Soil-gas surveying twhniques A new way to detect volatile organic contaminants in the subsurface
Y
q u i d to confirm and monitor subsurface contamination; however, quicker and less expensive techniques are. useful for prehinary site evaluations. Soil-gas surveying is.a technique that is applicable to a wide range of volatile organic compounds (VOCs) under a variety of geologic and hydrologic settings. The most common uses of soil-gas Delineation and remediation of subsurface contamhation have become a ma- data include planning monitoring well jor focus of environmental science dur- networks and defining plume boundaing the past five years. Conventional ries for remedial action. Preliinary technologies available for subsurface screening techniques are.effectivein seinvestigations (e.g., monitoring wells lecting locations for.detailed sampling and soil borings) always will be re- and analysis. Site investigators can use
DonnL. Marrin La Jolln, 92037 Henry B. Kerfoot LocwIeed Engineering and Management Senices COmpMy Las Egm, Nev. 89119
74a Emimn. Sci. TechnOl.. Vol. 22. No. 7,lSSn
results from a preliinary soil-gas survey to drill monitoring wells at locations withii the boundaries of a VOC plume. Soil-gas investigations also can be used to identify sources of VOCs and to distinguish between soil and groundwater contamination (1). Chemical analysis of soil gases has recently been used to monitor solvent and fuel leaks from underground storage tanks. In order to effectively design soil-gas surveys and interpret their results, the subsurface transport and fate of VOCs must be understd. These phenomena can have a pofound impact on the presence and concentrations of VOCs
0013936x1~22-0740501.5010 0 1988 American Chemical saciely
in the soil atmosphere. Physical, chemic&, and microbiological processes can be important in determining VOC concentrations in soil gas. Physical pmeesses Organic compounds can undergo a variety of equilibrium and transport in the subsurface (2-4). The most lmportant physical process affecting soil-gas surveys is solution/vapor equilibrium. Because of their relatively low solubilities and high vapor pressures, dissolved VOCs have a marked tendency to partition into the soil atmosphere. The physical law that quantitatively describes the solutiodvapor equilibrium of VOCs is Henry's law. Henry's law constants are calculated by dividing the vapor pressure of a pure compound by its water solubility. This constant, when multiplied by the dissolved concentration in a solution, PIC+ vides an estimate of the VOC concentration in the adjacent gas phase. Organic compounds with the highest Henry's law constants will partition most favorably from groundwater into the soil gases. The constants provide only a relative indication of solution/ vapor partitioning in the field, for VOC equilibrium acmss the capillary fringe is affected by solute and matrix properties. Table 1 shows the most frequently encountered substances at Superfund sites and their Henry's law constants; most of these are VOCs amenable to soil-gas surveying. The aromatic and aliphatic components of gasoline have been identified as good candidate compounds for soil-gas analysis (5, 6). Several theories have been proposed to describe the transport of volatile compounds from groundwater into the soil gas. Swallow and Gschwend (7) have formulated and laboratory-tested a steady-state model that predicts vertimimation of vocs in the saturated zone Lsed on transverse hydrodynamic dispersion. They hypothesized that VOCs are transported though the capiUary fringe by the combined'processes of disoersionand molecular diffusion. Othe; workers have considered the effectsof episodic factors on vertical VOC transport. For example, Lap pala and Thompson (8have &est& that water table fluctuations may provide an important mechanism for transporting VOCs from groundwater into soil gas. Once VOCs enter the soil gas, they ditfuse in response to a chemical concentration gradient. Although advective processes or vapor density effects may locally influence VOC migration, gaseous diffusion is the predominant transport mechanism. According to steadystate models, vertical contaminant flux is proportional to the air-iilledporosity
Trichloroethylene" Lead and comwunds Toluene" Benzenee
Polychlorinated biphenyls (PCeS) Chloroforma Tetrachlomethylened Phenol Arsenic and arsenic compoun Cadmium and cadmium compounds Chromium and chromium compounds
1.I ,l-Trichloroethaned
Zinc and zinc compouni Ethvlbenzene" Xylenesd
... NA
NA 30 NA 59
Methylene chloride" trans-l,2-Dichloroethylene.NA = not applicable.
b0as-phase concentration(ppbv) *Gas concentratmn IpgR) i liqui "Amenable to 8011.aas samnlino.
hase concentration(
of the vadose zone, the VOC diffusion coefficient, and the gas-phase concentration gradient. Vertical transport by diffusion predicts a linear increase in VOC concentration with depth, which has been confirmed in one field study (9). Subsurface geologic heterogeneities, soil porosity, moisture conditions, VOC concentrations in groundwater, and sorption equilibria can significantly affect VOC gradients in the vadose zone (2, 10-12). VOC concentrations in soil gas above any diffusion-limiting layer will be lower than concentrations below such a laver. Firmres 1B and 1C show how subshface &d surface diffision barriers can affect voc concentrations. Such barriers include saturated clay layers, perched water, and pavement.
Bio~'ehemical P m S Degradation of VOCs in the subsurface can have a negative effect on the remote detection caaabilities of soil-gas surveying. Oxidation can convert VOCs into nonvolatile or water-soluble compounds that art not amenable to soil-gas analysis. Hydrocarbon and h a l o c h n compounds have different susceptibilitiesto biological and chemical degradation. Hydrocarbons are readily oxidized under the aerobic conditions that are prevalent in the upper vadose zone. Aerobic microbiol 'cal oxidation in the subsurface has%n measured in environments where the rate-limiting process was oxygen h'ansport from the atmosphere (13).
Halocarbons are generally more resistant to aerobic degradation than hydrocarbons but can undergo anaerobic biodegradation. Compounds with minimal halogen substitution, such as cNodxnzene, can undergo aerohic as well as anaemhic degradation (14). Selective biodegradation of VOCs is probably related to envimnmentd factors such as pH, redox potential (pe), and the composition of microflora in any particular soil. Figure 1D illustrates the possible effect of subsurface biodegradation on VOC concentrations in soil gas. I
h ~ l i n and e Sampling techniques used in soil-gas surveying fall into two categories: grab samDhe and Dassive -line. Static gra6 & p h i gives an &&tanms oicture of the soil ahnosnhere at a whcular subsurface location, whekas passive sampling provides an integrated measure of VOC concentrations over time. Grab sampling. Static grab sampling is a technique whereby the samples are collected from a quiescent soilgas sample. Dynamic grab sampling, on the other hand, involves samples being collected from a moving stream of soil gas that is pumped through a hollow probe. The strategy behind grab sampling is to collect soil gas as quickly as possible from a specific depth. Grab samples typically are analyzed on-site in order to permit the use of real-time data in selecting sampling locations and Envimn. Sci.Technol.. Vol. 22,NO.7, I N 741
voc
concentration
(A) Homogeneous porous material with sufficienl air-filled poroslty (B) Impermeable Subsurlace layer (e.g., clay or perched water) (C) Impermeable surface layer (e.9.. pavement) (D) Zone 01 high microbiological activity (circles and wavy lines indicate dinerent compounds) (E) VOC source in the vadose zone
to m i n i the problems associated with handling and transporting gas samples. The advantage of grab-sampling techniques combined with on-site gas chromatography is that results are available in a matter of minutes. k r haps the greatest disadvantage associated with dynamic sampling is the perturbation of local VOC concentrations as a result of soil-gas pumping. In addition, the results obtained from any grab-sampling method are highly depthdependent. The selection of a p propnate sampling depths is based on site-specific factors (e.g., moisture conditions, air-filled porosity, VOC concentrations, and depth to groundwater) as well as compound-specific factors (e.g., solubility, volatility,,and degradability). On-site analysis of soil-gas samples can be performed either with mobile laboratories or with portable instruments. Mobile laboratories can provide detailed chemical data as well as convenience because packing and shipping of samples is not necessary. On the other hand, mobile laboratories require a large initial investment and the presence of an analytical chemist. Although portable instruments are less sensitive and versatile tban mobile labs, they are relatively small and inexpensive. Furthermore, they can be operated by a trained field technician. wssiie sampling. Passive sampling u t i l i a charcoal sorbent to trap con742 Emiron. Sci. Techno1 , MI. 22. No 7.lSSS
taminants that diffuse tbmugh the soil gases. Passive charcoal samplers are buried in the shallow soil for as long as one month and then retrieved for analysis. Once the VOCs are sorbed onto the charcoal, samples are transported to a laboratory where desorption and chemical analyses are performed. Both solvent desorption (15)and pyrolysis techniques (16) have been used to desorb soil-gas VOCs prior to analysis. The advantage of passive soil-gas sampling is that field operations require minimal training, and the technique averages out concentration fluctuations caused by changes in environmental conditions. The major disadvantage of passive samplers is that results are not available for days to weeks because desorption and laboratory analysis are both time consuming. Additionally, passive sampling may not be appropriate for VOCs that have boiling points below approximately 5 'C or for compounds that are prone to thermal decomposition during pyrolysis (I7). case studies A shallow soil-gas investigation of a solvent plume was conducted in a suburban area where the underlying groundwater (at a depth of approximately 30 m) was contaminated with several chlorinated solvents. The investigation was designed to delineate the areal extent of l,l,l-trichlomethane (TCA), which had been discovered at concentrations as high as 3000 pg/L in
groundwater wells located downgradient from the source. Prior to the soilgas survey, there were no data regarding plume characteristics or the downgradient extent of TCA contamination. The results of grab sampling at 1.5 m below ground surface and soilgas analysis by gas chromatography/ electron capture detection (GCIFCD) are shown in Figure 2. Figure 2 indicates that the most concentrated portion of the TCA plume (50 pg/L contour) lies just east of the source. The soil-gas plume is oriented along a definite north-south axis, indicating TCA migration in a predominantly southerly direction from the source. Boring logs indicate that the regional geology consists of sand and gravelly sand with isolated perched water zones that are underlain by clay. Coarse-grained soils (e.g., sands and gravels) are optimal for the diffusive movement of gaseous contaminants because they are well drained and have high air-filled porosities. Soil gas was sampled adjacent to 11 wells in the vicinity of the source to compare TCA concentrations in soil gas with those in groundwater (Figure 3). A correlation coefficient of 0.88 was calculated using seven of the 11 points (99%significance). The remain-' ing four points (w-5 through W-8)were excluded from the regression analysis because they were clustered in an area underlain by perched water. Perched water can act as a barrier to VOC diffision, creating low VOC concentrations in soil gas between the barrier and the ground surface (see Figure 1B). The soil-gadgroundwater concentration ratios of the four points in the area of perched water were low compared with the majority of points (Figure 3). Anomalously low soil-gaslgroundwater ratios usually indicate either the presence of a barrier to gaseous diffusion (e.g., perched water or clay lenses) or high levels of biodegradation in the vadose zone. Conversely, anomalously high soil-gadgroundwater ratios suggest that probes have been sampled near a spill or subsurface leak. The soil-gas survey provided resolution of plume characteristics that could not be achieved by sampling groundwater wells. The presence of several anomalous soil-gas concentrations did not preclude accurate mapping of subsurface contamination because of the numerous sampling points. Results were used to select optimal locations for monitoring wells and to confirm the association between TCA contamination at the source area and at wells l o cated as far as 2 km downgradient. A p proximately 130 soil-gas samples were collected and analyzed during seven days of field investigation. The entire
3 I
,
-
soil-gas survey was completed for a p proximately the same cost as that required to install and sample three monitoring wells. A second case history involves a soilgas survey that was performed to delineate subsurface contamination from a leaking underground gasoline storage tank. Service sation records indicated that approximately 70,000 L of unleaded gasoline leaked from the tank during a one-year period. Private wells and exploratory bore holes suggested the presence of subsurface gasoline contamination, which was migrating to the east. To evaluate the extent of subsurface migration, a soil-gas survey was performed and confirmatory bore holes were drilled. Groundwater was present at depths ranging from 8 to 30 m beneath the ground surface. Preliminary soil-gas samples were collected from depths of 0.67 m and 2.2 m below the ground surface to select an appropriate sampling depth. The 2.2411 depth was selected because hydrocarbons could not be detected in soil gas at the shallower depth; this phenomenon was attributed to aerobic biodegradation of the hydrocarbons in the shallow soil. Soil-gas sampling probes were installed on a grid pattern within a 400x700-m area and were analyzed on site with a portable gas chromatograph and photoionization detector. Figure 4 shows the soil-gas sampling locations. Figure 4a shows the areal extent of gasoline contamination as indicated by the analysis of soil core and groundwater samples. The results of soil-gas analyses for isooCtane and butane are shown in Figures 4b and 4c, respectively. Concentrations of butane and isooctane in soil-gas samples predicted the areal extent of the gasoline plume. These two compounds have been determined in laboratory studies to be more resistant to biodegradation than other components of gasoline (18). An anomalous low-concentration zone is located near the source of the soil-gas plumes (Figure 4b and 4c). Anomalously low soil-gas concentrations were observed for all hydrocarbon compounds in this zone, which is near a septic tank drain field. The drain field probably has higher biodegradation rates than other sites in the survey area because of its high bacterial concentrations. VOC concentrations in soil gas may be affected not only by increases in biological activity but also by high moisture levels and a significant organic phase in the soil underlying the drain field. Gasoline hydrocarbons partition out of the gaseous phase and into aqueous or organic phases of the soil in accordance with their physical properties. In addition, Environ. Sci. Technol.. Vol. 22, No. 7, iQs8 743
the effective diffusion of volatile hydrocarbons in soil gas decreases as the percentage of water-filled pores increases.
rlGURE 4
%~ubSurface contamination from a lealdng gasoline tank
summing up Sod-gas surveys serve as an effective reconnaissance tool because numerous
data points can be sampled over a short time period for a relatively low cost. Both grabsampling and passive-sampling techniques have been used to evaluate the magnitude and lateral extent of VOC contamination and to locate sources of subsurface contamination. Soil-gas surveying has an advantage over geophysical techniques for detecting shallow contamination because identifiable compounds rather than indirect physical changes in the subsurface are measured. Moreover. soil-@ techniques are sensitive both to dissolved VOCs and to pure product layers. Soil-gas surveying is applicable to the most volatile organic compounds, which pose a major threat to groundwater supplies. VOCs are present at a majority of hazardous-waste sites and are generally more mobile than the nonvolatile organic compounds (e.g., PCBs, polycyclic aromatic hydrocarbons, and pesticides). Theoretically, soil-gas concentrations of compounds produced by in situ degradation of less volatile contaminants can be used to detect subsurface organic contamination. In fact, use of such techniques for detection of groundwater contamination and fuel tank leakage have been recently reported (19, 20). Soil-gas techniques will never replace the monitoring functions served by groundwater wells and soil brings, and, in fact, were developed to enhance the effectiveness of these conventional methodologies. Similar to other remote detection techniques, soil-gas surveying is effective only for specific types of subsurface contamination and must be interpreted with careful regard to the physical chemistry of contaminantsand the hydrogeologic setting. Broad generalizations concerning flux rates, degradation potentials, or soil-gas/groundwater correlations should be avoided in favor of case-by-case interpretations. Although the theory behind sampling VOC concentrations in soil-gas is straightforward, the data interpretation associated with these surveys is o h complex.
References ( I ) Marrin, D. L. Gmund Hbter Monit. Rev. 1988. &2). 51-54. (2) Baucr, A. L. h r r Resour Res. 1987, 23(10). 1926-38. (3) MacKay, D.M.;Roben, 0. V ; Cherry, 1. A. Envimn. Sci. Tpchnol. 1985, 19. 38492. 744
Environ. Sci. Technol.. W. 22,NO.7. 1988
Sour1
:a) Product plume according to confirmatory borehole results
n
(b) ISwctane concentrations(ppb by volume) in soil gas at 2.2 meters
-
Legend Contour line Sampling point
Scale
0
50 100 (meters)
amination from well and exploratorv borehole data wwaminated groundwater prediction , Q Enclosed lowconcentration zone
.. .
W. A.; Spencer, W. F.; Parmer, (:!$?E "VITO". ' pml. 1983,12,558-64. ( 5 ) Thompson, G. M . ; Marrin, D. L. Ground M e r Monit. Rev. lWn, 7(3), 8893. (6) Spittler, T M.; Pitch, L.; Clifford, S. In Proceedings of the Annual .Qmposiwn o n . Characterizarion and Monitoring of rhe Vadore Zone; National Water Well Association: Warthington, Ohio, 1985;pp. 295-305. (7) Swallow, I. A,; Gschwend, P. M. In Proceedings orthe 3ni NatioMI Symposium on Aquifer Resromtion and Groundwater Modfaring; National Water Well Association: Wonhington, Ohio, 1983;pp. 327-33. (8) Lappala, E. 0.; Thorn son, 0. M. In Proceedings of rhe Annu! Symposium on
Characterization and Monitoring of rhe Vadose Zone: National Water akll Association: . Wonhington, Ohio, 1983;pp. 659-79. (9) Kerfoot, H. B. Inf. 1. Environ. Anal. Chern. 1987.30, 167-81. (IO) Marrin, D. L.; Thompson, 0. M. Ground M e r 1987,25, 21-27. (11) weeks, E. E; Earp, D. E.; Thompson, 0. M. WrerRcsour:Res. 1982,8, 13-55-78, (12) Kcrfool, H. B.; Miah, M. 1. Chemomefrics and Inrelligenr Laboratory Systems 1988,3,73-78. (13) Wilson, I. T. ei al. Environ. Exicol. Chern. 1985.4.721-26. (14) Bower, E.1. In Pefrolewn Hydrocarbons and Organic Chem'cnls in Groundwarec National Water Well Association:
Worthington, Ohio. 1984, pp. 66-81. (15) Kerfwt. H. B.: Mayer, C. L. Ground Wter Monit. Rev. 1986.6. 74-78. (16) Voorhees. K. 1.; Hickey, J. C.; Klusman, R. W. And. Chem. 1904.56. 2604-7. (17) lamison. V. W.; Raymond. R. L.; Hudson. 1. 0. I n Proceedings o the Awd lnrer
s'
narionol Biodqrodoriun ymposrum. . ' . Shar-piey. J. M.;Kaplan. A. M..Us.: Applied
Science: London. 1975. ((8) Kerfwt. H. B.; Barrows, L. 1. Soil Gas
M~osuremenrfor Derection of Subsurface Organic Contominorion: US. Environmen-
tal Protection Agency: Las Vegas, Nev.. 1986; p. 2. (19) Kerfoot. H. B. et ai. Ground Water
Monir. Rev. 1988,8(2), 67-71. (20) Diem, D. S.;Ross. B. E.; Kerfwt. H. B. In Proceedings of rhe Second Notionof Ourdoor Action Conference on Aquifer Resrorar i m , Ground Warer Monitoring ond Geophysical Methods; National Water Well
Association: Dublin. Ohio, in press.
Acknowledgment Although the research described in this article has been funded wholly or in part by the US. Environmental Protection Agency through Contract 68-03-3249 to Lockheed Engineering and Management Services Company, Inc.. it does not necessarily reflect the views of the agency and no official endorsement should be inferred. This article has been reviewed for suitability as an ES&T feature by Thomas E. Spinler, EPA Region 1 Laboratory, Lexington, Mass. 02173; and by David K. Kreamer. Arizona State University, Tempe, Aril. 85287.
IChemistry helps Alvin and Jason Jr. explore R.M.S. Titanic. IAn easy way to make diamond films. IBringing atoms and molecules into view. IAncient water helps tell time. IThe chemical basis of memory. I New approaches to cancer therapy. ISolar-powered pesticides. IKey bacterial toxin discovered. IOrganic chemicals challenge silicon's reign as king of semiconductors. These are among the most interesting and promising areas of chemical research discussed in the new edition of WHAT'S HAPPENING IN CHEMISTRY?, the American Chemical Society's annual compendium of reports from the cutting edge of the chemical sciences. This award winning booklet is researched and written by Joseph Alper, a biochemist by training and one of the nation's outstanding science writers, WHAT'S HAPPENING IN CHEMISTRY? is prepared for nontechnical readers. Anyone can he entertained and enlightened by its nearly two dozen articles.
Uonn L. Manin (I) is an irrdependm m -
sulrant and research scimrisr specio1i;ing in hydrology, aquatic/environmenroI chemistry, and the behavior of subsurface organic contaminants. He has designed and evaluated soil-gas investigations. vadose zone studies. and vapor extrarrionlmoniroring systems for in situ remediation of sires throughout the United Stares. He holds a Ph. D. in hydrology and water resourcesfrom the Universiry of Arizona as well as degrees in the biochemical and environmental sciencesfrom the Universiryof California.
Henry B. Kerfoot (r) is a project manager at Lockheed Engineering and Management Services Company in Los Vegas, Nev. His current research interests include vadose zone techniquesfor detection of subsurface contamination, indirect indicarors of groundwater contamination. and soil-gas cleanups. He has designed and evaluored soil-gas surveying systems for mapping plumes and for storage tank leak derecrion. In 1987 Kerfoor received Lockheed S Roberr E. Gross Award for his work in soil-gas surveying. He holds degrees in chemistryfrom n e Johns Hopkins Universiry and Florida State Universiry.
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EnviRHI. ScLTechnol..Vol. 22. No. 7,1988 745
