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Journal of the Air & Waste Management Association Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uawm18
The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil a
a
a
a
Dennis J. Paustenbach , Tibor T. Sarlos , Virginia Lau , Brent L Finley , David A. a
Jeffrey, & Michael J. Ungs
a
a
ChemRisk , Alameda , California , USA Published online: 06 Mar 2012.
To cite this article: Dennis J. Paustenbach , Tibor T. Sarlos , Virginia Lau , Brent L Finley , David A. Jeffrey, & Michael J. Ungs (1991) The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil, Journal of the Air & Waste Management Association, 41:10, 1334-1340, DOI: 10.1080/10473289.1991.10466930 To link to this article: http://dx.doi.org/10.1080/10473289.1991.10466930
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ISSN 1047-3289 J. Air Waste Manage. Assoc. 41:1334-1340
The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil
Downloaded by [University of Connecticut] at 05:44 11 October 2014
Dennis J. Paustenbach, Tibor T. Sarlos, Virginia Lau, Brent L Finley, David A. Jeffrey, and Michael J. Ungs ChemRisk Alameda, California
Mathematical models and field data were used to estimate the airborne concentrations of 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) vapor and participates which could originate from soil containing 100 ppb TCDD. The model of Jury et al. (1983) and the box approach were used to predict the concentration of TCDD vapor from soil. The daily soil temperature was assumed to vary between 20° C and 40° C for six months of the year to account for diurnal warming and cooling of the soil. The depth of contamination was 50 mm. The model predicted average vapor flux rate for TCDD from soil for this temperature profile was 1.5 x 10~ 14 mg/ sec-cm2. The upper-bound estimates of the TCDD vapor concentration on-site at 40° C and 20° C were 2.5 pg/m 3 and 1.8 pg/m 3 , respectively. Using a recently proposed unit risk value (URV) of 2.9 x lO" 6 ( p g / m 3 ) 1 [slope factor = 1.0 x 10" 14 (mg/kg-day)~ 1 ], the maximum plausible cancer risk is about 1 x 1 0 5 . If one accepts the EPA URV of 3.3 x 10~ 5 (pg/m 3 )- 1 (slope factor = 1.2 x 10~ 13 (mg/kg-day)~1), then the risk is no greater than 1 x 10~ 4 . A maximum TCDD vapor concentration of 0.21 pg/m 3 was predicted 100 meters downwind (for summer days). The on-site concentration of TCDD in suspended paniculate was estimated to be 1.4 pg/m 3 (based on a TSP level of 0.07 mg/m 3 from site soil). For persons exposed to vapors and particulates about 100 meters off-site, the exposure was about 10-fold 4ess. The model-predicted concentrations of TCDD vapor were compared with the results of field and laboratory studies conducted by Yanders et al. (1988). Our estimate of the maximum possible
Implications Dioxin contaminated soil has been a significant public health concern since 1980. At many contaminated sites, the predominant route of exposure is often perceived to be the inhalation of vapors and fugitive dusts from the site. This study used a popular environmental model to predict the inhalation hazard and the results were compared against laboratory results. The original (1983) model was unable to properly predict the actual field data, however, a modified version accurately predicted the results. This paper shows that due to its low volatility, at soil concentrations of 100 ppb TCDD, the emanation of vapor and the level of TCDD in airborne dust is very small. At soil concentrations at least as high as 100 ppb, the maximum plausible cancer risk due to inhalation is less than one in a million for persons who might live immediately adjacent to such sites.
TCDD vapor flux rate for their study was 4.6 x 10~ 17 mg/seccm 2 , These results indicate that the concentrations predicted by the Jury et al. model are about 1000 fold greater than which actually occur. The inability of the Jury model to accurately estimate the rate of volatilization of TCDD from soil is probably because the TCDD was applied to the soil in a formulated state where it could easily migrate below the surface. A recent modification of the Jury model (1990) suggests that a 5-50 mm layer of clean soil will significantly retard (or eliminate) the vapor hazard posed by TCDD contaminated soil. The risks due to fugitive dust will always be greater than the vapor hazard, but for soil concentrations of 100 ppb the cancer risk should be less than 10~ 6 . Since few sites have average soil concentrations as high as 100 ppb, this nearly worst case analysis indicates that inhalation will rarely, if ever, be a significant route of exposure to TCDD-contaminated soil.
Although 2,3,7,8-TCDD has been shown to have a negligible vapor pressure (1.5 x 10~9 mm Hg at 25° C)1, several risk assessments have claimed or suggested that airborne concentrations of TCDD from contaminated soil could be sufficient to pose a significant health risk. 2 ' 3 Other reports have indicated that the inhalation hazard posed by TCDD vapors and particulates is negligible at virtually all plausible environmental soil concentrations and that this route of exposure can be dismissed from most risk assessments.4 To quantitatively evaluate this issue, the highest plausible long-term airborne concentration of TCDD (vapor and particulate-bound) originating from a site with contaminated soil was predicted via models and the associated theoretical cancer risks were estimated. The modelpredicted concentrations were compared with the results of field studies of TCDD contaminated soil. Methods In this analysis, a one acre hypothetical site was assumed to contain soil uniformly contaminated with 100 ppb TCDD to a depth of 50 mm. It was also assumed that only 50 percent of the soil was exposed or vegetated. The other half was paved or covered by buildings or driveways. This hypothetical scenario is representative of many known dioxin sites in the United States. While it is true that TCDD soil concentrations in excess of 100 ppb have been observed at some former individual sites, such as in Newark, New Jersey, the vast majority of remediated sites (including Copyright 1991—Air & Waste Management Association
1334
J. Air Waste Manage. Assoc.
Downloaded by [University of Connecticut] at 05:44 11 October 2014
Newark) have had average soil concentrations much less than 100 ppb. Since it is known that diffusion of chemical vapors through cement or other hard building surfaces is negligible compared to the potential escape through cracks5 in the concrete, and since the percent of cracks or imperfections in the surface is only 0.01-0.1 percent of the total floor area,6 it was assumed that those paved portions of the hypothetical site do not release significant amounts of TCDD vapor. It has been suggested that TCDD volatilization will change during the diurnal warming and cooling of the soil. Experimental data indicate that for soil depths of 2.0 to 10 centimeters the soil temperature can vary significantly with depth, but below 48 centimeters the soil temperature decreases at a constant rate. 3 For this analysis, the daily soil temperature was assumed to vary from 20° C to 40° C corresponding to average nighttime and daytime temperatures, respectively, for six months of the year. For the remainder of the year, the soil temperature was assumed to be 0° C. The potential TCDD vapor emissions from site soils were estimated using the model of Jury et al.7 which predicts the time-weighted average vapor flux from contaminated soil. The model accounts for the soil characteristics and the physical properties of the chemical. It considers the contaminant's volatility (vapor pressure), strength of adsorption to soil particles (Krj), and upward leaching in the soil column due to evapotransporation (wicking). The model also accounts for the continual loss of contaminant from the soil; that is, the soil vapor flux decreases over time. The values for the parameters used in this analysis are presented in Tables I, II and III. 8 - 12 The results of the Jury et al. model were compared with actual field/laboratory data. The calculations used to interpret the field data are presented in the Appendix. The Jury model relies on a number of chemical- and soil-specific parameters. The soil desorption coefficient, Krj, is a measure of a substance's tendency to remain sorbed to soil versus existing as a free unbound liquid. Henry's Constant, H, reflects the tendency of the liquid to enter the vapor state. Thus, the model treats soil-bound contaminant emissions as a two-stage process. Other parameters needed for the Jury model include foo pa» Pw> PT> Pb> a n d Jw- All °f them, with the exception of Jw, are related to the characteristics of the soil. Tables I, II, and III present the Jury
parameters corresponding to temperatures at 0° C, 20° C and 40° C, respectively. Work conducted by Freeman and Schroy12 indicated that the vapor pressure of TCDD in soil can increase 25-fold when daily temperatures vary from 20° C to 40° C. Henry's constant will increase by 25-fold and the air diffusion coefficient, D^, will increase by a factor of 1.4 when the temperature is 40° C versus 20° C. All other parameters were assumed to remain unchanged with temperature. A 'box model' approach was used to predict the maximum plausible levels of TCDD vapor on site. This approach assumed that the air above the surface soils was contained in a "building" having two open walls and a 2 meter high ceiling, with wind blowing through it. The choice of a 2-meter ceiling was to represent the approximate breathing zone of an individual walking over exposed site soils; a conservative approach which neglects dispersion. This box model approach is depicted in Figure 1. The base of the box is the entire site (one acre), since individuals may be located anywhere within the site. However, as was previously discussed, only xh acre emits TCDD vapor into this box due to the buildings and other paved sections. Although the approach is simplistic and will only yield maximum plausible values, it is useful for a screening-level assessment. The box model assumes that the TCDD vapor travels only a very short distance from the soil prior to inhalation. Therefore, the model accounts for removal of TCDD vapor by a uni-directional windstream but ignores dispersion effects and photodegradation (these are important when estimating the risks to persons off site). Airborne contaminant concentrations determined from the box model are therefore only relevant for persons who live close to the emitting surface. To this extent, the box model produces Upper-bound estimates of the airborne contaminant concentration since most exposures to individuals will occur at off site locations, where air dispersion effects are important. The SCREEN air dispersion model13 was used to estimate a worst-case (downwind) airborne concentration of TCDD (vapor and particulate-bound) at a distance of 100 meters off site. As its name implies, SCREEN is a model which produces upper-bound estimates of the vapors emitted from an area source. It accounts for horizontal and vertical dispersion during worst-case meteorological conditions (that is, extremely slow, stable winds). For the
Table I. Values used in model to estimate the volatilization of TCDD from soil at 0°C. Parameter Koc
S H h P MW Kd *oc Dg 3 * Q water
Pd Pa Pw PT
Pb P
H2O ^erav
dw
[TCDD] t Z a Value for 22°C. b Value for 0°C.
October 1991
Description
Value 6
Organic soil carbon-liquid partition coefficient Water solubility
2.78xl0 ml/g 7.96 x 10~6 mg/l a
Henry's constant Dimensionless Henry's Constant Vapor pressure Molecular weight Solid-liquid partition coefficient Organic soil carbon fraction Air diffusion coefficient Water diffusion coefficient Soil particle density Volumetric air fraction of soil Volumetric water fraction of soil Total soil porosity Dry bulk density Density of water Gravimetric water fraction of soil Water evaporation rate Soil concentration of TCDD Time-span for average flux calculation Depth of contamination
9.1 x 1(T7 atm-m3/mol 4.06 x 10~5 2.27 x 10~9 pascalsb 322 g/mole 2.78 x 104 ml/g 0.01 0.043 cm3/cm-sec 5.6 x 10~6 cm3/cm-sec 2.65 g/cm3 0.35 0.15 0.50 1.3 g/cm3 1 g/cm3 0.12 g/g 10 in/year 100 |xg/kg (wet soil) 70 years 50 mm
Volume 41, No. 10
Source/Rationale
Marpleetal., 1987 Marple et al., 1987; Freeman and Schroy, 1986 Determined from p, MW, and S Determined from H Determined from T Determined from KoC and foc Assumed Freeman and Schroy, 1985 Freeman and Schroy, 1985 Assumed Assumed Assumed PT = Pa + P w Assumed Pgrav
=
Pw ' PlKo/Pb
Assumed Initial assumption Initial assumption Initial assumption
1335
Table II. Values used in model to estimate the volatilization of TCDD from soil at 20°C. Parameter
Value 6
Koc S
Organic soil carbon-liquid partition coefficient Water solubility
2.78 x 10 ml/g 7.96 x 10~6 mg/la
H h P MW
Henry's constant Dimensionless Henry's Constant Vapor pressure Molecular weight Solid-liquid partition coefficient Organic soil .carbon fraction Air diffusion coefficient Water diffusion coefficient Soil particle density Volumetric air fraction of soil Volumetric water fraction of soil Total soil porosity Dry bulk density Density of water Gravimetric water fraction of soil Water evaporation rate Soil concentration of TCDD Time-span for average flux calculation Depth of contamination
3.44 x 10~5 atm-m3/mol 1.43 x 10~3 8.61 x 10~8 pascalsb 322 g/mole 2.78 x 104 ml/g 0.01 0.045 cm3/cm-sec 5.6 x 10~6 cm3/cm-sec 2.65 g/cm3 0.35 0.15 0.50 1.3 g/cm3 1 g/cm3 0.12 g/g 10 in/year 100 fxg/kg (wet soil) 70 years 50 mm
Ka j ) water
Pd Pa ?w PT p b PH 2 O
P x
grav
a n d the number of hours of darkness per day. %avg = %day x (# daylight hours/24 hours) + % night x (# dark hours/24 hours) Assumptions:
= %day 50% %night = 0.0% # daylight hours = 12 hours # dark hours = 1 2 hours
To account for the loss due to photodegradation, the amount of TCDD needs to be scaled up. Even though none of the filters had detectable levels of TCDD, we assumed that the amount of TCDD captured in the filter (Amount detected) was at the analytical limit of detection or 10 ng. Therefore, the initial mass of TCDD that "could have volatilized" without being detected (Amount vo i at jij ze d) is: Amount v o i a t i l i z e d = (Amount det ected)/(1 ~ %avg) Amountdetected = 10 ng %avg = 25% Amount vo i ati i ize d = 1 0 n g / ( l - 0.25) Amount vo i ati i iZ ed = 13.3 ng The maximum annual percent loss of TCDD due to vaporization and particulate emission (%i0Ss) i s a function of the initial mass of TCDD in the soil and the average mass of TCDD captured in the niters.
x 100 Assumptions: Amountsoji
= 13.3 ng = 315000 ng %loss = (13.3ng/(315,000ng)) x 100 %loss = 0.004%
1340
To estimate the total loss of TCDD due to volatilization and particulate emission (Loss^otai) over 70 years is: Loss total = %ioss x Lifespan Assumptions: %\oss = 0.002% loss/year Lifespan = 70 years Loss total = 0.002% loss/year x 70 years Loss total = 0.14% Assuming that half an acre of soil is contaminated with 100 ppb TCDD to a depth of 50 mm, about 1.46 g TCDD is initially present in the soil (Calculation I). If 0.14% of this mass is emitted over 70 years, the mass at 70 years is (1.46 g)(0.14/100) = 2.04 x lO" 3 g. The TCDD emission rate (ER) is the total mass which volatilizes per unit time or 2.04 x 10~ 3 g/70 years = 2.92 x 10~ 5 g/year. The predicted average flux depends on the emission rate (ER) and the emitting area. Assumptions:
Flux = (ERX I/Area) ER = 2.92 x 10~ 5 g/yr Area =0.5 acre Conversion factor = 4 x 103 m 2 /acre Conversion factor = 3.17 x 10~ 8 yr/sec Conversion factor = 103 mg/g Conversion factor = 104 cm 2 /m 2
Flux = (2.92 x 10~5 g/yr) (1/0.5 acre) (1/4 x 103 m2/acre) (l/10 4 cm 2 /m 2 )(3.17 x 10~8 yr/sec) (1Q3 mg/g) Flux = 4.6 x 10~17mg/sec-cm2
% a v g = (0.5X12 hours/24 hours) + (0.0)(12 hours/24 hours) % a v g = 25%
Assumptions:
Assuming that the amount of TCDD in the niters was present at half the LOD of the assay or 5 ng, the amount of TCDD that could have volatized without being detected (LOSSsoii) isO.004%/2 = 0.002%.
The box model predicted airborne concentration is determined by the following factors: Assumption: Flux = 4.6 x 10~17mg/sec-cm2 Conversion factor = 104 cm 2 /m 2 Area = 0.5 acre = 2000 m 2 Wind speed = 2 m/sec Length = ^4000 m2 = 63 m Height = 2 m (average height of a person) Air Cone. = (4.6 x 10- 17mg/sec-cm2)(104 cm2/m2)(2OOOm2)/((2m/sec) (2m)(63m)) Air Cone. = 3.7 x 10~ 12 mg/m 3 = 3.7 x 10- 3 pg/m 3 Based on the results above, the lifetime cancer risk should be no greater than (3.7 x 10- 3 pg/m 3 )(2.9 x 10~6) = 1.1 X 1Q-8.
The authors are with ChemRisk, a Division of McLaren/ Hart, 1135 Atlantic Avenue, Alameda, CA 94501. This manuscript was submitted for peer review on March 7, 1991. The revised manuscript was received on August 1, 1991.
J. Air Waste Manage. Assoc.
Journal of the Air & Waste Management Association Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/uawm18
The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil a
a
a
a
Dennis J. Paustenbach , Tibor T. Sarlos , Virginia Lau , Brent L Finley , David A. a
Jeffrey, & Michael J. Ungs
a
a
ChemRisk , Alameda , California , USA Published online: 06 Mar 2012.
To cite this article: Dennis J. Paustenbach , Tibor T. Sarlos , Virginia Lau , Brent L Finley , David A. Jeffrey, & Michael J. Ungs (1991) The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil, Journal of the Air & Waste Management Association, 41:10, 1334-1340, DOI: 10.1080/10473289.1991.10466930 To link to this article: http://dx.doi.org/10.1080/10473289.1991.10466930
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
ISSN 1047-3289 J. Air Waste Manage. Assoc. 41:1334-1340
The Potential Inhalation Hazard Posed by Dioxin Contaminated Soil
Downloaded by [University of Connecticut] at 05:44 11 October 2014
Dennis J. Paustenbach, Tibor T. Sarlos, Virginia Lau, Brent L Finley, David A. Jeffrey, and Michael J. Ungs ChemRisk Alameda, California
Mathematical models and field data were used to estimate the airborne concentrations of 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) vapor and participates which could originate from soil containing 100 ppb TCDD. The model of Jury et al. (1983) and the box approach were used to predict the concentration of TCDD vapor from soil. The daily soil temperature was assumed to vary between 20° C and 40° C for six months of the year to account for diurnal warming and cooling of the soil. The depth of contamination was 50 mm. The model predicted average vapor flux rate for TCDD from soil for this temperature profile was 1.5 x 10~ 14 mg/ sec-cm2. The upper-bound estimates of the TCDD vapor concentration on-site at 40° C and 20° C were 2.5 pg/m 3 and 1.8 pg/m 3 , respectively. Using a recently proposed unit risk value (URV) of 2.9 x lO" 6 ( p g / m 3 ) 1 [slope factor = 1.0 x 10" 14 (mg/kg-day)~ 1 ], the maximum plausible cancer risk is about 1 x 1 0 5 . If one accepts the EPA URV of 3.3 x 10~ 5 (pg/m 3 )- 1 (slope factor = 1.2 x 10~ 13 (mg/kg-day)~1), then the risk is no greater than 1 x 10~ 4 . A maximum TCDD vapor concentration of 0.21 pg/m 3 was predicted 100 meters downwind (for summer days). The on-site concentration of TCDD in suspended paniculate was estimated to be 1.4 pg/m 3 (based on a TSP level of 0.07 mg/m 3 from site soil). For persons exposed to vapors and particulates about 100 meters off-site, the exposure was about 10-fold 4ess. The model-predicted concentrations of TCDD vapor were compared with the results of field and laboratory studies conducted by Yanders et al. (1988). Our estimate of the maximum possible
Implications Dioxin contaminated soil has been a significant public health concern since 1980. At many contaminated sites, the predominant route of exposure is often perceived to be the inhalation of vapors and fugitive dusts from the site. This study used a popular environmental model to predict the inhalation hazard and the results were compared against laboratory results. The original (1983) model was unable to properly predict the actual field data, however, a modified version accurately predicted the results. This paper shows that due to its low volatility, at soil concentrations of 100 ppb TCDD, the emanation of vapor and the level of TCDD in airborne dust is very small. At soil concentrations at least as high as 100 ppb, the maximum plausible cancer risk due to inhalation is less than one in a million for persons who might live immediately adjacent to such sites.
TCDD vapor flux rate for their study was 4.6 x 10~ 17 mg/seccm 2 , These results indicate that the concentrations predicted by the Jury et al. model are about 1000 fold greater than which actually occur. The inability of the Jury model to accurately estimate the rate of volatilization of TCDD from soil is probably because the TCDD was applied to the soil in a formulated state where it could easily migrate below the surface. A recent modification of the Jury model (1990) suggests that a 5-50 mm layer of clean soil will significantly retard (or eliminate) the vapor hazard posed by TCDD contaminated soil. The risks due to fugitive dust will always be greater than the vapor hazard, but for soil concentrations of 100 ppb the cancer risk should be less than 10~ 6 . Since few sites have average soil concentrations as high as 100 ppb, this nearly worst case analysis indicates that inhalation will rarely, if ever, be a significant route of exposure to TCDD-contaminated soil.
Although 2,3,7,8-TCDD has been shown to have a negligible vapor pressure (1.5 x 10~9 mm Hg at 25° C)1, several risk assessments have claimed or suggested that airborne concentrations of TCDD from contaminated soil could be sufficient to pose a significant health risk. 2 ' 3 Other reports have indicated that the inhalation hazard posed by TCDD vapors and particulates is negligible at virtually all plausible environmental soil concentrations and that this route of exposure can be dismissed from most risk assessments.4 To quantitatively evaluate this issue, the highest plausible long-term airborne concentration of TCDD (vapor and particulate-bound) originating from a site with contaminated soil was predicted via models and the associated theoretical cancer risks were estimated. The modelpredicted concentrations were compared with the results of field studies of TCDD contaminated soil. Methods In this analysis, a one acre hypothetical site was assumed to contain soil uniformly contaminated with 100 ppb TCDD to a depth of 50 mm. It was also assumed that only 50 percent of the soil was exposed or vegetated. The other half was paved or covered by buildings or driveways. This hypothetical scenario is representative of many known dioxin sites in the United States. While it is true that TCDD soil concentrations in excess of 100 ppb have been observed at some former individual sites, such as in Newark, New Jersey, the vast majority of remediated sites (including Copyright 1991—Air & Waste Management Association
1334
J. Air Waste Manage. Assoc.
Downloaded by [University of Connecticut] at 05:44 11 October 2014
Newark) have had average soil concentrations much less than 100 ppb. Since it is known that diffusion of chemical vapors through cement or other hard building surfaces is negligible compared to the potential escape through cracks5 in the concrete, and since the percent of cracks or imperfections in the surface is only 0.01-0.1 percent of the total floor area,6 it was assumed that those paved portions of the hypothetical site do not release significant amounts of TCDD vapor. It has been suggested that TCDD volatilization will change during the diurnal warming and cooling of the soil. Experimental data indicate that for soil depths of 2.0 to 10 centimeters the soil temperature can vary significantly with depth, but below 48 centimeters the soil temperature decreases at a constant rate. 3 For this analysis, the daily soil temperature was assumed to vary from 20° C to 40° C corresponding to average nighttime and daytime temperatures, respectively, for six months of the year. For the remainder of the year, the soil temperature was assumed to be 0° C. The potential TCDD vapor emissions from site soils were estimated using the model of Jury et al.7 which predicts the time-weighted average vapor flux from contaminated soil. The model accounts for the soil characteristics and the physical properties of the chemical. It considers the contaminant's volatility (vapor pressure), strength of adsorption to soil particles (Krj), and upward leaching in the soil column due to evapotransporation (wicking). The model also accounts for the continual loss of contaminant from the soil; that is, the soil vapor flux decreases over time. The values for the parameters used in this analysis are presented in Tables I, II and III. 8 - 12 The results of the Jury et al. model were compared with actual field/laboratory data. The calculations used to interpret the field data are presented in the Appendix. The Jury model relies on a number of chemical- and soil-specific parameters. The soil desorption coefficient, Krj, is a measure of a substance's tendency to remain sorbed to soil versus existing as a free unbound liquid. Henry's Constant, H, reflects the tendency of the liquid to enter the vapor state. Thus, the model treats soil-bound contaminant emissions as a two-stage process. Other parameters needed for the Jury model include foo pa» Pw> PT> Pb> a n d Jw- All °f them, with the exception of Jw, are related to the characteristics of the soil. Tables I, II, and III present the Jury
parameters corresponding to temperatures at 0° C, 20° C and 40° C, respectively. Work conducted by Freeman and Schroy12 indicated that the vapor pressure of TCDD in soil can increase 25-fold when daily temperatures vary from 20° C to 40° C. Henry's constant will increase by 25-fold and the air diffusion coefficient, D^, will increase by a factor of 1.4 when the temperature is 40° C versus 20° C. All other parameters were assumed to remain unchanged with temperature. A 'box model' approach was used to predict the maximum plausible levels of TCDD vapor on site. This approach assumed that the air above the surface soils was contained in a "building" having two open walls and a 2 meter high ceiling, with wind blowing through it. The choice of a 2-meter ceiling was to represent the approximate breathing zone of an individual walking over exposed site soils; a conservative approach which neglects dispersion. This box model approach is depicted in Figure 1. The base of the box is the entire site (one acre), since individuals may be located anywhere within the site. However, as was previously discussed, only xh acre emits TCDD vapor into this box due to the buildings and other paved sections. Although the approach is simplistic and will only yield maximum plausible values, it is useful for a screening-level assessment. The box model assumes that the TCDD vapor travels only a very short distance from the soil prior to inhalation. Therefore, the model accounts for removal of TCDD vapor by a uni-directional windstream but ignores dispersion effects and photodegradation (these are important when estimating the risks to persons off site). Airborne contaminant concentrations determined from the box model are therefore only relevant for persons who live close to the emitting surface. To this extent, the box model produces Upper-bound estimates of the airborne contaminant concentration since most exposures to individuals will occur at off site locations, where air dispersion effects are important. The SCREEN air dispersion model13 was used to estimate a worst-case (downwind) airborne concentration of TCDD (vapor and particulate-bound) at a distance of 100 meters off site. As its name implies, SCREEN is a model which produces upper-bound estimates of the vapors emitted from an area source. It accounts for horizontal and vertical dispersion during worst-case meteorological conditions (that is, extremely slow, stable winds). For the
Table I. Values used in model to estimate the volatilization of TCDD from soil at 0°C. Parameter Koc
S H h P MW Kd *oc Dg 3 * Q water
Pd Pa Pw PT
Pb P
H2O ^erav
dw
[TCDD] t Z a Value for 22°C. b Value for 0°C.
October 1991
Description
Value 6
Organic soil carbon-liquid partition coefficient Water solubility
2.78xl0 ml/g 7.96 x 10~6 mg/l a
Henry's constant Dimensionless Henry's Constant Vapor pressure Molecular weight Solid-liquid partition coefficient Organic soil carbon fraction Air diffusion coefficient Water diffusion coefficient Soil particle density Volumetric air fraction of soil Volumetric water fraction of soil Total soil porosity Dry bulk density Density of water Gravimetric water fraction of soil Water evaporation rate Soil concentration of TCDD Time-span for average flux calculation Depth of contamination
9.1 x 1(T7 atm-m3/mol 4.06 x 10~5 2.27 x 10~9 pascalsb 322 g/mole 2.78 x 104 ml/g 0.01 0.043 cm3/cm-sec 5.6 x 10~6 cm3/cm-sec 2.65 g/cm3 0.35 0.15 0.50 1.3 g/cm3 1 g/cm3 0.12 g/g 10 in/year 100 |xg/kg (wet soil) 70 years 50 mm
Volume 41, No. 10
Source/Rationale
Marpleetal., 1987 Marple et al., 1987; Freeman and Schroy, 1986 Determined from p, MW, and S Determined from H Determined from T Determined from KoC and foc Assumed Freeman and Schroy, 1985 Freeman and Schroy, 1985 Assumed Assumed Assumed PT = Pa + P w Assumed Pgrav
=
Pw ' PlKo/Pb
Assumed Initial assumption Initial assumption Initial assumption
1335
Table II. Values used in model to estimate the volatilization of TCDD from soil at 20°C. Parameter
Value 6
Koc S
Organic soil carbon-liquid partition coefficient Water solubility
2.78 x 10 ml/g 7.96 x 10~6 mg/la
H h P MW
Henry's constant Dimensionless Henry's Constant Vapor pressure Molecular weight Solid-liquid partition coefficient Organic soil .carbon fraction Air diffusion coefficient Water diffusion coefficient Soil particle density Volumetric air fraction of soil Volumetric water fraction of soil Total soil porosity Dry bulk density Density of water Gravimetric water fraction of soil Water evaporation rate Soil concentration of TCDD Time-span for average flux calculation Depth of contamination
3.44 x 10~5 atm-m3/mol 1.43 x 10~3 8.61 x 10~8 pascalsb 322 g/mole 2.78 x 104 ml/g 0.01 0.045 cm3/cm-sec 5.6 x 10~6 cm3/cm-sec 2.65 g/cm3 0.35 0.15 0.50 1.3 g/cm3 1 g/cm3 0.12 g/g 10 in/year 100 fxg/kg (wet soil) 70 years 50 mm
Ka j ) water
Pd Pa ?w PT p b PH 2 O
P x
grav
a n d the number of hours of darkness per day. %avg = %day x (# daylight hours/24 hours) + % night x (# dark hours/24 hours) Assumptions:
= %day 50% %night = 0.0% # daylight hours = 12 hours # dark hours = 1 2 hours
To account for the loss due to photodegradation, the amount of TCDD needs to be scaled up. Even though none of the filters had detectable levels of TCDD, we assumed that the amount of TCDD captured in the filter (Amount detected) was at the analytical limit of detection or 10 ng. Therefore, the initial mass of TCDD that "could have volatilized" without being detected (Amount vo i at jij ze d) is: Amount v o i a t i l i z e d = (Amount det ected)/(1 ~ %avg) Amountdetected = 10 ng %avg = 25% Amount vo i ati i ize d = 1 0 n g / ( l - 0.25) Amount vo i ati i iZ ed = 13.3 ng The maximum annual percent loss of TCDD due to vaporization and particulate emission (%i0Ss) i s a function of the initial mass of TCDD in the soil and the average mass of TCDD captured in the niters.
x 100 Assumptions: Amountsoji
= 13.3 ng = 315000 ng %loss = (13.3ng/(315,000ng)) x 100 %loss = 0.004%
1340
To estimate the total loss of TCDD due to volatilization and particulate emission (Loss^otai) over 70 years is: Loss total = %ioss x Lifespan Assumptions: %\oss = 0.002% loss/year Lifespan = 70 years Loss total = 0.002% loss/year x 70 years Loss total = 0.14% Assuming that half an acre of soil is contaminated with 100 ppb TCDD to a depth of 50 mm, about 1.46 g TCDD is initially present in the soil (Calculation I). If 0.14% of this mass is emitted over 70 years, the mass at 70 years is (1.46 g)(0.14/100) = 2.04 x lO" 3 g. The TCDD emission rate (ER) is the total mass which volatilizes per unit time or 2.04 x 10~ 3 g/70 years = 2.92 x 10~ 5 g/year. The predicted average flux depends on the emission rate (ER) and the emitting area. Assumptions:
Flux = (ERX I/Area) ER = 2.92 x 10~ 5 g/yr Area =0.5 acre Conversion factor = 4 x 103 m 2 /acre Conversion factor = 3.17 x 10~ 8 yr/sec Conversion factor = 103 mg/g Conversion factor = 104 cm 2 /m 2
Flux = (2.92 x 10~5 g/yr) (1/0.5 acre) (1/4 x 103 m2/acre) (l/10 4 cm 2 /m 2 )(3.17 x 10~8 yr/sec) (1Q3 mg/g) Flux = 4.6 x 10~17mg/sec-cm2
% a v g = (0.5X12 hours/24 hours) + (0.0)(12 hours/24 hours) % a v g = 25%
Assumptions:
Assuming that the amount of TCDD in the niters was present at half the LOD of the assay or 5 ng, the amount of TCDD that could have volatized without being detected (LOSSsoii) isO.004%/2 = 0.002%.
The box model predicted airborne concentration is determined by the following factors: Assumption: Flux = 4.6 x 10~17mg/sec-cm2 Conversion factor = 104 cm 2 /m 2 Area = 0.5 acre = 2000 m 2 Wind speed = 2 m/sec Length = ^4000 m2 = 63 m Height = 2 m (average height of a person) Air Cone. = (4.6 x 10- 17mg/sec-cm2)(104 cm2/m2)(2OOOm2)/((2m/sec) (2m)(63m)) Air Cone. = 3.7 x 10~ 12 mg/m 3 = 3.7 x 10- 3 pg/m 3 Based on the results above, the lifetime cancer risk should be no greater than (3.7 x 10- 3 pg/m 3 )(2.9 x 10~6) = 1.1 X 1Q-8.
The authors are with ChemRisk, a Division of McLaren/ Hart, 1135 Atlantic Avenue, Alameda, CA 94501. This manuscript was submitted for peer review on March 7, 1991. The revised manuscript was received on August 1, 1991.
J. Air Waste Manage. Assoc.
