Site Conditions Conducive for Potential Petroleum Vapor

Intrusion (PVI) RiskRobert E. Sweeney, Ph.D., R.G.

Intrusion (PVI) Risk

E & P Geochemistry

AEHS 21st Annual West Coast International Conference on Soil, Sediment, Water & Energy

Mission Valley, San Diego, CaliforniaMarch 14-17, 2011

Evaluation of BiodegradationPVI approach using methane field data

• All Sites are not the same for PVI risk– Discrepancy in evaluation for benzene – ‘fast’ reaction for biodegradation vs

‘large’ exclusion distance for sites in database must reflect site conditionslarge exclusion distance for sites in database - must reflect site conditions.

• Site conditions:– 1) limited oxygen flux;1) limited oxygen flux; – 2) inefficient biodegradation (oxygen and benzene both present); – 3) enhanced oxygen demand – fresh gasoline and methane; – 4) unidentified shallow source; ) ;– 5) sample interval.

• Methane study:y– 1) oxygen measurements at 4 or 5 depths; – 2) 891 measurements of oxygen and methane;– 3) 142 methane measurements > oxygen demand than fresh gasoline; – 4) no shallow sources; – 5) sample interval at 2 feet from surface to 8 or 10 feet.

Diffusion Dominated Vapor TransportEBI Study Guadalupe Oil FieldDil t (di l) l U i F t R ti l l ti 75% O

Install vapor points – field meter can be used to measure oxygen and methane concentrations at limit 0.1 %-v. Need m ltiple samples in aerobic one to define o gen

0

EBI Study - Guadalupe Oil FieldDiluent (diesel) plume - Using Fast Reaction calculation – 75% Oxygen Flux used to oxidize methane

multiple samples in aerobic zone to define oxygen concentration gradient.

0 20 40 fe

et)

60 80

100

Dep

th (f

Aerobic Zone

100 120

0 2 4 6 8 10 12 14 16 18 20 22 24

Anaerobic Zone – clean soil – TPH measurements

0 6 8 0 6 8 0Concentration Soil Vapor (%-v)

Methane Oxygen

Gasoline/Diesel smear zone extending downgradient from source – 56 locations with soil and soil vapor measurements

Typical cross section – silt layer over smear zone (red) – confinedyp y ( )aquifer above - but usually unconfined with top of smear at about7 – 10 feet-bgs. TPH/BTEX at top, middle and bottom of smear.

Soil Vapor AnalysesBenefit of soil gas profile data – can evaluated data quality andand thickness of biodegradation zone

70

Soil Vapor AnalysesSeasonal - 56 locations - 891 analyses

and thickness of biodegradation zone.

50

60 Biodegradation Zone – Oxygen & Methane

30

40

4 (%

-v) Bad Data ?

20

30

CH

4

No biodegradation

0

10

0 5 10 15 20 25 Oxygen (%-v)

Top of Contamination

Oxygen and Methane concentrations are valid.

Sample interval = 2 feetThickness biodegradation layer <= 4 feet

Oxygen Demand Methane 2 - 4 times fresh gasoline

0PZ-23Well #1 – soil vapor data from 1998; 2000; 2006

0

2

44

6

epth

(ft)

8

10

D Oxygen Flux 100 mg/m3-hr

Top of Contamination12

0 5 10 15 20 25 Soil Gas (%-v)

Top of Contamination

O2 (9/98) O2 (6/00) O2 (11/06)

CH4 (9/98) CH4 (6/00) CH4 (11/06)

0

PZ-23Well #1 – Methane Data Collected in 20090

2

4

6

epth

(ft)

8

10

De

12 0 1 2 3 4 5 6 7

Methane (%-v)Methane (%-v)

May 2009 August 11 2009y g

August 28 2009 November 2009

0PZ-23Well #1 – Benzene Data Collected in 2009

0

2

4

6

epth

(ft)

8

10

De

12 0 200 400 600 800 1,000 1,200

B ( / 3)Benzene (ug/m3)

May 2009 August 11 2009

August 28 2009 November 2009

Effectiveness of Biodegradation• Review of site with extensive gasoline/diesel smear zone. No near surface

contamination.

• 891 measurements made at 56 different locations and at 2 foot intervals from the surface to 8 or 10 feet depth.

• Methane measurements– 623 at < 0.1 %-v [all samples from < 6 feet] – 126 between 0.1 and 10 %-v [range for oxygen demand – gasoline in soil]

142 at 10 % v or greater [larger oxygen demand than for fresh gasoline]– 142 at 10 %-v or greater [larger oxygen demand than for fresh gasoline]

• Thickness of biodegradation zone (clean soil needed for attenuation) = 4 feet.– At 4 locations: < 0.1 %-v methane corresponded to benzene < 30 ug/m3p g– 0.1 %-v methane = 700,000 ug/m3 methane– How low does the methane concetration go in aerobic zone?

Low level methane measurement (FID) – Atmosphere = 1.7 ppm-v

- Roslev, et al. (1997) – Applied Envir Microbiology v 63 #3 p.874

- www.co2science.org/subject/m/summaries/methaneextract.php

Ohio Site - May 2009Soil Vapor Samples @ 2 foot Depth

2,500

Soil Vapor Samples @ 2-foot Depth

At h i M th L l2,000

b-v)

Atmospheric Methane Level

1,500

ne (p

pb

500

1,000

Met

han

0

500

. . .

PVI approaches to evaluate biodegradation i h dusing methane data

Th i di b bi d d i• There is no discrepancy between biodegradation models and exclusion rules.

• 891 measurements – all consistent with bi d d i d lbiodegradation model.

• Thickness of biodegradation layer is 4 feet for oxygen demand up to 4 times fresh gasoline.