Article pubs.acs.org/est
Examination of the Influence of Environmental Factors on Contaminant Vapor Concentration Attenuation Factors Using the U.S. EPA’s Vapor Intrusion Database Yijun Yao,† Rui Shen,† Kelly G. Pennell,†,‡ and Eric M. Suuberg*,† †
School of Engineering, Brown University, Providence, Rhode Island 02912, United States ABSTRACT: Those charged with the responsibility of estimating the risk posed by vapor intrusion (VI) processes have often looked to information contained in the U.S. Environmental Protection Agency (EPA)’s VI database for insight. Indoor air concentration attenuation factors have always been a key focus of this database, but the roles of different environmental factors in these attenuation processes are still unclear. This study aims to examine the influences of these factors in the context of the information in the VI database. The database shows that the attenuation factors vary over many orders of magnitude and that no simple statistical fluctuation around any typical mean value exists. Thus far, no simple explanation of this phenomenon has been presented. This paper examines various possible contributing factors to the enormous range of observed values, looking at which ones can plausibly contribute to explaining them.
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INTRODUCTION The U.S. EPA’s VI database1 is a collection of measured data from VI sites throughout U.S.2 The database includes all kinds of information involving building foundations types, contaminants, subsurface, and indoor air contaminant concentrations. The “attenuation factors” (AF) of indoor air concentration to subsurface soil vapor concentrations have always been a key focus of this database and related reports3−5 published by U.S. EPA’s Office of Solid Waste and Emergency Response (OSWER). In both database and reports, statistical analyses and evaluation were made of different contaminant vapor concentration attenuation factors and data-screening criteria, and a conclusion was reported that VI conceptual models are basically reliable.6 However, except for background indoor air concentration,4 there has been little quantitative discussion about the role of different environmental factors based on results in this database. The possible role of both spatial and temporal variability, and even nonrepresentative samples was mentioned, but leaves practitioners with little quantitative guidance. Figure 1 presents cumulative probability plots of the measured AF as reported in the EPA’s VI database. These kinds of plots, also presented by U.S. EPA in its analysis of the database,1,3,5,6 present cumulative probability distributions for the indicated AF ratios, i.e., (cin)/(cgw), (cin)/(css), and (css)/ (c gw ), where c in , c ss , and c gw are contaminant vapor concentrations from indoor air, subslab, and near groundwater surface, respectively. The value of cgw in the VI database is normally based on groundwater sampling and vapor concentration is estimated from Henry’s Law.1 © 2012 American Chemical Society
There are two types of contaminant source, polluted groundwater and contaminant vapor from nonaqueous phase vapor liquid (NAPL), in unsaturated media. In the EPA database,1 polluted groundwater was considered as the major contaminant vapor source and is the focus of this study. The contaminant vapor concentration attenuation process from groundwater source to indoor air consists of two parts, the first involving transport from groundwater to subslab and the second from subslab to indoor air. Figure 1 shows that the distributions of these different measured AF values are similar. They all show a large variation (6−7 orders of magnitude) which is why they are plotted on a log scale. This range is independent of biodegradation, since cumulative probability plots are also similar for nonbiodegradable contaminants, such as PCE and TCE. There, however, is not an apriori reason to see this large a variation in AF, according to the governing equations of vapor transport used in intrusion modeling. The results of an examination of the influence of various factors on the variation seen in the database are presented below. We consider first the commonly accepted equations governing contaminant transport from source through soil to the receptor building. Received: Revised: Accepted: Published: 906
August 24, 2012 November 20, 2012 December 19, 2012 December 19, 2012 dx.doi.org/10.1021/es303441x | Environ. Sci. Technol. 2013, 47, 906−913
Environmental Science & Technology
Article
Figure 1. Cumulative probability plots for contaminant vapor concentration attenuation factors (a: all contaminants; b: PCE and TCE; cin: indoor air contaminant vapor concentration; css: subslab contaminant vapor concentration; cgw: groundwater contaminant vapor concentration).1
Figure 2. Indoor air and subslab contaminant vapor concentration data from the U.S. EPA’s VI database.1
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GOVERNING EQUATIONS IN VI MODELS Soil Vapor Transport. In steady state, the nondimensionalized equation governing nonbiodegradable contaminant vapor transport can be written as 7 ⎛ *q ⎞ C w⎟ * * * 0 = −∇*·C* − ∇*·⎜⎜ ⎟ + ∇ · (D ∇ C ) H q ⎝ g ⎠
enclosed space of concern as continuous stirred tank(s) (CST)8 cin =
Js + VbAe catm VbAe + Q s
≈
Js VbAe
(2)
where Js is the contaminant mass flow rate into the enclosed space [M/T], Vb is the volume of the enclosed space [L3], catm is the contaminant concentration in atmosphere [M/L3], Qs is the volumetric flow rate of soil gas into the enclosed space [L3/ T], and Ae is the indoor air exchange rate [1/T]. In most cases, where depressurization of indoor air space determines the contaminant entry rate, the normalized indoor air concentration to subslab concentration is
(1)
where ∇* represents the nondimensional ∇ operator, nondimensionalized with respect to some characteristic length scale L, C* = (cg)/(cgw), and D* = (DLa)/(qg). cg is the concentration of contaminant in the gas phase [Mi/L3gas], cgw is the concentration of contaminant in the groundwater source vapor [Mi/L3gas], qg is the soil gas flow per unit area [L3gas/ L2soil /T], qw is the water flux in the vadose zone if relevant [L3water/L2/T], D is overall effective diffusion coefficient for transport of contaminant in porous media [L2/T], and H is the contaminant Henry’s Law constant [(Mi/L3gas)/(Mi/L3water)]. Note that this equation does not include terms with a logarithmic or exponential form involving concentration - it is a linear PDE. Entry into the Building. The common way to estimate indoor air contaminant concentration cin is to represent the
Qs cin c ≈ in ≈ cck css VbAe
(3)
where cck is the subslab crack contaminant soil vapor concentration [M/L3]. Equation 3 suggests that cin is linearly dependent on css, and this conclusion also works for scenarios involving a permeable wall contaminant entry or any other case for which Qs = 0. 907
dx.doi.org/10.1021/es303441x | Environ. Sci. Technol. 2013, 47, 906−913
Environmental Science & Technology
Article
Figure 3. Groundwater and subslab PCE and TCE vapor concentration data from the U.S. EPA’s VI database (duplicates of cgw were removed in (b)).1
Table 1. Summary of Influences of Environmental Factors in Normalized Subsurface Contaminant Soil Concentration Profile (oom Refers to Order(s) of Magnitude)a environmental factors building
soil
contaminant source
building foundations (basement, crawl space, and slab-on-grade) multiple buildings indoor pressurization indoor air exchange rate impermeable surface cover indoor pressure distribution (wind load) soil permeability effective diffusivity layered soil of different effective diffusivity or permeability (horizontal and vertical) biodegradation rate (for petroleum hydrocarbon) sorption source concentration source depth source-building separation multiple sources
a
influence Contributes
