Empirical Data to Evaluate the Occurrence of Sub-slab O 2 Depletion Shadow at Petroleum Hydrocarbon- Impacted Vapor Intrusion Sites Ravi Kolhatkar, Emma Hong Luo & Tom Peargin Chevron Energy Technology Company March 20, 2013
Outline Motivation Sub-slab soil vapor data Preliminary hypothesis for general lack of oxygen depletion shadow Conclusions 2
Conceptual Model of Petroleum Vapor Intrusion (PVI) Biodegradable PHVOC s are rapidly attenuated and cannot transport through significant thickness of vadose zone soil LNAPL floats, spreads laterally Short, rapidly attenuated dissolved plume Widespread aerobic biodegradation enables vertical screening distance approach to prioritize PVI investigations (EPA 2013, Lahvis et al 2013) Oxygen replenishment under the slab is a critical control on PVI Supply = f(soil type, surface cover, barometric effects, wind effects, slab construction, slab footprint??, building use) Demand = f(type and strength of vapor source, source proximity) Today s focus 3
Potential for Sub-slab O 2 Depletion (O 2 < 4%v/v) 3D numerical simulations (EPA 2012) suggest oxygen depletion and accumulation of petroleum vapors below slab for certain source conditions Slab at former refinery site (Patterson and Davis, 2009) Kerosene LNAPL at 10 ft bgs 2700 ft 2 building with 10 ft wide thick concrete apron Observed oxygen depletion (<1%v/v) and high TPH (2 x 10 7 µg/m 3 ) below slab Uncertainty about the PVI decisions using near-slab soil vapor data Empirically evaluate occurrence of oxygen depletion 4
Definition of Source Types (GW vs. VZ) <30 ft Vadose Zone Source Groundwater Source 5
Sub-slab Soil Vapor Data Compilation 260 sub-slab soil vapor samples at 50 UST sites (gasoline) 197 samples at 35 UST sites - acceptable data quality Conventional sub-slab probe construction Summa canister sampling Oxygen data available appropriate leak detection (He, DFA, IPA) 89 samples at 20 UST sites vapor source under the slab o 24 samples at 7 sites (GW source LNAPL or dissolved) o 65 samples at 13 sites (vadose zone source dirty soil or LNAPL under slab) 6
Sub-slab Oxygen vs. Distance from the Edge of Slab 25 Oxygen depletion: 20 not related to distance of sample location from edge of the slab Oxygen (% v/v) 15 10 VZ source GW source LNAPL under slab large slab (> 25 ft)* seen only for vadose zone sources (soil/lnapl) not observed for GW sources (LNAPL/dissolved) 5 0 0 10 20 30 40 50 Shortest Distance from Edge of Slab (ft) *Draft PVI guidance in Australia (Wright 2013) 7
Minimum Sub-slab Oxygen vs. Building Footprint Minimum Sub-slab Oxygen (% v/v) 25 20 15 10 5 0 100 1000 10000 100000 Buliding Footprint (ft 2 ) GW Source VZ source LNAPL under slab Oxygen depletion: not related to building footprint seen only for vadose zone sources (soil/lnapl) not observed for GW sources (LNAPL/dissolved) 8
Sub-slab Oxygen vs. Depth to GW Source 25 24 samples (7 sites) 20 No oxygen depletion seen for vapors from GW sources Oxygen (% v/v) 15 10 5 LNAPL (B > 1000 ug/l) Dissolved (B < 1000 ug/l) Even LNAPL at water table as shallow as 10 ft Note that this dataset does not have LNAPL sources shallower than 10 ft 0 0 2 4 6 8 10 12 14 16 18 Depth to Water (ft) 9
Sub-slab Oxygen vs. TPHg Oxygen (% v/v) 25 20 15 10 5 VZ source GW source LNAPL under slab 0 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 TPHg (ug/m3) 83 samples (13 sites) Soil vapor likely from zone straddling aerobic biodegradation zone Spatial variability in the distribution of LNAPL source below slab Oxygen depletion seen only for vadose zone (soil/lnapl) sources and not for GW sources 10
Sub-slab Oxygen vs. Benzene Oxygen (% v/v) 25 20 15 10 5 100 µg/m 3 VZ source GW source LNAPL under slab 111 samples (20 sites) Soil vapor likely from zone straddling aerobic biodegradation zone 5 of 111 samples (~5%) have benzene > 100 µg/m 3 and oxygen depletion, all for vadose zone sources 0 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Benzene (ug/m3) 11
Sub-slab Oxygen vs. Methane Oxygen (% v/v) 25 20 15 10 5 0 0.0001 0.001 0.01 0.1 1 10 100 CH4 (%v/v) VZ source GW source LNAPL under slab 0.5% CH 4 (10% LEL ) 98 samples (19 sites) Methane detections indicative of anaerobic biodegradation of hydrocarbons Methane > 0.5% v/v sourced only from vadose zone sources and LNAPL under slab No significant methane detected from GW sources 12
3D Numerical Simulations No permeable backfill below 8 slab 16 thick permeable backfill below slab HC Oxygen Airflow assumed only through perimeter crack (0.2 L/min) Oxygen depletion shadow and hydrocarbon vapors below slab Airflow through perimeter crack and backfill (ca 50 L/min) Sub-slab is oxygenated and no hydrocarbon vapor accumulate below slab
Summary Sub-slab oxygen depletion shadow not observed for GW sources Limited instances of sub-slab oxygen depletion linked to: Vadose zone sources below slab High source strength LNAPL source right beneath the slab Source proximity Sub-slab oxygen depletion not dependent on: Building footprint Distance from the edge of the slab Generic 3D simulations: likely underestimate the actual air flow under slabs 14
References Patterson, B.M., and G.B. Davis. 2009. Quantification of Vapor Intrusion Pathways into a Slab-on-ground Building Under Varying Environmental Conditions. Environmental Science and Technology 43(3):650 656. EPA 530-R-10-003, February 2012. Conceptual Model Scenarios for the Vapor Intrusion Pathway. EPA 510-R-13-001, January 2013. Evaluation Of Empirical Data To Support Soil Vapor Intrusion Screening Criteria For Petroleum Hydrocarbon Compounds. Lahvis, M. A., Hers, I., Davis, R. V., Wright, J. and G. E. DeVaull. 2013. Vapor Intrusion Screening at Petroleum UST Sites, Groundwater Monitoring & Remediation, accessed online on March 16 at http://onlinelibrary.wiley.com/doi/10.1111/gwmr.12005/pdf. Wright, J., February 2013. Petroleum Vapor Intrusion (PVI) Guidance, CRC for Contamination Assessment and Remediation of the Environment, Australia. 15