EPA Spreadsheet for Modeling Subsurface Vapor Intrusion
Johnson and Ettinger (1991) Model for Subsurface Vapor Intrusion into Buildings
Concerns have been raised about the potential for subsurface contamination in either soil or ground water adversely impacting indoor air quality. In September 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, revisions to the models have been made and a series of new models have been added. Data for any chemical may be edited, new chemicals added, or existing chemicals deleted from the Chemical Properties Lookup Table within the VLOOKUP worksheet. To begin an editing session, the user must unprotect (unseal) the worksheet (the password is "ABC" in capital letters); editing of individual elements or addition and deletion of chemicals may then proceed.
The models have been revised to incorporate the default values recommended in Appendix G of the EPA Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway (November, 2002). The revisions include new values of intermediate variables for estimating the soil vapor permeability and the degree of water saturation in the capillary zone above the water table. In addition, new human health benchmarks have been added for some chemicals and revised for others.
Finally, a series of automatic checks have been added to the models to prevent the use of inappropriate initial soil or ground water contaminant concentrations (i.e., soil concentrations greater than the soil saturation concentration or ground water concentrations greater than the solubility limit). The 3-phase soil contamination models listed below theoretically partition the contamination into three discrete phases:
- in solution with water,
- sorbed to the soil organic carbon, and
- in vapor phase within the soil air-filled pores.
The 3-phase models replace the old models previously available on this website. These models have been constructed in Microsoft® Excel and are applicable when NAPL is not present in subsurface soils or in ground water.
In addition to the 3-phase models discussed above, two new models have been added allowing the user to estimate vapor intrusion into buildings from measured soil gas data. These models are also constructed in Excel and are included in the 3-phase model zip files listed below. The User's Guide also shown below covers use of both the 3-phase models and the soil gas models.
When NAPL is present in soils, the contamination includes a fourth or residual phase. In such cases, the new NAPL models listed below can be used to estimate the rate of vapor intrusion into buildings and the associated health risks. The new NAPL models use a numerical approach for simultaneously solving the time-averaged soil and building vapor concentration for each of up to ten soil contaminants. This involves a series of iterative calculations for each contaminant. The NAPL models are available in Excel. The NAPL model User's Guide that accompanies the new models is also listed below.
Finally, a new fact sheet has been constructed that gives the procedures used to correct the Henry's law constant of a chemical for the actual soil or ground water temperature. The procedures in the fact sheet are the same as those used in the 3-phase models for correcting the Henry's law constant. These procedures and calculations can be used when adding new chemicals and associated chemical properties to those already included in the models.