Attenuation of Hydrocarbon Vapors from LNAPL Under Residential and Commercial/Industrial Buildings

Transcription

1 Attenuation of Hydrocarbon Vapors from LNAPL Under Residential and Commercial/Industrial Buildings Presented at: Vapor Intrusion Attenuation Workshop 14 th Annual West Coast Conference on Soils, Sediments and Water, San Diego, California March 15-16, 16, 2004 C.Y. Jeng, Stephen Song, and Stephen Washburn ENVIRON International Corporation

2 Site Background A former military supply center and adjacent areas located in South Philadelphia, PA Light non-aqueous aqueous-phase liquid (LNAPL) and dissolved-phase plumes under residential and commercial/industrial areas Chemicals of concern include BTEX, MTBE, naphthalene and 2-methylnaphthalene2 Ground water table at 5 to 7 m bgs 2

3 Site Map and Approximate Area with Subsurface Hydrocarbon Contamination 100 m Super Super Highways Highways Commercial Property Residential Housing Development 3 LEGEND Hydrocarbon Contamination Detected Hydrocarbon Contamination Not Detected Approximate Boundary of Dissolved Phase Approximate Boundary of Non-Aqueous Phase Liquid (NAPL) Plume

4 Conceptual Model for Vapor Intrusion Pathway Building types: Slab-on on-grade, two-story residential duplexes with cinder blocks atop concrete slabs Multiple-floor concrete manufacturing buildings (Building 9/13) with crawl space/basement Future residential buildings with basements and generic commercial/industrial buildings Ventilation: Residential - HVAC units (occupied); natural ventilation (unoccupied) Building 9/13 HVAC units and exhaust fans 4

5 Conceptual Model for Vapor Intrusion Pathway oil Layer Fill Silt ~5m Sand & Gravel Contaminated Soils Non-Aqueous-Phase Liquid (NAPL) Groundwater 5

6 Sampling Matrix Medium Sampling Locations Sampling Frequency Analytical Method LNAPL ~20 4 events over 5 years VOC (8260B) SVOC (8270) Groundwater ~40 5 events over 5 years VOC (8260B) SVOC (8270) Soil ~20 1 event VOC (8260B) SVOC (8270) Soil Gas (Depth Profile) ~40 1 event VOC (8260B) O2 and CO2 Soil Gas (Time Series) 3 4 events over 2 years VOC (TO-14) Indoor and Outdoor Air ~20 (each) 6 2 events over 2 years VOC (TO-14)

7 Other Relevant Information Soil geotechnical data (porosity, density, grain-size, hydraulic conductivity) to simulate steady-state state soil moisture profiles for strata overlying the plume O 2 and CO 2 soil gas analysis to identify evidence of hydrocarbon degradation Soil investigation to assess anomalous soil gas data (e.g., natural gas line leak) Indoor air-related related parameters (e.g., temperature, pressure, wind speed/direction, house conditions, ventilation rate) 7

8 Efforts to Identify Other Contributions Collected both indoor and outdoor samples at each location Collected soil gas samples Inspected utilities, sumps, floor and wall surfaces for biased sampling locations Building preparation before sampling in some cases (e.g., shutdown special ventilation for Buildings 9/13; shut off and purged gas lines for unoccupied residences) 8

9 Box Plots for Source Vapor Concentrations (mg/m 3 ) for Benzene NAPL(Ia) 1 SGd(Ia) Group NAPL(Ib) SGd(Ib) 9

10 Time-Series Soil Gas Concentrations 2500 Benzene Conc (mg/m 3 ) Mar-99 Apr-99 May-99 Mar-01 SGD4-Deep SGD4-Intermediate SGD4-Shallow 10

11 Benzene Concentrations (ug/m 3 ) in Residential Air Sampling Location On-Plume Off-Plume Detected/Count Minimum Mean Maximum Detected/Count Minimum Mean Maximum Indoor 7/8 ND(1.4) /8 ND(1.3) Outdoor 6/8 ND(1.3) /8 ND(1.3)

12 Indoor Air Attenuation Factor (α)( for Benzene Ind_B13 (NAPL) Ind_B9 (NAPL) Res_Ib (NAPL) Res_Ib (SGd) Res_Ia (NAPL) Res_Ia (SGd) 1E-07 1E-06 1E-05 Indoor Air α 12

13 Results - Multiple Lines of Evidence Air monitoring results: On-plume and off-plume indoor air data do not show discernible differences Indoor air concentrations were not discernibly higher than outdoor concentrations Measured concentrations in indoor and outdoor air are similar to background concentrations reported in the literature Indoor air data can be affected by sources unrelated to subsurface conditions 13

14 Results - Multiple Lines of Evidence Soil Gas Results: Deep soil gas data agree with source vapor concentrations estimated from reliable LNAPL and groundwater data Vertical profiles of measured soil gas concentrations support that the silt layer is an effective barrier for upward vapor migration Soil gas data have large spatial and temporal variations Oxygen and carbon dioxide profiles indicate favorable conditions for aerobic biodegradation 14

15 Results - Multiple Lines of Evidence Mathematical Modeling: Deep soil gas data are comparable to model- estimated source vapor concentrations Model-estimated estimated concentrations are within the range of measured indoor air concentrations Variations in soil properties for any given soil layer had little effect on estimated long-term soil moisture profiles Convection is generally less important than diffusion in affecting vapor attenuation at this site 15

16 Summary on Attenuation Factors (α)( LNAPL-IA α range from less than 5x10-7 to 6x10-6 for benzene and from less than 3x10-6 to 6x10-5 for T/E/X SGd-IA α values are similar to the corresponding LNAPL-IA α values Compared to LNAPL and groundwater data, soil gas data have large spatial/temporal variations and can be more affected by other sources Limited data on basement IA/crawl space gas α (~ 0.6) and 1st floor/basement IA α (0.5~1) 16

17 Acknowledgements Hasan Dogrul of the Defense Logistics Agency Mike Heffron and his staff at Foster Wheeler Environmental Corporation David Burke of the Pennsylvania Department of Environmental Protection 17