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Vapor Intrusion

EPA Spreadsheet for Modeling Subsurface Vapor Intrusion

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Model Spreadsheet and Documentation


Background

Because of concerns that subsurface contamination in either soil or groundwater can adversely impact indoor air quality, in 1998, EPA developed a series of models for estimating indoor air concentrations and associated health risks from subsurface vapor intrusion into buildings. These models were based on the analytical solutions of Johnson and Ettinger (1991) for contaminant partitioning and subsurface vapor transport into buildings.

Since that time, EPA has revised the models and added a series of new models. The spreadsheet tool below was updated in September 2017, and is version 6.0.

The spreadsheet tool implements the steady-state solution to vapor transport (infinite or non-diminishing source and steady-state vapor concentrations) described by Johnson and Ettinger in 1991. The spreadsheet tool allows the user to input a site-specific subsurface soil gas concentration and sampling depth, or a groundwater concentration and depth to groundwater.

The model provides default values of vadose zone characteristics, building characteristics and exposure parameters, and it allows the user to enter site-specific information. Model output includes:

  • Risk-based soil gas or groundwater concentrations below which associated health effects are considered unlikely; and
  • Estimates of the incremental risks associated with user-defined initial soil gas or groundwater concentrations.

The model reverse-calculates an “acceptable” soil gas or groundwater concentration given a user-defined risk level (i.e., target risk level or target hazard quotient). The model also forward-calculates an incremental cancer risk or hazard quotient based on an initial soil or groundwater concentration.

The models have been revised to incorporate the default values recommended in EPA’s:

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Version 6.0 Updates

In addition, version 6.0 of the model (updated September 2017) implements a number of updates to improve ease of use, transparency and interpretability. Updates include:

  • The groundwater and soil gas models have been combined into the same sheet, although only one model may be run at a time.
  • The DATAENTER, CHEMPROPS, INTERCALCS and RESULTS worksheets in the previous version have been combined into the MODEL worksheet in Version 4.0, and the screening and advanced models in the previous version have been combined into a single interface.
  • Chemicals, building characteristics, and soil types are selected by name via dropdown menus. The model automatically displays the relevant default values for chemical properties, building characteristics, and vadose zone characteristics once the user selects the appropriate inputs.
  • The model includes a blue-highlighted preview section at the top of the MODEL sheet which displays the indoor air attenuation factor (α) and the indoor air concentration due to vapor intrusion based on currently entered input values.
  • To be consistent with the 2015 OSWER Vapor Intrusion Guidance (PDF)(267 pp, 3 MB), EPA has removed the soil model option and non-aqueous phase liquids models.
  • The model can now calculate output for multiple chemicals simultaneously.
  • Risk-based target screening levels and incremental risk estimates are now included in the model predictions.
  • Three new foundation types are available: a basement with a dirt floor, a closed crawlspace with a dirt floor, and a closed crawlspace with slab.
  • The output now includes a graph of the predicted soil gas concentration by depth to allow users to compare values calculated by the model to measured soil gas values.
  • An interpretation section displays whether advection or diffusion is the dominant mechanism of transport across the foundation of the building or whether both diffusion and advection contribute to this process.
  • Error messages now indicate if values input by the user are outside of reasonable ranges. These warning messages appear next to the parameter entered as well as in the output section of the MODEL sheet.
  • The model now outputs a range for the predicted attenuation factors, indoor air concentration, and human health risk based on the range of values for Qsoil/Qbuilding reported in the literature.

For additional information, please review the user’s guide for detailed explanations of the model, default values, and running the workbook.

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