Temporal Variability of Soil Gas Concentrations and Pressure Under Natural Weather Conditions at Petroleum Hydrocarbon Impacted Sites

Transcription

1 Temporal Variability of Soil Gas Concentrations and Pressure Under Natural Weather Conditions at Petroleum Hydrocarbon Impacted Sites Emma Hong Luo, Ph.D. Chevron Energy Technology Company Houston, TX Presentation at 28 th AEHS, San Diego, CA 03/21/2018 1

2 Outline Purpose of this presentation Current understanding of temporal variability in soil gas Temporal variability in PVI soil vapor field data Summary 2

3 Purpose Examples of understanding of soil gas temporal variability in some current VI guidance documents: Data on temporal changes in temperature, barometric pressure, wind speeds/direction, precipitation can aid in correctly identifying trends (P103, US EPA OUST PVI guidance) permanent probes should be installed to evaluate the long-term behavior of contaminated soil gas. Soil gas may need to be monitored through time, in a fashion similar to groundwater to determine long-term stability of contaminant concentrations Conditions warrants in the installation of long-term soil gas monitoring well may include the following: barometric pressure (P15-16, CA DTSC VI guidance) 3

4 Purpose Questions: 1. How big are the effects of wind and barometric pressure on soil gas concentration and pressure at PVI sites? 2. Is the temporal variability in soil gas significant enough to warrant frequent and/or long-term sampling at PVI sites (i.e. changing risk)? 4

5 Common Understanding of Temporal Variability in CVI Temporal variability in CONC. observed at two CVI sites (Johnson et al) Soil gas conc. variation increases orders of magnitude from vapor source to IA; Factors including natural weather and preferential pathways 5

6 Soil Gas Temporal Variability at PVI Sites? Extreme case with extreme weather/geology , PVI research building-1 Sand/gravel from 0-6 ftbgs K soil = m 2 No GW encountered Strong winds and extremely cold winters LNAPL-impacted soil right under slab 6

7 Temporal Variability in Soil Gas Concentrations at PVI Sites ALL Little weather influence despite the permeable soils in a ALL situation: hot is always hot, variation <30% 7

8 Temporal Variability in Soil Gas Concentrations at PVI Sites NONE Little weather influence despite the permeable soils in a none situation: cold is always cold, variation <10% 8

9 Temporal Variability in Soil Gas Concentrations at PVI Sites Strong weather influence on soil gas conc. due to a big direct conduit 9

10 Temporal Variability in Soil Gas Pressure at PVI Sites Sub-slab soil gas pressure - Fluctuation of ΔP (-10 Pa to 30 Pa) at subslab correlates with wind over 4 m/s at this site - No good correlation with barometric pressure changes; - No significant changes in soil gas conc. without a direct conduit. 10

11 Temporal Variability in Soil Gas Concentrations at PVI Sites Near-slab soil gas concentrations Little weather effect (both barometric pressure and strong wind) on soil gas concentrations, variation <20% 11

12 Temporal Variability in Soil Gas Concentrations at PVI Sites Near-slab soil gas concentrations Little weather effect (both barometric pressure and strong wind) on soil gas concentrations 12

13 Temporal Variability in Soil Gas Pressure at PVI Sites Near slab soil gas pressure - Fluctuation of ΔP (-20 Pa to 40 Pa) outside the building correlate with wind (>4 m/s); - No good correlation with barometric pressure changes; - No significant changes in soil gas concentrations. 13

14 Temporal Variability in Soil Gas at PVI Sites A case with a more common weather and geology , PVI research building-2 Sandy silt soils beneath and around building; K soil = m 2 No GW encountered Milder weather and no extreme sustaining high winds LNAPL-impacted soil at 30 ft bgs and a skirt of clay around building from 0-5ftBGS (Luo, et al, 2013) 14

15 Temporal Variability in Soil Gas at PVI Sites Near-slab (location A) soil gas conc. and pressure Little weather effect on conc. concentrations at 2.5 ft bgs; Pressure diff. varied (-15 to 20Pa); No clear correlation with wind nor barometric pressure 15

16 Temporal Variability in Soil Gas Concentrations at PVI Sites Sub-slab soil gas conc. and pressure Little weather effect on conc.; Pressure diff. varied (-10 to 20Pa), similar to #A; No clear correlation with wind nor barometric pressure 16

17 Summary How big are the effects of wind and barometric pressure on soil gas concentrations and pressure at PVI sites? Near-slab shallow soil gas and subslab P SG-IA changes but soil gas concentration variation is not significant enough to change VI risk except in the direct VI conduits; Change in soil gas pressure Change in soil gas Conc. No good correlation between soil gas conc. and pressure with barometric pressure changes; Soil gas pressure and wind only correlates well with more permeable soils and strong wind. 17

18 Summary Is the temporal variability in soil gas significant enough to warrant more frequent soil gas sampling at PVI sites? No since hot being always hot and cold being always cold and small variation in soil gas conc. is insignificant to change the VI risk; 18

19 Acknowledgement ASU Colleagues from Dr. Paul C. Johnson (ASU-CSM) Dr. Paul Dahlen (ASU) And many other people who were involved with the two intensive PVI field studies from Thanks to all the colleagues that have reviewed/commented on my presentation 19

20 Thank you! Question? 20

21 Backup slides 21

22 Transient Modeling Data Extreme barometric pressure falling and rising scenarios Time 1 Time 2 K soil = m 2 Φ= 0.35 Φw= 0.07 R= 0.18/hr (Luo, 2009, ASU) 22

23 Transient Modeling Data Benzene - Extreme barometric pressure change (increasing/falling) can affect soil gas pressure difference between SS to IA or OA - Soil gas concentration DO NOT change significantly with soil gas pressure changes at all locations. Change in soil gas pressure Change in soil gas Conc. (Luo, 2009, ASU) 23

24 Effect of Water Table Fluctuation on Soil Gas Concentration Collaborated with Dr. Yuanming Guo at Arizona State Univ. 24

25 Modeling: soil gas conc vs. water table fluctuation Collaborated with Dr. Yuanming Guo at Arizona State Univ. Scenario: -Sandy soils -moderate water table fluctuation - Max. soil gas conc. varies about 2.5X (~3% of the source vapor concentration), and bigger changes in the deeper depth; - At 5 ft bgs, soil gas conc. varied less than 2X and ~1.5% of the vapor source conc. 25

26 Modeling: soil gas conc vs. water table fluctuation Collaborated with Dr. Yuanming Guo at Arizona State Univ. Scenario: -Sandy soils -slow water table fluctuation - Max. soil gas conc. varies about 2X; - At 5 ft bgs, soil gas conc. varied less than 2X; 26

27 Modeling: soil gas conc vs. water table fluctuation Collaborated with Dr. Yuanming Guo at Arizona State Univ. Scenario: -Sandy silt -moderate water table fluctuation - Smaller temporal fluctuation in soil gas conc. with sandy silt soils with max. changes about 1X; - Minimal soil gas concentration changes at shallower depth 5 ft above water table; 27

28 Modeling: soil gas conc vs. water table fluctuation Collaborated with Dr. Yuanming Guo at Arizona State Univ. Scenario: -Sandy silt -slow water table fluctuation - Smaller temporal fluctuation in soil gas conc. with sandy silt soils with max. changes <1X; - Minimal soil gas concentration changes at shallower depth 5 ft above water table; 28

29 Temporal Variability Soil Gas Concentrations at PVI Sites 29

30 Temporal Variability in Soil Gas Concentrations at PVI Sites Sub-slab soil gas concentrations Little weather effect (both barometric pressure and strong wind) on soil gas concentrations, variation <10% 30

31 Temporal Variability in Soil Gas Pressure at PVI Sites Near-slab soil gas pressure - Fluctuation of ΔP (-15 Pa to 15 Pa) outside building and does not correlates well with wind; - No good correlation with barometric pressure changes; - No significant changes in soil gas concentrations. 31

32 Temporal Variability Soil Gas Pressure at PVI Sites Sub-slab soil gas pressure - Fluctuation of ΔP (-10 Pa to 12 Pa) and no good correlation with with wind; - No apparent correlation with barometric pressure changes; - No significant changes in soil gas concentrations. 32

33 Scenario A PVI CSM Refresh EPA OUST VI guidance shows two scenarios with potential VI concerns: Scenario B Scenario A: soil impacts serving as vapor source under/near building footprint within 0-15 ft bgs.; with or without groundwater impacts. Scenario B: Impacted GW serving as vapor source within 0-6 ftbgs without localized impacted soils 33