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Evaluating Vapor Intrusion into Buildings from Contaminated Groundwater and Soils

Background

Migration of volatile chemicals from the subsurface into overlying buildings is called vapor intrusion (VI). Volatile organic chemicals in contaminated soils or groundwater can emit vapors, which may migrate through subsurface soils and may enter the indoor air of overlying buildings. Building depressurization may cause these vapors to enter the home through cracks in the foundation. Depressurization can be caused by a combination of wind effects and stack effects, which are the result of heating within the building and/or mechanical ventilation. In extreme cases, the vapors may accumulate in dwellings to levels that may pose near-term safety hazards, such as explosion. Typically, however, vapor concentrations are present at low levels, to which long-term exposure may pose increased risk for chronic health effects.


Figure 1. Conceptual diagram of vapor intrusion into overlying buildings.

This on-line calculator implements the Johnson and Ettinger (J&E) (Johnson and Ettinger, 1991) simplified model to evaluate the vapor intrusion pathway into buildings. This J&E model replicates the implementation that the US EPA Office of Solid Waste and Emergency Response (OSWER) used in developing its draft vapor intrusion guidance, but includes a number of enhancements that are facilitated by web implementation: temperature dependence of Henry's Law Constants, automatic sensitivity analysis of certain parameters, and others described below.

The J&E model has become increasingly popular with regulators and consultants over the last 10 years and several manuscripts have been published on its use (see citation list on the following page). Briefly, the model is a one-dimensional analytical solution, which incorporates both advection and diffusion transport mechanisms to produce a unit-less "attenuation factor". This attenuation factor is a measure of how soil and building properties limit the intrusion of organic vapors into overlying buildings and is defined as the concentration of the compound in indoor air divided by the concentration of the compound in soil gas or groundwater. Chemical concentrations in groundwater will attenuate more than chemicals in soil gas due to the added limitations imposed by mass-transfer across the capillary fringe.The larger the attenuation factor produced by the model, the greater the intrusion of vapors into indoor air.

The J&E model was based on a number of simplifying assumptions (e.g., homogeneity, diffusion-only through subsurface, uncontaminated capillary fringe, etc.). The reader is directed to Environmental Quality Management (2003) or U.S. Environmental Protection Agency (2002) for a discussion of these limitations. Conditions under which the J&E model should not be used include:

  • Presence or suspected presence of NAPLs;
  • Heterogeneous geology, fractured media, karst or macropores;
  • Sites where significant lateral flow of vapors may occur (e.g., utility conduits);
  • Very shallow groundwater that wets building foundation;
  • Very small building air exchange rates (e.g., <0.25/hr);
  • Buildings with crawlspaces, earthen floors, stone floors, etc.;
  • Contaminated groundwater sites with large fluctuations in water table elevations; and
  • Sites with time-varying flow rates and/or concentrations for which a steady state assumption is not conservative.


Evaluating Vapor Intrusion using this Web-Based Model

Input

Use of the on-line vapor intrusion model is intended to be intuitive and simple. The user enters a site name or description (optional), selects the building type, contaminant of concern and soil type beneath the building and enters the depth to the contamination and estimated bounds on this depth. Soil or groundwater temperature is also required to calculate the temperature-dependent Henry's Law Constant. Default soil, building and exposure parameters that populate the input sheet based on user selections are obtained from U.S. Environmental Protection Agency (2002) and the user is directed to this document for a discussion of their justification. Contaminants of concern are selected from a list of 108 chemicals that may be found at contaminated sites. Their default values are obtained from Environmental Quality Management (2003). Users may accept the default parameter values or overwrite them with site-specific information, if available.

Results

Upon clicking the "Calculate Results" button the on-line calculator performs two calculations and presents their results. First, the Johnson and Ettinger model is run to produce two attenuation factors for the contaminant-soil-building system: one for soil gas and one for groundwater. If a user has soil or groundwater concentration information for a particular site, then these attenuation factors can be used to estimate concentrations that can be expected in overlying structures.
Second, the on-line model calculates an indoor-air concentration based on a user-defined target cancer risk ranging from 1x10-6 to 1x10-4 (for cancer-causing contaminants) or a target hazard quotient of 1 (for non-cancer-causing contaminants). The model then reverse-calculates an "acceptable" soil-gas or groundwater concentration using this indoor air concentration and the attenuation factors described previously. Users select the risk value from a drop down menu and are immediately displayed the effect of their selected value on estimated risk-based soil-gas and groundwater cleanup goals.

Sensitivity Analysis

Secondary screening target concentrations require at least a rudimentary knowledge of subsurface conditions such as depth of contamination and soil type. A simple sensitivity analysis is produced with the output results, providing upper and lower bounds on the target concentrations based on these two parameters. Depth to contamination may contain uncertainty due to estimation error or to seasonal variability in water table depth. A user-defined bound on depth of contamination is included to accommodate this potential uncertainty. Additionally, a range of default water-filled-porosities in the unsaturated zone based on soil type is included in the input section. This moisture content range is taken from values presented in U.S. Environmental Protection Agency (2002).

Important Features of this Online Vapor Intrusion Model

This on-line implementation of the Johnson and Ettinger model utilizes the power of web-based modeling to incorporate the following features:

  • Automatic sensitivity analysis on user-supplied range of two important parameters - depth to contamination and vadose zone moisture content;
  • Information on transport limitations in modeled system through analysis of system parameters as developed by Paul Johnson (2002);
  • Formatted report feature allowing convenient one-page printouts of model input and results;
  • Automatic testing of soil and building parameter "reasonableness" with alert if data is outside of EPA recommended ranges;
  • Incorporation of compound-specific Henry's Law Constant, diffusivity in air and diffusivity in water values;
  • Inclusion of temperature dependence in Henry's Law Constant values
  • Instantly updated target concentrations for quick comparison of new user-specified cancer risk factors; and
  • Ability to incorporate differences in basement and slab-on-grade building types in attenuation calculation.

Citations

Environmental Quality Management, 2003.
for U.S. Environmental Protection Agency - Office of Emergency and Remedial Response, Washington, D.C.
User's Guide for Evaluating Subsurface Vapor Intrusion into Buildings.
http://www.epa.gov/superfund/programs/risk/airmodel/johnson_ettinger.htm

Hers, I., Zapf-Gilje, R., Johnson, P.C. and Li, L., 2003.
Evaluation of the Johnson and Ettinger Model for Prediction of Indoor Air Quality.
Ground Water Monitoring and Remediation, 23(2): 119-133.

Johnson, P.C., 2002.
Identification of Critical Parameters for the Johnson and Ettinger (1991) Vapor Intrusion Model.
American Petroleum Institute Technical Bulletin Number 17: 38.

Johnson, P.C. and Ettinger, R.A., 1991.
Heuristic Model for Predicting the Intrusion Rate of Contaminant Vapors into Buildings.
Environmental Science and Technology, 25(8): 1445-1452.

Johnson, P.C., Kemblowski, M.W. and Johnson, R.L., 1999.
Assessing the Significance of Subsurface Contaminant Vapor Migration to Enclosed Spaces:
Site-Specific Alternatives to Generic Estimates. Journal of Soil Contamination, 8(3): 389-421.

U.S. Environmental Protection Agency, 2002.
USEPA Office of Solid Waste and Emergency Response, Washington, D.C.
Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils.
http://www.epa.gov/correctiveaction/eis/vapor.htm

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