Comparison of the Johnson-Ettinger vapor intrusion screening model predictions with full three-dimensional model results

Abstract

The Johnson-Ettinger vapor intrusion model (J-E model) is the most widely used screening tool for evaluating vapor intrusion potential because of its simplicity and convenience of use. Since its introduction about twenty years ago, the J-E model has become a cornerstone in guidance related to the potential for significant vapor intrusion-related exposures. A few papers have been published that claim it is a conservative predictor of exposure, but there has not been a systematic comparison in the open literature of the J-E model predictions with the results of more complete full three-dimensional descriptions of the phenomenon. In this paper, predictions from a three-dimensional model of vapor intrusion, based upon finite element calculations of homogeneous soil scenarios, are directly compared with the results of the J-E model. These results suggest that there are conditions under which the J-E model predictions might be quite reasonable but that there are also others in which the predictions are low as well as high. Some small modifications to the J-E model are also suggested that can bring its predictions into excellent agreement with those of the much more elaborate 3-D models, in some specific cases of homogeneous soils. Finally, both models were compared with actual field data.

Figure 1

Cross sectional view of (a) full domain of interest (b) boundary conditions of modeled quarter domain (c) details of modeled perimeter crack

Figure 2

(a) The influence of foundation depth and depth of source on the soil gas entry flow through a perimeter crack based on 3-D simulation. (b) Estimates of the soil gas flow through the crack using different calculational methods.

Figure 3

The comparison of the EPA implementation of the J-E model (curves) with the full 3-D simulation (points) for a 2m depth foundation (a) and a 0.1m depth foundation (b); the comparison of the revised J-E model (flow and diffusivity corrections) results with those of the full 3-D simulation, for a 2 m depth foundation (c) and a 0.1m depth foundation (d); The comparison of the revised J-E model (curves, which include flow, diffusivity and mass conservation corrections) on indoor air concentration attenuation factor with detailed simulation results (points) for a 2m deep foundation (e) and a 0.1m depth foundation (f).

Figure 4

The relationship between indoor air concentration attenuation factor and effective diffusivity for a source at 8m bgs for the J-E model. (Here, the modified Q_ck and D^crack corresponding to gas-phase diffusivity have been used; Lines are from the J-E model, and points are from 3-D simulation).

Figure 5

(a) The influence of soil permeability on contaminant release rate from the source based on 3-D simulation for 2m (solid points) and 0.1m (open points) depth foundation. (b) The influence of soil permeability on mass conservation ratio from 3-D simulation for 2m (solid points) and 0.1m (open points) depth foundation. The values in the legends refer to source depth (m).