A numerical investigation of vapor intrusion — The dynamic response of contaminant vapors to rainfall events

Abstract

The U.S. government and various agencies have published guidelines for field investigation of vapor intrusion, most of which suggest soil gas sampling as an integral part of the investigation. Contaminant soil gas data are often relatively more stable than indoor air vapor concentration measurements, but meteorological conditions might influence soil gas values. Although a few field and numerical studies have considered some temporal effects on soil gas vapor transport, a full explanation of the contaminant vapor concentration response to rainfall events is not available. This manuscript seeks to demonstrate the effects on soil vapor transport during and after different rainfall events, by applying a coupled numerical model of fluid flow and vapor transport. Both a single rainfall event and seasonal rainfall events were modeled. For the single rainfall event models, the vapor response process could be divided into three steps: namely, infiltration, water redistribution, and establishment of a water lens atop the groundwater source. In the infiltration step, rainfall intensity was found to determine the speed of the wetting front and wash-out effect on the vapor. The passage of the wetting front led to an increase of the vapor concentration in both the infiltration and water redistribution steps and this effect is noted at soil probes located 1 m below the ground surface. When the mixing of groundwater with infiltrated water was not allowed, a clean water lens accumulated above the groundwater source and led to a capping effect which can reduce diffusion rates of contaminant from the source. Seasonal rainfall with short time intervals involved superposition of the individual rainfall events. This modeling results indicated that for relatively deeper soil that the infiltration wetting front could not flood, the effects were damped out in less than a month after rain; while in the long term (years), possible formation of a water lens played a larger role in determining the vapor intrusion risk. In addition, soil organic carbon retarded the transport process, and damped the contaminant concentration fluctuations.

Influence of Soil Moisture on Soil Gas Vapor Concentration for Vapor Intrusion

“…Soil gas or moisture advection in transient processes, such as with atmospheric pressure change (Picone et al, 2012), groundwater level fluctuation (Picone et al, 2012), and rainfall (Tillman and Weaver, 2007;Shen et al, 2012), have been considered in previous work and are ignored here. An experimental study (McCarthy and Johnson, 1993) of soil moisture distribution in sandy soil is considered as a basis for a 1D modeling scenario.…”

“…The vapor intrusion modeling studies related to soil moisture were mostly focused on transient effects (Sanders and Talimcioglu, 1997;Tillman and Weaver, 2007;Picone et al, 2012;Shen et al, 2012). A previous study coupled transient soil water and gas flow processes with the vapor transport process.…”

“…A previous study coupled transient soil water and gas flow processes with the vapor transport process. Such models are complicated to solve in two-or three-dimensions; therefore, previous work (Sanders and Talimcioglu, 1997;Tillman and Weaver, 2007;Picone et al, 2012;Shen et al, 2012) either simplified the scenarios into one-dimensional (1D) scenarios, or assumed quasiequilibrium of either the flow or transport process. A full description of transient vapor transport processes must include the role of soil moisture, even in a hydrostatic state (Bear, 1988).…”

Role of the source to building lateral separation distance in petroleum vapor intrusion

“…It is worth noting that the higher attenuation, i.e. the lower lateral screening distance, obtained in the case of groundwater contamination is due to the presence of the capillary fringe that, as known, strongly limits the vapor migration due to the low effective air-phase porosity (Shen et al, 2012;Picone et al, 2012). The lateral exclusion criteria described above can be drastically reduced depending on the source concentration.…”

On-Site Gas Chromatography/Mass Spectrometry (GC/MS) Analysis to Streamline Vapor Intrusion Investigations

“…These factors include soil moisture content, rainfall events, building pressurization, and air exchange rates (Shen et al, 2012;Folkes et al, 2009). Manipulation of building pressure can be used as a tool to reduce the uncertainties from these and other factors by controlling and measuring building pressure while the building is being sampled.…”