Environ. Sci. Technol. 1984, 18, 973-974
Screening for Organic Contamination of Groundwater: Ethylene Dibromide in Georgia Irrigation Wellst Luz R. Martl,z John de Kanel, and R. C. Dougherty"
Department of Chemistry, Florida State University, Tallahassee, Florida 32306 This paper describes a screening procedure for organic contaminants of groundwater based on purge-and-trap concentration followed by gas chromatography and mass spectrometry. Liquid-liquid extraction followed by gas chromatography-mass spectrometry or electron capture gas chromatographywas used to analyze for pesticides with low volatility. We have validated these methods for a series of pesticides that have been extensively used in agriculture in the southeast. Over a 3-year period, the only detected organic pesticide in groundwater obtained from three different irrigation wells was ethylene dibromide (EDB). The concentration of this compound was determined by calibration against benzene-d,. The concentration of EDB varied between 1 and 94 pg/L. H
Introduction Southwest Georgia is an area of intensive irrigation agriculture where two to four crops per year are harvested from a given field. Current agriculture practice makes extensive use of groundwater for irrigation and of pesticides for control of plant and insect pests. We have developed methods for assessing the impact these agricultural practices have on groundwater quality. Because of the large number of pest control chemicals that are used in intensive agriculture, a molecularly specific detector is essential for a screening application. The most powerful detector available to us was gas chromatography-mass spectrometry (GC/MS). In addition to screening for unknown organic contaminants at parts per billion and higher levels, we specifically validated the techniques for the presence of ppb levels of seven plant protection chemicals. These chemicals are atrazine (l),chlorothalonil (2), chloropicrin (3), alachlor (4), ethoprop (5), pendimethalin (6), and ethylene dibromide (7). These compounds were selected because of their current and past usage in agriculture in southwest Georgia. Experimental Section Gas Chromatography-Mass Spectrometry. Electron impact gas chromatography mass spectra were obtained with a Finnigan 4510 GC-mass spectrometer equipped with a fused silica capillary column. The capillary column (30 m, 0.32 mm i.d., 1.0 pm film thickness, DB-5) was passed directly through the capillary transfer line and into the mass spectrometer ion source. The mass spectrometer was operated under the control of an INCOS CIS 2000 data system which scanned the mass range of m/z 50-500. A complete spectrum was recorded every second during the gas chromatographic run. Purge-and-Trap Sample Preparation. The purgeand-trap system used to collect volatile organic compounds was similar to that described by Bellar and Lichtenberg (1). The method sensitivty was improved by increasing the size of the purging flask by 0.5 L. The absorbent was This work was taken in part from the M.S. Thesis (Florida State University, 1983) of L.R.M. Present address: USDA ARS, Southwest Watershed Research Laboratory, Tifton, GA 31793.
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0013-936X/84/0918-0973$01.50/0
Tenax-GC (poly(2,6-diphenyl-p-phenylene oxide)). The absorbent was conditioned by extraction in a Soxhlet with ethanol followed by similar extraction with pentane. The absorbent was heated to 200 "C for 1 h under a nitrogen flow and then dried under vacuum at room temperature for 2 h. Absorption cartridges were prepared by packing 0.8 g of the prepared Tenax-GC into a 1.6 cm 0.d. Pyrex tube. After packing, cartridges were thoroughly desorbed at 280 "C for 8 h with a 30 mL/min flow of nitrogen (grade 0).
Water samples (500 mL), which had been stored under refrigeration in completely full Teflon-sealed glass bottles, were purged at a temperature of 90 "C with a flow of 30 mL/min of nitrogen (grade 0) for 1 h. The purging nitrogen was passed through liquid nitrogen cooled traps to remove any volatile contaminants. The Tenax tube with absorbed volatiles was attached to the gas chromatograph-mass spectrometer system through a four-port two-position high-temperature valve. The cartridge was purged at room temperature with high purity helium to eliminate oxygen and moisture present in the trap. The gas chromatograph oven was cooled to -50 "C, and the Tenax cartridge was heated to 280 "C under a flow of high purity helium (grade 4.5). After 10 min of desorption the gas chromatograph carrier gas was returned to normal operation, and the oven temperature was increased to 50 "C at 30 "C/min and then programmed at 5 "C to a final temperature of 240 "C. Liquid-Liquid Extraction Cleanup. Nonvolatile organics were extracted from water samples (pH 7.4-8.0) with 15% methylene chloride-85% pentane (Burdick & Jackson; distilled in glass). Each sample was extracted with three 100-mL portions of the solvent. The combined extracts were filtered through anhydrous sodium sulfate and concentrated to a volume of 10 mL in a KudernaDanish evaporator attached to a three-ball Snyder column. A 100-mL portion of pentane was added and the volume again reduced to remove all traces of methylene chloride. The final volume was adjusted to 10 mL with hexane. Gas Chromatography-Electron Capture Detection. Quantitation of nonvolatile analytes extracted with methylene chloride-pentane was accomplished with a Hewlett-Packard 5880 GC equiped with a 63Nielectron capture detector and a splitless capillary injector. A 25-m fused silica capillary was used (0.2 mm id., SE-54 silicon gum, 0. ll-pm film thickness). The temperature program was started at 35 "C with a 5-min delay followed by a linear temperature rise to 200 "C. Sampling. Groundwater samples were collected from three 12-in. irrigation wells located in Seminole County, Georgia between Oct 1981and Aug 1983. Each of the wells was more than 150 feet deep and sampled water from the Ocala Limestone Aquifer. Each well was pumped for at least 10 min prior to sampling. Results and Discussion Of the seven compounds in the experimental design, only three could be satisfactorily analyzed by using purge-and-trap methodology. GC/MS analysis was accomplished in purified laboratory water spiked with
0 1984 American Chemical Society
Environ. Sci. Technol.,
Vol. 18, No. 12, 1984 973
Table I. Recoveries of Pesticides and Volatiles from Water Standards by Purge-and-Trap GC/MS recoverv (%) compound
100 pg/L
benzene-d6 chloropicrin
100 63
EDB pendimethalin
50 wg/L
100 85 100 62
89 50
10 pg/L
100 90 105 35
I "
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6 1 ; ;
_ _ _ _ _. 5n/ h i ; ; ; h ; 1982 DATE
b I ; b i : ;
L i b ; ;
i
I983 SAMPLE@
Flgure 2. EDB found In three contaminated wells between 1981 and 1983.
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1396
sampled. During the course of this study the application range varied from 3 qt/acre for alachlor to 30 lb/acre for ethoprop. We did not detect any of these compounds in any of the samples examined. Figure 2 illustrates the concentrations of ethylene dibromide found in water samples from the three irrigation wells sampled in this study in the time frame 1981-1983. The concentrations varied from approximately 1pg/L to greater than 90 pg/L. The application rates for ethylene dibromide above wells BH 1and BH 2 increased from 1.5 gal/acre in 1980 (one application per year) to 3 gal/acre (two applications per year) in 1982. The application rates near wells BH 1and BH 2 in 1981 were 2 and 1.5 gal/acre, respectively. The change in concentration of ethylene dibromide in water in the aquifer does not appear to be directly related to the rate of application of the compound to the surface above the sampled wells. The ethylene dibromide concentration in these wells may reflect application of ethylene dibromide to surface sites at some distance from the wells. This would be consistent with the known fact that there is a significant horizontal movement of water in limestone aquifers. A thorough study of the relationship between application rates for nonpolar organic compounds and their concentrations in the aquifers in the region should give some detailed information concerning recharge of the acquifers from surface sources. EDB is surprisingly soluble in water (4300 ppm at 25 "C), and it is a nonpolar molecule that would not be expected to be strongly adsorbed on clay minerals. These two facts coupled with the relatively high use rate of EDB as a soil nematocide are probably responsible for the presence of the compound in the Ocala Aquifer. The fact that EDB is known to be a carcinogen (2), a mutagen (3), and a teratogen (4) suggests that this compound may have posed a human health hazard when it was used as a soil fumigant. Registry No. EDB, 106-93-4; atrazine, 1912-24-9; chlorothalonil, 1897-45-6; chloropicrin, 76-06-2; alachlor, 15972-60-8; ethoprop, 13194-48-4;pendimethalin, 40487-42-1;water, 7732-18-5.
Literature Cited (1) Bellar,T. A.; Lichtenberg, J. J. J. Am. Water Works Assoc. 1974, 66, 739-744. (2) Powers, M. B.; Voelker, R. W.; Page, N. P.; Weisburger, E. K.; Kraybill, H. F. Toxicol. Appl. Pharmacol. 1976,33, 171-172. (3) Strickman, D.; Drawbaugh, R. B. J. Enuiron. Health 1982, 45,74-77. (4) Am. Conf. Gov. Ind. Hyg. 1980, 180. Received for review February 6, 1984. Accepted May 29, 1984.
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