Exclusion Distance Criteria for Assessing Potential Vapour Intrusion at Petroleum Hydrocarbon Sites Matthew Lahvis, Shell Global Solutions (UK), Thornton, England George DeVaull, Shell Global Solutions (US), Houston, Texas AEHS Foundation Annual Meeting March 15-18, 2010 San Diego, California
Overview Background Development of Exclusion Distance Criteria Field Validation Conclusions 2
Benzene Screening Levels in Ground Water 200 K-BASE ED G LEVEL (ug/l) 10-6 RIS EENING 1 SCR 180 160 140 120 100 80 60 40 20 0 EPA 1 CO 2 NJ 3 IN 4 MN 5 CT 6 WA 7 AK 8 OR 9 OH 10 OK 11 KEY regulatory screening levels l for benzene in groundwater can be POINT highly conservative and do not consider biodegradation 3
Regulatory Screening Approaches: Hydrocarbons Bio-attenuation Factor (10x): - New Jersey DEP - Massachusetts DEP, - New Hampshire DES - San Francisco RWCB - California DTSC - Utah DEP - Health Canada KEY POINT additional factor of 10x assumes bioattenuation is constant across unsaturated zone (no scientific basis, not representative of field observations) 4
Biodegradation Under Aerobic Conditions tion Facto or ce io r / subsurfac entration rati indoor conce Attenua 1E01 1.E-01 Dissolved phase NAPL no degradation limit 1.E-02 1.E-03 1.E-03 1.E-05 1.E-04 1E07 1.E-07 L=1m 1.E-05 aerobic limit 1.E-09 1.E-06 1.E-11 1E13 1.E-13 1.E-15 1.E-17 1E19 1.E-19 from DeVaull (2007) increased oxygen 1.E-21 0.01 0.1 1 10 100 source to foundation distance (m) ttenuation Fac ctor A 1.E-07 1.E-08 1.E-09 L = 2 m L = 3 m L = 5 m L = 10 m 1.E-10 01 0.1 1 10 100 1000 Vapor Source Concentration (mg/l) L = 1 m, λ = 0.79 (1/h) L = 2 m bgs, λ = 0.79 (1/h) L = 3 m, λ = 0.79 (1/h) L = 5 m, λ = 0.79 (1/h) L = 10 m, λ = 0.79 (1/h) L = 1 m, No Biodegradation L = 10 m, No Biodegradation from Abreu et al. (2008) RE ELATIVE DISTA ANCE ABOVE S OURCE 1 0 HC O 2 sharp reaction front 0 1 RELATIVE SOURCE CONCENTRATION KEY POINT under aerobic conditions, rapid attenuation either see it or you don t (noted by ITRC, 2007) bio-attenuation factors rather meaningless for DP sites where aerobic biodegradation is anticipated sharp attenuation interface amenable to exclusion criteria
Exclusion Distances: Atlantic PIRI (2006) EPA (2002) 100 ft some states and ASTM (2008) propose lateral/vertical separation (30 100 ft) KEY POINT exclusion distances do not account for biodegradation Atlantic PIRI, 2006, ATLANTIC RBCA, Risk-Based Corrective Action, Version 2.0 For Petroleum Impacted Sites in Atlantic Canada User Guidance, Appendix 9, Guidance for Soil Vapour and Indoor Air Monitoring Assessments, July 2006, 65 pp. 6
Objectives develop a rational, risk-based screening criteria based for petroleumhydrocarbons based on sound science and supported by field data shallow sources
General Classes of VI Sites divide and conquer 8
Dissolved Phase (DP) Sites Characteristically Different (Effect of Biodegradation Rates) LOW CONC. SOURCE HIGH CONC. SOURCE 9 from API (2009) from API (2009)
Aerobic Conditions Observed at DP Sites SOIL GAS CONCENTR RATION IN (P ERCENT) O2 25 20 15 10 5 0 O2 Data SAMPLE SIZE = 76 (excludes suspect data) O2 (Suspect) (O2 + CO2 < 18%) 10th Percentile 1 10 100 1000 10000 100000 BENZENE CONCENTRATION IN GROUNDWATER (ug/l) aerobic conditions (> 5 % O2) observed in KEY unsaturated zone for virtually all cases POINT insufficient source mass to drive conditions anaerobic R. Davis Pet. HC dbase Dissolved-phase only sites Filtered out: 1. GW wells with current or historic evidence of free product 2. GW /vapor wells with residual phase source inferred (GRO > 100x BTEX) 3. presence of aliphatics in soil gas 4. designated source area wells 20 sites,35 locations, 84 measurements 10
Aerobic Conditions Observed at DP Sites upper quartile O2 CONCEN NTRATION N IN SOIL GAS (PERC CENT) 22 20 18 16 14 12 10 8 6 SAMPLE SIZE = 83 SAND O2 median lower quartile SAMPLE SIZE = 19 SILT/CLAY O2 outlier KEY POINT only a slight dependence of O2 on soil type at dissolved-phase sites SUSPECT DATA REMOVED 11
BioVapor* API 1-D Steady State VI Model 3 advection, diffusion, and dilution through building foundation Oxygen aerobic zone 2 diffusion & 1 st order biodegradation in aerobic zone Vapor Source Hydrocarbon anaerobic zone 1 diffusion i only in anaerobic zone Algebra Solution for: Oxygen demand = Oxygen Supply *Available at: http://www.api.org/vi
Model Application exclusion distance criteria for benzene as ƒ (gw concentration) base case (conservative): - building parameters (basement) - 10-5 risk (3.1 ug/m 3 ) indoor air - homogeneous unsaturated zone (low moisture sandy soil) - k w = 0.79 /hr (benzene) Oxygen aerobic zone permutations ti - biodegradation rate others - f oc - Q soil - soil moisture saturations anaerobic zone Hydrocarbon Vapor Source 13
Model Input (Biodegradation Rate Selection) de egradation rate, kw (1 / hr) 1E+3 1.E+3 1.E+2 1.E+1 1.E+0 1.E-1 1.E-2 1.E-3 aliphatic branched aromatic 0.0 01 0.0 02 0.0 05 0. 1 n-c6-12 E N B BTEX 2 3 4 5 6 7 8 0. 0. 0. 0. 0. 0. 0. X cyclic T k w = 0.79 /hr (BTEX) k w = 42 /hr (aliphatics) 0. 9 0.9 95 0.9 98 0.9 99 FIR RST-ORDER R RATE CO ONSTANT (k w ) (hr -1 ) 10 1 0.1 0.01 0.001 k (benzene) w k w = 0.1/hr k w = 0.09 hr -1 (median) R. Davis dbase 17 sites 45 wells sand, silt, clay 00001 0.0001.1 1 5 10 2030 50 7080 9095 99 99.9 from DeVaull (2007) cumulative data fraction from Lahvis (2009) from Lahvis (2009) KEY POINT Significant data exists on 1st -order rate constants for biodegradation modeling applications PERCENT 14
Model Results (Sensitivity to Bio. Rate - k w ) SEPARATION DISTANCE (m) SEPARATION DISTANCE (m) (RBSL) L) NING LEVEL ( WATER (mg/l ASED SCREEN GROUNDW RISK BA IN 100 10 10 1 0.1 0.01 0.001 0 2 4 6 8 10 rapid biodegradation slow biodegradation average biodegradation no biodegradation BENZENE base case no biodegradation base case with average biodegradation k w = 0.79/hr (BTEX) k w = 42/hr (all other compounds) base case with slow biodegradation k w = 0.16/hr (BTEX) k w = 8.4/hr (all other compounds) base case with rapid biodegradation k w = 3.7/hr (BTEX) k w = 195/hr (all other compounds) KEY POINT for very conservative input, source to foundation separation distances of > 2 m are generally sufficient to screen out most sites with gw benzene concentrations less than 1 mg/l even a slow rate of aerobic biodegradation (e.g., k w < 0.16/hr) can have a significant impact on VI potential 15 2009 Shell Global Solutions (US) Inc. All rights reserved. Do not reproduce without the express written permission of copyright owner.
Model Validation R. Davis Pet. HC dbase BENZENE CONCENTRATIO ON IN SOIL GAS (ug/m3) 1000.00 100.00 10.00 1.00 0.10 soil-gas screening-level assuming: benzene indoor air = 2 μg/m 3 foundation AF = 0.05 1 10 100 1000 10000 100000 BENZENE CONCENTRATION IN GROUNDWATER (ug/l) SAMPLE SIZE = 116 Measured Non Detect 90th Percentile Dissolved-phase only sites Filtered out: 1. GW wells with current / historic evidence of free product 2. GW /vapor wells with residual phase source inferred (GRO > 100x BTEX) 3. presence of aliphatics in soil gas 4. designated source area wells 20 sites,39 locations, 116 measurements KEY POINT field data validate exclusion distance criteria benzene attenuated below soil-gas screening levels at dissolvedphase only sites (few exceptions - sites where NAPL is inferred) 16 2009 Shell Global Solutions (US) Inc. All rights reserved. Do not reproduce without the express written permission of copyright owner.
Benzene in Soil Gas f (distance above source) BE ENZENE CO ONCENTR RATION IN SOIL GAS (ug/ /m3) 1000.00 100.00 10.00 1.00 sub foundation screening-level assuming: benzene indoor air = 2 μg/m 3 foundation AF = 0.05 KEY POINT Measured Non Detect 90th Percentile bioattenuation at dissolved-phase sites is independent SAMPLE of distance above SIZE = 116water table 0.10 0 10 20 30 40 50 DISTANCE ABOVE WATER TABLE (FEET) 17 2009 Shell Global Solutions (US) Inc. All rights reserved. Do not reproduce without the express written permission of copyright owner.
GW-Soil Gas Attenuation Factor (Benzene) 10-2 ION FAC CTOR BENZ ZENE ATT TENUAT 10-3 SAND SILT 10-4 10-5 10-6 10-7 1 5 10 20 30 50 70 80 90 95 99 KEY POINT slight dependence of bioattenuation on soil type likely linked to O2 availability PROBABILITY (PERCENT) 2009 Shell Global Solutions (US) Inc. All rights reserved. Do not reproduce without the express written permission of copyright owner. 18
Exclusion Criteria: Groundwater to Overlying Soil Vapor Supporting evidence Criteria for Evaluating Data Set Dissolved sources at known depth to groundwater Clean soil overlies groundwater Groundwater & soil vapor data collected at about same time Complete attenuation of soil vapors defined by shallow soil vapors = 0, <DL (which may vary; full attenuation verified by samplers/authors) Majority of soil vapor measurements from multi-depth soil vapor points LUST sites & refineries included LNAPL sites excluded Benzene in Groundwater (ug/l) Robin V. Davis, Evaluating the Vapor Intrusion Pathway: Studies of Natural Attenuation of Subsurface Petroleum Hydrocarbons & Recommended Screening Criteria, 21st Annual National Tanks Conference, Sacramento, California, Vapor Intrusion Workshop & Session, March 30-April 1, 2009.
In Regulatory Context define separate approach for dissolved-phase hydrocarbon sites relevant questions to ask: is BTEX (benzene) present? does the dissolved-phase concentration exceed RBSL within X meters vertically of building foundation? approach requires the site meets the following criteria: absence of preferential pathways no free or residual product present no groundwater in contact with building foundation no relatively continuous impervious surface cover 20
Conclusions meaningful screening approaches for hydrocarbon sites require consideration of biodegradation approach requires some understanding of CSM (source type - NAPL vs. dissolved phase) exclusion criteria i developed d through h model (API BioVapor) application indicate many current RBSLs are far too conservative (e.g., ppm not ppb) soil-gas data from dissolved phase only sites validates exclusion distance criteria and suggests that VI not an issue if there is any source/receptor separation distance future evaluation to include additional data sets (e.g. Australia) and focus on NAPL sites 21 2009 Shell Global Solutions (US) Inc. All rights reserved. Do not reproduce without the express written permission of copyright owner.