byRobin V. Davis, P.G.Project ManagerUtah Department of Environmental QualityLeaking Underground Storage Tanksrvdavis@utah.gov 801-536-4177

Methods for Developing and Applying Screening Criteria for the

Petroleum Vapor Intrusion PathwayWorkshop 7

Tuesday March 24, 20156:30 pm – 9:30 pm

Association for Environmental Health & Sciences (AEHS)25th Annual International Conference on Soil, Sediment, Water & Energy

San Diego, California

OBJECTIVES Understand why petroleum vapor intrusion (PVI) is

very rare despite so many petroleum LUST sites

Show mechanisms, characteristics, degree of vapor bioattenuation

Avoid unnecessary additional investigation, soil gas/air sampling

Show distances of vapor attenuation, apply as Screening Criteria, screen out low-risk sites

Understand causes of PVI

SCOPE Field studies published by work groups, individuals

Source strength: LNAPL in soil and GW, dissolved-phase Associated soil gas measurements from 1000s of sample points

at 100s of sites Extensive peer review and quality control checks

Data compiled to an empirical database:

Some US States Australia 2012 EPA draft PVI April 2013 ITRC October 2014 EPA ORD Issue Paper 2014

Guidance Documents Issued:

EPA Petroleum Vapor Database Jan. 2013

124/>1000

PerthSydney

Tasmania

Australia

Davis, R.V., 2009-2011McHugh et al, 2010Peargin and Kolhatkar, 2011Wright, J., 2011, 2012, Australian dataLahvis et al, 2013EPA Jan 2013, 510-R-13-001

REFERENCES

4/13

70/816

Canada

United States

MAP KEY# geographic locations evaluated

# paired concurrent measurements of subsurface benzene soil vapor & source strength

70

Petroleum Vapor Database of Empirical Studies EPA OUST Jan. 2013

Australian sites evaluated separately

816

CAPILLARY ZONE

a) LNAPL SOURCE

UNSATURATED ZONE

SATURATED ZONE

sharp reaction

front

O2

VOCs

b) DI SSOLVED- PHASE SOURCE

CAPILLARY ZONE

UNSATURATED ZONE

SATURATED ZONE

high massflux

limited mass flux sharp

reaction front

constituent distributions

O2

VOCs

constituent distributions

Conceptual Characteristics of Petroleum Vapor Transport and Biodegradation

After Lahvis et al 2013 GWMR

O2/Hydrocarbon Vapor Profile

O2/Hydrocarbon Vapor Profile

KEY POINTS

•Aerobic biodegradation of vapors is rapid, occurs over short distances

•LNAPL sources have high mass flux, vapors attenuate in longer distances than dissolved sources

•Sufficient oxygen supply relative to its demand, function of source strength

0 1

0 1

>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

Bioattenuation Study ResultsSubsurface Petroleum Vapor

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

LNAPLLNAPL

LNAPLLNAPL

41

3LNAPLLNAPL

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

UST system

Dissolved contamination

Clean 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

LNAPL

Signature Characteristics of Aerobic Biodegradation of Subsurface Petroleum Vapors

• Vapors aerobically biodegraded by oxygen-consuming microbes, waste product carbon dioxide

• Vapors attenuate in short distances

Vapor Bioattenuation Limited by Contaminated Soil

LNAPL in Soil(sand, silty sand)

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

VW-11 Hal’s, Green River, Utah

No attenuation within contaminated soil zone

EPA OUST Jan. 2013

Results of Empirical Studies

http://www.epa.gov/oust/cat/pvi/PVI_Database_Report.pdf

• Thickness of clean soil required to attenuate vapors associated with LNAPL and dissolved sources

• Screening Criteria

4 feetBenzene in GW 3180/ ug/L

Benzene in GW 12,000 ug/L

4.94 feet

Dissolved Sources

= Distance between top of source and deepest clean vapor point

Thickness of clean soil needed to attenuate vapors

8 feet

LNAPL Sources

= Distance between top of LNAPL and deepest clean vapor point

Thickness of clean soil needed to attenuate vapors

Screening Distances

95%-100% Confidence

Dissolved SourcesBenzene Vapors vs. Distance of Attenuation

LNAPL Sources (small sites)Benzene Vapors vs. Distance of Attenuation

5 ft 15 ft

13 feet, 95% Confidence

Lahvis et al 2013Results of Vapor Attenuation from LNAPL Sources

• Different analysis, similar results

• 13 ft vertical separation attenuates LNAPL source vapors

LNAPL Indicators

17

LNAPL INDICATOR MEASUREMENTS

Current or historic presence of LNAPL in groundwater or soil

Visual evidence: Sheen on groundwater or soil, soil staining, measurable thickness

Groundwater, dissolved-phase PHCs >0.2 times effective solubilities (Bruce et al. 1991)

Benzene >3-5 mg/L TPH-gro >20-30 mg/L TPH-dro >5 mg/L

Soil, adsorbed-phasePHCs >effective soil saturation (Csat)

Benzene >10 mg/kg TPH-gro >250-500 mg/kg

Soil field measurements Organic vapor analyzer/PID/OVA of soil cores

Gasoline-contaminated soil: >500 ppm-v Diesel-contaminated soil: >10 ppm-v

Soil Gas measurements

- O2 shows no increase and CO2 shows no decrease with increasing distance from source

- Elevated aliphatic soil gas concentrations, eg Hexane >100,000

ug/m3

(after EPA 2013; Lahvis et al 2013)

Results of Empirical Studies for Developing Screening Criteria

Various methods of data analysis yield similar results

Dissolved Sources require 5 feet separation distance:• Benzene <5 mg/L• TPH <30mg/L

LNAPL Sources require 15 feet separation distance:• Benzene >5 mg/L, >10 mg/kg• TPH >30mg/L, >250-500 mg/kg• 18 feet separation required for large industrial sites

Soil within separation distance:

• LNAPL-free soil contains sufficient oxygen to bioattenuate vapors• “Clean” (non-source), biologically active, sufficient oxygen and moisture• EPA: <100 mg/kg TPH “clean” soil

Field Example:

Deep SV Benzene, ug/m3

Shallow SV Benzene, ug/m3

=AF

~7,000,000x contaminant reduction

~1 ug/m3

145,000 ug/m3AF = = 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 25

O2 & CO2 (% V/V)

1.E+00 1.E+02 1.E+04 1.E+06 1.E+08

Benzene (ug/m3)

Oxygen

Carbon Dioxide

Benzene

Benzene in GW16,000 ug/L

0

20

40

60

80

100

1.E-021.E-03<1.E-04

Subsurface Vapor Attenuation Factors

Nu

mb

er

of

So

il V

ap

or

Sa

mp

le E

ve

nts

Benzene TPH

0

30

60

90

120

150

Reason 1: NoClean

Overlying Soil

Reason 2: LowSource

Strength

Reason 3:Rapid

AttenuationNear High-StrengthSource

Nu

mb

er o

f S

oil

Vap

or

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

Nu

mb

er

of

So

il V

ap

or

Sa

mp

le E

ve

nts

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

http://www.epa.gov/oswer/vaporintrusion/documents/vi-cms-v11final-2-24-2012.pdf

EPA Modeling StudiesVertical and Lateral Attenuation Distances

Lateral Distance, meters

Ver

tica

l D

ista

nce

Bel

ow

Gra

de,

met

ers

10 20 30 40 80 9050 60 70

0

2

4

6

8

0

2

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

U.S. and AustraliaScreening Criteria

Reference

Screening Distance

(feet)

Screening Concentration

Benzene, TPH (ug/L)Other

Criteria

EPA OUST Petroleum Database Report

515

<5000, <30,000LNAPL

LNAPL UST sites. 18 ft for large sites. Clean soil <250 mg/kg TPH

Wright, J., Australia, 2011

5 <1000  Includes large industrial sites

  30 LNAPL  

California5 <100 SG Oxygen not required

5 <1000 SG Oxygen required >4%

  10 <1000 SG Oxygen not required

  30 LNAPL  

Indiana

5 <1000 SG Oxygen not requiredDistances apply vertically & horizontallyAFs for GW & SG30

LNAPL

New Jersey5 <100 SG Oxygen not required

5 <1000 SG Oxygen required >2%

  10 <1000 SG Oxygen not required

  100 LNAPL Distances apply vertically & horizontally

Wisconsin5 <1000 Distances apply vertically & horizontally

20 >1000  

  30 LNAPL  

THANK YOU