropriate and adequate for generic and semi-site-specific screening? Site-specific modeling? VI Guidance Precluding Factors: • Signs for immediate action: wet buildings, NAPL, … • Sources of vapors above the water table, or • Shallow GW (<5 ft below building foundation), or • Relatively shallow groundwater sources )<15 ft) and • Crawlspace homes (without liners*) • Very permeable geology • Significant preferential pathways • Mobile gas plumes (landfill gas, vapor clouds) • Very low air exchange rates or • Very high (negative) pressure differentials

  8. 7: Shallow Sources • Is it technically justifiable to allow (for SG or GW): • Semi-site-specific models within 5 ft of building base? • Site-specific models within 5 ft of building base? • Do you recommend we modify VI Guidance to allow it? • Note: Basis for this policy in VI Guidance was largely public relations

  9. 8: Q4 and Q5 Screening • Do we need both Q4 and Q5? Could we combine them? • Q4 = Generic (observation-based screening • Q5 = Semi-site-specific (model-based screening • Should we continue to use different Henry’s law constant temperatures (approximately 2x factor)? • 25 C in Q4 Table 2 • 10 C in Q5 Table 3

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