1、 STD-API/PETRO PUBL 4b74-ENGL 3198 II 0732270 ObL458b 275 American Petroleum Institute ASSESSING THE SIGNIFICANCE OF SUBSURFACE CONTAMINANT VAPOR MIGRATION TO ENCLOSED SPACES SITE-SPECIFIC ALTERNATIVES TO GENERIC ESTIMATES HEALTH AND ENVIRONMENTAL SCIENCES DEPARTMENT PUBLICATION NUMBER 4674 DECEMBER
2、 1998 vadose zone source zone aquifer American Petroleum Institute American Petroleum Institute Environmental, Health, and Safety Mission and Guiding Principles -b- MISSION The members of the American Petroleum Institute are dedicated to continuous efforts to improve the compatibility of our operati
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10、ansport or dispose of similar raw materials, petroleum products and wastes. STD.API/PETRO PUBL 4b74-ENGL 1778 0732290 Ob14588 O48 Assessing the Significance of Subsurface Contaminant Vapor Migration to Enclosed Spaces Site-Specific Alternative to Generic Estimates Health and Environmental Sciences D
11、epartment API PUBLICATION NUMBER 4674 PREPARED UNDER CONTRACT BY: PAUL C. JOHNSON, PH.D. ARIZONA STATE UNIVERSITY DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING MARIUSH W. KEMBLOWSKI, PH.D. UTAH WATER RESEARCH LABORATORY UTAH STATE UNIVERSITY RICHARD L. JOHNSON, PH.D. OREGON GRADUATE INSTITUTE DE
12、PARTMENT OF ENVIRONMENTAL SCIENCE AND ENGINEERING DECEMBER 1998 American Petroleum Institute STD.API/PETRO PUBL qb7q-ENGL 1778 0732290 ObLLi589 TBq FOREWORD API PUBLICATIONS NECESSARILY ADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPECT TO PARTICULAR CIRCUMSTANCES, LOCAL, STATE, AND FEDERAL LAWS AND
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16、isher. Contact the publisher, API Publishing Services, 1220 L Street, N.W. Wshington, D.C. 20005. Copyright O 1998 American Petroleum Institute iii ACKNOWLEDGMENTS THE FOLLOWING PEOPLE ARE RECOGNIZED FOR THEIR CONTRIBUTIONS OF TIME AND EXPERTISE DURING THIS STUDY AND IN THE PREPARATION OF THIS REPOR
17、T API STAFF CONTACTS Roger Claff, Health and Environmental Sciences Department Harley Hopkins, Health and Environmental Sciences Department MEMBERS OF THE VAPOR MIGRATION WORKGROUP Phil Bartholomae, BP Oil Company Tim E. Buscheck, Chevron Research data from Ostendorf and Kampbell(l991). Lines show e
18、xpected concentration profiles in homogeneous settings at near steady conditions for no degradation, and first-order degradation .3 9 Predicted vapor concentration profiles for a homogeneous system at steady-state with a first-order reaction using Equation (8) 40 Attenuation coefficient predicted by
19、 Equation (1 O) for the case of a homogeneous medium at steady-state with a first-order degradation reaction. . .4 1 Schematic of dominant layer model bio-attenuation scenario .42 Comparison of dominant layer model with data from Fischer et al. (1 996) . 43 Hypothetical plot showing conditions neces
20、sary for significant bio-attenuation .44 STD.API/PETRO PUBL 4b74-ENGL 1778 0732270 b14593 Y05 List of Tables Table 1 Refinement options and associated data collection and analysis needs . 32 Table 2 Sample use of field data (data from BP 1997) to determine site-specific effective vapor-phase diffusi
21、on coefficients . .33 Table 3 Inputs used in generating Figure 1 O using the dominant layer model . 33 Executive Summary The move toward more structured risk-based corrective action (RBCA) approaches has led to an interest in better understanding vapor migration to enclosed spaces. The significance
22、of this pathway is currently the subject of intense debate, with many believing that existing non-site-specific risk-based (“generic” or “Tier 1 ”) screening levels are too conservative. As little data are available to justi generic approaches, this pathway must be addressed on a more site-specific
23、basis. This document provides options for addressing the vapor migration pathway on a more site-specific basis. Vapors in enclosed spaces pose two levels of concern. First, enclosed-space vapors may be found at concentrations near those that pose immediate flammability andor health risks. These site
24、s warrant immediate attention and response as required by most state and federal regulatory guidance. In the second class of sites, concentrations are lower and the concern is for longer term health risks. This report focuses exclusively on his second class of sites, where advection and diffusion oc
25、cur through a soil layer and into an enclosed space and time is available to adequately address the problem on a site-specific basis. The options considered in this document for refining generic vapor migration calculations and assessing the significance of this pathway on a more site-specific basis
26、 include: a) direct measurement through sampling of enclosed-space vapors, b) use of near-foundation or near-surface soil gas sampling results, c) use of site-specific homogeneous and layered soil diffusion coefficient estimates in generic algorithms, and d) assessment of bio-attenuation potential.
27、Data requirements, data presentation, and data interpretation are discussed and illustrated for each option. As discussed in the document, it is envisioned that options (c) and (d) will be used much more often than options (a) and (b) for the assessment of longer term impacts, due to a variety of te
28、chnical and practical considerations. - ES-1 - Also, a vision for a simpler site-specific assessment approach is presented and accompanied by a discussion of the steps necessary to progress toward that goal. This improved approach considers bio-attenuation, but requires only soil moisture content me
29、asurements, or alternatively, in-situ diffusion coefficient measurements, to screen sites. - ES-2 - STD.API/PETRO PUBL 4S74-ENGL 1998 0732290 ObL459b LI4 W 1 .O INTRODUCTION When soils are impacted by leaks or spills, or wastes are placed in impoundments, the potential exists for contaminant vapor m
30、igration to enclosed spaces (buildings, conduits, etc .) and leachate migration to groundwater. Regulations have historically considered the leachate impacts on groundwater; however, the issue of vapor migration has only recently been formally and quantitatively considered. This focus has been broug
31、ht about in large part by the move toward more structured risk-based corrective action (RBCA) approaches (e.g., ASTM 1995), coupled with an increased awareness of this pathway. The significance of the vapor intrusion pathway and natural attenuation of vapors in the vadose zone is currently the subje
32、ct of intense debate. When common screening-level algorithms (e.g., Johnson and Ettinger 1991, Little et al. 1992) are combined with conservative soil properties, geometries, and exposure assumptions, the resulting risk- based screening levels (RBSLs) are very low. In fact they are often one-tenth t
33、o one- thousandth the existing cleanup guidelines in many states. For example, the sample calculation in the ASTM RBCA Standard (ASTM, 1995) suggests that benzene concentrations in excess of 5 pgkg-soil could be of concern if one wishes to be protective to a lo6 excess cancer risk level. Many intuit
34、ively feel that the current generation of screening-level predictive models is too conservative and leads to unnecessarily low cleanup levels. Some point to the fact that the algorithms generally do not account for biodegradation and other possible vadose zone attenuation mechanisms. It is reasonabl
35、e to expect that some chemicals of interest degrade as they migrate, especially those originating from petroleum spills (e.g., benzene). If this is true, then these chemicals should be found at concentrations much less than those predicted by the current generation of screening level algorithms. Thi
36、s hypothesis is supported to some degree by the Fitzpatrick and Fitzgerald (1 997) Massachusetts indoor air survey, the data of Fischer et al. (1 996), and others who have observed and reported on petroleum hydrocarbon biodegradation in the vadose zone under natural conditions (e.g., Ostendorf and K
37、ampbell 1991). Unfortunately, little data exist to refute or support existing algorithms, or to quantify the degree of over-conservatism. This lack of data is a result of many factors, including the fact that interest in this pathway is relatively new. From a comparison of model predictions with pub
38、lished radon intrusion data, Johnson et al. (1 99 1) and Little et al. -1- STD-API/PETRO PUBL 4b7g-ENGL 1998 0732270 Ob14597 050 W (1 992) conclude that the screening algorithms should predict reasonable results when contaminants are present in soil gas immediately adjacent to a basement (e.g., Naza
39、roff et al. 1987). Yet to be reported are rigorous comparisons of model predictions and measurements for well-characterized sites where the contaminant sources are located at a distance from the buildings. Recently, Fitzpatrick and Fitzgerald (1 997) presented their conclusions from a study of Massa
40、chusetts sites where indoor air samples were collected. Their goal was to review site characteristics and then identie specific trends and field conditions that most influence vapor migration and vapor intrusion into buildings. They also were interested in assessing the validity of generic state gui
41、dance derived from use of the Johnson and Ettinger (1 99 1) algorithm. In summary, they noted that the generic Massachusetts guidelines overestimated vapor intrusion impacts for petroleum fuel hydrocarbon sites; however, they also found that the generic screening guidelines sometimes under-predicted
42、 indoor concentrations at sites where chlorinated organic vapors were present. Contrary to the popular belief that the models are overly conservative, the authors concluded that the generic Massachusetts guidelines were not conservative enough for site screening purposes, at least for Chlorinated co
43、mpound sites. Given limited data and limited understanding, the potential for high sensitivity to site- specific conditions, and the tendency to lean toward conservativeness when developing regulations, it seems unlikely that technically defensible alternatives for developing generic screening level
44、s will surface in the near-term. The inevitable consequence is that many sites containing volatile carcinogens are unlikely to satis9 generic RBSLs for this pathway. Thus, this pathway will need to be addressed on a more site-specific basis, and options are needed to ensure that this is done in a te
45、chnically defensible manner. Some state-level regulatory agencies are already struggling with developing site-specific guidance for assessing this pathway. In answer to this need, options for addressing the vapor migration pathway on a more site-specific basis are proposed here. These include more r
46、efined use of existing screening algorithms for layered geologic settings, as well as use of updated algorithms that consider biodegradation. These options stem from consideration of available data, existing algorithms, theoretical considerations, and empirical experience (Jury et al. 1983, Kampbell
47、 et al. 1987, Nazaroff et al. 1987, Garbesi and Sextro 1989, Jury et aE . 1990, Johnson et al. 1990, Loueiro et al. 1990, Ostendorf and Kampbell 1990, Johnson and Ettinger 199 1, Johnson and Perrot 1991, Hodgson et al. 1992, Little et al. 1992, Unlu et al. 1992, Ostendorf 1993, Jin 1994, Acomb et al
48、. 1996, Auer et al. 1996, Fischer et al. -2- STD.API/PETRO PUBL 4b74-ENGL 1778 0732270 Ob14578 T77 m 1996, Jeng et al .1996, Lahvis and Baehr 1996, Smith et al. 1996, Uchrin 1996, BP 1997, DeVaull et al. 1997, Li 1997, Sextro 1997, Stout 1997). The data collection and data reduction activities can e
49、asily be arranged in a sequence of increasing complexity, increasing data requirements, and likely increasing cost. Whether or not this approach is reasonable and defensible can only be determined by application to actual field sites followed by review of the results and experiences. It is recognized that with application, knowledge will continue to grow, and opinions and recommended practices are likely to evolve and become refined over the next few years. In order to provide insight to the technical challenges, the reader is first provided an introduction to curr
