Environ. Sci. Technol. 1992, 26, 185-192
Petroleum Contamination of an Elementary School: A Case History Involving Air, Soil-Gas, and Groundwater Monitoring Clifford L. Moseley*st and Michael R. Meyer’
Division of Federal Employee Occupational Health, Health Resources and Services Administration, US. Public Health Service, Atlanta, Georgia, 30323, and Geotek Engineering and Testing Services, Inc., Sioux Falls, South Dakota 57104
A case history of petroleum contamination migration with the groundwater is presented. Air, soil-gas, and groundwater monitoring were used to investigate the source and extent of petroleum contamination in an elementary school located adjacent to a gasoline station and a petroleum tank farm. The school had been closed when students and teachers reported noxious odors. Groundwater and soil-gas data indicated that a plume of contamination following the local groundwater gradient had appeared between a petroleum tank known to have leaked 20000 gal of gasoline and the school building. Air samples revealed contamination beneath the school and within the school itself. Caution is warranted when soil-gas surveys are used to determine the location of a groundwater contaminant plume. Groundwater recharge from precipitation can markedly influence the migration rate of a contaminant plume and can increase the ability of vaporous components to percolate upward. The elementary school affected by this environmental episode was never reoccupied. Introduction The transport of chemicals via migration in environmental media occurs frequently, sometimes with adverse environmental and human health consequences. Examples are the Bophal, India, tragedy (atmospheric transport), the Exxon Valdez, Valdez, AK, oil spill (surface water dispersion), and Kepone contamination of the James River and Chesapeake Bay in Virginia (sediment transport). This report describes environmental air, soil-gas, and groundwater monitoring used in an investigation of gasoline migration via groundwater into a school. Site Description and History Figure 1illustrates the environment setting in which this episode took place. An elementary school, located in the Midwest United States, was surrounded by a large petroleum and liquid-fertilizer tank farm, a gasoline station, small commercial establishments, and a residential area. Originally constructed in the 1940s in a rural area, development over time extended to surround the facility. Historically, the tank farm handled jet fuel, gasoline, fuel oil, and liquid fertilizer. The northern boundary of the school is separated from the tank farm by a two-lane asphalt road. West of the school is a service station with two dispensing islands and seven (five 10000-gal and two 12 000-gal) underground storage tanks (USTs). Single-familyhousing is east and a vacant field is south of the school. The immediate area around the school is paved, as indicated by the shaded area in Figure 1. The tank farm is diked. The tank farm property and the nearest aboveground gasoline storage tank are approximately 150 and 400 ft, respectively, from the school building. This cylindrical steel storage tank, constructed in 1950, has a bottom thickness of 0.25 in. and a capacity U S . Public Health Service. Geotek Engineering and Testing Services. 0013-936X/92/0926-0185$03.00/0
of approximately 1.5 million gal. The topsoil was removed during construction and the bottom of the tank rests on a bed of crushed rock placed directly over the natural sand and gravel sediments. The service station’s USTs and the boundary of the residential area are approximately 200 and 170 f t from the school building, respectively. Part of the elementary school was constructed in 1953, and consisted of 11 classrooms, a library, and a general office area. Additions were made 1966 (five rooms), 1974 (gymnasium and three rooms), and 1985 (four rooms). The original structure is closest to the tank farm and has a 3-ft by 4-ft tunnel or crawl space underneath portions of the building, which provides access to the electrical and plumbing lines and a now-defunct, forced-air, heating system. The ductwork for this heating system was installed directly into the concrete-slab floor and was left in place when the current heating system was installed. There is a 10000-gal, No. 2 fuel oil, storage tank located on school property for this purpose. Until 1984, the school’s water was supplied by a 62-ft well on site. The school was connected to the city water system in 1984 after a well-water sample revealed 5 parts per million (ppm) total hydrocarbon (THC) contamination. A sanitary sewer system, flowing east-west under the paved road and consisting of 6-ft sections of 10-in.-diameter clay tile, serves the school, the private residence, and other establishments along the paved road. This sewer line lies approximately at the depth of the water table, and depending on the fluctuation, the line may lie above, or be submerged in, the water table. On May 30th, a resident whose property was approximately 150 f t southwest of the gasoline storage tank reported gasoline vapors in the home to the local fire department after having noticed increasingly strong odors over the past several weeks. Explosive levels of hydrocarbon vapors were measured in the home. In response, the resident was evacuated and a wider investigation began. Inventory control measures used by the tank farm operator consisted of daily height-of-liquid and temperature measurements. The height gauges were accurate to l/*th in. or f442 gal in the tank in question, and were recorded daily (1). With this measuring system, a leak of 442 gal or less per day had the potential to go undetected. On the basis of the presence of several inches of free product in a number of monitoring wells, a consultant hired by the tank farm demonstrated that the tank closest to the home had lost an undetermined amount (eventually approximated to be 20 000 gal) of unleaded gasoline during an unknown time period preceding the vapor problem in the home. Several groundwater monitoring wells and a recovery well were installed in June and August to monitor and control the loss from the tank. Other monitoring wells and recovery wells, installed previously in response to earlier leaks or spills, apparently had failed to detect the loss. In September, some staff and students in the school became ill from noxious odors. The local fire department measured levels of airborne vapors up to 40% of the lower explosive limit (LEL, pentane calibration), and the school
0 1991 American Chemical Society
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was evacuated. The LEL for gasoline is 1.3% (2), and the levels measured represent values in the range of 5200 ppm THCs. Follow-up sampling over the next week indicated levels ranging from 2 to 10% LEL in various locations within the school. Levels in sewer manholes in the school parking lot and playground were 5-20% LEL. Two potential sources of the vapors were suspected initially-the service station's USTs, and the tank farm's most recent 186 Environ. Sci. Technol., Vol. 26, No. 1, 1992
leak or spill. On the basis of historical data, other leaks or spills were believed to have occurred in the tank farm. The tank farm was the strongest candidate because of the suspected direction of groundwater flow in the area and the temporal and spatial relationship of the tank. The school board requested additional assistance from government agencies to determine the nature and cause of the noxious odors in the school. In addition, the tank farm
conducted an investigation using their own consultant.
Methods and Procedures The objectives of the organizations interested in environmental protection were to determine the source, areal extent, and concentration of the groundwater plume and responsibility for the contamination and to determine the magnitude of any remedial actions that might be required. The public health organizations’ objectives were to measure the degree of contamination within the school and decide whether the school could be reoccupied and to estimate the potential for continued contamination of the school by the groundwater plume. Groundwater Investigation. The groundwater portion of this investigation was extensive and has been reported elsewhere (1). The major findings of the groundwater investigation, as they pertain to the soil-gas and environmental air investigation, will be summarized here. Although petroleum products are complex hydrocarbon mixtures, only benzene, toluene, xylene, and THC components will be discussed. Forty-seven groundwater samples were collected in June, August, and September of that year, and in January of the following year, from wells installed specifically for the investigation and other wells that had been installed previously. Well locations are shown in Figure 1. Twoinch-diameter wells were completed to depths of 9-27 f t (average depth 19 ft) by either the hollow-stem or mudrotary method. Well placement was guided by the primary need to determine the groundwater gradient and subsequently to estimate the direction, rate of migration, and boundaries of the plume. Samples for benzene, toluene, and xylene and THCs (as gasoline) were analyzed by U.S. Environmental Protection Agency method 602 (purgeand-trap gas chromatography with a photoionization detector, also referred to as the modified California method) (3). Soil-Gas Investigation. Soil-gas sampling is a more rapid and economical method than well installation for determining the presence of subsurface hydrocarbon contamination and was used in this investigation to supplement groundwater monitoring. Soil-gas samples were collected during October from 43 locations (Figure 1). These locations were along the hypothesized north-south center line of the plume, along the road, around the school building, and between the school property and the gasoline station to determine the extent of vertical migration in those areas. A 4-ft metal rod with a sliding weight-a slam-bar-was used to punch through the soil to a depth of approximately 4 ft below ground surface. A 0.5-in. hollow steel tube with a removable plug tip was then inserted in the hole. The plug (to keep soil out of the hollow tube) was removed and the tube lifted approximately 6 in. to create the sampling space. The sampling probe was immediately connected to the hollow tube with a rubber grommet. Initial screening for total ionizable hydrocarbons was conducted with a H-Nu photoionization detector (10.2-eV lamp, benzene calibration). Peak and steady readings were recorded. Based on H-Nu results, 16 locations were selected for additional sampling and analysis in the field using a Photovac lOAlO portable gas chromatograph. This sampling was accomplished by placing a l-L Tedlar bag (SKC Products, Inc.) in a desiccator and drawing a vacuum around the bag to inflate it. Five Tedlar bag samples were adsorbed onto Supelco, Inc., Tenax/carbon molecular sieve tubes, by pulling the sample from the bag and through the tube with a syringe, for subsequent laboratory confirmation by gas chromatography/mass spectrometry (GC/MS) (4).
Air Investigation. Environmental air sampling was conducted during October in the tank farm, in areas between the school and the tank farm, and in the school itself, in order to estimate the present level of contamination within the school, compare values to outdoor or background values, and assess the potential for further degradation of fiir quality in the school from the groundwater plume. Fifteen air samples were collected, with sampling times ranging from 7 to 10 h, beginning approximately 6:OO p.m. The school was completely closed and the ventilation system shut off. To estimate the concentrations of hydrocarbons directly underneath the school that could be a reservoir of continuing contamination of the school, 0.5-in.-diameter holes were drilled through the slab floor of the school at various locations, either to the earth below or into the old heating ductwork. Samples were obtained according to standard, solid-sorbent sampling and analytical methods (5). Laboratory analysis by GC/MS was confined to benzene and THCs.
Results and Discussion Site Hydrogeology and Groundwater Monitoring. The investigation area lies on glacial terrace deposits along the northern edge of a flood plain associated with a large creek. This creek is located approximately 0.5-mi south of the school. The northern edge of the tank farm is bounded by a hill composed of glacial till (sandy clay). The terrace deposits underlying the school and tank farm include very poorly sorted sands and gravel and clay lenses derived from the retreat of melting ice sheets. There did not appear to be a continuous clay layer. Sediments ranged in size from colloidal to large cobbles. These sandy and gravelly sediments are underlain at depth by glacial till and a Precambrian-age orthoquartzite. The terrace deposits and flood plain overlie a shallow sand-and-gravelaquifer which is the major source of water for the area. The water table is 9-17 f t below ground surface. Using an arbitrary reference datum of 100 ft below ground surface, groundwater elevations were determined to range from 92 f t at the aboveground gasoline storage tank to 85 ft at the school, indicating a hydraulic gradient from north to south. The topography is rather flat with a gentle slope to the south. Table I contains the groundwater monitoring data. Quality control data showed relative differences in duplicate analyses of less than lo%, spiked sample recoveries for toluene and benzene between 89 and 102% , and surrogate standard recoveries of p-bromofluorobenzene between 86 and 115%. Coeluting interferences on some samples contributed to the high recoveries. Although additional wells were included in the monitoring program, data from seven wells are not reported due to incomplete information (such as well location and contaminant concentration). Figure 1has been overlaid on a grid referenced to a horizontal and vertical center line on the aboveground storage tank. Each box represents approximately 100 ft2. For example, zone -Y1 is 100 f t downgradient or south of the horizontal center line; zone +X1 extends 100 ft to the right of the tank. Zone +Y 1 has remained essentially uncontaminated through out the sampling period, supporting the belief that the groundwater flow direction is south, and likely eliminates any source of contamination directly upgradient of the aboveground tank. It is interesting to note that the highest concentrations found at any time during the investigation were recorded in zones -Y5 and -Y6 in September, the month that the school was closed because of high vapor levels. The difference in concentrations between zones -Y7 and -Y9 and -YlO-na wells were placed Environ. Scl. Technol., Vol. 26, No. 1, 1992
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in zone -Y&is striking. Zones -Y9 (two well points) and -Y10 (one well point) may have defined the southern boundary of the plume at the time of sampling, although this supposition is based on a small number of data points. Zones -X4 and -X3, in general, are free of substantial hydrocarbon contamination, suggesting that it is unlikely that the contamination originated from a source located to the west of either the tank farm or the service station USTs. Zonea +X4 and +X5 contain some contamination and, at the time of sampling, may have defmed the leading eastern boundary of the plume. Absence of contamination in wells W26 and W27 and the presence of contamination in W36 suggests a change in plume migration direction from south to southeast. Figure 2 illustrates three-dimensionally the spatial relationship of peak benzene concentration locations to the tank, the private residence, and the school, over the 6-7month sampling period. Peak benzene locations a shown in the z plane. The somewhat b i o d a l nature of the plume (especially Figure 2d) is likely a function of the nonnniform distribution of sampling locations (from Table I, note the almost complete lack of data from wells W17-W22 in January) and resultant computer software interpolation more so than a source-related phenomenon, although the recovery wells’ zones of influence may have retarded the trailing portion of the plume. The change in peak benzene location along the direction of groundwater flow over t i e coincides with the report of odors in the school (Figure 24. It is noteworthy that the well locations sampled in August-W12-W20were unable to confirm the magnitude of the spill. (Compare parts a and c of Figure 2 to part b.) The depth of these wells ranged from 9 to 24 ft
and, with the water ranging from 9 to 17 ft, should have been at a depth to detect the spill. In only two of the eight boring logs were gasoline odors mentioned. However, a review of the National Oceanic and Atmospheric administration data (7) for this period indicates an abnormal amount of rainfall for the month of September (9.26 in., compared to a 29-year mean of 4.09 in. for that month), which may have raised the water table to levels above the customary sampling points of these wells. Petroleum products such as gasoline are known to be “floaters” and stay primarily on top of the water layer. According to Iles et al. (I),water table levels rose approximately 2 ft during the June 23 to September 21 period and declined approximately 2 ft during the September 21 to November 16 period, this provides evidence of the significant and rapid influence rainfall has on the water table in this area. Also, a sanitary sewer line running from the residence and school to the main line may have channeled the migration path around the monitoring wells. The relatively low levels reported in August may have mislead early investigators in thinking that the plume migration had been stopped by the recovery wells (RW2, RW3, and RW4). It is also noteworthy that rainfall for the month of April of that year was 5.15 in. (compared to a the 29-year mean of 2.5 in.) and may have been a fador in the initial reporting of the problem by the resident. Figure 3 compares the aqueous phase of one sample from W41, located on the school property, to the aqueous phase of one sample from RW2, located on the tank farm. Figure 3a shows the ion chromatograms for W41; Figure 3b shows the ion chromatograms for RW2. Although the concentration in RW2 is almost twice that of W41 (note the scale Environ. Sci. Technol., Val. 26. No. 1,
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differences between the diagrams), the ion chromatograms are virtually identical. This, along with the groundwater flow direction, suggests that the petroleum product found in the tank farm recovery well and the petroleum product 190
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from the groundwater beneath the school have the same source (8). A more meaningful comparison could have been made if additional samples had been taken and, in particular, similar samples from one or more wells located
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clase to the service station’s USTs. However, these samples were not collected, and possible contribution from the service station’s USTs cannot be ruled out. Soil-Gas Monitoring. Laboratory analysis of the Tedlar bag samples by GC/MS confmed the presence of benzene and toluene. Peak H-Nu readings of soil gas (total hydrocarbons or THCs) illustrate a pattern of readings above background suggestive of a plume encompassing the school, the m a between the serive station tank and the school, and the area between the school and the tank farm. Readings varied depending upon sampling location (Figure l),with the highest readings found closest to the leaking tank and diminishing southward toward the school in the direction of groundwater flow. East of the school (zones +X4 and +X5), readings were not measurable; readings on the west side toward the service station’s UST were increased above background. Background soil-gas readings in the area were less than 10 H-Nu units (actual concentrations were not calculated). The data illustrate that both the service station and the tank farm
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may be contributors to the plume. However, the sampling points lying hydraulicdy upgradient of the service station USTs suggest substantidy greater contribution from the tank farm. Figure 4a depicts the peak soil-gas readings from October three-dimensionally. Figure 4b shows the locations of the THC concentrations in groundwater in September, 7-12 days earlier. Comparison of Figure 4a with Figure 4b illustrates that the soil-gas plume lags behind the groundwater plume. Poasible reasons for thisphenomenon include the difference in sampling depths from ground surface (groundwater, 9-15 ft; soil gas, 4 ft), depth to groundwater, and groundwater velocity. Along the north and south sides of the paved road, the ditches apparently had been backfilled with clayey sediments. Soil-gas sampling was limited to 4-ft depths and found very minor contamination in these meas. Additional soil-gas sampling (9),performed in the following January to help determine the location for new monitoring wells (some of which were installed with an auger rig), verified the presence of contamination in these ditches and confirmed the presence of a 2 ppm benzene plume encompassing the suspect storage tank, the private residence, gas station USTs, and the school. Ambient Air. Environmental air concentrations measured during Odober are shown in Table II. Samples and blanks were desorbed with carbon disulfide and analyzed by capillary column (30-m DB-1 fused silica) gas chromatography (limit of detection, 1pg/sample); representative samples were also analyzed by GC/MS for compound identification (10). Indoor school concentrations of benzene are slightly elevated over outdoor values. The EPA has reported that the median benzene level in urban and suburban areas of the United States is 2.8 parts per billion (11). The background level recorded here is not appreciably different. Higher values were anticipated, based on the high groundwater contaminant levels and plume location in September. Indoor THC levels are up to 40 times outdoor levels. However, many common sources of hydrocarbons found in indoor environments (paint, printer’s ink, clothing, gas and oil heating systems, cleaning agents, and so forth) may be a contributor to these higher THCs. Several studies reviewed by Wallace et al. (12) support this. Hence, the contribution to these levels from the groundwater plume cannot be determined quantitatively. Concentrations found in areas in the school below flwr level are significantly higher. The crawl space had no barrier to the soil; contaminants were free to migrate upward into this space. Access to the crawl space can be gained through the boiler room, which may explain the sligbtly higher totalhydrocarbon values in the boiler room (although vapors from the No. 2 fuel oil used for heating may be an additional source). The soil/ductwork/floor interface, a very confined space, had lower concentrations than did the crawl space, a somewhat confined, but much Environ. Sci. Technol.. Voi. 26. NO. 1, 1992
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larger, area. The explanation for this is unclear. Higher humidity at the floor interface interfering with the sample collection efficiency (13), and the lower elevation of the crawl space bringing it closer to the contaminated groundwater, are two possible explanations. Conclusions
Contaminant isopleths determined from groundwater and soil-gas data are similar and describe a plume of subsurface contamination encompassing the area south of the leaking tank and portions of the school property, including the school building itself. Water table elevations and peak contaminant concentrations suggest that the plume may have originated north of the private residence and migrated south with the groundwater. At the time of sampling, the leading edge of the plume was located approximately beneath the school. Although there are similarities between the material found in the groundwater on the tank farm and beneath the school, these data and the contamination found in W30 and W35 do not rule out the possibility that the service station USTs may have contributed to the plume. The data demonstrate that soil-gas sampling can be a viable method for determining the location of volatile hydrocarbons in groundwater and subsurface soil, offering rapid information about potential well placement at low cost. On the basis of these limited data, the soil-gas plume appears to lag behind the groundwater plume, and one should consider this when using soil-gas data to locate a groundwater contaminant plume or to place monitoring wells. Possible factors in this relationship, such as the depth of sampling, the depth to groundwater, and the velocity of groundwater, need further investigation. The technique used here was a shallow soil (4-ft) procedure which has limitations if impermeable or semipermeable clay layers are encountered between the end of the probe and the contaminant-bearing layer. Volatile hydrocarbon contaminants may migrate beneath dwellings and other structures and result in potentially hazardous concentrations within those structures. In this case, the decision was made to not reoccupy the school in light of the reservoir of contamination present beneath the school. Occupants of buildings, particularly buildings without tight barriers to the soil below and close to USTs, should be aware of potential indoor air contamination from leaks or spills that migrate in the subsurface. This phenomenon has been previously reported (14). The inventory control measures in use at the time failed to detect a large leak of an estimated 20000 gal of gasoline. More sensitive inventory control methods should be considered. Monitoring wells in place prior to the incident failed to detect the presence of extensive contamination that eventually affected the school. Short-circuiting of the contaminant pathway may have occurred along sewer lines and other subsurface soil disturbances, thus circumventing the monitoring wells. Investigators should be aware of the possibility of utility lines and other sources affecting migration pathways. Sampling at more than one location in the water column may be necessary to prevent missing the presence of a floating or dissolved contaminant, and the influence of rainfall on water table fluctuations should be considered. Groundwater recharge and resultant water table levels may influence the rate of migration of contaminants and soil gas.
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Acknowledgments
Several sources of data were used in compiling this report. We acknowledge all those persons who participated in the collection and analysis of the data-the tank farm owner, contractors, the school board, and federal, state, and local health and environmental officials. We also thank Joseph Hugart, P.G., for his technical advice in the preparation of this paper. Registry No. Benzene, 71-43-2; toluene, 108-88-3; xylene, 1330-20-7.
Literature Cited (1) Iles, D. L.; Meyer, M. R.; Baron, R.; Markley, E. Assessment of Hydrogeologic and Groundwater Contamination Data in the Vicinity of the Hayward Elementary School; Report No. 44-UR; Division of Geological Survey and Division of Environmental Quality, Department of Water and Natural Resources, South Dakota, 1988. (2) Sax, I. N. Dangerous Properties of Industrial Materials, 6th ed.; Van Nostrand Reinhold Co.: New York, 1979; p 1471. (3) Methods for Organic Chemicals Analysis of Municipal and Industrial Wastewater;Appendix A to Part 136. Fed. Regist. 1984 (October 26), 49 (No. 209). (4) Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air; EPA-600/4-84-041; U.S. Environmental Protection Agency, Washington, DC, 1984. (5) Manual of Analytical Methods, 3rd ed.; NIOSH-84-100; National Institute for Occupational Safety and Health, Cincinnati, OH, 1984; Method 1501. (6) SURFER (version 4). Golden Software, Inc., 807 14th St., Box 281, Golden, CO 80402, 1988. (7) National Oceanic and Atmospheric Administration. Local Climatological Data; Annual Summaries for 1989, Part 111-Central Region. National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, National Climatic Data Center, Ashville, NC, 1989. (8) Senn, R. B.; Johnson, M. S. Interpretation of Gas Chromatographic Data in Subsurface Hydrocarbon Investigations. Ground Water Monit. Rev. 1987, 7, 58-62. (9) Iles, D. L.; Meyer, M. R.; Baron, R.; Markley, E. Assessment of Hydrogeologic and Groundwater Contamination Data i n the Vicinity of the Hayward Elementary School; Report No. 44-UR Division of Geological Survey and Division of Environment Quality, Department of Water and Natural Resources, South Dakota, 1988. (10) National Institute for Occupational Safety and Health. Division of Physical Sciences and Engineering, Measurementa Research Support Branch laboratory report, HHE 87-002 (seq no. 5664), October 28, 1986. (11) Volatile Organic Chemicals in the Atmosphere: A n Assessment of Available Data; EPA-600/3-83-027(A); U.S. Environmental Protection Agency, Washington, DC, 1983. (12) Wallace, L. A.; Pellizzari, E. D.; Gordon, S. M. Organic Chemicals in Indoor Air: A review of Human Exposure Studies and Indoor Air Quality Studies. In Indoor Air and Human Health, Proceedings of the 7th Annual Life Sciences Symposium, Gammage, R. B., Kaye, S. V., Eds.; Lewis Publishers: Chelsea, MI, 1985. (13) Rudling, J.; Bjorkholm, E. Effect of Adsorbed Water on Solvent Desorption of Organic Vapors Collected on Activated Carbon. Am. Ind. Hyg. Assoc. J. 1986,47,615-620. (14) Kullman, G.; Hill, R. Indoor Air Quality Affected by Abandoned Gasoline Tanks. Appl. Occup. Hyg. 1990,5(1), 36-37. Received for review March 12,1991. Revised manuscript received August 2, 1991. Accepted August 5, 1991.
