Evaluation Of Vapor Intrusion Risks In Contaminated Sites Based On Three- Dimensional Numerical Simulation And Quantitative Analysis

Contaminated sites as a global environmental problem has posed a serious threat to human health and the environment, especially for vapor intrusion (VI) which is a process for the contaminant soil gas being released from sources and entering the concerned buildings on the surface. It is critical to evaluate the VI risks and take measures to prevent the VI threat to people’s health by adopting more direct and effective methods. Some studies showed that environmental factors may play an important role in VI process, but not included in analytical model established. Therefore, based on sites contaminated by chlorinated hydrocarbon and petroleum hydrocarbon, the effect of environment factors in VI was investigated by means of the three-dimensional numerical model, and then the analytical models established to accurately assess the risks of VI. The primary conclusions were as followed:(1) At sites contaminated by chlorinated compounds, U.S. EPA (Environmental Protection Agency) proposed a general buffer zone of approximately 100 feet (30 m) laterally to determine which buildings to include in VI investigations in 2002. However, this screening distance can be threatened by factors such as extensive surface pavements. Under such circumstances, EPA recommended investigating soil vapor migration distance on a site-specific basis. To serve this purpose, we present an analytical model-AAMLPH (Analytical Approximation Method involving Lateral soil gas transport, Paved ground surface and Heterogeneous soil) as an alternative to estimate lateral VI screening distances at chlorinated compound-contaminated sites. Based on a previously introduced model-AAML (Analytical Approximation method with Lateral distance), AAMLPH is developed by considering the effects of impervious surface cover and soil geology heterogeneities, providing predictions consistent with the three-dimensional (3-D) numerical simulated results. By employing risk-based and contribution-based screening levels of subslab concentrations (50 and 500μg/m3, respectively) and source-to-subslab attenuation factor (0.001 and 0.01, respectively), AAMLPH suggests that buildings greater than 30 m from a plume boundary can still be affected by VI in the presence of any two of the three factors, which are high source vapor concentration, shallow source and significant surface cover. This finding justifies the concern that EPA has expressed about the application of the 30 m lateral separation distance in the presence of physical barriers (e.g., asphalt covers or ice) at the ground surface.(2) At petroleum vapor intrusion (PVI) sites, to identify the buildings that can be threatened by such hydrocarbon vapors, the EPA recently proposed a screening criteria for vertical source-building separation distance. But in Such scenarios where is significant methane generation, upward advective soil gas transport may be observed, which may increase the health risks for PVI and explosion risk for MI (Methane Vapor Intrusion). To evaluate the health and explosion risks that may exist under such scenarios, a one-dimensional analytical model describing these processes is introduced in this study. This new model accounts for both advective and diffusive transport in soil gas and couples this with a piecewise first-order aerobic biodegradation model, limited by oxygen availability. The predicted results from the new model are shown to be in good agreement with the simulation results obtained from a three-dimensional numerical model. These results suggest that this analytical model is suitable for describing cases involving open ground surface beyond the foundation edge, serving as the primary oxygen source. This new analytical model indicates that the major contribution of upward advection to indoor air concentration could be limited to the increase of soil gas entry rate, for example, the upward soil gas flow entry rate can cause about one order of magnitude increase in soil gas entry rate when the Peclet number changes from 0-20, since the oxygen in soil might already be depleted owing to the associated high methane source vapor concentration. Thus the 6.1-meter vertical screening distance suggested by EPA may not long enough to eliminate the risk of PVI for buildings overlying the sites with high methane source concentration.