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Gasoline vapor transport through a high-water-content soil
Journal of Contaminant Hydrology, 8 (1991) 317-334
317
Elsevier Science Publishers B.V., Amsterdam
Gasoline vapor transport through a high-water-content soil R i c h a r d L. J o h n s o n and M a t t h e w Perrott
Oregon Graduate Institute, Department of Environmental Science and Engineering, Beaverton, OR 97006-1999, USA (Received March 29, 1990; revised and accepted May 17, 1991)
ABSTRACT Johnson, R.L. and Perrott, M,, 1991. Gasoline vapor transport through a high-water-content soil. J. Contain. Hydrol., 8: 317-334. The diffusive transport of gasoline vapors in a fine-grained, high-water-content soil was examined in a combined field, laboratory and modeling study. Subsurface vapor concentrations of a variety of gasoline-derived compounds were monitored for one year at a gasoline service station. Effective diffusion coefficients were measured in the laboratory using soil columns. Data indicate that diffusive transport in the soil is quite slow. Numerical modeling, using the parameters determined in the field and laboratory, suggests that the vapors would spread only a few meters per year. The implications are that vapor-phase monitoring for gasoline leaks in such soils will not allow rapid detection and that even small releases of product will cause high vapor concentrations for prolonged time periods.
INTRODUCTION
Subsurface monitoring of hydrocarbon vapors is widely used to detect leaks from underground storage tanks (UST's) as well as other sources of groundwater contamination. In coarse porous media, which is often used as backfill material around UST's, vapors can move by molecular diffusion and/or advective flow. As a result, vapors from leaking UST's can be detected within hours or days of the initiation of a release. Because many of the most rapidly moving gasoline compounds are also very volatile, vapor concentrations can quickly rise to tens of thousands of parts-per-million by volume (ppmv). Thus, vapor detection within engineered backfill zones is often a rapid and sensitive way to detect leaks. Gaseous advection can result from external pressure gradients or densitydifferences between the vapors and the pore air. Density-driven advection occurs because the molecular weights of most organic compounds are greater than the average molecular weight of air. Thus, air containing significant
318
R.L. JOHNSON AND M. PERROTT
quantities of gasoline components is more dense than uncontaminated air. The relative vapor density (Pr = Pair+organics/Pair) of gasoline can be /> 1.2, which is ample for significant gravity-driven advective flow in coarse media (e.g., pea gravel). In less-permeable soils (e.g., permeabilities < 10 l l c m 2 ) gravity-driven flow has been shown to be small compared to diffusion. (Mendoza and McAlary, 1989; Mendoza and Frind, 1990a, b). In soils where the water content is high, diffusive pathways may be limited and tortuous and air permeability will be low. Thus, vapor diffusion will be slow and density-driven flow will probably not occur. These conditions often occur when UST's are backfilled with fine-grained soils. As a consequence, vapor monitoring is generally not a good option at these sites. Slow transport in high-water-content soils occurs for at least four reasons: (1) the gasoline components will partition to the pore water, making them less available for gas-phase diffusion; (2) the gasoline components may also partition to the soil; (3) the pore water blocks many pathways for gas-phase diffusion, increasing the length of the path along which the vapors must move; and (4) densitydriven advective transport is minimized in fine-grained soils, thus diffusion is the dominant mechanism. Numerous field studies have shown that vapor transport by simple diffusion dominates in many subsurface environments (e.g., Kreamer, 1982; Weeks et al., 1982; Kreamer et al., 1988). These studies demonstrated that subsurface diffusion can be adequately described by Fick's second law. For the case where the effective diffusion coefficient (De) and retardation factor (R) are constant, Fick's second law can be expressed in one dimension as: ~Ca
De ~2C a =
~t
R ~2x
(1)
where Ca = concentration in the gas phase (mass/length3). D e is often given as"
De =
-~Dai r
(2)
where T is the tortuosity of the medium in a three-phase system (i.e., soil, water and air) and D~ir is the free-air diffusion coefficient. A number of expressions for z have been reported (e.g., Penman, 1940; Marshall, 1959; Reible and Shair, 1982; Kreamer et al., 1988). The relationship most often used in the groundwater context is (Millington, 1959):
Oa/3 -
2
0t
(3)
where 0~ = air-filled porosity and 0t = total porosity. Using eq. 3, the effect of water content on tortuosity for a soil with 0t = 0.4 is seen in Fig. 1. Clearly, at high water contents • becomes small and
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
319
0.15
(D 0.1
~)~0.05
0.05
0.1
0.15
0.2
AIR-FILLED POROSITY
Fig. 1. Vapor tortuosity factor calculated using eq, 3 for a soil with a total porosity of 0.4.
correspondingly so does De. De and z values can also be determined experimentally. The soil in this study had a very high water content, thus diffusion was strongly influenced by z, and tortuosity was determined experimentally. The retardation factor, R, can be expressed as (Baehr, 1987): R
=
1 + -O-w K n + pbKHgp O~ Oa
(4)
where K . = dimensionless water-air partition coefficient, 0w = water-filled porosity (dimensionless), Pb = soil bulk density (m/length 3) and Kp = soilwater partition coefficient. It is implicit in eq. 4 that the values of both KH and Kp are independent of concentration (i.e., linear Freundlich isotherms) and always at equilibrium. These assumptions are thought to be generally applicable; however, cases where substantial deviations were observed have been reported (Peterson et al., 1988). This is particularly the case in low water-content soils (Chiou and Shoup, 1985; Houston et al., 1989). KH and Kp values for a wide range of environmentally-relevant compounds are available from the literature (Mabey et al., 1982; Montgomery and Welkom, 1990). Kn can also be estimated from solubility [S (moles/l)] and vapor pressure [Pv (atm)] data as the ratio (Mackay and Wolkoff, 1973): H.
=
S Pv Rg Ta
(5)
where Rg = ideal gas constant = 0.08211-atm/mole°K and Ta = absolute temperature. Because subsurface temperatures are often below 20°C, and because vapor pressure is a strong function of temperature, Pv in eq. 5 should be adjusted to the ambient temperature. Kp can be determined experimentally, or it can be estimated using the relationship (Karickhoff et al., 1979; Karickhoff, 1984): Kv =
KoJo¢
(6)
320
R.L. JOHNSON AND M. PERROTT
TABLE l Physical properties of selected gasoline components Compound
Vapor pressure (atm)"
Solubility (moles/l)~
Henry's constant (m 3atm/mole)d
/~
K~p
Butane Pentane Hexane Benzene Toluene
2.0 0.54 0.16
0.0033 0.0018 0.0008 0.0228 0.0058
0.61 0.30 0.20 0.0044 0.0066
0.04 0.08 0.12 5.54 3.70
2.18 4.9 2.70 10.4 3.77 20.2 0.64 193.0 1 . 2 4 218.0
0.10 b
0.038 b
Retardation factor g
a from API (1966) unless otherwise noted (20°C) bfrom Mabey et al. (1982) c from API (1966) dcalculated using P~ IS (20°C) calculated using eq. 5 calculated using eq. 6 and 7, withfo~ = 0.009 gcalculated using eq. 4, p = 1.6, 0, = 0.04, 0w = 0.36
where Koc is the organic carbon partition coefficient (volume of water/mass organic carbon) and foe is the fractional organic carbon content of the soil (mass organic carbon/mass soil). Koc values are available in the literature (Mabey et al., 1982; Montgomery and Welkom, 1990) and foc can be readily determined for the soil of interest. Alternately, Koc can be estimated from other physical property data. Mabey et al. (1982) review a number of correlation equations for determining Koc. One based on aqueous solubility has been reported by Kenaga and Goring (1980): logKo~ =
- 0 . 5 5 l o g S + 3.64
(7)
where S is the solubility in mg/1. Because Ko¢ values for some of the compounds of interest are not known, eq. 7 has been used here. Physical properties for a range of compounds of interest are listed in Table 1. EXPERIMENTAL
Field site description Soil vapor concentrations were measured for a period of approximately one year at an operating gasoline service station in Portland, OR. The underground storage tanks at the site are 10 to 15 yr old and were backfilled with native soil, which is a fine silty loam. No major leaks are known to have occurred at the site. However, a small leak in the piping between the tanks and
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
321
8
a)
;Z4
'13 "25
~
DnG[
~SL~D
'22
"11
21]
TANKS
"24
"12
SERVICE BAYS
AN ] 28
"~
~7
4 2 rn
b) G R O U N D SURFACE
() ( Fig. 2. (a) Schematic plan view of the gasoline service station showing the locations of vapor sampling wells. (b) Schematic cross-section view of the gasoline service station showing the depths of the tanks and sampling probes and the range of water depths.
the service island was identified during the study and repaired. Plan and cross-section views of the site are seen in Fig. 2. The topography of the site generally slopes downward to the northwest. The asphalt extends approximately 30 m to the north of the tanks, 50 m to the west and south, 10 m to the east. The asphalt is underlain by ,-~ 0.2 m of drain rock. The site has a history of small releases from overfilling the tanks and as spills on the tarmac. The asphalt covering the site is cracked and product spilled on the surface was frequently washed down through the asphalt to minimize fire hazard. This probably contributed to subsurface vapor concentrations. In addition, the drain rock beneath the asphalt may have acted as a conduit to distribute gasoline over a wider area.
322
R.L. JOHNSON AND M. PERROTT
Field vapor sampling equipment Vapor samples were collected into evacuated stainless steel (SS) canisters through SS tubes at a depth of 1.2m below ground surface. The tubes consisted of a 1 m section of 6 . 4 m m (0.25inch) O.D. SS tube with a 0.2 m section of 13mm O.D. tube welded to the top of it. To install the tubes, a 25 m m diameter hole was drilled through the asphalt and underlying drain rock. A 2 0 m m O.D. rod was then driven to a depth of about 1 m and removed. The sampling tube, with a 3 m m O.D. SS rod insert, was then pushed to the final depth, such that the top of the tube was just below ground surface. The tube was then sealed into the asphalt with caulk and the insert was removed. Considerable care was taken to prevent leaks from the surface along the outside of the tube, however the extent of leaks was not assessed in the field. (Vapor analyses of the samples showed oxygen concentrations which were substantially lower than atmospheric values, indicating the leaks were not a problem.) Between sampling events the tops of the tubes were sealed with a disk of aluminum fitted with a compressible rubber seal (Fig. 3a). This provided an air-tight seal in a nearly flush-mounted assembly which required only a 25 m m diameter hole to be drilled in the asphalt. The canisters were connected to the tubes by a manifold (Fig. 3b) which consisted of an inflatable packer through which the sample line passed. After connecting the canisters to the sampling tubes, the sampling tube was purged using a hand-operated vacuum pump (Peters and Russell, Inc., Springfield, OH). Vapor samples were then collected in the evacuated SS canisters by opening the valves on the canisters and allowing them to fill to ambient pressure.
Analytical parameters The canister contents were analyzed for a range of gasoline hydrocarbons, as well as COz, CH4 and 02. Prior to sampling, the canisters were cleaned by heating at 100°C under vacuum. The canisters were then partially filled to 0.2atm with an internal standard (1% propylene in nitrogen). Hydrocarbon analyses were by capillary gas chromatography (GC) (HP 5890A, Hewlett Packard, Inc., Sunnyvale, CA). Prior to their analyses, eight canisters were placed in an oven at 70°C and connected to a canister selection valve which was connected to a sampling valve (both of which were also in the oven, which was adjacent to the GC). When the sampling valve was switched, sampled air flowed from the selected canister through a loop on the valve. When the valve returned to its original position the sample was injected onto a 0.53 m m I.D. DB-1 capillary column (J & W Scientific, Folsom, CA). Detection was by flame ionization (FID) and photo
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
a)
~ - -
323
SCREWS =
ALUMINUM CAP
THREADED PLATE
10 mm
b)
VACUUM GAUGE
EVACUATED SAMPLE CANISTER
'QUICK-CONNECT' FOR PURGING PUMP 'QUICK-CONNECT' FOR PACKERINFLATION PACKER
GROUND SURFACE SAMPLING TUBE
Fig. 3. (a) Schematic drawing of the cap for sealing the vapor-sampling probes. (b) Schematic drawing of
the manifold for collecting samples from the vapor probes.
ionization (PID) and for some samples by mass spectrometry. Vapor concentrations were determined by comparison to multi-component vapor standards. Permanent gas concentrations (CO2, CH4 and 02) were also determined by GC. Following the hydrocarbon analyses, the canisters were pressurized to 0.68 atm gauge pressure with nitrogen and connected to a sampling valve. In this case the valve contained two loops, and simultaneously injected samples onto a Spherocarb column (for CO2 and C H 4 determinations) and a molecular sieve column (for 02 analyses). CO2 and C H 4 concentrations were determined by FID (after conversion of the CO2 to CH4). 02 was determined by thermal
324
R.L. JOHNSON AND M. PERROTT
conductivity (TCD). Calibration was again made by comparison to standard gas mixtures.
Soil properties determinations As discussed above, soil properties (grain size, total porosity, water content, organic carbon content) play an important role in the rate at which vapors diffuse through the soil. As a consequence, each of these was determined experimentally. Grain size of the soil was determined by sieving and hydrometer analysis. Water content was determined gravimetrically by weighing a known volume of soil before and after drying. The total porosity was determined by measuring the bulk volumes of portions of soil, as well as the particle volumes for those same portions. Total porosity was calculated using the equation: 0t
~-
volum%.lk - - volumeparticle volumebu~k
(8)
Organic carbon content of the soil (foe) was determined using a combustion apparatus which converts the organic carbon to CO2 and finally to C H 4. The C H 4 is measured using an FID. The apparatus was calibrated using soils with known organic carbon content. Accuracy of the method has been demonstrated in an inter-laboratory comparison with several other techniques (Powell et al., 1989).
Effective diffusion coefficient determinations It is implicit in eq. 1 that diffusion occurs through a continuous medium (i.e. connected air pores). However, due to the very high water content of the soil at this site, it is unclear if continuous air-filled pathways exist in the soil. To investigate this, experimental measurements of steady-state diffusive flux through soil cores were made using the apparatus shown in Fig. 4. These data were used to determine z, which was used to estimate D c for the compounds of interest. Five centimeter diameter cores of 1-2 cm thickness were obtained by pressing stainless steel rings into the soil at the desired depth. Each core was then sealed between the two end reservoirs. One ml of C H 4 w a s injected into the lower reservoir, resulting in a CH4 concentration of ,-~ 1% by volume. A stream of humidified nitrogen (12ml/min) was passed through top reservoir. At regular intervals the nitrogen stream exiting the apparatus was sampled by an automated GC controlled by a data system. Samples were collected until the concentration in the effluent stream reached steady state. (The volume of the source reservoir was sufficiently large so the source concentration was not substantially decreased during the experiment.)
325
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
NEEDLE VALVE VENTTO ATMOSPHERE
DiFFUSION~cELL 1~~ [PUMP
FLOW METER
F ~
NEEDLE ~ - I
J E ~ [ FLOW END/ ~ . ]~ METER CAPS ~ \SOIL I COLUMN HUMIDIFIER
SOURCE INLET
Fig. 4. Schematic drawing of the steady-state diffusion apparatus used to measure effective diffusion coefficient in the high-water-content soils.
Absolute concentrations in the lower reservoir and in the effluent stream were determined by comparison to a series of dilution standards. Flux through the column was calculated as: F -
-ACV A
(9)
where AC = the concentration difference across the column (g/cm3), A = cross-sectional area of the column (cm 2) and V = volumetric flow rate of the nitrogen (cm3/s). The effective diffusion coefficient for methane was estimated using a rearranged form of Fick's first law: De -
-FAX
OaAC
(10)
where AX = the thickness of the column. Tortuosity was calculated using eq. 2 and the free-air diffusion coefficient for methane (24mm2/s; Cussler, 1979).
Environmental parameters A number of meteorological and hydraulic parameters were continuously monitored on-site or at locations within 10km of the site. These included: daily rainfall, wind speed, air temperature, soil temperature, groundwater level, barometric pressure and differential pressure between the atmosphere and the soil below the asphalt. Water level, the differential pressure and soil and air temperatures were recorded on-site with a TERRA-8 data logging system (Terra Science Systems, Ltd, Vancouver, B.C.). Rainfall, barometric
326
R.L. JOHNSON A N D M. PERROT'I
.,.
'1o . . . . . . . . . "',..
~0
/ ,
__
BENZENE
HEXANE
...'"'"....... ,...%........,.
.J
i 8ERVII~E
.,,"
s
METHANE 22
~
BUTANE
~~ ,, ! \ \~.
30 m
",.
-'~
"',...,'"
42 m
Fig. 5. Plan view isoconcentration contours of methane, butane, hexane, and benzene for 9 Jan. 1990. Contours correspond to 90, 80, 70, 60, 50, 40, 30, 20, 10 and 2% (dashed) of the maximum observed concentrations.
pressure, wind speed, and soil and air temperatures were measured both at the Oregon Graduate Institute and at several National Weather Service locations nearby. RESULTS AND DISCUSSION
Vapor concentrations Data for three gasoline compounds (butane, hexane and benzene) plus methane at all of the sampling locations are shown in Fig. 5. These three compounds were chosen because they represent a range of physical and chemical properties (Table 1). The isoconcentration lines in Fig. 5 were generated with data from the sampling locations shown in the figure using a
327
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
LTM1A
LTM5
~E',o~TSI~V10~C ~ ,~'~IM~"iA~R~I~Y,'UIJULN ,AUG 1~ ]~~EN~'~P IO~,1~C~.....~,.Ap~91~y I~NIJ~L,~~
40 I
' BENZEN~
O_
0/ 5o0
' HEXANE
a0.0oq
DAYS
,
:
'
0
o
,
350
,
'
0L
DAYS
I
350
Fig. 6. Vapor concentrations of butane, hexane and benzene as a function of time during the period Sept. 1988-July 1989 at a depth of 1.2m at sampling points L T M I and LTM5.
linear kriging algorithm (Golden Software, Golden, CO). Considering the range of physical properties of the compounds (e.g., R values from ,-~ 5-220), it is somewhat surprising that the distributions of the chemicals are so similar. The reasons for the similarities and the implications for vapor-phase leak detection in high water content soils are examined below. Vapor concentrations of butane, hexane and benzene at two sampling locations (LTM1 and LTM5) are plotted versus time in Fig. 6. D a t a from LTM5 show no strong trends with time. LTM5 represents the highest measured concentrations for a number of compounds and is thought to be near the source of the vapors. (The small leak in the piping mentioned earlier was located between LTM5 and the nearest corner of the service bays.) Vapor concentrations at LTM 1 show a general increase with time, which is probably the result of diffusive transport. As with the areal contours, the time series data indicates that the behaviors of the different compounds are relatively similar. Despite the general similarities in shape of the vapor plumes, the ratios of concentrations observed at LTM1 and LTM5 clearly reflect differences between the compounds. The maximum concentration for benzene observed at LTM1 is ~ 10% of the value at LTM5. For hexane the LTM1 value is ,-- 25% of LTM5 and for butane it is nearly 100%. It should again be noted that the concentrations observed at LTM5 may not be the maximum values
328
R.L. JOHNSON AND M. PERROTT
TABLE 2 Soil properties at the study site Total porosity Air-filled porosity L~ G r a i n size distribution - - d~0, dgo Bulk density
~ 0.4 0.02-0.06 ~ 0.009 0.012 ram, 0.07 m m ~ 1.6 g/cm 3
at the site, especially for the less mobile compounds. Thus it could be expected that the concentration ratio for benzene could be even lower. Permanent gas concentrations indicated substantial biological activity in the soil. CO2 concentrations were elevated throughout the sampling area (5-9% by volume), but were not substantially higher in the contaminated zone. Marrin (1989) and others have found CO2 concentrations in the unsaturated zone to be a good indirect indicator of hydrocarbon contamination. Due to the generally high levels of CO2, this does not appear to be the case for the high-water-content soil at this site. Oxygen concentrations were substantially depleted in the contaminated zone (e.g., < 1% by volume), this is probably due to biological activity. C H 4 concentrations ranged from nearatmospheric values (--~ 1 ppm) to ,-~ 5900 ppm near the tanks. As seen in Fig. 5, the distribution of C H 4 w a s quite similar to the hydrocarbon vapors, thus it appears to represent the best biological indicator of the subsurface contamination.
Soil properties The backfill material, as well as the undisturbed native soil, is a fine silty loam. Grain sizes range from clay-sized particles to small pebbles, with dj0 = 0.012mm and dg0 = 0.07mm. Other physical properties of the soil are listed in Table 2. Total porosity of the soil is estimated to be ,~ 0.40. The percent water saturation at ,-~ 1 m depth was measured to be in the range 85-95%. The soil fraction organic carbon content (foe) is ~ 0.009. These data have been used to estimate the retardation factor for the compounds of interest using eq. 4-6 (Table 1). Based on these values, benzene would be expected to have a retardation factor R ,-~ 190 and R for butane would be ~ 5. Again, this should result in substantially different distributions for the contaminants.
Effective diffusion coefficient Laboratory experiments indicate that De for CH4 in the native soil is approximately 0.06 mm2/s. This value is much smaller than would be the case
G A S O L I N E V A P O R T R A N S P O R T T H R O U G H A H I G H - W A T E R - C O N T E N T SOIL
329
in drier soils, however it is significantly larger than the expected aqueousphase diffusion coefficient in the soil, which is approximately 0.002 mmR/s. Therefore, it appears likely that continuous air-filled pores exist and that diffusion occurs primarily within those pore spaces in the soil. The physical appearance of the soil suggests that many of these continuous pores could be macropores which are the result of soil structure. Furthermore, our ability to collect vapor samples from this soil suggests that there were at least some continuous airways. (Substantially-reduced oxygen concentrations in many of the vapor samples confirm that direct leaks to the atmosphere were minimal.) The soil tortuosity estimated from these experiments using eq. 2 is z = 0.0025. The value estimated from eq. 3 is 0.0034. Both of these values are quite sensitive to air-filled porosity, thus the uncertainties are relatively large. For the purposes of this study, the agreement between experimental and calculated values is considered good. The experimental value, r -- 0.0025, was used in the numerical modeling of gasoline-vapor transport in the soil. E n v i r o n m e n tal p a r a m e t e r s
Compared to many leaking underground storage tank (UST) sites, vapor concentrations at this site were relatively stable over a 1-year period. This observation suggests that environmental factors (other than soil properties) played a secondary role in vapor behavior. The frequency of change of many of the parameters was rapid by comparison to vapor concentration changes. For example, barometric pressure fluctuated on a daily or weekly basis and its effect on concentration distributions was observed to be small in comparison to other factors. Similarly, there did not appear to be any significant external pressure gradients at the site. The water level fluctuated ___1 m intermittently and appeared to be controlled by some unknown local source(s). These fluctuations are not thought to have contributed significantly to the lateral spreading of the contaminants. One parameter which probably has an influence on vapor concentrations was subsurface soil temperature. Figure 7 shows daily soil temperatures at 1 m depth for the study period. As expected, soil temperature dropped by ~ 10°C in the winter and some effect of this may be visible in the data from LTM5 (Fig. 6). Trends in the data from LTM 1 and other sampling points at the periphery of the plume suggest that the slow spreading of the vapors was not strongly dependent on temperature changes at the site. The similar distributions of gasoline components whose physical properties vary so substantially suggest that the vapors probably resulted from an areally-extensive source. This source is likely to have resulted from a long history of small releases in the vicinity of the tanks and p u m p island. It may
330
R.L.
1988 1989 30 !SEPOCTNOVDEC!JANFEB MAR,APRMAYJUN JUL AUG
TEMPERATURE 1.I MBELOWSURFACE
SOIL
SEPT88- AUG89
s
UJ O
O CO LL o iii O rr
80
0
DAYS
200
300
M.
PERROTT
~
40-
20-
4
02O
J
O 15-
100
AND
I a) R=I
60 -
Z !
JOHNSON
b) R=10
~
~
¢r t0FZ LU 5 0 Z
0 o
2g 0
l~
0
2
4
I
I
6
F
[
I
8
10
TIME (YEARS) Fig. 7. Subsurface soil temperature at 1 m depth in Portland, OR, for the period Sept. 1988-July 1989. Fig. 8. (a) Numerical model results showing vapor concentrations as a function of time at 1, 2, 4 and 7 meters from a source when the retardation factor is R = I. (b) Numerical model results showing vapor concentrations as a function of time at 1 and 2 meters from a source when R = 10.
also have been facilitated by washing of spilled gasoline into the soil through cracks in the asphalt tarmac.
Numerical diffusion modeling Because little is known of the temporal and/or spatial distribution of the source of vapors, it is difficult to reproduce conditions at the site with a numerical model. Nevertheless, it is useful to model vapor diffusion in the high-water-content soil to examine the general behavior of the gasoline components in the soil. Both the source of the vapors and the sampling points are located above the UST's; thus, vapor transport is assumed to be radially symmetrical. Diffusion is considered to be the dominant transport mechanism. A two-dimensional, axis-symmetric finite-element model developed by Mendoza and Frind (1990a, b) was used for these simulations. In the model, effective diffusion coefficients are estimated using eqs. 2 and 3. Partitioning to the water and soil is assumed to be instantaneous, following eq. 4. For these simulations, the upper and lower boundary conditions were set to be impermeable, with 3 m of"unsaturated" zone between them. Because there was no history of large releases, a small source (1 m high and 1 m in diameter) was used for these simulations. The tanks had been in place 10-15 yr, thus the simulations were conducted for a period of 10 yr. Figure 8a shows how simulated vapor concentrations vary with time for a non-soluble, non-sorbing compound at 1, 2, 4 and 7 m from the edge of the
G A S O L I N E V A P O R T R A N S P O R T T H R O U G H A H I G H - W A T E R - C O N T E N T SOIL
33 !
source. Again, these are not intended to replicate the distribution pattern of the contaminants at the site. Rather, they indicate the rate at which vapors would be expected to spread by diffusion. Several features of the plots are significant. First, long times (i.e., years) are required before significant concentrations of vapor reach 7 m from the source. Second, the increase in concentration at each distance is very gradual, requiring many years to approach source levels. Figure 8b shows the breakthrough at 1 and 2 m of a hypothetical compound with R = 10. Vapor transport in this case is extremely slow and almost no mass reaches the 4 and 7 m distances during the 10-yr simulation. The estimated retardation value for benzene was 190. Clearly, under conditions present at the site, benzene would be expected to move very slowly. The slow transport rates observed in these simulations are somewhat accentuated because a relatively small source size was used. The contrast between the slow, compound-dependent radial spreading in the simulations and the broad, compound-independent plumes at the site again suggest that the source of the vapors at the site must have been areally large. CONCLUSIONS
Subsurface gasoline vapor concentrations were monitored for a period of 1 yr. During that time, concentrations at sampling points near the "center" of the vapor plume remained relatively stable, while those at greater distances showed long-term increases. The principal reasons for the relatively stable concentrations were that diffusive transport of the vapors was very slow and diffusive fluxes were small. This was primarily due to the high water content of the soil, which was approximatley 90% of saturation. The long-term increases observed at some sampling locations are probably due to diffusive spreading of the gasoline vapors in the soil. However, it was not possible to eliminate all other potential mechanisms. The effective diffusion coefficient of CH4 in the soil was measured experimentally. Using eq. 2, the tortuosity of the soil was estimated to be 0.0025. This compares well with • = 0.0034 calculated from eq. 3. Other environmental parameters (e.g., water level fluctuations, temperature, rainfall) played only a secondary role in controlling vapor transport. Due to the high water content of the soil, and the range of Henry's constants for the gasoline compounds, it was expected that the distributions of chemicals would be substantially different from one another. This was not observed. Diffusion model results indicate that diffusion distances of less than a few meters per year are to be expected, even for the most mobile compounds. Because equilibrium partitioning is implicit in the mathematical model used here, compounds with appreciable water solubility were strongly retarded. In
332
R.L. J O H N S O N A N D M. P E R R O T T
this context, the stable concentration observed at LTM5 are not surprising, considering the slow diffusion. The rapid increases in concentration observed at LTM1 are not explained by the model. The increases could be due to continuing small releases at the site. In general terms, however, the model predicts an even slower spread of the vapors than was observed at the site. Biodegradation was active at the site, although it appears that oxygen may be limiting in the vicinity of the source. Common biological indicators (CO2, C H 4 and 02) were all affected by the spill. CO 2 could not be used to map the plume because it was generally high throughout the subsurface. C H 4 resulting from anaerobic degradation was the best biological indicator for mapping the contaminated zone. A number of important implications regarding vapor-phase leak detection systems can be drawn from these results. First, vapor-phase leak detection will not be effective at sites with high-water-content soils because vapor diffusion is very slow. Given a typical distance of 6-7 m from a leak to an observation well, years would be required for vapors to reach the detection point. This is in dramatic contrast to engineered backfills (e.g., pea gravel) where detection can occur in a matter of hours. Second, the slow diffusion means that once vapors do get into these soils, they will persist for a long period of time. Thus, at sites where past leaks or spills have occurred, future leak detection using vapor sampling may not be possible. Finally, the areal distribution of gasoline vapors within the soil may be strongly influenced by factors other than the location of the release (e.g., cracks in the tarmac, the presence of drain rock under an impermeable cap). Thus, mapping soil vapors at sites similar to the one studied here may not accurately assess the presence and location of gasoline leaks in the subsurface. DISCLAIMER
Although the research described in this article has been supported by the United States Environmental Protection Agency through Cooperative Agreement No. CR-81-4561-01-01, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. ACKNOWLEDGEMENTS
We would like to acknowledge Dr. Philip Durgin, formerly of the U.S. EPA, EMSL-Las Vegas, for his support of our research program. REFERENCES API, 1966. Technical Data Book - - Petroleum Refining. Am. Pet. Inst., New York, NY. Baehr, A.L., 1987. Selectrive transport of hydrocarbons in the sunsaturated zone due to aqueous and vapor phase pationing. Water Resour. Res., 23: 1926-1938.
GASOLINE VAPOR T R A N S P O R T T H R O U G H A HIGH-WATER-CONTENT SOIL
333
Chiou, C.T. and Shoup, T.D., 1985. Soil sorption of organic vapors and effects of humidity on sorptive mechanism and capacity. Environ. Sci. Technol., 19:1196-1200. Cussler, E.L., 1985. Diffusion: Mass Transfer in Fluid Systems. Cambridge University Press, 525 pp. Houston, S.L., Kreamer, D.K. and Marwig, R., 1989. A batch-type testing method for determination of adsorption of gaseous compounds on partially saturated soils. Geotech. Testing J., GTJODJ, 12: 3-10. Karickhoff, S.W., 1984. Organic pollutant sorption in aquatic systems. J. Hydraul. Eng., 110: 707-735. Karickhoff, S.W., Brown, D.S. and Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res., 13: 241-248. Kenaga, E.E. and Goring, C.A.I., 1980. Am. Soc. Test. Mater., Washington, DC, ASTM Spec. Publ. No. 707. Kreamer, D.K., 1982. In situ measurement of gas diffusion characteristics in unsaturated porous media by means of tracer experiments. Diss., Univ. of Arizona, Tucson, AZ. Kreamer, D.K., Weeks, E.P. and Thompson, G.M., 1988. A field technique to measure the tortuosity and sorption-affected porosity for gaseous diffusion of materials in the unsaturated zone with experimental results from near Barnwell, South Carolina. Water Resour. Res., 24: 331-341. Mabey, W.R., Smith, J.H., Podoll, R.T., Johnson, H.L., Mill, T., Chou, T.-W., Gates, J., Waight partridge, I., Jaber, H. and Vandenberg, D., 1982. Aquatic fate process data for organic priority pollutants. U.S. Environ. Prot. Agency, EPA 440/4-81-014, 434 pp. Mackay, D. and Wolkoff, A.W., 1973. Rate of evaporation of low-solubility contaminants from water bodies to atmosphere. Environ. Sci. Technol., 7:611-614. Marrin, D.L., 1989. Detection of non-volatile hydrocarbons using a modified approach to soil-gas surveying. In: Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Groundwater Conference, Houston, TX, Nov. 15-17, 1989. Marshall, T.J., 1959. The diffusion of gases through porous media. J. Soil Sci., 10: 79-82. Mendoza, C.A. and Frind, E.O., 1990a. Advectice-dispersive transport of heavy organic solvent vapours in the unsaturaetd zone, 1. Model development. Water Resour. Res., 26: 379-387. Mendoza, C.A. and Frind, E.O., 1990b. Advective-dispersive transport of heavy organic solvent vapours in the unsaturated zone: 2. Sensitivity analysis. Water Resour. Res., 26: 388-398. Mendoza, C.A. and McAlary, T.A., 1989. Modeling of groundwater contamination caused by organic solvent vapours. J. Ground Water, 28: 199-206. Millington, R.J., 1959. Gas diffusion in porous media. Science, 130: 100-102. Millington, R.J. and Quirk, J.P., 1961. Permeability of porous solids. Trans. Faraday Soc., 57: 1200-1207. Montgomery, J.H. and Welkom, L.M., 1990. Groundwater Chemicals Desk Reference. Lewis Publishers, Chelsea, MI, 640 pp. Penman, H.L., 1940. Gas and vapor movements in the soil, I, The diffusion of vapours through porous solids. J. Agric. Sci., 30: 437-462. Peterson, M.S., Lion, L.W. and Shoemaker, C.A., 1988. Influence of vapor-phase sorption and diffusion on the fate of trichloroethylene in an unsaturated aquifer system. Environ. Sci. Technol., 22: 571-578. Powell, R.M., Bledsoe, B.E., Curtis, G.P., Johnson, R.L., 1989. Interlaboratory methods comparison for the total organic carbon analysis of aquifer materials. Environ. Sci. Technol., 23: 1246-1249.
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Reible, D.D. and Shair, F.H., 1982. A technique for the measurement of gaseous diffusion in porous media. J. Soil Sci., 33: 165-174. Weeks, E.P., Earp, D.E. and Thompson, G.M., 1982. Use of atmospheric fluorocarbons F-11 and F-12 to determine the diffusion parameters of the unsaturated zone in the southern high plains of Texas. Water Resour. Res., 18: 1365-1378.
317
Elsevier Science Publishers B.V., Amsterdam
Gasoline vapor transport through a high-water-content soil R i c h a r d L. J o h n s o n and M a t t h e w Perrott
Oregon Graduate Institute, Department of Environmental Science and Engineering, Beaverton, OR 97006-1999, USA (Received March 29, 1990; revised and accepted May 17, 1991)
ABSTRACT Johnson, R.L. and Perrott, M,, 1991. Gasoline vapor transport through a high-water-content soil. J. Contain. Hydrol., 8: 317-334. The diffusive transport of gasoline vapors in a fine-grained, high-water-content soil was examined in a combined field, laboratory and modeling study. Subsurface vapor concentrations of a variety of gasoline-derived compounds were monitored for one year at a gasoline service station. Effective diffusion coefficients were measured in the laboratory using soil columns. Data indicate that diffusive transport in the soil is quite slow. Numerical modeling, using the parameters determined in the field and laboratory, suggests that the vapors would spread only a few meters per year. The implications are that vapor-phase monitoring for gasoline leaks in such soils will not allow rapid detection and that even small releases of product will cause high vapor concentrations for prolonged time periods.
INTRODUCTION
Subsurface monitoring of hydrocarbon vapors is widely used to detect leaks from underground storage tanks (UST's) as well as other sources of groundwater contamination. In coarse porous media, which is often used as backfill material around UST's, vapors can move by molecular diffusion and/or advective flow. As a result, vapors from leaking UST's can be detected within hours or days of the initiation of a release. Because many of the most rapidly moving gasoline compounds are also very volatile, vapor concentrations can quickly rise to tens of thousands of parts-per-million by volume (ppmv). Thus, vapor detection within engineered backfill zones is often a rapid and sensitive way to detect leaks. Gaseous advection can result from external pressure gradients or densitydifferences between the vapors and the pore air. Density-driven advection occurs because the molecular weights of most organic compounds are greater than the average molecular weight of air. Thus, air containing significant
318
R.L. JOHNSON AND M. PERROTT
quantities of gasoline components is more dense than uncontaminated air. The relative vapor density (Pr = Pair+organics/Pair) of gasoline can be /> 1.2, which is ample for significant gravity-driven advective flow in coarse media (e.g., pea gravel). In less-permeable soils (e.g., permeabilities < 10 l l c m 2 ) gravity-driven flow has been shown to be small compared to diffusion. (Mendoza and McAlary, 1989; Mendoza and Frind, 1990a, b). In soils where the water content is high, diffusive pathways may be limited and tortuous and air permeability will be low. Thus, vapor diffusion will be slow and density-driven flow will probably not occur. These conditions often occur when UST's are backfilled with fine-grained soils. As a consequence, vapor monitoring is generally not a good option at these sites. Slow transport in high-water-content soils occurs for at least four reasons: (1) the gasoline components will partition to the pore water, making them less available for gas-phase diffusion; (2) the gasoline components may also partition to the soil; (3) the pore water blocks many pathways for gas-phase diffusion, increasing the length of the path along which the vapors must move; and (4) densitydriven advective transport is minimized in fine-grained soils, thus diffusion is the dominant mechanism. Numerous field studies have shown that vapor transport by simple diffusion dominates in many subsurface environments (e.g., Kreamer, 1982; Weeks et al., 1982; Kreamer et al., 1988). These studies demonstrated that subsurface diffusion can be adequately described by Fick's second law. For the case where the effective diffusion coefficient (De) and retardation factor (R) are constant, Fick's second law can be expressed in one dimension as: ~Ca
De ~2C a =
~t
R ~2x
(1)
where Ca = concentration in the gas phase (mass/length3). D e is often given as"
De =
-~Dai r
(2)
where T is the tortuosity of the medium in a three-phase system (i.e., soil, water and air) and D~ir is the free-air diffusion coefficient. A number of expressions for z have been reported (e.g., Penman, 1940; Marshall, 1959; Reible and Shair, 1982; Kreamer et al., 1988). The relationship most often used in the groundwater context is (Millington, 1959):
Oa/3 -
2
0t
(3)
where 0~ = air-filled porosity and 0t = total porosity. Using eq. 3, the effect of water content on tortuosity for a soil with 0t = 0.4 is seen in Fig. 1. Clearly, at high water contents • becomes small and
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
319
0.15
(D 0.1
~)~0.05
0.05
0.1
0.15
0.2
AIR-FILLED POROSITY
Fig. 1. Vapor tortuosity factor calculated using eq, 3 for a soil with a total porosity of 0.4.
correspondingly so does De. De and z values can also be determined experimentally. The soil in this study had a very high water content, thus diffusion was strongly influenced by z, and tortuosity was determined experimentally. The retardation factor, R, can be expressed as (Baehr, 1987): R
=
1 + -O-w K n + pbKHgp O~ Oa
(4)
where K . = dimensionless water-air partition coefficient, 0w = water-filled porosity (dimensionless), Pb = soil bulk density (m/length 3) and Kp = soilwater partition coefficient. It is implicit in eq. 4 that the values of both KH and Kp are independent of concentration (i.e., linear Freundlich isotherms) and always at equilibrium. These assumptions are thought to be generally applicable; however, cases where substantial deviations were observed have been reported (Peterson et al., 1988). This is particularly the case in low water-content soils (Chiou and Shoup, 1985; Houston et al., 1989). KH and Kp values for a wide range of environmentally-relevant compounds are available from the literature (Mabey et al., 1982; Montgomery and Welkom, 1990). Kn can also be estimated from solubility [S (moles/l)] and vapor pressure [Pv (atm)] data as the ratio (Mackay and Wolkoff, 1973): H.
=
S Pv Rg Ta
(5)
where Rg = ideal gas constant = 0.08211-atm/mole°K and Ta = absolute temperature. Because subsurface temperatures are often below 20°C, and because vapor pressure is a strong function of temperature, Pv in eq. 5 should be adjusted to the ambient temperature. Kp can be determined experimentally, or it can be estimated using the relationship (Karickhoff et al., 1979; Karickhoff, 1984): Kv =
KoJo¢
(6)
320
R.L. JOHNSON AND M. PERROTT
TABLE l Physical properties of selected gasoline components Compound
Vapor pressure (atm)"
Solubility (moles/l)~
Henry's constant (m 3atm/mole)d
/~
K~p
Butane Pentane Hexane Benzene Toluene
2.0 0.54 0.16
0.0033 0.0018 0.0008 0.0228 0.0058
0.61 0.30 0.20 0.0044 0.0066
0.04 0.08 0.12 5.54 3.70
2.18 4.9 2.70 10.4 3.77 20.2 0.64 193.0 1 . 2 4 218.0
0.10 b
0.038 b
Retardation factor g
a from API (1966) unless otherwise noted (20°C) bfrom Mabey et al. (1982) c from API (1966) dcalculated using P~ IS (20°C) calculated using eq. 5 calculated using eq. 6 and 7, withfo~ = 0.009 gcalculated using eq. 4, p = 1.6, 0, = 0.04, 0w = 0.36
where Koc is the organic carbon partition coefficient (volume of water/mass organic carbon) and foe is the fractional organic carbon content of the soil (mass organic carbon/mass soil). Koc values are available in the literature (Mabey et al., 1982; Montgomery and Welkom, 1990) and foc can be readily determined for the soil of interest. Alternately, Koc can be estimated from other physical property data. Mabey et al. (1982) review a number of correlation equations for determining Koc. One based on aqueous solubility has been reported by Kenaga and Goring (1980): logKo~ =
- 0 . 5 5 l o g S + 3.64
(7)
where S is the solubility in mg/1. Because Ko¢ values for some of the compounds of interest are not known, eq. 7 has been used here. Physical properties for a range of compounds of interest are listed in Table 1. EXPERIMENTAL
Field site description Soil vapor concentrations were measured for a period of approximately one year at an operating gasoline service station in Portland, OR. The underground storage tanks at the site are 10 to 15 yr old and were backfilled with native soil, which is a fine silty loam. No major leaks are known to have occurred at the site. However, a small leak in the piping between the tanks and
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
321
8
a)
;Z4
'13 "25
~
DnG[
~SL~D
'22
"11
21]
TANKS
"24
"12
SERVICE BAYS
AN ] 28
"~
~7
4 2 rn
b) G R O U N D SURFACE
() ( Fig. 2. (a) Schematic plan view of the gasoline service station showing the locations of vapor sampling wells. (b) Schematic cross-section view of the gasoline service station showing the depths of the tanks and sampling probes and the range of water depths.
the service island was identified during the study and repaired. Plan and cross-section views of the site are seen in Fig. 2. The topography of the site generally slopes downward to the northwest. The asphalt extends approximately 30 m to the north of the tanks, 50 m to the west and south, 10 m to the east. The asphalt is underlain by ,-~ 0.2 m of drain rock. The site has a history of small releases from overfilling the tanks and as spills on the tarmac. The asphalt covering the site is cracked and product spilled on the surface was frequently washed down through the asphalt to minimize fire hazard. This probably contributed to subsurface vapor concentrations. In addition, the drain rock beneath the asphalt may have acted as a conduit to distribute gasoline over a wider area.
322
R.L. JOHNSON AND M. PERROTT
Field vapor sampling equipment Vapor samples were collected into evacuated stainless steel (SS) canisters through SS tubes at a depth of 1.2m below ground surface. The tubes consisted of a 1 m section of 6 . 4 m m (0.25inch) O.D. SS tube with a 0.2 m section of 13mm O.D. tube welded to the top of it. To install the tubes, a 25 m m diameter hole was drilled through the asphalt and underlying drain rock. A 2 0 m m O.D. rod was then driven to a depth of about 1 m and removed. The sampling tube, with a 3 m m O.D. SS rod insert, was then pushed to the final depth, such that the top of the tube was just below ground surface. The tube was then sealed into the asphalt with caulk and the insert was removed. Considerable care was taken to prevent leaks from the surface along the outside of the tube, however the extent of leaks was not assessed in the field. (Vapor analyses of the samples showed oxygen concentrations which were substantially lower than atmospheric values, indicating the leaks were not a problem.) Between sampling events the tops of the tubes were sealed with a disk of aluminum fitted with a compressible rubber seal (Fig. 3a). This provided an air-tight seal in a nearly flush-mounted assembly which required only a 25 m m diameter hole to be drilled in the asphalt. The canisters were connected to the tubes by a manifold (Fig. 3b) which consisted of an inflatable packer through which the sample line passed. After connecting the canisters to the sampling tubes, the sampling tube was purged using a hand-operated vacuum pump (Peters and Russell, Inc., Springfield, OH). Vapor samples were then collected in the evacuated SS canisters by opening the valves on the canisters and allowing them to fill to ambient pressure.
Analytical parameters The canister contents were analyzed for a range of gasoline hydrocarbons, as well as COz, CH4 and 02. Prior to sampling, the canisters were cleaned by heating at 100°C under vacuum. The canisters were then partially filled to 0.2atm with an internal standard (1% propylene in nitrogen). Hydrocarbon analyses were by capillary gas chromatography (GC) (HP 5890A, Hewlett Packard, Inc., Sunnyvale, CA). Prior to their analyses, eight canisters were placed in an oven at 70°C and connected to a canister selection valve which was connected to a sampling valve (both of which were also in the oven, which was adjacent to the GC). When the sampling valve was switched, sampled air flowed from the selected canister through a loop on the valve. When the valve returned to its original position the sample was injected onto a 0.53 m m I.D. DB-1 capillary column (J & W Scientific, Folsom, CA). Detection was by flame ionization (FID) and photo
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
a)
~ - -
323
SCREWS =
ALUMINUM CAP
THREADED PLATE
10 mm
b)
VACUUM GAUGE
EVACUATED SAMPLE CANISTER
'QUICK-CONNECT' FOR PURGING PUMP 'QUICK-CONNECT' FOR PACKERINFLATION PACKER
GROUND SURFACE SAMPLING TUBE
Fig. 3. (a) Schematic drawing of the cap for sealing the vapor-sampling probes. (b) Schematic drawing of
the manifold for collecting samples from the vapor probes.
ionization (PID) and for some samples by mass spectrometry. Vapor concentrations were determined by comparison to multi-component vapor standards. Permanent gas concentrations (CO2, CH4 and 02) were also determined by GC. Following the hydrocarbon analyses, the canisters were pressurized to 0.68 atm gauge pressure with nitrogen and connected to a sampling valve. In this case the valve contained two loops, and simultaneously injected samples onto a Spherocarb column (for CO2 and C H 4 determinations) and a molecular sieve column (for 02 analyses). CO2 and C H 4 concentrations were determined by FID (after conversion of the CO2 to CH4). 02 was determined by thermal
324
R.L. JOHNSON AND M. PERROTT
conductivity (TCD). Calibration was again made by comparison to standard gas mixtures.
Soil properties determinations As discussed above, soil properties (grain size, total porosity, water content, organic carbon content) play an important role in the rate at which vapors diffuse through the soil. As a consequence, each of these was determined experimentally. Grain size of the soil was determined by sieving and hydrometer analysis. Water content was determined gravimetrically by weighing a known volume of soil before and after drying. The total porosity was determined by measuring the bulk volumes of portions of soil, as well as the particle volumes for those same portions. Total porosity was calculated using the equation: 0t
~-
volum%.lk - - volumeparticle volumebu~k
(8)
Organic carbon content of the soil (foe) was determined using a combustion apparatus which converts the organic carbon to CO2 and finally to C H 4. The C H 4 is measured using an FID. The apparatus was calibrated using soils with known organic carbon content. Accuracy of the method has been demonstrated in an inter-laboratory comparison with several other techniques (Powell et al., 1989).
Effective diffusion coefficient determinations It is implicit in eq. 1 that diffusion occurs through a continuous medium (i.e. connected air pores). However, due to the very high water content of the soil at this site, it is unclear if continuous air-filled pathways exist in the soil. To investigate this, experimental measurements of steady-state diffusive flux through soil cores were made using the apparatus shown in Fig. 4. These data were used to determine z, which was used to estimate D c for the compounds of interest. Five centimeter diameter cores of 1-2 cm thickness were obtained by pressing stainless steel rings into the soil at the desired depth. Each core was then sealed between the two end reservoirs. One ml of C H 4 w a s injected into the lower reservoir, resulting in a CH4 concentration of ,-~ 1% by volume. A stream of humidified nitrogen (12ml/min) was passed through top reservoir. At regular intervals the nitrogen stream exiting the apparatus was sampled by an automated GC controlled by a data system. Samples were collected until the concentration in the effluent stream reached steady state. (The volume of the source reservoir was sufficiently large so the source concentration was not substantially decreased during the experiment.)
325
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
NEEDLE VALVE VENTTO ATMOSPHERE
DiFFUSION~cELL 1~~ [PUMP
FLOW METER
F ~
NEEDLE ~ - I
J E ~ [ FLOW END/ ~ . ]~ METER CAPS ~ \SOIL I COLUMN HUMIDIFIER
SOURCE INLET
Fig. 4. Schematic drawing of the steady-state diffusion apparatus used to measure effective diffusion coefficient in the high-water-content soils.
Absolute concentrations in the lower reservoir and in the effluent stream were determined by comparison to a series of dilution standards. Flux through the column was calculated as: F -
-ACV A
(9)
where AC = the concentration difference across the column (g/cm3), A = cross-sectional area of the column (cm 2) and V = volumetric flow rate of the nitrogen (cm3/s). The effective diffusion coefficient for methane was estimated using a rearranged form of Fick's first law: De -
-FAX
OaAC
(10)
where AX = the thickness of the column. Tortuosity was calculated using eq. 2 and the free-air diffusion coefficient for methane (24mm2/s; Cussler, 1979).
Environmental parameters A number of meteorological and hydraulic parameters were continuously monitored on-site or at locations within 10km of the site. These included: daily rainfall, wind speed, air temperature, soil temperature, groundwater level, barometric pressure and differential pressure between the atmosphere and the soil below the asphalt. Water level, the differential pressure and soil and air temperatures were recorded on-site with a TERRA-8 data logging system (Terra Science Systems, Ltd, Vancouver, B.C.). Rainfall, barometric
326
R.L. JOHNSON A N D M. PERROT'I
.,.
'1o . . . . . . . . . "',..
~0
/ ,
__
BENZENE
HEXANE
...'"'"....... ,...%........,.
.J
i 8ERVII~E
.,,"
s
METHANE 22
~
BUTANE
~~ ,, ! \ \~.
30 m
",.
-'~
"',...,'"
42 m
Fig. 5. Plan view isoconcentration contours of methane, butane, hexane, and benzene for 9 Jan. 1990. Contours correspond to 90, 80, 70, 60, 50, 40, 30, 20, 10 and 2% (dashed) of the maximum observed concentrations.
pressure, wind speed, and soil and air temperatures were measured both at the Oregon Graduate Institute and at several National Weather Service locations nearby. RESULTS AND DISCUSSION
Vapor concentrations Data for three gasoline compounds (butane, hexane and benzene) plus methane at all of the sampling locations are shown in Fig. 5. These three compounds were chosen because they represent a range of physical and chemical properties (Table 1). The isoconcentration lines in Fig. 5 were generated with data from the sampling locations shown in the figure using a
327
GASOLINE VAPOR TRANSPORT THROUGH A HIGH-WATER-CONTENT SOIL
LTM1A
LTM5
~E',o~TSI~V10~C ~ ,~'~IM~"iA~R~I~Y,'UIJULN ,AUG 1~ ]~~EN~'~P IO~,1~C~.....~,.Ap~91~y I~NIJ~L,~~
40 I
' BENZEN~
O_
0/ 5o0
' HEXANE
a0.0oq
DAYS
,
:
'
0
o
,
350
,
'
0L
DAYS
I
350
Fig. 6. Vapor concentrations of butane, hexane and benzene as a function of time during the period Sept. 1988-July 1989 at a depth of 1.2m at sampling points L T M I and LTM5.
linear kriging algorithm (Golden Software, Golden, CO). Considering the range of physical properties of the compounds (e.g., R values from ,-~ 5-220), it is somewhat surprising that the distributions of the chemicals are so similar. The reasons for the similarities and the implications for vapor-phase leak detection in high water content soils are examined below. Vapor concentrations of butane, hexane and benzene at two sampling locations (LTM1 and LTM5) are plotted versus time in Fig. 6. D a t a from LTM5 show no strong trends with time. LTM5 represents the highest measured concentrations for a number of compounds and is thought to be near the source of the vapors. (The small leak in the piping mentioned earlier was located between LTM5 and the nearest corner of the service bays.) Vapor concentrations at LTM 1 show a general increase with time, which is probably the result of diffusive transport. As with the areal contours, the time series data indicates that the behaviors of the different compounds are relatively similar. Despite the general similarities in shape of the vapor plumes, the ratios of concentrations observed at LTM1 and LTM5 clearly reflect differences between the compounds. The maximum concentration for benzene observed at LTM1 is ~ 10% of the value at LTM5. For hexane the LTM1 value is ,-- 25% of LTM5 and for butane it is nearly 100%. It should again be noted that the concentrations observed at LTM5 may not be the maximum values
328
R.L. JOHNSON AND M. PERROTT
TABLE 2 Soil properties at the study site Total porosity Air-filled porosity L~ G r a i n size distribution - - d~0, dgo Bulk density
~ 0.4 0.02-0.06 ~ 0.009 0.012 ram, 0.07 m m ~ 1.6 g/cm 3
at the site, especially for the less mobile compounds. Thus it could be expected that the concentration ratio for benzene could be even lower. Permanent gas concentrations indicated substantial biological activity in the soil. CO2 concentrations were elevated throughout the sampling area (5-9% by volume), but were not substantially higher in the contaminated zone. Marrin (1989) and others have found CO2 concentrations in the unsaturated zone to be a good indirect indicator of hydrocarbon contamination. Due to the generally high levels of CO2, this does not appear to be the case for the high-water-content soil at this site. Oxygen concentrations were substantially depleted in the contaminated zone (e.g., < 1% by volume), this is probably due to biological activity. C H 4 concentrations ranged from nearatmospheric values (--~ 1 ppm) to ,-~ 5900 ppm near the tanks. As seen in Fig. 5, the distribution of C H 4 w a s quite similar to the hydrocarbon vapors, thus it appears to represent the best biological indicator of the subsurface contamination.
Soil properties The backfill material, as well as the undisturbed native soil, is a fine silty loam. Grain sizes range from clay-sized particles to small pebbles, with dj0 = 0.012mm and dg0 = 0.07mm. Other physical properties of the soil are listed in Table 2. Total porosity of the soil is estimated to be ,~ 0.40. The percent water saturation at ,-~ 1 m depth was measured to be in the range 85-95%. The soil fraction organic carbon content (foe) is ~ 0.009. These data have been used to estimate the retardation factor for the compounds of interest using eq. 4-6 (Table 1). Based on these values, benzene would be expected to have a retardation factor R ,-~ 190 and R for butane would be ~ 5. Again, this should result in substantially different distributions for the contaminants.
Effective diffusion coefficient Laboratory experiments indicate that De for CH4 in the native soil is approximately 0.06 mm2/s. This value is much smaller than would be the case
G A S O L I N E V A P O R T R A N S P O R T T H R O U G H A H I G H - W A T E R - C O N T E N T SOIL
329
in drier soils, however it is significantly larger than the expected aqueousphase diffusion coefficient in the soil, which is approximately 0.002 mmR/s. Therefore, it appears likely that continuous air-filled pores exist and that diffusion occurs primarily within those pore spaces in the soil. The physical appearance of the soil suggests that many of these continuous pores could be macropores which are the result of soil structure. Furthermore, our ability to collect vapor samples from this soil suggests that there were at least some continuous airways. (Substantially-reduced oxygen concentrations in many of the vapor samples confirm that direct leaks to the atmosphere were minimal.) The soil tortuosity estimated from these experiments using eq. 2 is z = 0.0025. The value estimated from eq. 3 is 0.0034. Both of these values are quite sensitive to air-filled porosity, thus the uncertainties are relatively large. For the purposes of this study, the agreement between experimental and calculated values is considered good. The experimental value, r -- 0.0025, was used in the numerical modeling of gasoline-vapor transport in the soil. E n v i r o n m e n tal p a r a m e t e r s
Compared to many leaking underground storage tank (UST) sites, vapor concentrations at this site were relatively stable over a 1-year period. This observation suggests that environmental factors (other than soil properties) played a secondary role in vapor behavior. The frequency of change of many of the parameters was rapid by comparison to vapor concentration changes. For example, barometric pressure fluctuated on a daily or weekly basis and its effect on concentration distributions was observed to be small in comparison to other factors. Similarly, there did not appear to be any significant external pressure gradients at the site. The water level fluctuated ___1 m intermittently and appeared to be controlled by some unknown local source(s). These fluctuations are not thought to have contributed significantly to the lateral spreading of the contaminants. One parameter which probably has an influence on vapor concentrations was subsurface soil temperature. Figure 7 shows daily soil temperatures at 1 m depth for the study period. As expected, soil temperature dropped by ~ 10°C in the winter and some effect of this may be visible in the data from LTM5 (Fig. 6). Trends in the data from LTM 1 and other sampling points at the periphery of the plume suggest that the slow spreading of the vapors was not strongly dependent on temperature changes at the site. The similar distributions of gasoline components whose physical properties vary so substantially suggest that the vapors probably resulted from an areally-extensive source. This source is likely to have resulted from a long history of small releases in the vicinity of the tanks and p u m p island. It may
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TIME (YEARS) Fig. 7. Subsurface soil temperature at 1 m depth in Portland, OR, for the period Sept. 1988-July 1989. Fig. 8. (a) Numerical model results showing vapor concentrations as a function of time at 1, 2, 4 and 7 meters from a source when the retardation factor is R = I. (b) Numerical model results showing vapor concentrations as a function of time at 1 and 2 meters from a source when R = 10.
also have been facilitated by washing of spilled gasoline into the soil through cracks in the asphalt tarmac.
Numerical diffusion modeling Because little is known of the temporal and/or spatial distribution of the source of vapors, it is difficult to reproduce conditions at the site with a numerical model. Nevertheless, it is useful to model vapor diffusion in the high-water-content soil to examine the general behavior of the gasoline components in the soil. Both the source of the vapors and the sampling points are located above the UST's; thus, vapor transport is assumed to be radially symmetrical. Diffusion is considered to be the dominant transport mechanism. A two-dimensional, axis-symmetric finite-element model developed by Mendoza and Frind (1990a, b) was used for these simulations. In the model, effective diffusion coefficients are estimated using eqs. 2 and 3. Partitioning to the water and soil is assumed to be instantaneous, following eq. 4. For these simulations, the upper and lower boundary conditions were set to be impermeable, with 3 m of"unsaturated" zone between them. Because there was no history of large releases, a small source (1 m high and 1 m in diameter) was used for these simulations. The tanks had been in place 10-15 yr, thus the simulations were conducted for a period of 10 yr. Figure 8a shows how simulated vapor concentrations vary with time for a non-soluble, non-sorbing compound at 1, 2, 4 and 7 m from the edge of the
G A S O L I N E V A P O R T R A N S P O R T T H R O U G H A H I G H - W A T E R - C O N T E N T SOIL
33 !
source. Again, these are not intended to replicate the distribution pattern of the contaminants at the site. Rather, they indicate the rate at which vapors would be expected to spread by diffusion. Several features of the plots are significant. First, long times (i.e., years) are required before significant concentrations of vapor reach 7 m from the source. Second, the increase in concentration at each distance is very gradual, requiring many years to approach source levels. Figure 8b shows the breakthrough at 1 and 2 m of a hypothetical compound with R = 10. Vapor transport in this case is extremely slow and almost no mass reaches the 4 and 7 m distances during the 10-yr simulation. The estimated retardation value for benzene was 190. Clearly, under conditions present at the site, benzene would be expected to move very slowly. The slow transport rates observed in these simulations are somewhat accentuated because a relatively small source size was used. The contrast between the slow, compound-dependent radial spreading in the simulations and the broad, compound-independent plumes at the site again suggest that the source of the vapors at the site must have been areally large. CONCLUSIONS
Subsurface gasoline vapor concentrations were monitored for a period of 1 yr. During that time, concentrations at sampling points near the "center" of the vapor plume remained relatively stable, while those at greater distances showed long-term increases. The principal reasons for the relatively stable concentrations were that diffusive transport of the vapors was very slow and diffusive fluxes were small. This was primarily due to the high water content of the soil, which was approximatley 90% of saturation. The long-term increases observed at some sampling locations are probably due to diffusive spreading of the gasoline vapors in the soil. However, it was not possible to eliminate all other potential mechanisms. The effective diffusion coefficient of CH4 in the soil was measured experimentally. Using eq. 2, the tortuosity of the soil was estimated to be 0.0025. This compares well with • = 0.0034 calculated from eq. 3. Other environmental parameters (e.g., water level fluctuations, temperature, rainfall) played only a secondary role in controlling vapor transport. Due to the high water content of the soil, and the range of Henry's constants for the gasoline compounds, it was expected that the distributions of chemicals would be substantially different from one another. This was not observed. Diffusion model results indicate that diffusion distances of less than a few meters per year are to be expected, even for the most mobile compounds. Because equilibrium partitioning is implicit in the mathematical model used here, compounds with appreciable water solubility were strongly retarded. In
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this context, the stable concentration observed at LTM5 are not surprising, considering the slow diffusion. The rapid increases in concentration observed at LTM1 are not explained by the model. The increases could be due to continuing small releases at the site. In general terms, however, the model predicts an even slower spread of the vapors than was observed at the site. Biodegradation was active at the site, although it appears that oxygen may be limiting in the vicinity of the source. Common biological indicators (CO2, C H 4 and 02) were all affected by the spill. CO 2 could not be used to map the plume because it was generally high throughout the subsurface. C H 4 resulting from anaerobic degradation was the best biological indicator for mapping the contaminated zone. A number of important implications regarding vapor-phase leak detection systems can be drawn from these results. First, vapor-phase leak detection will not be effective at sites with high-water-content soils because vapor diffusion is very slow. Given a typical distance of 6-7 m from a leak to an observation well, years would be required for vapors to reach the detection point. This is in dramatic contrast to engineered backfills (e.g., pea gravel) where detection can occur in a matter of hours. Second, the slow diffusion means that once vapors do get into these soils, they will persist for a long period of time. Thus, at sites where past leaks or spills have occurred, future leak detection using vapor sampling may not be possible. Finally, the areal distribution of gasoline vapors within the soil may be strongly influenced by factors other than the location of the release (e.g., cracks in the tarmac, the presence of drain rock under an impermeable cap). Thus, mapping soil vapors at sites similar to the one studied here may not accurately assess the presence and location of gasoline leaks in the subsurface. DISCLAIMER
Although the research described in this article has been supported by the United States Environmental Protection Agency through Cooperative Agreement No. CR-81-4561-01-01, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. ACKNOWLEDGEMENTS
We would like to acknowledge Dr. Philip Durgin, formerly of the U.S. EPA, EMSL-Las Vegas, for his support of our research program. REFERENCES API, 1966. Technical Data Book - - Petroleum Refining. Am. Pet. Inst., New York, NY. Baehr, A.L., 1987. Selectrive transport of hydrocarbons in the sunsaturated zone due to aqueous and vapor phase pationing. Water Resour. Res., 23: 1926-1938.
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Chiou, C.T. and Shoup, T.D., 1985. Soil sorption of organic vapors and effects of humidity on sorptive mechanism and capacity. Environ. Sci. Technol., 19:1196-1200. Cussler, E.L., 1985. Diffusion: Mass Transfer in Fluid Systems. Cambridge University Press, 525 pp. Houston, S.L., Kreamer, D.K. and Marwig, R., 1989. A batch-type testing method for determination of adsorption of gaseous compounds on partially saturated soils. Geotech. Testing J., GTJODJ, 12: 3-10. Karickhoff, S.W., 1984. Organic pollutant sorption in aquatic systems. J. Hydraul. Eng., 110: 707-735. Karickhoff, S.W., Brown, D.S. and Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Res., 13: 241-248. Kenaga, E.E. and Goring, C.A.I., 1980. Am. Soc. Test. Mater., Washington, DC, ASTM Spec. Publ. No. 707. Kreamer, D.K., 1982. In situ measurement of gas diffusion characteristics in unsaturated porous media by means of tracer experiments. Diss., Univ. of Arizona, Tucson, AZ. Kreamer, D.K., Weeks, E.P. and Thompson, G.M., 1988. A field technique to measure the tortuosity and sorption-affected porosity for gaseous diffusion of materials in the unsaturated zone with experimental results from near Barnwell, South Carolina. Water Resour. Res., 24: 331-341. Mabey, W.R., Smith, J.H., Podoll, R.T., Johnson, H.L., Mill, T., Chou, T.-W., Gates, J., Waight partridge, I., Jaber, H. and Vandenberg, D., 1982. Aquatic fate process data for organic priority pollutants. U.S. Environ. Prot. Agency, EPA 440/4-81-014, 434 pp. Mackay, D. and Wolkoff, A.W., 1973. Rate of evaporation of low-solubility contaminants from water bodies to atmosphere. Environ. Sci. Technol., 7:611-614. Marrin, D.L., 1989. Detection of non-volatile hydrocarbons using a modified approach to soil-gas surveying. In: Proceedings of the Petroleum Hydrocarbons and Organic Chemicals in Groundwater Conference, Houston, TX, Nov. 15-17, 1989. Marshall, T.J., 1959. The diffusion of gases through porous media. J. Soil Sci., 10: 79-82. Mendoza, C.A. and Frind, E.O., 1990a. Advectice-dispersive transport of heavy organic solvent vapours in the unsaturaetd zone, 1. Model development. Water Resour. Res., 26: 379-387. Mendoza, C.A. and Frind, E.O., 1990b. Advective-dispersive transport of heavy organic solvent vapours in the unsaturated zone: 2. Sensitivity analysis. Water Resour. Res., 26: 388-398. Mendoza, C.A. and McAlary, T.A., 1989. Modeling of groundwater contamination caused by organic solvent vapours. J. Ground Water, 28: 199-206. Millington, R.J., 1959. Gas diffusion in porous media. Science, 130: 100-102. Millington, R.J. and Quirk, J.P., 1961. Permeability of porous solids. Trans. Faraday Soc., 57: 1200-1207. Montgomery, J.H. and Welkom, L.M., 1990. Groundwater Chemicals Desk Reference. Lewis Publishers, Chelsea, MI, 640 pp. Penman, H.L., 1940. Gas and vapor movements in the soil, I, The diffusion of vapours through porous solids. J. Agric. Sci., 30: 437-462. Peterson, M.S., Lion, L.W. and Shoemaker, C.A., 1988. Influence of vapor-phase sorption and diffusion on the fate of trichloroethylene in an unsaturated aquifer system. Environ. Sci. Technol., 22: 571-578. Powell, R.M., Bledsoe, B.E., Curtis, G.P., Johnson, R.L., 1989. Interlaboratory methods comparison for the total organic carbon analysis of aquifer materials. Environ. Sci. Technol., 23: 1246-1249.
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Reible, D.D. and Shair, F.H., 1982. A technique for the measurement of gaseous diffusion in porous media. J. Soil Sci., 33: 165-174. Weeks, E.P., Earp, D.E. and Thompson, G.M., 1982. Use of atmospheric fluorocarbons F-11 and F-12 to determine the diffusion parameters of the unsaturated zone in the southern high plains of Texas. Water Resour. Res., 18: 1365-1378.