Chapter 4: Results and Discussion
4.5 Field Sample Analyses
4.5.4 Depth Profile of Gasoline Vapor
Figures 25 a,b,c,d,e show the target compound
concentrations as a function of depth from the surface of the ground surface. There is considerable variability in
concentration among the four sampling locations at each of the two lower depths. Nonetheless, taking the average concentration at each depth shows that concentration decreases towards the surface as would be expected if
(a) if
^§
3 (b)I
1 3 1 0.9 H 0.8 0.7 0.6 ^ Q.5 0.4 - 0.3 - 0.2 0.1 - 35 30 25 20 - 15 - 10Toluene/TMP Depth Effects
a RD-2 O RD- 3 DRD-l nRD- 2 CJRD-4 DRD- 3 nRD-1 DRD- 4 1^ a 10 Depth— feet
Octane Depth Effects
a RD-2 nRD-4 "T" 8 —I— 10 nRD-3 RD- 2 aRD-4 URD-l 12 DRD-1 RD-3 dRD-2 'RD-4 a RD-2 D RD-4 PRO-3 DRD-X 12 Depth— feet
Figure 2 5. Vapor Concentration Measurements of Target
(c) 0 0> (d) 3 70 60 - SO - 40 - 30 - 20 - 10 -
Ethylbenzene Depth Effects
JU D RD-4' 28 - ORD- 2 DRD-2 26 - 24. -
^^^..-'-^'^^n RD-3
22 - ^^^ -"^ 20 - ͤRD- a.^--"^^ DRD-l 18 - ^^^-^-^ 16 -^^^-^-""^
14 - DRD-4 D RD- 1 12 " DRD-ͣ2,,.'-^''''^ 10 - DRD-3: a - DRD-4 6 - ORD-1. 4 - 2 - 1 I 1 1 1 1 10 Depth— feetnn.p—Xylene Depth Effects
D RD- 2 DRD-2 ^RD-1 D RD-4 RD- 3 RD-4 DRD-1 G \ 10 12 DRD-2 a RD-4 nRD-3 ͤ RD- 12 Depth— feet
Figure 25. Vapor Concentration Measurements of Target
(e) a 3 20 19 18 17 - 18 - 15 - 14. - 13 - 12 - 11 - 10 - 9 8 - , 7 - e 5 ^ 4 3 2 1 - 0 6
o —Xylene Depth Effects
D RD-2 ͤ RD-3 RD-1 dRD-2 RD-4 n RD-3 D RD- 2 RD-3 Q RD-4 D RD- 1 D RD-4 D RD-1 8 —1— 10 12 Dapth— faat
Figure 25. Vapor Concentration Measurements of Target
verticle diffusion from the NAPL was important.
Biodegradation near the ground surface may contribute to the concentration gradient. Evans and Thompson recently reported a similar observation and indicate that petroleum
hydrocarbons are particularly susceptible to biodegradation in the shallow regions of the soil profile (12). Aerobic biodegradation is more likely near the ground surface because there is more oxygen available to the petroleum degrading microorganisms. The authors point out the importance of depth profiling as a means of determining adequate sampling depths in order to insure significant hydrocarbon
concentration while minimizing probe emplacement time.
Subsurface hydrocarbon vapors are most effectively detected
when samples are collected as close to the water table as
^possible if the vapors originate from the water.
Concentration measurements from samples used to
construct the vapor contours were taken at depths of 7-10
feet. Considering the concentration gradient revealed in the aforementioned study, these concentration measurements can be expected to have an inherent window of variance of less than
100%.
4.6 Contaminant Distribution in the Three Phase System
4.6.1 Soil Mass/Vapor-Phase Distribution
the total mass of target compound present per volume of soil in the unsaturated zone. If the system is equilibrated, the ratio of soil vapor-phase concentration to soil-mass
concentration is described by a partition coefficient between
these two phases. At low concentrations, this ratio may be a
constant, the specific value of which depends on chemical properties of the compound. An analysis of the fraction of each target compound found in the vapor phase is given in Table 6. These fractions were calculated from vapor-phase and soil mass (solid, vapor and aqueous phases combined) taken at locations MW3, MW7, and MW8 in Figure 11. The mass of any target compound in the vapor-phase is:
M^ = C„ e„ V„'V V T
3
where C^ is the concentration (ug/M ) m the vapor-phase; e ,
the fraction of voids filled with vapor-phase (0.24 has been estimated based on field measuremnts); and V^ is the total unit volume of the soil mass (1 M in Table 6). The total mass, Mrn, in all three phases is calculated from measurement
of mass in the soil sample:
M^ = C^ Pg (1 - e)
where C is is the concentration (ug/kg) of target compound per dry weight of soil; p , the density of dry soil (2.65
3
per m-
in Vapor Vapor-Phage
Cv (ugZ(?!ii )
Soil Mass Vapor-Phase Soil Mass -Phase
Target Well C^ (ug/kg) M (L^) M™ (uq) Conpound No. ^x 10-^ X 10- X 10-^ F Toluene/TMP 3 1960 203 233 470 349 1.35 7 1206 151 168 289 402 259 1.17 1. 12 8 143 221 290 1.00 168 34 > 381 289 0.09 0.12 n-Octane 3 98 72 62 24 123 0.19 7 90 64 74 22 108 109 0.22 0.20 8 13 63 83 3 t 127 109 144 0.17 0.03 0.02 Ethylbenzene 3 117 284 291 28 488 0.06 7 59 ͣ 388 247 14 501 669 0.06 0.02 8 3 259 278 0.7 \ 426 447 479 0.03 0.001 0.001 »fP-Xylene 3 231 35 19 55 \ 60 0.92 7 ͣ 73 18 18 18 33 32 1.66 0.55 8 5 17 ND 1.3 31 30 ND 0.56 0.04 -.. ͣͣͣͣ' --...---- --.---_---..._ 00
measurements). The fraction of target compound, F, found in the vapor-phase is:
F = M^M^
When all of the target compound is in the vapor-phase, the
maximum possible value of F is unity. However, Table 6 shows
that four values larger than unity were obtained. These
anomalous values occurred because F is calculated from
independent measurements - one from the vapor-phase directly, and the other of the soil sample. The value of F for any given target compound decreases roughly an order of magnitude from MW3 to MW8, which also is in increasing distance from the source of contamination. Toluene/TMP and m,p-xylene show the greatest tendency to be in the vapor-phase i.e. they have
the largest F values.
Theoretically, a linear equilibrium relationship
exists between M^ and M_ regardless of concentration.
However, Table 6 shows values of F which vary markedly. This immediately suggests that the system being tested was not at equilibrium. A non-equilibrium condition could exist due to aerobic biodegradation or to the kinetics of sorption/
desorption. Biodegradation has been shown to significantly reduce the level of hydrocarbon contaminants sorbed on the soil particles in unsaturated systems. As an example, Kuhlmeier and Sunderland (27) recently showed that non- nutrient enhanced, natural microbial flora decreased the levels of toluene, ethylbenzene, and m,p-xylene from 214,
11.7, 92.4 ppb contamination levels to below detection limits
(Ippb) for a respirometer vessel study of naturally
contaminated soil. The kinetics of sorption may also limit
the sorptive uptake of contaminant vapors from the soil atmosphere to soil particles and associated water. Even
considering these factors, the concentration in the soil-
sorbed phase should not be completely independent of the vapor-phase concentrations. Thus, considering the limited amount of this data, a strong conclusion is difficult to
make.
4.6.2 Ground Water/ Vapor-Phase Distribution
Figure 26 shows the relationship observed between the
subsurface vapor-phase and ground water concentrations for selected compounds at three monitoring well locations
(locations MW3, MW7, MW8 on Figure 11) likely located within the NAPL region. A logarithmic plot was necessary to
accommodate the full range of data. If the expected results
were a constant ratio of vapor-phase to ground water phase concentration, at equilibrium, the relationship is described
by Henry's Law:
C =Kh C .
V w
Logarithmic transformation to facilitate data presentation
gives:
log C = log Kh + log C
ͤ 4 ^O «>. Q. 4) 2 5 « c « 3 _ V 4) C N V «> X •> I >x t o 00 s o DO 0 o 00 00
|i 11 I I I—I—I---|ii I I I 1—I—I---|ii I I I I—I—I---["111 lO I—r
O o o ;:f
(jodoA ijos |/6n) uoipjjusouoQ ssDqd jodo/y
0) o M (Q •H "^ Vi 0 M M W Q) C C Q) C-H K 0) h) Q) >i ^ͣ0^ T ͣP-H •O 4, a) rH TJ Oo CO 0 0) !X 5 0) u 3 "0 -H :) M 0) >-• 0 4) M a-P P4 C 6 (0 o 0 s « v* o c cs O 0 ͣ P T3-H 0) C -P o 0>3 3 c M O A o (0 ^l-H o &H a vi +J 1-1 TJ W -4-' 0 C-H o lOQ ? c 0 0) a O"? •H M 3 T— 3 4J RJ-H o 3£ M I- i3 a< A o •H 1 -H M h rH +J O-H ffl 0,3 •H (0 0< Q > H ,_ • ca <u u g -H c^
For Henry's Law to hold, the slope of the plots shown in
Figure 26 should all equal one; theoretical partitioning lines with slopes of one and Kh values from the literature are
2
included for comparison. Table 7 lists the slopes and R values calculated from the data used in constructing Figure
26, in addition to literature vaues for Henry's constants.
Table 7. Comparison of Vapor / Water Partitioning Data to Henry's Law
2
Slope R Value Henry's (50) Constant (unitless)
Toluene/THP 0.54 0.89 0.28 n-Octane (not detected in water) 136
Ethylbenzene 0.23 0.81 0.37 m,p-Xylene 0.86 0.84 0.22 o-Xylene 0.79 0.71 0.22
* Henry's Constant for Toluene/TMP was calculated for toluene. Trimethyl Pentanes, like other alkanes would have a larger value (e.g. n-octane)
As shown by Figure 26 most of the data for ground
water/soil vapor-phase partitioning were fairly close to that predicted by Henry's Law, although the logarithmic scales may distort the extent of agreement in some instances. The most
notable discrepancies from equilibrium predictions were for
ethylbenzene (at MW8), m,p-xylene (at MW8) and o-xylene (at
MW7). As shown in Figure 23, MW7 and MW3 were located on the
edge of the predicted region of NAPL contamination, while MW8
is well outside of the NAPL region. Thus, contaminant vapors
at MW3 and MW7 vapors may have originated from the ground water and transverse diffusion of vapors from the NAPLregion, while at MW8 likely originated from contaminated
ground water only.
Much of the non-compliance of the data in Figure 2 6 with Henry's Law may be due to the violation of the
assumption of dilute solutions (i.e. the concentrated mass of
contaminant existing in the NAPL layer) and the distance that the vapor samples were taken from the water table(approximately 10 feet). Contrary to the results reported
here, Evans and Thompson reported vapor-phase concentrations
1-2 orders of magnitude less than ground water concentrations
as predicted by Henry's Law (12) in a similar field study
comparison.
4.7 Cost and Efficiency Comparison of Vapor-Phase vs. Ground
Water Assessment
Table 8 lists the estimated costs and efficiency of
using this vapor-phase sampling method versus a typical ground
water monitoring application. These figures show that soil
vapor-phase sampling is much less expensive and time
consuming means of detecting and delineating the distribution
of subsurface contamination than obtaining similarinformation from ground water samples.
Table 8. Estimated Cost and Efficiency Comparison: Vapor-Phase Sampling vs. Monitoring Wells Monitoring Wells
23-ft X 2" Stainless Steel Casings (8) . $1120
Well Screens ... 600
Well Installation (8 wells) ... 8000
Teflon Water Bailer ... 100
Riser and Iron Caps ... 200
Grout, Bentonite, and Concrete ... 240
Development Costs ... 160
Total ... 10520
Wells per Day ... 2
Vapor-Phase Sampling 12-ft Stainless Steel Tubing ... $ 75
Inner Rod and Welding Supplies . 25 Stainless Steel Fittings ... 30
3/8" Stainless Tubing ... 10
Teflon-diaphram Air Pump... 275
Flow Meter ... 50
Construction Costs ... 120
Carbon Traps per 50 ... 50
Total ... 635
The carbon sorption technique can serve as a rapid, inexpensive means of detecting and delineating the
distribution of subsurface gasoline contamination. With a two-person sampling crew, the amount of data that can be collected per day of vapor-phase sampling would take several days with conventional drilling and sampling techniques.
Vapor-phase sampling is even more cost effective as the depth to ground water increases because the costs of conventional drilling methods increase while the cost of vapor-phase sampling remains relatively constant. Remediation efforts would benefit by using the technique as an aid for effective
location of monitoring and recovery wells.
The addition of an activated carbon trap to the
conventional ground probe apparatus provides the following
advantages over conventional vapor sampling methodologies:
(1) a reduction in necessary field equipment, (2) the convenience of in-lab analyses, (3) the opportunity for
replicate analyses of samples, and (4) the possibility for
increased sensitivity. The importance of increasedsensitivity may not be as apparent in the case of a gross
contamination, however, its utility can be realized in the
case of a small leakage over a period of time or when used todefine plume boundaries. In addition to the carbon trap advantages, fitting the ground probe with a removeable inner rod assures unrestricted flow of vapors from the subsurface, which is not always guaranteed with other probe designs.
While the methodology is a very practical and useful tool for detecting and delineating subsurface contaminant
vapors, two disadvantages are worthy of mention. First, is
the possibility of puncturing underground utilities if their
locations are not known presents a safety hazard to the field
crew; it is essential to have records of these utilities
prior to sampling. Second, this vapor sampling procedure
requires return of the field samples to the laboratory for
analyses. Therefore, it does not allow for selection oralteration of sampling locations as would be possible with a
real time measurement. The disadvantage could be overcome
with a portable gas chromatograph and a mobile laboratory in
which to perform extractions and analyses. However, the
simplicity of the field procedure as it is now designed is
well suited to use by field crews lacking laboratory oranalytical skills such as would be found in small, regional
offices of state agencies.
Soil vapor-phase measuremnets provided a convenient way
to map the extent of contamination from a leaking underground
storage tank at the Camp Lejeune site. Concentrations of
target compounds ranged from 10,00 ug/1 to less than 10 ug/1
of soil vapor. The highest concentrations were found above
the NAPL as was expected. The method also showed that
contamination exists well beyond the NAPL, although the concentrations drop off precipitously. This type of information is especially valuable when deciding on the locations of monitoring and recovery wells.
This research also showed vapor concentrations of
toluene, as revealed by GC/MS analysis, that were much lower
than expected from compositional analyses of toluene in
gasoline. Therefore, monitoring only of aromatic hydrocarbon
mixtures such as the widely reported BTX (benzene, toluene,
xylenes) mixture may not truly represent the distribution and
amount of the bulk of gasoline contamination present.Indeed, using this aromatic hydrocarbon monitoring mixture to
describe the status of remedial progress may lead to theconclusion that remedial efforts are complete. Curtailing
clean-up efforts based on this information could result inaccumulations of straight and branched chain alkane vapors in
overlying structures that could be explosive.Attempts to relate vapor-phase concentrations to those
in the soil proved difficult. Soil concentration, which
includes the solid, vapor and aqueous phases, was roughly the
same regardless of whether the sample was taken within or
outside the NAPL region; however, vapor-phase contribution to
the total decreased outside the of the NAPL. Although, therewere exceptions, the vapor-phase/ground water data appeared
to support equilibrium partitioning predeicted by Henry's
Law, despite the fact that vapor-phase samples were obtained
10 feet or more above the ground water. Thus it may bepossible to use Henry's Law and vapor-phase concentrations to
estimate ground water concentrations.
Recommendations for further investigation include using
a larger carbon trap for monitoring hydrocarbons that are
less effectively sorbed onto activated carbon (e.g. butanes,
pentanes, hexanes). Because of the high volatility and lowwater solubility of these compounds, they would likely be
seen near the surface soon after the spread of contamination.
Although these compounds were not monitored in this study,
they were seen in much higher concentrations than those that
were monitored.
In addition, much more work is required in order to
explain why toluene was seen in such low concentrations
relative to the other compounds, which chromatographically
elute at similar retention times.Additional research into multi-depth sampling would be
beneficial for indicating optimal sampling depth. This would
minimize the time needed for placement of the probe while
assuring adequate sensitivity for delineating the extent of
REFERENCES
1. Alexander M. Introduction to Soil Microbiology. 2nd Edition pp.208-221. Chapter 14: Transformations of Hydrocarbons. John Wiley and Sons. 1977.
2. Bafour, W.D., and C.E. Schmidt. "Sampling Approaches for Measuring Emmission Rates from Hazardous Waste Disposal
Facilities." Presented at the 77th Annual Meeting of the Air Pollution Control Association. San Francisco. California.
June 24 - 29. 1984.
3. Bruell, C.J. and G.E Hoag. "The Diffusion of Gasoline- Range Hydrocarbon Vapors in Porous Media, Experimental
Methodologies" in Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater-
Prevention. Detection and Restoration. Houston. Texas
November 12-14. 1986. pp.420-443.
4. Chiou, C.T. and T.D. Shoup. "Soil Sorption of Organic Vapors and Effects of Humidity on Sorptive Mechanism and Capacity." Environmental Science and Technology. Vol.19 pp.1196 - 1200,
(1985) .
5. Choui, C.T. "Roles of Organic Matter, Minerals and
Moisture in Sorption of Nonionic Compounds and Pesticides by Soils." U.S.G.S. Unpublished Manuscript.
6. Colenutt, B.A. and D.N. Davies. "The Sampling and Gas Chromatographic Analysis of Organic Vapours in Landfill Sites." International Journal of Environmental Analytical Chemistry. Vol.7 pp.223 -229, (1980).
7. Colenutt, B.A. and S. Thorburn. International Journal of Environmental Analytical Chemistry 7 (1980) p.231.
8. Dietz, W.A. "Response Factors for Gas Chromatographic
Analyses" Journal of Gas Chromatography February, 1967.
9. Eklund, B.M. and C.E. Schmidt. "Review of Soil Gas Sampling Techniques." in Soil Gas Sampling Techniques of Chemicals for Exposure Assessment. Radian Corporation, Interim Report. EPA-EMSL 68-02-3513, Work Assignment 32,
1983.
10. Eklund, B.M., "Detection of Hydrocarbons in Groundwater by Analysis of Shallow Soil Gas Vapor" API Publication
No.4394 (May 1985).
11. Evans, O.D. Personal Correspondence, October 7, 1987. 12. Evans, O.D. and G.M. Thompson. "Field and Interpretation
Techniques for Delineating Subsurface Petroleum Hydrocarbon
Spills Using Soil Gas Analysis." in the Proceedings of the NWWA/API Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater- Prevention. Detection and
Restoration. Houston. Texas November 12-14. 1986. pp. 444-
455.
13. Everett, L.G. et al. "Constraints and Categories of Vadose Zone Monitoring Devices." Ground Water Monitoring Review Winter, 1984.
14. Environmental Protection Agency. Seminar Publication: Protection of Public Water Supplies from Ground-Water
Contamination EPA/625/4-85/016 pg. 115.
15. Grob, K.Jr. and S. Rennhard. Journal of High Resolution
Chromatography Communication Vol.3 pp. 627, 1980.
16. Grob K. Journal of Chromatography. Vol.84 255. 1983
17. Goring, C.A. and J.W. Hamaker. Organic Chemicals in the Soil Environment. Vol.1 Marcel Dekker, Inc., N.Y., 1972.
18. Hance, R.J. "Observations on the Relationships Between
the Adsorption of Diuron and the Nature of the Adsorbent."