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1.
Subslab soil gas sampling and analysis is a common line of evidence for assessing human health risks associated with subsurface vapor intrusion to indoor air for volatile organic compounds; however, conventional subslab sampling methods have generated data that show substantial spatial and temporal variability, which often makes the interpretation difficult. A new method of monitoring has been developed and tested that is based on a concept of integrating samples over a large volume of soil gas extracted from beneath the floor slab of a building to provide a spatially averaged subslab concentration. Regular field screening is also conducted to assess the trend of concentration as a function of the volume removed to provide insight into the spatial distribution of vapors at progressive distances away from the point of extraction. This approach minimizes the risk of failing to identify the areas of elevated soil vapor concentrations that may exist between discrete sample locations, and can provide information covering large buildings with fewer holes drilled through the floor. The new method also involves monitoring the extraction flow rate and transient vacuum response for mathematical analysis to help interpret the vapor concentration data and to support an optimal design for any subslab venting system that may be needed.  相似文献   

2.
Shallow trichloroethene (TCE) groundwater and soil contamination associated with a Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) Superfund Site in Michigan resulted in a vapor intrusion (VI) investigation of overlying condominium units. Units with data suggesting a complete VI pathway received subslab depressurization systems (SSDs). Performance monitoring following the installation of an SSD at one unit indicated that the indoor air TCE concentrations remained elevated, despite pressure field extension tests that showed the system should effectively reduce VI from soil gas. Therefore, a cost-efficient and incremental investigation was launched to identify other potential source(s) of TCE using a field-portable gas chromatograph/mass spectrometer (GS/MS). The combination of room-by-room air sampling, potential VI entry point sampling, and emission tests of potential sources were used, which resulted in successfully identifying a bedroom furniture set as an indoor source of TCE for the unit. Although many common household products are recognized as indoor sources of TCE, emissions from finished furniture products have not been widely discussed in the VI literature. The findings of this study indicate that off gassing from furniture can lead to TCE concentrations in indoor air that exceed regulatory guidelines.  相似文献   

3.
Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.  相似文献   

4.
The vapor intrusion impacts associated with the presence of chlorinated volatile organic contaminant plumes in the ground water beneath residential areas in Colorado and New York have been the subject of extensive site investigations and structure sampling efforts. Large data sets of ground water and indoor air monitoring data collected over a decade-long monitoring program at the Redfield, Colorado, site and monthly ground water and structure monitoring data collected over a 19-month period from structures in New York State are analyzed to illustrate the temporal and spatial distributions in the concentration of volatile organic compounds that one may encounter when evaluating the potential for exposures due to vapor intrusion. The analysis of these data demonstrates that although the areal extent of structures impacted by vapor intrusion mirrors the areal extent of chlorinated volatile organic compounds in the ground water, not all structures above the plume will be impacted. It also highlights the fact that measured concentrations of volatile organic compounds in the indoor air and subslab vapor can vary considerably from month to month and season to season. Sampling results from any one location at any given point in time cannot be expected to represent the range of conditions that may exist at neighboring locations or at other times. Recognition of this variability is important when designing sampling plans and risk management programs to address the vapor intrusion pathway.  相似文献   

5.
A value of 0.001 is recommended by the United States Environmental Protection Agency (USEPA) for its groundwater‐to‐indoor air Generic Attenuation Factor (GAFG), used in assessing potential vapor intrusion (VI) impacts to indoor air, given measured groundwater concentrations of volatile chemicals of concern (e.g., chlorinated solvents). The GAFG can, in turn, be used for developing groundwater screening levels for VI given target indoor air quality screening levels. In this study, we examine the validity and applicability of the GAFG both for predicting indoor air impacts and for determining groundwater screening levels. This is done using both analysis of published data and screening model calculations. Among the 774 total paired groundwater‐indoor air measurements in the USEPA's VI database (which were used by that agency to generate the GAFG) we found that there are 427 pairs for which a single groundwater measurement or interpolated value was applied to multiple buildings. In one case, up to 73 buildings were associated with a single interpolated groundwater value and in another case up to 15 buildings were associated with a single groundwater measurement (i.e., that the indoor air contaminant concentrations in all of the associated buildings were influenced by the concentration determined at a single point). In more than 70% of the cases (390 of 536 paired measurements in which horizontal building‐monitoring well distance was recorded) the monitoring wells were located more than 30 m (and one up to over 200 m) from the associated buildings. In a few cases, the measurements in the database even improbably implied that soil gas contaminant concentrations increased, rather than decreased, in an upward direction from a contaminant source to a foundation slab. Such observations indicate problematic source characterization within the data set used to generate the GAFG, and some indicate the possibility of a significant influence of a preferential contaminant pathway. While the inherent value of the USEPA database itself is not being questioned here, the above facts raise the very real possibility that the recommended groundwater attenuation factors are being influenced by variables or conditions that have not thus far been fully accounted for. In addition, the predicted groundwater attenuation factors often fall far beyond the upper limits of predictions from mathematical models of VI, ranging from screening models to detailed computational fluid dynamic models. All these models are based on the same fundamental conceptual site model, involving a vadose zone vapor transport pathway starting at an underlying uniform groundwater source and leading to the foundation of a building of concern. According to the analysis presented here, we believe that for scenarios for which such a “traditional” VI pathway is appropriate, 10?4 is a more appropriately conservative generic groundwater to indoor air attenuation factor than is the EPA‐recommended 10?3. This is based both on the statistical analysis of USEPA's VI database, as well as the traditional mathematical models of VI. This result has been validated by comparison with results from some well‐documented field studies.  相似文献   

6.
Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.  相似文献   

7.
Different types of data can be collected to evaluate whether or not vapor intrusion is a concern at sites impacted with volatile organic compound (VOC) contamination in the subsurface. Typically, groundwater, soil gas, or indoor air samples are collected to determine VOC concentrations in the different media. Sample results are evaluated using a “multiple lines of evidence” approach to interpret whether vapor intrusion is occurring. Data interpretation is often not straightforward because of many complicating factors, particularly in the evaluation of indoor air. More often than not, indoor air sample results are affected by indoor or other background sources making interpretation of concentration‐based data difficult using conventional sampling approaches. In this study, we explored the practicality of compound‐specific isotope analysis (CSIA) as an additional type of evidence to distinguish between indoor sources and subsurface sources (i.e., vapor intrusion). We developed a guide for decision‐making to facilitate data interpretation and applied the guidelines at four different test buildings. To evaluate the effectiveness of the CSIA method for vapor intrusion applications, we compared the interpretation from CSIA to interpretations based on data from two different investigation approaches: conventional sampling and on‐site GC/MS analysis. Interpretations using CSIA were found to be generally consistent with the other approaches. In one case, CSIA provided the strongest line of evidence that vapor intrusion was not occurring and that a VOC source located inside the building was the source of VOCs in indoor air.  相似文献   

8.
Soil vapor extraction (SVE) is effective for removing volatile organic compound (VOC) mass from the vadose zone and reducing the potential for vapor intrusion (VI) into overlying and surrounding buildings. However, the relationship between residual mass in the subsurface and VI is complex. Through a series of alternating extraction (SVE on) and rebound (SVE off) periods, this field study explored the relationship and aspects of SVE applicable to VI mitigation in a commercial/light-industrial setting. The primary objective was to determine if SVE could provide VI mitigation over a wide area encompassing multiple buildings, city streets, and subsurface utilities and eliminate the need for individual subslab depressurization systems. We determined that SVE effectively mitigates offsite VI by intercepting or diluting contaminant vapors that would otherwise enter buildings through foundation slabs. Data indicate a measurable (5 Pa) influence of SVE on subslab/indoor pressure differential may occur but is not essential for effective VI mitigation. Indoor air quality improvements were evident in buildings 100 to 200 feet away from SVE including those without a measurable reversal of differential pressure across the slab or substantial reductions in subslab VOC concentration. These cases also demonstrated mitigation effects across a four-lane avenue with subsurface utilities. These findings suggest that SVE affects distant VI entry points with little observable impact on differential pressures and without relying on subslab VOC concentration reductions.  相似文献   

9.
A detailed seasonal study of soil vapor intrusion at a cold climate site with average yearly temperature of 1.9 °C was conducted at a house with a crawlspace that overlay a shallow dissolved‐phase petroleum hydrocarbon (gasoline) plume in North Battleford, Saskatchewan, Canada. This research was conducted primarily to assess if winter conditions, including snow/frost cover, and cold soil temperatures, influence aerobic biodegradation of petroleum vapors in soil and the potential for vapor intrusion. Continuous time‐series data for oxygen, pressure differentials, soil temperature, soil moisture, and weather conditions were collected from a high‐resolution monitoring network. Seasonal monitoring of groundwater, soil vapor, crawlspace air, and indoor air was also undertaken. Petroleum hydrocarbon vapor attenuation and biodegradation rates were not significantly reduced during low temperature winter months and there was no evidence for a significant capping effect of snow or frost cover that would limit oxygen ingress from the atmosphere. In the residual light nonaqueous phase liquid (LNAPL) source area adjacent to the house, evidence for biodegradation included rapid attenuation of hydrocarbon vapor concentrations over a vertical interval of approximately 0.9 m, and a corresponding decrease in oxygen to less than 1.5% v/v. In comparison, hydrocarbon vapor concentrations above the dissolved plume and below the house were much lower and decreased sharply within a few tens of centimeters above the groundwater source. Corresponding oxygen concentrations in soil gas were at least 10% v/v. A reactive transport model (MIN3P‐DUSTY) was initially calibrated to data from vertical profiles at the site to obtain biodegradation rates, and then used to simulate the observed soil vapor distribution. The calibrated model indicated that soil vapor transport was dominated by diffusion and aerobic biodegradation, and that crawlspace pressures and soil gas advection had little influence on soil vapor concentrations.  相似文献   

10.
A portable gas chromatograph‐mass spectrometer (GC/MS) was used to investigate sources of chlorinated volatile organic compound (cVOC) contamination in indoor air at 46 residences around Hill AFB, Utah, that were potentially affected by vapor intrusion. Analytical methods were developed to allow sample turnaround times of less than 10 min and method detection limits (MDLs) generally less than 1 μg/m3 for a selected list of cVOCs. Area‐by‐area sampling was used to identify the likely vapor source locations. In many cases, individual container/enclosure sampling and subsequent field emission rate measurements from isolated consumer products were used to determine if identified products were likely to be the primary source of vapors in the residence. The portable GC/MS was also used to characterize vapor intrusion in two residences. In one of these two residences, building pressure control was used to enhance vapor entry in order to facilitate the investigation resulting in confirmation of vapor intrusion and identification of a primary route of vapor entry. cVOCs were identified in 42 of the 46 homes investigated, subsurface vapor intrusion was identified in two homes, and two homes had inconclusive results.  相似文献   

11.
Aerobic biodegradation can contribute significantly to the attenuation of petroleum hydrocarbons vapors in the unsaturated zone; however, most regulatory guidance for assessing potential human health risks via vapor intrusion to indoor air either neglect biodegradation in developing generic screening levels or allow for only one order of magnitude additional attenuation for aerobically degradable compounds, which may be overly conservative in some cases. This paper describes results from three-dimensional numerical model simulations of vapor intrusion for petroleum hydrocarbons to assess the influence of aerobic biodegradation on the attenuation factor for a variety of source concentrations and depths for residential buildings with basements and slab-on-grade construction. The simulations conducted in this study provide a framework for understanding the degree to which bioattenuation will occur under a variety of scenarios and provide insight into site conditions that will result in significant biodegradation. This improved understanding may be used to improve the conceptual model of contaminant transport, guide field data collection and interpretation, and estimate semi-site-specific attenuation factors for combinations of source concentrations, source depth, oxygen distribution, and building characteristics where site conditions reasonably match the scenarios simulated herein.  相似文献   

12.
The attenuation factor (AF) of 0.03 recommended by the U.S. Environmental Protection Agency (USEPA) is increasingly being used by regulatory agencies for the development of subsurface vapor screening levels for vapor intrusion (VI). There are concerns, however, over the database used to derive the AF and the AF's applicability to building types and geographies not included in USEPA database. To derive a more technically defensible AF for subsurface vapor screening in California, a database consisting of 8415 paired indoor and subsurface vapor samples collected from 485 buildings at 36 sites in California was compiled. Filtering was applied to remove data of suspect quality that were potentially affected by background (non-VI) sources. Filtering reduced the size of the database to 788 indoor air and subsurface vapor pairs, 80% of which were trichloroethylene (TCE) measurements. An AF of 0.0008 was derived from only TCE vapor data, based on the ability of the AF to reliably identify buildings with indoor air concentrations above screening levels in 95% of cases where subsurface vapor screening levels were exceeded. The AF derived from this study demonstrated limited sensitivity to the variables typically considered important in VI characterization, which was partially attributed to relatively weak correlation of indoor air and subsurface vapor concentration data. The results of this study can be used to improve VI screening in California and other states and help focus limited resources on sites posing the greatest potential risk.  相似文献   

13.
The United States Environmental Protection Agency (USEPA) is finalizing its vapor intrusion guidelines. One of the important issues related to vapor intrusion is background concentrations of volatile organic compounds (VOCs) in indoor air, typically attributed to consumer products and building materials. Background concentrations can exist even in the absence of vapor intrusion and are an important consideration when conducting site assessments. In addition, the development of accurate conceptual models that depict pathways for vapor entry into buildings is important during vapor intrusion site assessments. Sewer gas, either as a contributor to background concentrations or as part of the site conceptual model, is not routinely evaluated during vapor intrusion site assessments. The research described herein identifies an instance where vapors emanating directly from a sanitary sewer pipe within a residence were determined to be a source of tetrachloroethylene (PCE) detected in indoor air. Concentrations of PCE in the bathroom range from 2.1 to 190 µg/m3 and exceed typical indoor air concentrations by orders of magnitude resulting in human health risk classified as an “Imminent Hazard” condition. The results suggest that infiltration of sewer gas resulted in PCE concentrations in indoor air that were nearly two orders of magnitude higher as compared to when infiltration of sewer gas was not known to be occurring. This previously understudied pathway whereby sewers serve as sources of PCE (and potentially other VOC) vapors is highlighted. Implications for vapor intrusion investigations are also discussed.  相似文献   

14.
Screening level models are now commonly used to estimate vapor intrusion for subsurface volatile organic compounds (VQCs). Significant uncertainty is associated with processes and models and, to date, there has been only limited field-based evaluation of models for this pathway. To address these limitations, a comprehensive evaluation of the Johnson and Ettinger (J&E) model is provided through sensitivity analysis, comparisons of model-predicted to measured vapor intrusion for 11 petroleum hydrocarbon and chlorinated solvent sites, and review of radon and flux chamber studies. Significant intrusion was measured at five of 12 sites with measured vapor attenuation ratios (αm's) (indoor air/source vapor) ranging from ∼1 × 10−6 to 1 × 10−4. Higher attenuation ratios were measured for studies using radon, inert tracers, and flux chambers; however, these ratios are conservative owing to boundary conditions and tracer properties that are different than those at most VOC-contaminated sites. Reasonable predictions were obtained using the J&E model with comparisons indicating that model-predicted vapor attenuation ratios (αp's) were on the same order, or less than the αm's. For several sites, the αm were approximately two orders of magnitude less than the αp's indicating that the J&E model is conservative in these cases. The model comparisons highlight the importance in using appropriate input parameters for the J&E model. The regulatory implications associated with use of the J&E model to derive screening criteria are also discussed.  相似文献   

15.
Screening level models are now commonly used to estimate vapor intrusion for subsurface volatile organic compounds (VOCs). Significant uncertainty is associated with processes and models and, to date, there has been only limited field-based evaluation of models for this pathway. To address these limitations, a comprehensive evaluation of the Johnson and Ettinger (J&E) model is provided through sensitivity analysis, comparisons of model-predicted to measured vapor intrusion for 11 petroleum hydrocarbon and chlorinated solvent sites, and review of radon and flux chamber studies. Significant intrusion was measured at five of 12 sites with measured vapor attenuation ratios (αm's) (indoor air/source vapor) ranging from ∼1 × 10−6 to 1 × 10−4. Higher attenuation ratios were measured for studies using radon, inert tracers, and flux chambers; however, these ratios are conservative owing to boundary conditions and tracer properties that are different than those at most VOC-contaminated sites. Reasonable predictions were obtained using the J&E model with comparisons indicating that model-predicted vapor attenuation ratios (αp's) were on the same order, or less than the αm's. For several sites, the (m were approximately two orders of magnitude less than the a 's indicating that the J&E model is conservative in these cases. The model comparisons highlight the importance in using appropriate input parameters for the J&E model. The regulatory implications associated with use of the J&E model to derive screening criteria are also discussed.  相似文献   

16.
The California Department of Toxic Substances Control (DTSC) gathered empirical data from sites contaminated with chlorinated volatile organic compounds and generated a vapor intrusion database. The database includes 52 sites across California with 213 buildings, of which, 53% are residential, and 47% are commercial/industrial (nonresidential). DTSC's objective is to improve its knowledge and understanding of vapor intrusion and derive empirical attenuation factors that are representative of the climatic conditions and types of buildings commonly found in the state. Filtering was applied to remove data of suspect quality that were potentially affected by background sources. After filtering to 600 pairs from 32 sites across California, a subslab attenuation factor (AF) was calculated yielding a 95th percentile of 0.005. After filtering to 2926 paired measurements from 39 sites across California, a soil vapor AF was calculated yielding a 95th percentile of 0.0009. The groundwater data was not analyzed due to the small size of the dataset. The AFs from this study are similar to AFs in a California-specific study by Lahvis and Ettinger (2021). Accordingly, converging lines of evidence suggest that vapor attenuation in California is different from what is observed nationwide by the United States Environmental Protection Agency (USEPA), where an AF of 0.03 was empirically derived and later recommended for initial screening of buildings for potential vapor intrusion exposure (USEPA, 2015).  相似文献   

17.
In this study, we present a petroleum vapor intrusion (PVI) tool implemented in Microsoft® Excel® using Visual Basic for Applications and integrated within a graphical interface. The latter helps users easily visualize two‐dimensional soil gas concentration profiles and indoor concentrations as a function of site‐specific conditions such as source strength and depth, biodegradation reaction rate constant, soil characteristics and building features. This tool is based on a two‐dimensional explicit analytical model that combines steady‐state diffusion‐dominated vapor transport in a homogeneous soil with a piecewise first‐order aerobic biodegradation model, in which rate is limited by oxygen availability. As recommended in the recently released United States Environmental Protection Agency's final PVI guidance, a sensitivity analysis and a simplified Monte Carlo uncertainty analysis are also included in the spreadsheet.  相似文献   

18.
Vapor intrusion (VI) occurs when volatile contaminants in the subsurface migrate through the vadose zone into overlying buildings. The 2015 U.S. EPA petroleum VI guidance recommends that additional investigation of the VI risk from gasoline hydrocarbons at the underground storage tank (UST) sites is not necessary where the vertical distance between a building and a vapor source exceeds a recommended vertical screening distance. However, due to the lack of soil-gas data on the attenuation of ethylene dibromide (EDB), additional VI investigations to evaluate VI risk from EDB are recommended at UST sites with leaded gasoline releases containing EDB. We analyzed soil-gas and groundwater concentrations of EDB from eight petroleum UST sites using a new analytical method with soil-gas detection limit <0.16 μg/m3 EDB (VI screening level at the 10−6 risk level). The analysis included (1) assessing the frequency of EDB detections ≤0.16 μg/m3 at various vertical separation distances and (2) predicting vertical screening distances for EDB using the U.S. EPA PVIScreen model for different soil types in the vadose zone above dissolved-phase and LNAPL sources. Ranges of estimated aerobic biodegradation rate constants for EDB, air exchange rates for residential buildings, and source vapor concentrations for other constituents were combined with conservative estimates of EDB source concentrations as model inputs. Concentrations of EDB in soil-gas indicated that the U.S. EPA recommended vertical screening distances are protective of VI risk from EDB. Conversely, vertical screening distances predicted by modeling were >6 ft (1.8 m) for sites with sand and loam soil above dissolved phase sources and >15 ft (4.6 m) for sites with sand soil above LNAPL sources. This predicted dependence on the vapor source type and soil type in the vadose zone highlights the importance of soil characterization for VI screening at sites with EDB sources.  相似文献   

19.
Building pressure cycling (BPC) is becoming an increasingly important tool for studying vapor intrusion. BPC has been used to distinguish subslab and indoor sources of vapor intrusion as well as to define reasonable worst case volatile organic compound mass discharge into a structure. Analyses have been performed both semi-quantitatively with concentration trends and quantitatively with more rigorous flux calculation and source attribution methods. This paper reviews and compares the protocols and outcomes from multiple published applications of this technology to define the key variables that control performance. Common lessons learned are identified, including those that help define the range of building size and type to which BPC is applicable. Differences in test protocols are discussed, recognizing that the complexity of the test protocol required depends on the particular objectives of each project. Research gaps are identified and tabulated for future validation studies and applications.  相似文献   

20.
Temporal and spatial variability of indoor air volatile organic compound (VOC) concentrations can complicate vapor intrusion (VI) assessment and decision-making. Indicators and tracers (I&T) of VI, such as differential temperature, differential pressure, and indoor radon concentration, are low-cost lines of evidence to support sampling scheduling and interpretation of indoor air VOC sampling results. This study compares peak indoor air chlorinated VOC concentrations and I&T conditions before and during those peak events at five VI sites. The sites differ geographically and in their VI conceptual site models (CSM). Relative to site-specific baseline values, the results show that cold or falling outdoor temperatures, rising cross slab differential pressures, and increasing indoor radon concentrations can predict peak VOC concentrations. However, cold outdoor air temperature was not useful at one site where elevated shallow soil temperature was a better predictor. Correlations of peak VOC concentrations to elevated or rising barometric pressure and low wind speed were also observed with some exceptions. This study shows how the independent variables that control or predict peak indoor air VOC concentrations are specific to building types, climates, and VI CSMs. More I&T measurements at VI sites are needed to identify scenario-specific baseline and peak related I&T conditions to improve decision-making.  相似文献   

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