Screening Criteria for the Petroleum Vapor Intrusion Pathwayneiwpcc.org/tanks2015old/tanks2015presentations/1-Sunday... · 2018. 11. 3. · Robin V. Davis, P.G. Project Manager
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by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality Leaking Underground Storage Tanks [email protected]801-536-4177 Developing and Applying Screening Criteria for the Petroleum Vapor Intrusion Pathway Petroleum Vapor Intrusion Workshop Sunday September 13, 2015 1:00 pm – 5:00 pm 25th National Tanks Conference Phoenix, Arizona
by Robin V. Davis, P.G. Project Manager Utah Department of Environmental Quality Leaking Underground Storage Tanks [email protected] 801-536-4177
Developing and Applying Screening Criteria for the
Petroleum Vapor Intrusion Pathway
Petroleum Vapor Intrusion Workshop Sunday September 13, 2015
1:00 pm – 5:00 pm
25th National Tanks Conference Phoenix, Arizona
Understand why petroleum vapor intrusion (PVI) is very rare despite so many petroleum LUST sites
Show mechanisms, characteristics, degree of vapor bioattenuation
Show distances of vapor attenuation relative to source strength
Understand causes of PVI
Apply as Screening Criteria, screen out low-risk sites, avoid unnecessary investigation, soil gas/air sampling
Presenter
Presentation Notes
The objective of studying and evaluating the behavior of subsurface petroleum vapors is to understand why, with so many LUST sites worldwide, the PVI pathway is rarely complete. Using basic, routine field data enables us to develop screening criteria to determine when PVI investigations are necessary. Detailed discussion of the Petroleum Vapor Database is provided in R.V. Davis 2009, LUSTLine #61 and EPA Jan. 2013.
Field Data from 3 Countries, Published Field Studies
Paired, concurrent measurements of source strength and associated soil gas measurements
Source strength: LNAPL in soil and GW, dissolved-phase 1000s of sample points and measurements at 100s of sites Extensive peer review and quality control checks Distances of vapor attenuation quantified
EPA Database Report of Empirical Studies, Jan. 2013
Some US States
Australia 2012
ITRC October 2014
EPA final PVI June 2015
Guidance Documents Issued:
Presenter
Presentation Notes
The scope of evaluating the PVI pathway involved collecting and compiling basic site data (soil, GW, vapor data) to the EPA Petroleum Vapor Database. A LUST site must be fully characterized by collecting basic, good-quality data wherein the nature, extent and degree of contamination and contaminant sources are fully defined (required by 40 CFR Part 280), and provide knowledge of contaminant distribution in soil and groundwater, including temporal effects such as fluctuating DTW. Once these subsurface characteristics are understood, LUST PMs can better understand if the PVI pathway may be complete and if VI investigations are really necessary. VI investigations are costly and highly invasive to properties and their occupants, and unnecessary work should be avoided. Time Line of Studies and EPA Publications 2002: EPA OSWER Draft Guide for Vapor Intrusion Evaluations 2003-2005: EPA OUST/States Petroleum Vapor Intrusion Work Group 2005-2009: Work Group hiatus, continued independent data compilation, analysis 2009-2011: EPA OUST/States PVI Work Group Revived 2011: EPA OUST Petroleum Hydrocarbons and Chlorinated Hydrocarbons Differ in Their Potential for Vapor Intrusion 2012: EPA OUST Approach for Developing Lateral and Vertical Distances 2013: EPA OUST Evaluating Empirical Data for Screening Petroleum Sites (“Petroleum Vapor Database Report”) 2014: EPA OUST Approach Using Hydrocarbon Concentrations in Soil Gas to Evaluating Potential for PVI 2015: EPA OUST publishes final PVI Guidance
124/>1000 Perth
Sydney
Tasmania
Australia
Davis, R.V., 2009-2011 McHugh et al, 2010 Peargin and Kolhatkar, 2011 Wright, J., 2011, 2012, Australian data Lahvis et al, 2013 EPA Jan 2013, 510-R-13-001
EPA OUST Petroleum Database Report - Data analyzed for 74 Sites, 829 paired measurements of concurrent contaminant source strength and associated vapor data, additional 64 benzene vapor measurements without specific source strength data. Source strengths are divided into dissolved-phase, LNAPL in groundwater, and residual LNAPL in vadose zone soil. Australian data analyzed separately because of limited access to raw data.
January 2013
Petroleum Database Report
• Compilation of field data: LNAPL in soil & GW, & dissolved sources, and concurrent associated vapor data
The Database Report is a compilation and analysis of field data from Davis 2009-2011, and more data compiled by EPA OUST from various US states and Australia. The database consists of thousands of measurements of soil gas and concurrent associated source strengths from hundreds of sites.
Conceptual Characteristics of Petroleum Vapor Transport and Biodegradation
After Lahvis et al 2013 GWMR
O2/Hydrocarbon Vapor Profile
O2/Hydrocarbon Vapor Profile
KEY POINTS • LNAPL sources have high
mass flux, vapors attenuate in longer distances than dissolved sources
• Aerobic biodegradation of vapors is rapid, occurs over short distances
• Oxygen demand is a function of source strength
0
1
0
1
Presenter
Presentation Notes
Figure 1 Lahvis et al 2013
UST system
Dissolved contamination
Clean, Non-Source Soil
High vapor concentrations, high mass flux
from LNAPL & soil sources
Low vapor concentrations, low
mass flux from dissolved sources
Define extent & degree of contamination Apply Screening Criteria
Building
Collect Basic Data, Characterize Site, Construct Conceptual Site Model
LNAPL in soil
LNAPL in soil & GW
Soil Boring/MW Soil Boring/MW
Utility line
Presenter
Presentation Notes
Site characterization is routine and necessary in order to know if receptors are impacted. Title 40 of the Code of Federal Regulations, Part 280 for leaking USTs requires that sites be fully characterized by conducting subsurface investigations wherein the full extent and degree of contamination are defined. The presence and thickness of “clean” or non-source soil above contaminant sources are generally known in the early phases of site characterization by advancing soil borings, logging the borings, and completing as GW monitoring wells. Non-source soil, also called “Clean Soil.” is free of LNAPL and contains the necessary oxygen for biodegrading PHCs. Clean, non-source soil is easily characterized by logging borings/MWs, field PID measurements , visual and olfactory observations of soil cores, and collecting soil samples and analyzing for constituents of concern. Build a Conceptual Site Model based on site-specific data. Volatile compounds associated with LNAPL, contaminated soil, and very high dissolved contaminant concentrations can generate very high vapor concentrations that, when in close proximity to buildings or utilities, can cause PVI. Those conditions are the only known cases of petroleum vapor intrusion: There are no known or reported cases of petroleum vapor intrusion associated with low dissolved-phase concentrations at or near buildings or utilities. Contaminants partition to vapor phase from soil and LNAPL source according to Raoult’s Law. Contaminants dissolved in GW partition to vapor phase according to Henry’s Law Constant.
>100 years of research proves rapid vapor biodegradation by 1000s of indigenous microbes
Studies show vapors biodegrade and attenuate within a few feet of sources
No cases of PVI from low-strength sources
Causes of PVI are well-known
Causes of Petroleum Vapor Intrusion
Preferential pathway: sumps, elevator shafts
High-strength source in direct contact with building (LNAPL, high dissolved, adsorbed)
Groundwater-Bearing Unit
BUILDING
Unsaturated Soil
Affected GW
LNAPL
LNAPL
4 1
3 LNAPL
High-strength source in close proximity to building, within GW fluctuation zone
2
Drawing after Todd Ririe, 2009
High-Strength Sources Direct contact or close proximity to buildings Preferential pathways: engineered & natural
Preferential pathway: bad connections of utility lines; natural fractured and karstic rocks
Presenter
Presentation Notes
Practical field experience and published literature of field studies show that petroleum vapor intrusion impacts are generally associated with: Direct contact of LNAPL or very high dissolved concentrations to a building foundation Close proximity of LNAPL or very high dissolved concentrations to a building foundation that fluctuation GW levels bring contamination in direct contact with building Direct contact of LNAPL or very high dissolved concentrations to building sumps, elevator shafts Direct contact of LNAPL or very high dissolved concentrations with preferential pathway (e.g., improperly-sealed utility lines) Key Points: Field data confirm that petroleum vapor intrusion impacts are associated with high contaminant concentrations, and that vapor intrusion does not occur with low concentrations of petroleum hydrocarbons dissolved in groundwater or adsorbed in soil.
(ITRC PVI Tech Reg, 2014)
Preferential Pathway: Engineered
Petroleum Source
(ITRC PVI Tech Reg, 2014)
Preferential Pathway: Natural
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20
Benzene (ug/m3)
O2 & CO2 (% V/V)
Coachella, CA COA-2 (Ririe, et al 2002)
1.E+001.E+021.E+041.E+061.E+08
-5
0
5
10
15
20
0 5 10 15 20 25
Benzene (ug/m3)
Salina Cash Saver VMW-1 (UDEQ 7/27/07)
OA IA
LNAPL
LNAPL
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20 25
Benzene (ug/m3)
Dep
th, f
eet b
elow
gra
de
O2 & CO2 (% V/V)
Beaufort, SC NJ-VW2 (Lahvis, et al., 1999)
OxygenCarbon DioxideBenzene
Benzene in GW 16,000 ug/L
Signature Characteristics of Aerobic Biodegradation of Subsurface Petroleum Vapors
Vapor profiles of multi-depth vapor probes showing the signature characteristics of aerobic biodegradation of petroleum hydrocarbon (PHC): Aerobic soil microbes use oxygen in the process of capturing the carbon as food from the petroleum hydrocarbon. The resulting waste product is carbon dioxide. Therefore, near the contaminant source, O2 is depleted and CO2 is enriched. As the PHC is biodegraded, PHC vapor concentrations decrease, and O2 and CO2 rebound to near-atmospheric concentrations. Typical O2, CO2, PHC vapor profiles: petroleum vapors naturally biodegrade & attenuate with sufficient thickness of “clean” or non-source vadose zone soil 1000’s of such measurements yield consistent, predictable results Extent and magnitude of vapor attenuation can be quantified, and Screening Criteria developed and applied
Vapor Bioattenuation Limited by Contaminated Soil
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20
Benzene (ug/m3)
Dep
th, f
eet b
elow
gra
de
O2 & CO2 (% V/V)
Conneaut, OH VMP-1 (Roggemans, 1998; Roggemans et al., 2001)
OxygenCarbon DioxideBenzene
LNAPL in Soil (sand, silty sand)
Presenter
Presentation Notes
PHC vapors cannot attenuate because there is insufficient thickness of “clean” or non-source soil overlying the contaminated soil source. Without shallower vapor completion points, there is no way of knowing if vapors decrease and soil oxygen concentrations increase before reaching an overlying receptor.
8/26/06 6/27/07
Importance of Shallow Vapor Completion Points
Shallower point confirms attenuation above contaminated soil zone
Shallow completion too deep
• Example of apparent non-attenuation until shallow vapor point installed in non-contaminated soil
• Good vapor point completion ensures atmospheric influences are none/minimal
VW-11 Hal’s, Green River, Utah
No attenuation within contaminated soil zone
Presenter
Presentation Notes
This slide shows benzene and TPH multi-depth vapor profiles of vapor well VW-11 from two different sampling dates, the earlier 8/06 event having no shallow completion above the LNAPL soil, thus fails to determine if vapors attenuate and if PVI pathway is complete. Vapor concentrations are very high within the contaminated soil zone (red shading) and, from the 8/26/06 sampling event where the shallowest vapor sample was obtained from 4 feet deep, vapors appear to not attenuate below the overlying paved road. However, on 6/27/07, vapor samples obtained from 2.5 feet deep showed nearly complete vapor attenuation. Leak testing confirmed the good integrity of each completion point. Some practitioners maintain that vapor completion points set too shallow (some say <5 feet deep) may be subject to short-circuiting or otherwise drawing in atmospheric air, causing a false-negative effect on vapor analyses. Others argue that this effect is not occurring at most sites because, according to standard sampling practices, vapor samples are obtained relatively quickly (“grab samples”) and draw vapor in from the area directly around the completion point. Studies in Utah show that only faulty completion points or unnecessarily long sampling times result in drawing in atmospheric air. VW-11 is an example that shows the benefits of shallow completion points dispelling the notion that short-circuiting might occur.
Probe A3 (TCE - Normalized)
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0.2
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0.6
0.8
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1.6
1.8
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13:5
0:09
10:5
0:26
7:50
:44
4:51
:04
1:51
:43
22:5
3:46
5:43
:29
2:43
:47
23:4
4:04
20:4
4:22
17:4
4:39
14:4
4:57
11:4
5:15
8:45
:32
16:0
2:01
13:0
2:18
10:0
2:36
7:02
:55
4:03
:12
1:43
:56
22:4
4:15
7:06
:41
4:06
:59
1:07
:19
22:0
7:58
19:1
1:25
Time (3/16/07 to 4/10/07)
Norm
alize
d Con
cent
ratio
n
Probe A3-3' (Port 9)
Probe A3-8' (Port 10)
Probe A3-17' (Port 5)
Soil Gas Temporal Study, EPA-ORD
3’ bgs
8’ bgs
15’ bgs
• >500 points per probe collected once per hour over 4 week period • Soil gas concentrations varied by <10% even for probes only 3 feet below the surface.
Presenter
Presentation Notes
This is a plot of data recently collected for an EPA funded study by an automated instrument at at Vandenberg AFB site from three probes at the same location and at different depths (3’, 8, & 17’ bgs). This plot consists of over 500 points per probe collected once per hour over a 4 week period from mid-March to mid-April 2007. The soil gas concentrations varied by less than 10% over these four days even for probes only 3 feet below the surface.
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20 25
Benzene (ug/m3)
O2 & CO2 (% V/V)
Santa Clara, UT VW-4 1/19/2009
4 feet Benzene in GW 3180/ ug/L
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06
0
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4
6
8
10
0 5 10 15 20 25
Benzene (ug/m3)
Dep
th, f
eet b
gs
O2 & CO2 (% v/v)
Jackson’s, UT VMW-4 9/29/08
Oxygen, %Carbon Dioxide, %Benzene, ug/m3
Benzene in GW 12,000 ug/L
4.94 feet
Method for Determining Thickness of Clean Soil Required to Attenuate Vapors Associated with Dissolved Sources
Thickness of clean soil needed to attenuate vapors = Distance between top of dissolved source and deepest clean vapor point
Presenter
Presentation Notes
Vapor Profiles Left Panel: Benzene vapor concentrations emanating from a very high-strength dissolved source attenuate with 4.94 feet of overlying soil. Right Panel: Benzene vapor concentrations emanating from a moderate-strength dissolved source are very low, and attenuate with 4 feet of clean overlying soil and only 4 feet above the groundwater surface.
TPH: 73 exterior/near-slab + 24 sub-slab = 97 total Benzene: 199 exterior/near-slab + 37 sub-slab = 236 total
2009-2011 Analysis of Petroleum Vapor Database for Dissolved Sources
5 feet of clean soil attenuates vapors associated with dissolved sources: Benzene 1,000 ug/L, TPH 10,000 ug/L
Presenter
Presentation Notes
- The Petroleum Vapor database (Davis 2009, 2011) contains high-quality data from hundreds of paired field measurements of dissolved-phase and associated vapor-phase benzene and TPH. - Different methods of data analysis yield consistent results of the thickness of non-source overlying soil necessary to fully attenuate vapors from their respective dissolved source strengths (Davis 2009; EPA 2013; Lahvis et al 2013). Criteria for Evaluating Data Set - Non-source soil overlies groundwater (dissolved sources) - Known depth to groundwater and dissolved sources - Groundwater & soil vapor data collected at about same time (concurrent) from close proximity - Complete attenuation of soil vapors defined by shallow soil vapors = 0 or <DL (which does vary; full attenuation verified by samplers/authors) - Majority of soil vapor measurements from multi-depth soil vapor points, representative sub-slab events included - LUST sites AND refineries -LNAPL sites evaluated separately by EPA and Lahvis et al 2013
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20
Benzene (ug/m3)
O2 & CO2 (% V/V)
Coachella, CA COA-2 Ririe, et al 2002
9.5 feet
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08
0
5
10
15
0 5 10 15 20 25
Benzene (ug/m3)
Dep
th, f
eet b
elow
gra
de
O2 & CO2 (% V/V)
Beaufort, SC NJ-VW2 Lahvis, et al., 1999
OxygenCarbon DioxideBenzene
Benzene in GW 16,000 ug/L
8 feet
Method for Determining Thickness of Clean Soil Required to Attenuate Vapors Associated with LNAPL Sources
Thickness of clean soil needed to attenuate vapors = Distance between top of LNAPL source and deepest clean vapor point
Presenter
Presentation Notes
Vapor Profiles Left Panel: Benzene vapor concentrations emanating from a very high-strength dissolved source attenuate with 8 feet of overlying soil. The water table symbol is colored pink indicates that there is probably LNAPL at this site base on the very high dissolved concentration. Right Panel: Benzene vapor concentrations emanating from an LNAPL source are high at depth, and attenuate with 9.5 feet of clean overlying.
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7
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9
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Th
ick
ne
ss o
f Cle
an
So
il O
ve
rly
ing
LN
AP
LR
eq
uir
ed
to A
tte
nu
ate
Va
po
rs, f
ee
t
TPH SV Sample Event over LNAPL & Soil Sources
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9
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Thic
knes
s of C
lean
Soi
l Ove
rlyin
g LN
APL
Requ
ired
to A
ttenu
ate
Vapo
rs, f
eet
Benzene SV Sample Event over LNAPL & Soil Sources
Benzene TPH 48 exterior/near-slab + 23 sub-slab = 71 total 17 exterior/near-slab + 19 sub-slab = 36 total
1 Refinery Site
Sites Sites
2009-2011 Results of Vapor Attenuation from LNAPL and Soil Sources
8 feet of clean soil attenuates vapors associated with LNAPL and Soil Sources
Presenter
Presentation Notes
This plot shows all LNAPL UST and non-UST sites in the database that were evaluated using the Method shown on the previous slide. The maximum soil thickness of non-source soil, required to attenuate vapors associated with LNAPL sources and contaminated-only soil sites is 8 feet, including refinery sites. Criteria for Evaluating Data Set - Known/suspected depths and intervals of uncontaminated soil, top of contaminated soil and LNAPL smear zone, and deep contaminated SV - Complete attenuation of soil vapors defined by shallow soil vapors = 0, or 30-100 ug/m3, <DL (which may vary; full attenuation verified by samplers/authors)
Screening Distances
95%-100% Confidence for LUST Sites 93% Confidence for Refineries, Terminals
Dissolved Sources Benzene Vapors vs. Distance of Attenuation
LNAPL Sources (small sites) Benzene Vapors vs. Distance of Attenuation
6 ft 15 ft
Presenter
Presentation Notes
EPA 2013 database report Figures 16a and 18a, left-to-right.
Lahvis et al 2013 Results of Vapor Attenuation from LNAPL Sources
• Slightly different analysis, similar results
• 95% Confidence
that 13 ft vertical separation attenuates LNAPL source vapors
13 ft
Presenter
Presentation Notes
Figure 6 Lahvis et al 2013
LNAPL Indicators
24
LNAPL INDICATOR MEASUREMENTS
Current or historic presence of LNAPL in groundwater or soil
Visual evidence: Sheen on groundwater or soil, soil staining, measurable product thickness
Groundwater, dissolved-phase PHCs >0.2 times effective solubilities (Bruce et al. 1991)
Soil Gas measurements - O2 depleted, CO2 enriched with increasing distance from source - Elevated aliphatic soil gas concentrations (eg Hexane >100,000ug/m3)
(after EPA 2015; Lahvis et al 2013)
Presenter
Presentation Notes
Indicators of LNAPL based on field and numerical studies.
Various methods of data analysis yield similar results
and moisture to bioattenuate vapors • EPA 2015: <100 mg/kg TPH “clean” soil
Dissolved Sources require 5-6 feet separation:
Field Example:
Deep SV Benzene, ug/m3
Shallow SV Benzene, ug/m3 = AF
~7,000,000x contaminant reduction
~1 ug/m3
145,000 ug/m3 AF = = 7E-06
Measuring Magnitude of Subsurface Vapor Attenuation
Subsurface Attenuation Factor (AF) = Ratio of shallow to deep vapor concentration
Beaufort, SC NJ-VW2(Lahvis, et al., 1999)
0
5
10
15
0 5 10 15 20 25O2 & CO2 (% V/V)
1.E+00 1.E+02 1.E+04 1.E+06 1.E+08Benzene (ug/m3)
OxygenCarbon DioxideBenzene
Benzene in GW16,000 ug/L
Presenter
Presentation Notes
Magnitude of subsurface AFs is expressed by dividing the shallow soil vapor concentration by the deep soil vapor concentration. The lower the AF, the greater attenuation and contaminant reduction. Significant attenuation is most often represented by AF<0.001 (>1000-fold attenuation and reduction in contaminant concentration). AF>0.01 (<100-fold attenuation) are common and represent both significant and insignificant attenuation depending on the source characteristics. Significant attenuation is observed when the petroleum contaminant source has as little as 2 feet of clean overlying soil. This example (Beaufort, SC, Matt Lahvis et al, 1999) shows that vapors associated with a very high source strength are fully attenuated with 7.6 feet of clean overlying soil, and the AF is very low, <1E-06).
0
20
40
60
80
100
1.E-021.E-03<1.E-04
Subsurface Vapor Attenuation Factors
Num
ber o
f Soi
l Vap
or S
ampl
e Ev
ents
Benzene TPH
0306090
120150
Reason 1: NoClean
Overlying Soil
Reason 2: LowSource
Strength
Reason 3:Rapid
AttenuationNear High-StrengthSource
Num
ber
of S
oil V
apor
Sam
ple
Eve
nts Benzene TPH
3 Reasons for Insignificant AFs 10x-100x
0
40
80
120
160
200
>1.E-011.E-011.E-021.E-03<1.E-04
Subsurface Vapor Attenuation Factors
Num
ber o
f Soi
l Vap
or S
ampl
e Ev
ents
Benzene TPH
Distribution of Magnitude of Subsurface Petroleum Vapor Attenuation Factors
Screen these out Reasonable Screening
AF 100x-1000x
Most events exhibit Significant AFs >10,000x
Presenter
Presentation Notes
Vapor data in the Petroleum Vapor Database are plotted to show the magnitude of subsurface vapor attenuation. and can determine the need for PVI investigations. The data set contains the following number of sites: Benzene 237, TPH 189, including many sample events beneath buildings (benzene 53, TPH 41). Results: Weak attenuation of petroleum vapors are represented by Insignificant (or High) Attenuation Factors: AF>1E-02 (<100-fold contaminant reduction). There are 3 Reasons for Insignificant Attenuation: No clean soil surrounding the source. Clean soil contains the necessary oxygen to bio-degrade petroleum hydrocarbons. Without clean surrounding soil, vapors cannot bio-degrade and attenuate. Low source strengths. For weak vapor concentrations, there is very little hydrocarbon to degrade and AFs will be high. Vapors attenuate directly above even very strong sources. A large number of sites in the database have their deepest soil vapor completion point set about 1 foot above the water table (e.g., Newport Beach, Ririe et al; Jackson’s UDEQ) where dissolved benzene concentrations up to 20 mg/L are observed yet very low-to-non-detectable vapor concentrations are detected in those deepest vapor sample points. This phenomenon demonstrates the strength of the vadose zone as a “natural bio-reactor” and attenuation over very short distances above sources. Considerations: - Of the soil vapor sample events exhibiting significant attenuation, the majority show AF <1E-04 (>10,000-fold reduction contaminant vapor concentration). Applying AF of 1E-03 (1000-fold) is therefore conservative. This phenomenon serves to emphasize the value of considering and applying subsurface AFs. California permits a 1000-fold subsurface vapor AF.
EPA 2012, Lilian Abreu primary author. Results indicate that distance of petroleum vapor attenuation in the lateral direction is less than or the same as attenuation in the vertical direction.
Lateral Distance, meters
Vert
ical
Dis
tanc
e B
elow
Gra
de, m
eter
s
10 20 30 40 80 90 50 60 70
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8
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4
6
8
LNAPL Vapor Source 200,000,000 ug/m3
8 m deep (26 ft)
Lateral Attenuation 5m (16 ft)
Building with Basement
Vertical Attenuation 6m (20 ft)
Oxygen
Model: 20 ft vertical 16 ft lateral Field Data: 15 ft vertical 8 ft lateral Conclusions: - Models under-predict attenuation - Vapors attenuate in shorter distances laterally than vertically
Oxygen
Presenter
Presentation Notes
EPA 2012, Figure 35: Effect of source depth and source-building lateral separation distance on the distribution of hydrocarbon and oxygen in soil gas for a high-strength source (LNAPL). Hydrocarbon and oxygen concentration contour lines are normalized by source and atmospheric concentrations, respectively. The source vapor concentration is 200 mg/L at 8 meters deep (26 feet). Biodegradation rate (lamda) = 0.18h-1. The source concentration is attenuated by 100,000-fold within 20 feet in the vertical direction and 16 feet in the lateral direction. The slightly shorter distance of lateral attenuation is due to the larger area of clean, oxygen-rich soil.
Benzene in GW 16,000 ug/L
TPH in GW 67,100 ug/L
RESULTS BioVapor Model under-predicts subsurface attenuation by 100x to 10,000x
Compared to BioVapor Model Beaufort, South Carolina (Lahvis et al 1999)
Presenter
Presentation Notes
Vapor profiles of field data compared to BioVapor Model using dissolved source and three scenarios, each using the same attenuation factor (AF) and O2 and foc concentrations: 1) overlying bare earth; 2) overlying pavement (actual on-site condition, however the bare earth scenario most accurately reflects the field data), and; 3) specified depth of aerobic zone, which in this case is known from multi-depth SV points, and can also be determined from routine soil boring logs.
Reference
Screening Distance
(feet)
Screening Concentration
Benzene, TPH (ug/L) Other
Criteria
EPA OUST Petroleum Database Report
5 15
<5000, <30,000 LNAPL
LNAPL UST sites. 18 ft for large sites. Clean soil <250 mg/kg TPH
Wright, J., Australia, 2011 5 <1000 Includes large industrial sites
