Comparison of Sampling Methods to Document the Contribution of Aerobic Biodegradation to the Attenuation of Vapor Concentrations at UST Sites

Transcription

1 Comparison of Sampling Methods to Document the Contribution of Aerobic Biodegradation to the Attenuation of Vapor Concentrations at UST Sites Cynthia J. Paul, John T. Wilson, Dominic DiGiulio, and Ken Jewell U.S. EPA Office of Research and Development National Risk Management Research Laboratory Ground Water & Ecosystems Research Laboratory Ada, OK Robin Davis and John Menatti, Utah DEQ National Tanks Conference September 20-22, 2010 Boston, MA

2 Objective Determine whether the use of conventional sampling methods for hydrocarbons in soil gas overestimates risk at some UST sites, particularly in tight textured soils and when you pull a lot of gas during sampling.

3 Methods Used existing TPH and benzene values from soil core extracts to calculate surrogate benzene values: Pore water Predicted in soil gas Expected from diffusion Compared these values with measured benzene values Traditional active soil gas sampling UDEQ Vapor Probes New passive diffusion sampler PDS Stable carbon isotope ratio - SCIR

4 Field Site Hal s Chevron Green River, Utah Several thousand gallons of gasoline leaked from USTs and migrated down through the vadose zone soils (silty clays, clayey silts, and silts) to the ground water table at about 18 feet bgs and migrated laterally 300 ft.

5 Utah DEQ used StainlessSteel Soil Vapor Sampling Screen Polyethylene Tubing to Ground Surface direct buried vapor probes

6 Soil Gas Sample from UDEQ Multi-Depth Vapor Wells Using a plastic syringe, purged 4 tubing volumes from each sampling tube. Vapor samples were collected in 1-liter Summas. Did not use flow controllers, but had vacuum gauges on each canister syringe

7 Passive Diffusion Sampler (PDS) 40 ml VOA vial where the Teflon-lined septa is replaced by a permeable membrane (0.2 µm Supor) Trisodium phosphate (TSP) is added for preservation Diffusion membrane o-ring Vial VOA vials filled w/deionized water and capped

8 PDS and Messenger for 2-inch well Messenger (made of HDPE) PDS

9 Messenger and PDS within well casing

10 VW7 EPA1 VW10 EPA5 VW11 EPA3

11 2-in monitoring wells Vapor probes 3 ft 7 ft 2-in screen 11 ft 15 ft 18 ft

12 Results

13 VW10/EPA 5 Benzene in Soil Gas (mg/m 3 ) TPH gasoline range (mg/kg) Depth (feet) Depth (feet) Expected from Diffusion 18 PDS June 2007 UDEQ Vapor Probe June 2007

14 VW10/EPA Benzene in Soil Gas (mg/m 3 ) TPH gasoline range (mg/kg) Depth (feet) Depth (feet) Expected from Diffusion PDS June 2007 UDEQ Vapor Probe June 2007 Calculated Core Samples

15 VW10/EPA Benzene in Soil Gas (mg/m 3 ) TPH gasoline range (mg/kg) Depth (feet) Depth (feet) Expected from Diffusion PDS Sept UDEQ Vapor Probe Sept Calculated Core Samples

16 VW10/EPA Benzene in Soil Gas (mg/m 3 ) TPH gasoline range (mg/kg) Depth (feet) Depth (feet) Expected from Diffusion PDS June 2007 UDEQ Vapor Probe June 2007 Calculated Core Samples

17 Effect of Air Filled Porosity

18 VW-10 (EPA 5) Volume sampled 1.3 liters Θ air TPH as GRO (mg/kg) Green River Site (Clay) Asphalt Road Base Silty Clay Clay Depth (feet) Silt Silty fine Sand Silty Clay Clayey Silt Clay with (black stain) 16 Silty Clay 18

19 VW-10 (EPA 5) Volume sampled 1.3 liters Θ air TPH as GRO (mg/kg) Green River Site (Clay) Asphalt Road Base Silty Clay Clay Depth (feet) Silt Silty fine Sand Silty Clay Clayey Silt Clay with (black stain) 16 Silty Clay 18

20 VW-10 (EPA 5) Volume sampled 1.3 liters Θ air TPH as GRO (mg/kg) Green River Site (Clay) Asphalt Road Base Silty Clay Clay Depth (feet) Silt Silty fine Sand Silty Clay Clayey Silt Clay with (black stain) 16 Silty Clay 18

21 VW-10 (EPA 5) Volume sampled 1.3 liters Θ air TPH as GRO (mg/kg) Green River Site (Clay) Asphalt Road Base Silty Clay Clay Depth (feet) Silt Silty fine Sand Silty Clay Clayey Silt Clay with (black stain) 16 Silty Clay 18

22 14 12 Vertical Interval Sampled 10 8 Feet Air Filled Porosity (fraction total volume)

23 VW-10 (EPA 5) Volume sampled 1.3 liters Θ air Benzene in Soil Gas (mg/m 3 ) ,000 10, Asphalt Road Base Silty Clay Clay Depth (feet) Silt Silty fine Sand Silty Clay Clayey Silt Clay with (black stain) 16 Silty Clay 18

24 40 7 feet at VW-10 (EPA-5) Real Concentration Sampled Concentration Benzene (mg/m 3 ) Air Filled Porosity (fraction total volume)

25 Stable Isotopes to Evaluate Biodegradation

26 Expected Values Benzene δ 13 C o / oo % degraded Only 10% left Fraction Remaining (C/Co)

27 Benzene δ 13 C o / oo ~98% removal Fraction Remaining (C/Co)

28 Benzene δ 13 C o / oo Utah TO-15 Vapor Probe Samples Fraction Remaining (C/Co)

29 PDS Samplers Benzene δ 13 C o / oo Fraction Remaining (C/Co)

30 Conclusions Vertical profile of concentrations provides a mechanism to determine zones where active aerobic biodegradation of hydrocarbons is occurring allowing for a more accurate risk assessment from vapor intrusion. Biodegradation would be expected in shallow zones with adequate oxygen, however, SCIR showed benzene in shallow probes had same isotopic signature as deeper samples. The vapor probes over estimates concentrations and underestimates biodegradation because they included material from deeper in the profile.

31 Conclusions Active sampling methods pulls in gas from surround zones, which can result in inaccurate contaminant concentrations (see Schumacher et al., 2009) Air filled porosity, purge volume and purge/sampling rate greatly impact concentrations. Vertical profile concentrations can change radically across a small depth interval.

32 Recommendations Minimize amount of gas collected into samples, particularly for tight formations. The less gas you pull in from the formation, the more representative the sample is. Consider passive sampling methods for tight formations.

33 References Fischer, A. I. Herklotz, S. Herrmann, M. Thullner, S.A.B. Weelink, A.J.M. Stams, M. Schlomann, H. Richnow and C. Vogt. et al Environ. Sci. & Technol., 2008, 42 (12), Schumacher, B.A., J.H. Zimmerman, C.R. Sibert, I.E. Varner, and L.A. Riddick Macro- and Micro- Purge Soil-Gas Sampling Methods for the Collection of Contaminant Vapors. Ground Water Monitoring & Remediation 29, no. 1/ Winter 2009/pages

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35 Predicted Benzene in Pore Water using Soil Core Data where C w is the concentration in the ground water Co NAPL is the concentration in the gasoline that was spilled θ NAPL is the porosity filled with gasoline θw is the water-filled porosity C w = K K NAPL is the distribution coefficient between gasoline and water NAPL o, NAPL Henry s Law constant was used to calculate the vapor concentration. C θw + θ NAPL

36 Predicted Benzene Concentrations in Soil Gas using Soil Core Data C air = ( θ air * H ) + θ water + ( θ NAPL H * θ0, NAPL * C0, * K gasoline to water NAPL ) + ( RV solids * K oc * f oc * ρ ) b C 0,NAPL was estimated as the concentration of benzene in the methanol extract of the sediment (mg/kg) divided by the concentration of TPH in the extract (mg/kg). Assume θ NAPL = θ 0,NAPL As a conservative estimate, the total porosity (θtotal) of the sediment was assumed to be 0.40.

37 RVsolids was estimated as RVsolids = total ( 1 θ ) = 0.6 θwater was estimated as θ water = M water /( M water + ( M dry soil /(0.6* 2.64)) θair was estimated as θ air = θ θ = 0. 4 θ total water water H was calculated from a mean annual temperature of 12 oc using the EPA On-line Tools for Site Assessment Calculation. The value of H was Kgasoline to water was selected from Cline, et al. Environmental Science & Technology 25(5): (1991). The value was 350. Koc was selected from EPA On-line Tools for Site Assessment Calculation. The value is 88. foc was measured on the sediment samples. ρb was calculated as where ρs was assumed to 2.65 gm/cm3.