Article pubs.acs.org/est
Proof-of-Concept Study of an Aerobic Vapor Migration Barrier Beneath a Building at a Petroleum Hydrocarbon-Impacted Site Hong Luo,†,‡ Paul R. Dahlen,† Paul C. Johnson,*,† and Tom Peargin§ †
School of Sustainable Engineering and the Built Environment, Ira A. Fulton Schools of Engineering, Arizona State University, Tempe, Arizona 85287, United States ‡ Chevron Energy Technology Company, 3901 Briarpark Drive, Houston, Texas 77042, United States § Chevron Energy Technology Company, 6001 Bollinger Canyon Road, San Ramon, California 94583, United States S Supporting Information *
ABSTRACT: A proof-of-concept study was conducted to evaluate an alternative to traditional extraction-based subslab vapor mitigation systems at sites with petroleum hydrocarbon and/or methane vapor impact concerns. The system utilizes the slow delivery of air beneath a foundation to attenuate vapor migration to the building via aerobic biodegradation. The study was conducted at a site having elevated hydrocarbon plus methane and depleted O2 vapor concentrations (160 mg/L and 5% v/v soil gas O2 concentrations beneath the foundation and spanning a 1.5 m vertical interval. It was within 3× of the pretest stoichiometric requirement estimate of 3.8 L/min. This resulted in reductions in subslab hydrocarbon plus methane concentrations from 80 to C10
0.5 1.3
C9−C10 C8−C9
3.8 18.5 46.5
C7−C8 C5−C6
28.3 13.7
C4−C5 C3−C4
1.7 0.0
C2−C3 C1−C2 CH4
0.1
C6−C7 1980
35.5
below and adjacent to the building. These results were obtained by collecting soil gas samples in Tedlar bags using a vacuum box and analyzing them on-site by gas chromatography using flame ionization and thermal conductivity detectors (FID and TCD, respectively) for hydrocarbons (FID) and O2, CO2, CH4, and N2
carbon ranges
Table 1. Minimum Air Delivery Rate Design Calculation Inputs and Resultsa
Figure 3. Pretest soil gas distribution along left (west) to right (east) vertical cross-section. The sampling points are (from left to right): J, D, 7, 9, 10, 13, E, and F.
Ci,source [g-i/m3- vapor] Diair × 104 [m2/s] Dieff/Diair
3, 7, 8, 9, 11, 12, 13, 16, 17, 18, 19, 20, 22, B, C, D, E, and J). The differential pressures were referenced to Location F at 3.2 m BGS. Based on soil coring around the building, the shallow (5% v/v concentrations
well was capped. For the work presented here, only the western well was used and the south end of the eastern well was capped. The air injection system consisted of a GAST 75R645 Series pump that could generate in excess of 100 LPM at 20 psig, a compressed air tank with pressure relief for service pressure management, and Omega FMA5528 mass flow controllers with flow totalizers. This provided the flexibility to control the flow through a series of steps. At each injection rate (1, 5, and 10 L/min), the flow rate was held steady until the O2 distribution stabilized.
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RESULTS AND DISCUSSION Air injection rates of 1, 5, and 10 L/min were tested with flow delivered only to the western well shown in Figure 2. Each rate was held constant for periods ranging from about 40 to 60 d, until the O2 distribution stabilized as measured by the in situ O2 sensors and real-time data acquisition system (Figure S2 presents sample O2 sensor data). Figures 4−6 present transitional and steady-state soil gas O2 concentration distributions at 1.5 and 3 m BGS for the 1, 5, and 10 L/min air injection rates, respectively. Initially the vertical zones being monitored in detail were oxygen-deficient as shown in Figure 4 contour plots. The O2 concentrations remained unchanged after 46 days of 1 L/min air injection. For reference, 46 d of air injection at 1 L/min equates to a cumulative injected volume that is roughly equivalent to a 1982
dx.doi.org/10.1021/es3045532 | Environ. Sci. Technol. 2013, 47, 1977−1984
Environmental Science & Technology
Article
Figure 6. Plan view of soil gas O2 distribution at 1.5 m BGS and 3 m BGS after 6 days and 60 days (steady O2 distribution) of air injection from west well at 10 L/min.
resulting downward oxygen flux was performed. Using a diffusion coefficient for O2 in air of 0.23 cm2/s (Yaws16), the (Dieff/Diair) = 0.046 value from Table 1, and site data from the 10 L/min injection to estimate a concentration gradient of about (280 g-O2/m3)/ (3 m) and O2-increased area of about 500 m2, one can calculate a downward O2 diffusion rate of 3 g-O2/min, which is within 6% of the oxygen delivery for a 10 L/min injection rate. It is also within 20% of the required O2 delivery estimate, if the 195 m2 footprint in Table 1 is replaced with the 500 m2 O2-increased area measured during 10 L/min air injection. Subslab CO2 concentrations during 10 L/min injection (Table S4), averaged about 3% v/v, which is comparable to 4% v/v expected from a mass balance mixing calculation. These analyses are supportive of the premise that the AVMB system accomplishes mitigation via aerobic biodegradation rather than just vapor displacement and dilution. Detailed TO-15 lab analyses were performed on 11 soil gas samples collected from 10 locations beneath the foundation after reaching steady conditions at the 10 L/min air delivery rate (locations 1, 2, 7, 10, 12, 13, 14, 17 at 1.5 m BGS and locations 10 and 13 at 3 m BGS; all other GC-FID data presented in Table S5). The concentrations of compounds most frequently of interest at petroleum sitesbenzene, toluene, ethylbenzene, and xylenes were less than detection levels (0.5−0.74 ppbv). The petroleum hydrocarbon compounds detected were 2,2,4 trimethylpentane (9 samples, ranging 16−300 μg/m3), isopentane (5 samples, ranging
achieved beneath the building at all 1.5 and 3 m BGS monitoring locations. For reference, 60 d of air injection at 10 L/min equates to a cumulative injected volume that is roughly equivalent to seven 3-m thick pore volumes beneath the foundation with 0.3 m3-gas-filled voids/m3-soils. Figure 7 presents the vertical soil gas TPH and O2 concentration contour plots for a west−east transect through the center of the building. These are similar qualitatively to the AVMB conceptualization presented in Figure 1. The data suggest that there is about a 1.5-m thick air sweep/near-atmospheric O2 concentration zone beneath the foundation. From about 3 to 5 m BGS, there is a decreasing downward O2 concentration gradient that is mirrored by an upward decreasing hydrocarbon vapor concentration gradient from about 8 to 5 m BGS beneath the building foundation. TPH concentrations were about 80 mg/L immediately beneath the foundation prior to the test (Figure 3), and in contrast they were at nondetect (
