Aerobic Biodegradation in the Vadose Zone Workshop 1: Assessment and Evaluation of Vapor Intrusion at Petroleum Release Sites Conceptual Model: Aerobic Biodegradation in the Vadose Zone The Association for Environmental Heath and Sciences Foundation The 27th Annual International Conference on Soils, Sediments, Water, and Energy Amherst, MA, October 17-20, 2011 1 1

Overview of Petroleum VI n General VI Conceptual Model n Vadose Zone Attenuation of Petroleum Vapors n Oxygen Below Building Foundations

Groundwater-Bearing Unit Conceptual Model for Vapor Intrusion: Regulatory Framework BUILDING Building Attenuation Due to Exchange with Ambient Air 3 Air Exchange Advection and Diffusion Through Unsaturated Soil and Building Foundation Unsaturated Soil 2 Affected Soil Affected GW Partitioning Between Source and Soil Vapor Groundwater-Bearing Unit Slide Topic: Conceptual model for vapor intrusion Diffusion from source and advection into building does not match industry experience. The standard conceptual model of vapor intrusion has the following key elements: Partitioning between the source and soil vapors. An infinite source of VOCs is assumed (i.e., no mass flux or mass balance considerations) Diffusion and advection (but not biodegradation) through unsaturated soils and the building foundation Attenuation in the building due to exchange with ambient air. This conceptual model is based on the J&E model. The U.S. EPA VI guidance was developed using this conceptual model of vapor intrusion. Slide Presentation: Describe the conventional conceptual model for vapor intrusion. Key Points: This conceptual model does not distinguish between petroleum and chlorinated VOCs. The model assumes an infinite source of VOCs The model does not account for biodegradation in the vadose zone. The model does not reflect industry experience with petroleum vapor intrusion impacts (see next slide). 1 KEY POINT: Regulatory guidance focused on building impacts due to vapor migration.

Physical Barriers to Vapor Intrusion Possible Barriers No Barrier Building Foundation: (A) Low permeability foundation with no cracks or unsealed penetrations; (B) Positive building pressure A B Vadose Zone Vadose Zone: KEY POINT: Vapors (A) High moisture content or clay layer; (B) Aerobic biodegradation Presence of subsurface source does not always mean vapor intrusion is occurring. A B A B Groundwater interface: (A) Clean water lens (B) Clay layer Source Area GW Aquifer

Overview of Petroleum VI n General VI Conceptual Model n Vadose Zone Attenuation of Petroleum Vapors n Oxygen Below Building Foundations

Petroleum Biodegradation Conceptual Model CHmin Comax Aerobic Biodegradation Possible Co>Comin  Oxygen L No Aerobic Biodegradation Co<Comin Hydrocarbon Comin CHmax Hydrocarbon Source Vapor Concentration KEY POINT: Correlation between oxygen consumption and hydrocarbon attenuation. From Roggemans et al., 2001, Vadose Zone Natural Attenuation of Hydrocarbon Vapors: An Empirical Assessment of Soil Gas Vertical Profile Data, API’s Soil and Groundwater Technical Task Force Bulletin No. 15.

VOC Concentration vs. Depth Biogenic Gases vs. Depth Petroleum Biodegradation: Real Site Data Diesel Release Site, North Dakota VOC Concentration vs. Depth Biogenic Gases vs. Depth

Correlation Between Groundwater Concentration and Indoor Air?? IA ?? Petroleum Hydrocarbons Chlorinated Solvents GW Indoor Air Concentration ( ug/m3) Indoor Air Concentration ( ug/m3) CORRELATION ? YES (p <0.001) CORRELATION ? NO (p = 0.11) USEPA data base compiled by EPA, clumped together chlorinated and petroleum hydrocarbons. GW Concentration (ug/L) GW Concentration (ug/L) Observable Relationship Cia vs. Cgw ? n Petroleum Hydrocarbons: No n Chlorinated Solvents: Yes - Direct Cgw = COC conc. In groundwater; Cia = COC conc. In indoor air; (p = 0.11) = Probability = 11% that slope of best-fit line = 0 (I.e., no trend).

Overview of Petroleum VI n General VI Conceptual Model n Vadose Zone Attenuation of Petroleum Vapors n Oxygen Below Building Foundations

Oxygen Under Building Foundation Key Question n Is there enough oxygen below building foundations to support aerobic biodegradation? aerobic zone Ct anaerobic zone Cs Vapor Source

Oxygen Under Foundation: Model Prediction Numerical model predicts oxygen shadow below building, but….. n Very strong vapor source (200,000,000 ug/m3) n All flow into building is through perimeter crack n No advective flow below building Model does not account for key oxygen transport processes. KEY POINT: From Abreu and Johnson, ES&T, 2006, Vol. 40, pp 2304 to 2315.

Petroleum Hydrocarbons Aerobic Biodegradation: Oxygen Mass Balance bacteria Hydrocarbon + Oxygen Carbon dioxide + Water 1 kg CxHy + 3 kg O2 3.4 kg CO2 + 0.7 kg H2O New Cells Electrons & Carbon + Petroleum Hydrocarbons Energy Electrons Electron Acceptor (e.g., O2)

Aerobic Biodegradation: Oxygen Mass Balance n Atmospheric air (21% Oxygen) = 275 g/m3 oxygen > Provides capacity to degrade 92 g/m3 hydrocarbon vapors (= 92,000,000 ug/m3) KEY POINT: Even limited migration of oxygen into subsurface is sufficient to support aerobic biodegradation.

Bi-Directional Flow Across Foundation Transport of Oxygen Under Foundation Bi-Directional Flow Across Foundation Wind Driven Advection +/- +/- KEY POINT: Advection drives oxygen below building foundation.

Transport of Oxygen Under Foundation Conceptual Model Field Data 0.0 Depth (m) 0.5 Wind-driving building ventilation isoP 1.0 CH4 CO2 Wind Loading 02 1.5 Advection of subslab soil gas into bldg. 0.0 0.5 Biodegradation 1.0 Upwind-downwind advection in soil gas Diffusion from deep sub-slab soil gas 1.5 0.0 Subslab VOC source 0.01 0.1 1 10 100 1000 Concentration (g m-3) KEY POINT: Conceptual model and field data indicate common presence of oxygen under building foundation. From Fisher et al., 1996 Environmental Science and Technology, Vol. 30 No. 10, p. 2948.

Transport of Oxygen Under Foundation Soil Column Attenuation Transport of Oxygen Under Foundation Nitrogen Flooding Experiment: Purge sub-foundation soils with nitrogen gas and observe oxygen recovery Low Oxygen Time = 0 Time > 0 3 m N 1.1 0.8 1.0 0.9 concrete garage Injection wells % O2 (shallow) % O2 After Flood Oxygen Recovery Below Building Slide Topic: Transport of Oxygen Under Foundation, Case Study This slide and the next slide present the results of a field experiment conducted to measure the transport of oxygen below a building foundation. The test building was a residential structure in Santa Maria, CA built on fill material. For the experiment, nitrogen was pumped under the building, displacing the air (which contained relatively high oxygen levels). Following the nitrogen flooding, monitoring points (separated from the N2 injection points) showed that the air had been displaced, resulting in very low oxygen levels below the building. Oxygen sensors were then used to monitor the recovery of oxygen under the slab. The sensors (shown on the graph) indicate that oxygen levels recovered at many locations in less than two days and at all locations within two weeks. The sudden recovery in oxygen levels at just under time = 2 days corresponds to a high wind event, supporting the conceptual model that wind facilitates oxygen transport under the building foundation. Slide Presentation: Describe the experiment and the results. Key Points: The results of this experiment indicate rapid oxygen transport under the building foundation and support the conceptual model of wind-driven oxygen transport. Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

Transport of Oxygen Under Foundation Soil Column Attenuation Transport of Oxygen Under Foundation Nitrogen Flooding Experiment: Purge sub-foundation soils with nitrogen gas and observe oxygen recovery Low Oxygen Time = 0 High Oxygen Time = 2 weeks KEY POINT: 1.1 0.8 1.0 0.9 concrete 3 m N garage 16.6 18.4 19.4 15.4 14.0 15.2 12.2 14.5 13.7 15.9 3 m N garage concrete Rapid recovery of oxygen below building foundation supports petroleum biodegradation. % O2 After Flood Injection wells % O2 (shallow) Data from Lundegard, Johnson, and Dahlen. “Sub-slab Nitrogen Flood-Oxygen Re-entry Test.”

Advective Transport Processes Lower building pressure Residence in winter (chimney effect); bathroom, kitchen vents EXAMPLES Gas flow from subsurface into building Higher building pressure Building HVAC designed to maintain positive pressure Gas flow from building into subsurface Variable building pressure Barometric pumping; variable wind effects Bi-directional flow between building and subsurface Flow in Flow out Reversible flow Low Pressure High Pressure UPWARD TRANSPORT High Pressure Low Pressure DOWNWARD TRANSPORT

Pressure Gradient Measurements: School Building, Houston, Texas KEY POINTS: Pos. Pressure (Flow out of Bldg) • Pressure gradient frequently switches between positive and negative within a single day. • Continuous inward flow does not occur. Differential Pressure (Pascals) Neg. Pressure (Flow into Bldg) Time (July 14-15, 2005)

Advection Through Building Foundation: Field Evidence n VOCs from indoor air typically detected in sub-slab samples: - alpha pinene - limonene - p-dichlorobenzene n Oxygen transported below foundation by same mechanism INDOOR AIR S BELOW SLAB KEY POINT: Reversing pressure gradient drives air (and VOCs and oxygen) through building foundation. S

Fill, “crust,” sandy silt Fill, dredged river sand Chatterton Research Site, British Columbia, Canada (Hers, et al 2000) Building Feet Below Grade SG-BC, 10/1/97 SG-BR 5/14/97 <1000 Fill, “crust,” sandy silt 11% 10 % <1000 80,000 10% 8 % 1.0% 50,000,000 Oxygen, % Benzene, ug/m3 Sub-Slab vapor sample point Sub-Surface vapor sample point KEY SG-BC Vapor sample point identifier 55,000 3% 6% 25,000,000 50,000,000 Fill, dredged river sand 1.0% 50,000,000 5 60,000,000 Small building lies ~8 feet above benzene-rich LNAPL. Vapors fully attenuated with ~7 feet clean soil. Strong source puts strong demand on oxygen regardless of building presence. Building does not occlude oxygen. 1.0% 20 Feet, horizontal 10 LNAPL, benzene-rich Slide from Robin Davis, UDEQ

Hal’s, Green River, Utah (Utah DEQ, 8/26/06) Feet Below Grade Motel Office Breezeway Café/Bar VW-7 Asphalt 8.4 850 Basement Basement 14% Clayey Silt 7.0 380 VW-4 VW-5 20% 51 22 10 2800 1600 7.7 9.5% 18% Silt 250 12% 570 87 70,000 12,000 260,000 4.1% 11% 33,000,000 2.5% Benzene in GW 1,000-5,000 ug/L 20 Small- and Medium-sized buildings overlie very strong source of soil and groundwater gasoline contamination. Aerobic biodegradation of vapors with ~5 feet of clean overlying soil regardless of building presence. Buildings do not occlude oxygen or impede aerobic biodegradation if sufficient thickness of clean soil overlies the source. Vapor intrusion not likely. Exterior soil vapor sample points are adequately representative of sub-building conditions. KEY 260,000 2.5% Benzene, ug/m3 Oxygen, % VW-7 Multi-depth vapor monitoring well Sub-Surface vapor sample point TPH-gro, ug/m3 LNAPL, gasoline Feet, horizontal 20 33,000,000 Slide from Robin Davis, UDEQ

Perth, Australia (B. Patterson & G. Davis, 2009) Very Large Building 410 Feet Below Grade Uncovered open ground <2 19,000,000 <50,000 <50,000 <50,000 <0.5% 10.7% 19.9% 18.8% Lateral Extent of Oxygen & Biodegradation Sand 35,000,000 <50,000 <50,000 <50,000 5 <0.5% 8.2% 14.5% 15.9% 35,000,000 1,200,000 <50,000 <50,000 <0.5% 8.2% 4.5% 4.6% Very large building overlies very strong source of kerosene LNAPL. For ~30 feet laterally from center of building to building edge, ~5 feet of clean overlying attenuates vapors associated with very strong source as evidenced by low soil vapor and high Oxygen concentrations. Building >30 feet wide/deep occludes oxygen (“casts a shadow”) and impedes aerobic biodegradation. Interior and Exterior soil vapor sample points may be necessary for sites with very large buildings. NOTE: Indoor air TPH concentration does not exceed health-based criteria for kerosene VOCs. Conclusion: Very strong source in close vertical proximity to a very large building produces very high vapor concentrations that cannot degrade perhaps due to the size of the building and occlusion of oxygen. 10 KEY LNAPL Kerosene (very low BTEX) Outdoor air sample Indoor air sample Sub-slab vapor sample Sub-surface vapor sample 20 Feet, horizontal 1,200,000 Total Petroleum Hydrocarbons, ug/m3 8.2% Oxygen, % Slide from Robin Davis, UDEQ

Oxygen Under Building Foundation Summary n Wind and building pressure drive atmospheric air below building foundation n Even modest oxygen transport sufficient aerobic biodegradation