Screening Distances for Petroleum Vapor Intrusion Risk Assessment

Transcription

1 Screening Distances for Petroleum Vapor Intrusion Risk Assessment 23nd Annual International Conference on Soil, Water, Energy, and Air and AEHS Foundation Annual Meeting San Diego, California March 18-21, 2013 Matthew Lahvis LNAPL SOURCE UNSATURATED ZONE Use this area for cover image high hydrocarbon (height 6.5cm, mass flux width 8cm) hydrocarbon LNAPL CAPILLARY ZONE SATURATED ZONE DISSOLVED PHASE O relative soil gas concentrations aerobic reaction front 1

2 OVERVIEW Background Empirical Studies Implementation Regulatory Uptake Conclusions 2

3 SEPARATION DISTANCES VS. ATTENUATION FACTORS Attenuation Factor 1.E-01 1.E-03 1.E-05 1.E-07 1.E-09 1.E-11 1.E-13 1.E-15 1.E-17 1.E-19 aerobic limit from DeVaull (2007) no degradation limit increased oxygen 1.E KEY POINT MODELING Source to Foundation Distance (m) RELATIVE DISTANCE ABOVE SOURCE 1 0 FIELD DATA HC O RELATIVE SOURCE CONCENTRATION September 2011 AF ~ 0 sharp reaction front AF ~ 1 attenuation factor is depth dependent for reactive VOCs (hydrocarbons) under aerobic conditions, rapid attenuation either see it or you don t (noted by ITRC, 2007) sharp attenuation interface amenable to exclusion criteria 3

4 SCREENING DISTANCES EMPIRICAL STUDIES Reference Database Dissolved Phase (ft) US EPA OUST (2013) Lahvis et al. (2013) Davis (2009) + 11 Maine and 1 Canada site 74 sites 829 data pairs US EPA (2013) Wright (2012) LNAPL (ft) UST: non UST: 18 Benzene Soil-Gas Screening Level ( g/m 3 ) LNAPL Indicator Concentration Criteria ( g/l) 50, 100 C gw benzene > 5,000 C gw TPH > 30,000 0 UST: 13 30, 50, 100 C gw benzene > 15,000 Davis (2009) Peargin and Kolhatkar (2011) Wright (2012) 62 sites 735 data pairs 25 sites 218 data pairs 124 sites 1080 data pairs 5 UST: 8 non UST: 30 complete attenuation C gw benzene > 1,000 C gw TPH > 30, (no sub-slab data) 50 1,000

5 KEY CONCLUSIONS FROM EMPIRICAL STUDIES vertical screening distances derived for benzene are longest of any hydrocarbon (i.e., can be used to conservatively define screening distances) dissolved-phase petroleum hydrocarbon sources pose little risk vertical screening distances for LNAPL sources - petroleum UST (e.g., retail): 13 to 15.4 ft - non-ust (e.g., terminal, pipeline, refinery): 18 ft (LIMITED DATA SET) soil type has a negligible on vertical screening distances (for the range of data evaluated) vertical screening distances are only slightly sensitive to the presence of building foundations and pavement (effects are inconclusive) vertical screening distances are deemed broadly suitable for most sites & recent gasoline fuel types vertical screening distances are expected to apply laterally in the absence of preferential pathways or significant hydrogeologic barriers use of O 2 soil-gas measurements alone to validate screening distances should be exercised with caution 5

6 SCREENING PROCESS NO ENTER 1a Do preferential pathways exist between source and building? NO 1b Is the building within the lateral inclusion distance? YES 1c Any other precluding conditions? YES YES NO LEGEND 1 APPLICABILITY 2b NO 2a Is LNAPL likely present below building? 2b YES 2 APPLICATION NO DISSOLVED PHASE Source-building separation distance sufficient? LNAPL Source-building separation distance sufficient? NO 3 ADDITIONAL ASSESSMENT YES YES 3 Further PVI Investigation or mitigation required 6

7 SCREENING PROCESS lateral offset 7

8 SCREENING PROCESS 8

9 SCREENING PROCESS 9

10 LEAD SCAVENGERS (1,2 DCA) soil-gas data (6 sites, 21 locations, 31 samples) 1,2 DCA in soil vapor (µg/m3) DRAFT Non detect (<RL) Detect Soil gas screening level 9.4 µg/m 3 from Chevron Separation Distance between Probe and DTW (ft) PROBABILITY (%) soil gas concentration < 9.4 ug/m DISTANCE ABOVE SOURCE WATER TABLE (ft) KEY POINT preliminary analysis indicates that 15 ft of vertical separation distance is adequately protective for 1,2-DCA when the vapor source is at the water table 10

11 SCREENING PROCESS 11

12 IMPLICATIONS FOR SITE INVESTIGATION ACT THE SAME LOOK THE SAME (a) (b) (c) UNSATURATED ZONE UNSATURATED ZONE UNSATURATED ZONE high hydrocarbon mass flux high hydrocarbon mass flux low hydrocarbon mass flux CAPILLARY ZONE SATURATED ZONE LNAPL CAPILLARY ZONE SATURATED ZONE NO LNAPL CAPILLARY ZONE SATURATED ZONE NO LNAPL MW MW MW LNAPL DISSOLVED PHASE MW = Monitoring Well from Lahvis et al. (2013) KEY POINT good source-type characterization (i.e., identification of residual-phase LNAPL) essential during initial site investigations 12

13 LNAPL INDICATORS from Lahvis et al. (2013) June

14 SCREENING PROCESS soil-gas sampling - 1,000x bioattenuation factor (Cal EPA 2012) modeling (e.g., BioVapor) from Cal EPA (2012) 1,000x bioattenuation factor 14

15 GROUNDWATER CONCENTRATION MEASUREMENTS SOIL GAS CONCENTRATION LOWERMOST PROBE ( g/m 3 ) BENZENE Detect Soil Gas (53 samples) Non Detect Soil Gas (89 samples) GROUNDWATER CONCENTRATION ( g/l) from Lahvis et al. (2013) KEY POINT groundwater concentrations have limited value in VI risk assessment at hydrocarbon sites (vapor concentrations << predicted by Henry s Law) poor correlation likely attributed to biodegradation in the capillary zone and inability to measure water table concentrations May

16 SCREENING DISTANCES REGULATORY VALUES Guidance References Australia (CRC Care) (2013) J. Wright (2011); US EPA (2013); Lahvis et al. (2013); R Davis (2009) EPA OUST (in prep) NONE States (CA, NJ, IN, WI) various references (R.Davis, McHugh et al.) include O2 requirement Screening Distance (ft) 6.5 : benzene Cgw < 1,000 g/l; TPH < 10,000 g/l 26 : benzene Cgw > 1,000 g/l; LNAPL 5 : benzene Cgw < 100 g/l (NJ, CA) < 1,000 g/l (IN, WI) 10 : benzene Cgw < 1,000 g/l (CA, NJ, IN) 20 : benzene 1 ppm < Cgw < LNAPL (WI) 30 : LNAPL

17 CONCLUSIONS source-building separation distance a more meaningful metric than attenuation factors for PVI screening empirical studies suggest: - 5 ft - dissolved-phase - 13 ft - LNAPL method simple to apply (source-building separation distance & source type are key factors) regulatory uptake by US EPA, Australia (CRC Care), states implementation strategy being documented in ITRC PVI guidance 17

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19 CONSIDERATIONS / ADDITIONAL STUDY buffer distances - dissolved-phase sources uncertainties in water-table elevation (e.g., 5 10 ft) - LNAPL sources to account for uncertainties in delineating the edge of LNAPL (residual phase) (e.g., ft ) factors that affect biodegradation (O 2 availability) - large building foundations (e.g., associated with industrial/commercial, apartment complexes) - high-ethanol content fuel (e.g., E85) releases - pavement - large-volume releases (non-ust sites) 19

20 O2 SOIL-GAS MEASUREMENTS TPH CONCENTRATION SOIL GAS ( g/m 3 ) 3.5E E E E E E E E+00 Detect (307 Samples) Non Detect TPH (88 Samples) Sum of Partial Pressures < 95% atm (45 Samples) Hal's Chevron OXYGEN CONCENTRATION SOIL GAS (%) CUMULATIVE FRACTION OF SOIL GAS CONCENTRATION DATA (dimensionless) from Lahvis et al. (2013) KEY POINT O 2 /TPH concentrations in soil gas generally support the aerobic biodegradation reaction paradigm O 2 soil-gas concentration measurements not necessary for site screening May

21 O 2 SOIL-GAS MEASUREMENTS CUMULATIVE FRACTION OF SOIL GAS CONCENTRATION DATA (dimensionless) BENZENE AND TPH (HYDROCARBON) SOIL GAS CONCENTRATIONS Benzene (54 detect; 16 nondetect samples) TPH (60 detect; 0 nondetect samples) ANAEROBIC UNSATURATED ZONE (O 2 SOIL GAS CONCENTRATIONS < 4 % vol/vol) 1E+00 1E+02 1E+04 1E+06 1E+08 anaerobic unsaturated zone conditions (e.g., O 2 < 4% v/v) - ~25% of benzene soil-gas concentrations < 30 g/m 3 CUMULATIVE FRACTION OF SOIL GAS CONCENTRATION DATA (dimensionless) HYDROCARBON CONCENTRATION IN SOIL GAS ( g/m 3 ) BENZENE AND TPH (HYDROCARBON) SOIL GAS CONCENTRATIONS HIGHLY AEROBIC UNSATURATED ZONE (O 2 SOIL GAS CONCENTRATION > 17% vol/vol) Benzene (99 detect; 130 non detect samples) TPH (118 detect; 72 non detect samples) 1E+00 1E+02 1E+04 1E+06 1E+08 HYDROCARBON CONCENTRATION IN SOIL GAS ( g/m 3 ) highly aerobic unsaturated zone conditions (e.g., O 2 >17% v/v) - ~10% of benzene soil-gas concentrations >1,000 g/m 3 KEY POINT O 2 soil-gas concentration measurements may have limited value in validating site-screening criteria September