Environ. Sci. Technol. 1990,24, 1824-1831
Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1976. Hoffmann, M. R. Enuiron. Sci. Technol. 1977, 11, 61. Millero, F. J.;LeFerriere, A.; Fernandez, M.; Hubinger, S.; Hershey, J. P. Enoiron. Sci. Technol. 1989, 23, 209. Leung, P. K.; Hoffmann, M. R. J . Phys. Chem. 1985,89, 5267. Hoffmann, M. R.; Edwards, J. 0. Inorg. Chem. 1977, 16,
(17) Cristol, S. J.; Reynolds, R. D. J. Am. Chem. SOC. 1955, 77,
1284.
(18) Pandis, S. N.; Seinfeld, J. H. J . Geophys. Res. D 1989,94,
12911. (19) Schwarzenbach, G.;Fisher, A. Helu. Chim. Acta 1960,169,
1365. (20) Haag, W.; Mill, T. Enoiron. Tonicol. Chem. 1988, 7, 917.
3333.
Edwards, J. 0. Inorganic Reaction Mechanisms; Benjamin, New York, 1965.
Received for review January 30, 1990. Revised manuscript received June 1, 1990. Accepted June 11, 2990.
Production and Transport of Carbon Dioxide in a Contaminated Vadose Zone: A Stable and Radioactive Carbon Isotope Study Karen Hohe Suchomel+ Environmental Radioisotope Center, University of Arizona, Tucson, Arizona 8572 1
David K. Kreamer**t Water Resources Management Program, University of Nevada, Las Vegas, Nevada 8 9 154-4029
Austin Long Environmental Radioisotope Center, University of Arizona, Tucson, Arizona 8572 1
Analyses of soil gas compositions and stable and radioactive carbon isotopes in the vadose zone above an alluvial aquifer were conducted at an organic solvent disposal site in southeast Phoenix, AZ. The study investigated the source and movement of carbon dioxide above a plume of organic solvent contamination. Two soil gas monitor wells, each screened and grouted at four discrete depths above the water table, provided sampling access. One well penetrated the uncontaminated vadose zone, the other penetrated a contaminated area now covered with asphalt. Carbon dioxide concentrations in the uncontaminated area range from 1.45% at 8 ft to 3% at 19 ft below land surface. Isotopic evidence suggests root respiration and minor oxidation of organic matter as C 0 2 sources at this site. Carbon dioxide in soil gas samples from the contaminated area exceeded 15% while O2 levels were as low as 1%. Carbon dioxide concentrations and carbon isotope values are consistent with in situ aerobic biodegradation of the organic pollutants.
Introduction Analysis of volatile organic gases in the soil near underground contaminant plumes has become a popular field method to quickly assess the extent of subsurface pollution. Delineation of groundwater and vadose zone contamination through the use of volatile compounds in soil vapors has increased in recent years (1-4). This potentially powerful field technique suffers from an inability to detect nonvolatile organic compounds, such as those compounds that make up the major components of diesel and aviation fuels and heating and lubricating oils. Carbon dioxide gas, resulting from biodegradation of organic compounds, has been suggested as an additional indicator of subsurface pollution ( 5 ) . Bioproduced carbon dioxide would be logical Present address: Hydro-Search, Inc., 5250 S. Virginia Street, Ste. 280, Reno, NV 89502. *Former address: Department of Civil Engineering,Arizona State University, Tempe, AZ 85282. 1824
Environ. Sci. Technol., Vol. 24, No. 12, 1990
for helping to locate both volatile and nonvolatile hydrocarbons in the subsurface, provided other major sources of carbon dioxide do not exist, and that the dissipation or migration of the gas is not rapid. Because carbon dioxide can be produced by plant roots and other sources that are not linked to vadose zone or aquifer contamination, this field study was carried out to test the potential use of carbon dioxide vapor as an indicator of contamination. The isotopic composition of the carbon dioxide was analyzed to distinguish between possible sources. Supportive information was obtained from one-dimensional gaseous diffusion simulations (steady and transient state) (6),but this mathematical modeling is not discussed as part of this report. To investigate the behavior of carbon dioxide and other common soil gases in a contaminated vadose zone, an industrial site was chosen for analysis. A t several locations at this industrial site, many organic chemicals and acids leaked or were disposed of in a variety of ways. These chemical excursions into the subsurface happened, for the most part, near the "courtyard" area of the site, in the form of leaks from storage tanks, releases from dry wells, and loss from septic tank leach fields. As part of a site investigation, hundreds of sampling points were established and soil cores, soil gas samples, and groundwater from the site were analyzed. These sampling locations were not only horizontally distributed, but several vertical depths were sampled throughout the site. The major organic species was the industrial solvent trichloroethylene (TCE), and its degradation products [including isomers of dichloroethylene (DCE)], which comprised a majority of the contaminants and was thought to be released between the years 1963 and 1974. Also, large quantities of l,l,l-trichloroethane (TCA) were inadvertently leaked from an underground storage tank at the site, in approximately 1982. A worst case scenario estimated that 757 000 L of chlorinated solvents was released at the site (7). Generally, the most reliable samples showed a high of slightly less than 1OOOOOO ppb total volatile organic compounds (VOCs) in bedrock wells below the courtyard,
0013-936X/90/0924-1824$02.50/0
0 1990 American Chemical Society
Table I. Compilation of Selected blSC a n d "C Values area
water table, m
ref Fritz et al. (36)
Pearch Lake Basin, NE Ontario
Bandelier Tuff, NV
Wood and Petraitis (23)
Southern High Plains, TX
Thorstenson et al. (24)
Haas et al. (21)
Turin (22)
Southern High Plains, TX
Gascoyne, ND
Caranza Site, Tucson, AZ
month
5
0.5-1.5
-23.4 f 1.8'
forest
2-3
1.5
-21.0 i 1.2
Glenn
51
Lamb
77
Glenn
51
Lamb
77
no. 6
15.9
site 4
19.4
desert
95 ft
6.8-7.8 24 86 6.1 24.4 2.7 16.8 21.6
7.0 13.4 19.8 25.3 36.3 44.5 5.8 11.0 17.1 21.4 3.0 5.8 8.5 15.9 2.7 5.8 9.1 12.8 5 ft 10 ft 25 ft 45 ft 71 f t
"b13C values reported per mil (%o) with respect to PDB standard. 'Average of 11values.
to region of known contamination. -
Conclusions
and movement of soil gas carbon dioxide directly above an aquifer contaminated with organic solvents and use this information to determine possible in situ biodegradation of organic solvents in the subsurface. Carbon dioxide concentrations in the contaminated vadose zone greatly exceed concentrations in the uncontaminated areas in this study. Carbon dioxide concentrations and carbon isotope values are consistent with in situ aerobic biodegradation of the organic pollutants found in this study area. Carbon dioxide concentrations also show a correlation to total volatile organic compounds measured in soil cores taken from the contaminated vadose zone. This supports the use of C 0 2 as an indicator of organic contamination in subsurface environments. Care should be exercised when relating the concentration of carbon dioxide in the vadose zone with biodegradation of subsurface contamination. Quantitative analyses of soil gas constituents are necessary to determine the source of carbon dioxide. In addition, a thorough understanding of soil gas dynamics is necessary, as soil gas migration may be due to the sensitivity of diffusion to moisture content and drained porosity. High concentrations of carbon dioxide in soil gas may be due to increased moisture content or abiotic sources and not necessarily reflect increased biologic activity. Additional research is needed in the area of in situ biodegradation of contaminants. In field studies, carbon analyses can be incorporated into these studies to determine the chemical reactions occurring in the vadose zone, distinguish carbon dioxide sources, and trace the progress of in situ biodegradation in the vadose zone.
Analyses of soil gas concentration can yield information on geochemical processes occurring in the vadose zone. The objectives of this study were to investigate the source
Glossary DCE 1,l-dichloroethene ETB ethylbenzene
water containing the microbial cells, indicating that if 14C was in the organic chemicals undergoing biodegradation, it would be found in the C02released. Indeed, 14C-labeled C02 was bioproduced in laboratory experiments (29-31). Therefore, I 4 C depletion in soil gas at DM 102SG may be due to dilution of naturally occurring soil gas by COP produced via bacterial degradation of 14C-free organic solvents. Although different bacterial species will degrade organic chemicals in different ways, the extremely 14C depleted soil gas is strong evidence for in situ organic chemical biodegradation. Nitrogen is also of interest at DM 102SG since its concentration is greater than was observed in this area, and it varies with depth (17). A possible source of nitrogen is atmosphere diffusing into the vadose zone. However, horizontal mass flux of atmosphere across a distance of approximately 150 ft beneath the asphalt courtyard would also result in higher O2 and lower C 0 2 levels, a condition not observed. Denitrification may have produced the elevated N2 concentrations. Vadose zone conditions at DM 102SG are conducive to denitrification since oxygen concentrations are low, an organic food source is present, and a nitrate source is present [nitric acid was a component of the wastes disposed of in the courtyard (7)]. Free nitrogen is the end product of the denitrification pathway. The main mechanism of O2 consumption at DM 102SG is probably organic matter oxidation. In addition, asphalt weathering consumes 02,forming a hydroxyl ion (35). COz and Nzare unaffected by asphalt weathering.
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Environ. Sci. Technol., Vol. 24, No. 12, 1990
MEC
PCE PCO,
PDB PMC TCA TCE TOL
voc
methylene chloride tetrachloroethene partial pressure of COP gas PeeDee belemnite percent modern carbon l,1,1-trichloroethene trichloroethene toluene volatile organic compound
Acknowledgments
We t h a n k the following: A. J. T. Jull, T. W. Linick, L. J. Toolin and D. J. Donahue, NSF-University of Arizona National Science Foundation Accelerator Facility for Radioisotope Analysis, for providing 14C activities, James Hussey and Steve S m i t h of Dames and Moore, Robert Lee of Motorola, Inc., and James McNabb of the EPA. L i t e r a t u r e Cited Kerfoot, H. B. Int. J . Environ. Anal. Chem. 1987, 30, 167-181. Kerfoot, H. B.; Barrows, L. J. Soil Gas Measurements for Detection of Subsurface Organic Contamination. U S . EPA Contract No. 68-03-3245; Las Vegas, NV, 1987. Marrin, D. L.; Thompson, G. M. Groundwater 1987, 25, 21-27. Evans, 0. D.; Thompson, G. M. Proceedings of 1987 Petroleum Hydrocarbons and Organic Chemicals i n Groundwater: Prevention, Detection and Restoration; American Petroleum Institute: Houston, TX, 1987, pp 444-455. Kerfoot, H. B.; Mayer, C. L.; Durgin, P. B.; D’Lugosz, J. J. Ground Water Monit. Rev. 1988, 3, 67-71. Dames and Moore, Inc. Remedial Investigation/Feasibility Study, Motorola, Inc., Interim Study Draft Report. Phoenix, AZ, June 26, 1986. Dames and Moore, Inc. Remedial Investigation/Feasibility Study, Draft Report. Phoenix, AZ, June 1987. Dames and Moore, Inc. Remedial Investigation/Feasibility Study, Motorola, Inc., Stratigraphic Borings/Monitoring Wells Draft Report. Phoenix, AZ, July 2, 1985. Hendricks, D. M. Arizona Soils; University of Arizona Press: Tucson, AZ, 1985; p 244. Reeter, R. W.; Remick, W. H. Maps Showing Groundwater Conditions in the West Salt River, East Salt River, Lake Pleasant, Carefree, and Fountain Hills Sub-basins of the Phoenix Active Management Area, Maricopa, Pinal, and Yavapai Counties, AZ--1983. State of Arizona Dept. of Water Resources, Hydrologic Map Series Report No. 12. Wood, W. Technical Memo. U S . Geological Survey, January 9, 1987. Kramer, J. B.; Everett, L. G. Proceedings of the Spring 1990 Meeting of the American Institute of Hydrology, March 12-16, 1990, Las Vegas, NV; Kendall Hunt Publications: Falls Church, VA, in press. Hillel, D. Introduction to Soil Physics; Academic Press: San Diego, CA, 1982. Kreamer, D. K.; Weeks, E. T.; Thompson, G. M. Water Resour. Res. 1988, 24, 331-341.
(15) Petraitis, M. J. Carbon Dioxide in the Unsaturated Zone in the Southern High Plains of Texas. Masters Thesis, Texas Tech University, 1981; p 134. (16) Craig, H.; Keeling, C. D. Geochim. Cosmochim. Acta 1963, 12, 133-149. (17) Suchomel, K. H. Carbon Isotopes, and Carbon Dioxide Behavior in the Uncontaminated and Contaminated Unsaturated Zone, Phoenix, Arizona. Prepublication manuscript for M.S. Thesis, University of Arizona, Tucson, AZ, 1987; p 81. (18) Rightmire, C. T. A Radiocarbon Study of the Age and Origin of Caliche Deposits. M.A. Thesis, University of Texas, Austin, TX, 1967. (19) Rightmire, C. T.; Hanshaw, B. Water Resour. Res. 1973, 9,958-967. (20) Dorr, H.; Munnich, K. Radiocarbon 1986, 28, 338-345. (21) Haas, H.; Fisher, D. W.; Thorstenson, D. C.; Weeks, E. P. Radiocarbon 1983, 25, 301-314. (22) Turin, H. J. Carbon Dioxide and Oxygen Profiles in the Unsaturated Zone of the Tucson Basin. Prepublication manuscript for M.S. Thesis, University of Arizona, Tucson, AZ, 1986; p 67. (23) Wood, W.; Petraitis, M. Water Resour. Res. 1984, 20, 1193-1208. (24) Thorstenson, D. C.; Weeks, E. P.; Haas, H.; Fisher, D. W. Radiocarbon 1983,25, 315-346. (25) Kari, W. J.; Santucci, L. E. Proc.-Assoc. Asphalt Paving Technol., Tech. Sess 1963, 32, 148-163. (26) McLaughlin, J. F.; Goetz, W. H. Proc. Highw. Res. Board 1955, 34, 274-286. (27) McClellan, K. Biodegradation of Trichloroethylene by Bacteria Indigenous to a Contaminated Site. M.S. Thesis, University of Arizona, Tucson, AZ, 1986; p 77. (28) Bouwer, E.; Rittman, B. E.; McCarty, P. L. Enuiron. Sci. Technol. 1981, 15, 596-599. (29) Vogel, T. M.; McCarty, P. L. Appl. Environ. Microbiol. 1985,49, 1080-1083. (30) Nelson, M. J. K.; Montgomery, S. 0.; O’Neill, E. J.; Pritchard, P. H. Appl. Enuiron. Microbiol. 1986, 52, 383-384. (31) Wilson, J. T.; Wilson, B. H. Appl. Enuiron. Microbiol. 1985, 49, 242-243. (32) Parsons, F.; Wood, P. R.; DeMarco, J. J.-Am. Water Works Assoc. AWWA 1984 (February). (33) Stahl, W. J. Geochim. Cosmochim. Acta 1980, 44, 1903-1907. (34) Alexander, M. Enuiron. Sci. Technol. 1985, 18, 106-111. (35) Jiminez, R., University of Arizona, Tucson, personal communication, 1986. (36) Fritz, P.; Reardon, E. J.; Barker, J.; Brown, R. M.; Cherry, J. A.; Killey, R. W. D.; McNaughton, D. Water Resour. Res. -1978, 14, 1059-1067. (37) Kunkler, J. L. US. Geol. Surv. Prof. Pap. 1969, No. 650-B, B 185-B 188. (38) Parada, C. B.; Long, A.; Davis, S. N. Isot. Geosci. 1983,1, 219-236.
Received for review October 20, 1989. Revised manuscript received May 16, 1990. Accepted July 12, 1990. T h e research project was supported in part by E P A Grant CR-812583-01-0 and by Motorola, Inc. Preparation of the manuscript was supported in part by Hydro-Search, Inc.
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