Environ. Sci. Technol. 2001, 35, 4592-4596
Emission of Chiral Organochlorine Pesticides from Agricultural Soils in the Cornbelt Region of the U.S. ANDI D. LEONE,† SAM AMATO,† AND R E N E E L . F A L C O N E R * ,‡ Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, and U.S. EPA, National Exposure Research Lab, Research Triangle Park, North Carolina 27711
Several organochlorine pesticides are chiral molecules manufactured as racemic mixtures. Past research has shown that selective degradation of pesticide enantiomers by microorganisms occurs resulting in nonracemic signatures in soils. In this work, volatilization of chiral pesticides from soil was investigated to determine if enantioselective breakdown in soils could be used as a source signature to track releases of chiral pesticides to the atmosphere. Air samples were taken directly above agricultural soils at several sites, and enantiomeric signatures were found to be nonracemic following patterns found in the soil. A follow up study at one site showed that for most compounds concentration decreased with increasing height above the soil, while enantiomer fractions for chiral pesticides were similar to that found in the soil, signifying the soil as a source to the air. The enantiomer fractions of ambient air samples from rural nonagricultural areas in the region were also found to be nonracemic.
Introduction Organochlorine (OC) pesticides were used heavily on farmlands in the United States and Canada during the 1960s and early 1970s. Because OC pesticides and their metabolites are highly persistent, residues remain in many soils. Volatilization from reservoirs of accumulation, including soils, may be a significant source of these “old” pesticides to the atmosphere for decades after their usage has been stopped. Several OC pesticides are chiral and manufactured as racemic mixtures of enantiomers. Some chiral OC pesticides include R-hexachlorocyclohexane (R-HCH), trans- and cischlordane (TC, CC), heptachlor (HEPT), and o,p′-DDT. The pesticide metabolites oxychlordane (OXY), heptachlor exoepoxide (HEPX), and o,p′-DDD are also chiral. Pesticides are lost from soils primarily by physical processes (volatilization, leaching, erosion), chemical breakdown, and microbial attack. The last mechanism can result in enantioselective degradation in soil and other media. Since enantiomers have identical chemical and physical properties, abiotic processes are nonenantioselective. Thus, if degradation in the environment is dominated by nonenzymatic processes, the fraction of each enantiomer will remain the same (i.e. a racemic mixture). Chiral pesticides that are degraded by biological systems, however, often produce nonracemic residues. Past * Corresponding author phone: (412)365-1166; e-mail: rfalconer@ chatham.edu. Current address: Department of Chemistry, Chatham College, Woodland Rd., Pittsburgh, PA 15232. † Youngstown State University. ‡ U.S. EPA. 4592
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research has shown that selective enzymatic degradation of chiral OC pesticides by microorganisms does occur and can result in nonracemic signatures of chiral pesticides in soils (1-3). These signatures may be useful for tracking releases of chiral pesticides to the atmosphere. Recent research in the southern U.S. has shown the importance of southern soils as a source to regional ambient air for several OC compounds (4). The current work was done to determine if soil is also an important contributor of OCs to ambient air in the cornbelt region where both physical conditions and historical usage patterns were very different. For this study, air was collected directly above agricultural soils to determine whether enantiomeric signatures of OCs were altered upon volatilization from the soil into the atmosphere. This was followed by a soil-to-air volatilization study to determine how concentration and the chiral signature change with height above the soil. Ambient air samples were also taken in rural, nonagricultural locations to obtain background signals and for comparison of enantiomer signature and concentration with ambient air from the Great Lakes and indoor air from the cornbelt region. By investigating concentrations and enantiomer fractions (EFs) of chiral pesticides, conjectures to the importance of soil emissions to ambient air in the Great Lakes and other sensitive regions can be made. These data may also be useful for determining regional mass fluxes as well as for use in global models of OC chemical recycling.
Experimental Section Sample Collection. Three different types of air samples were collected during fall of 1996, spring/fall of 1997, and throughout 1998 in Pennsylvania, Ohio, Indiana, and Illinois as well as one sample in Missouri. Six above-soil air samples were collected (for enantiomeric analysis only) at agricultural sites where soils had been previously analyzed and found to contain detectable levels of several chiral OC pesticides (1). The samples were collected for 4-8 h approximately 15 cm directly above the soils using a high volume sampling train (Rotron DR-313 pump; flow rate ) 500 L/min). Air was passed through a glass fiber filter (to remove particulate matter) and target compounds were collected on a single 8 × 8.5 cm polyurethane foam (PUF) plug. Above-soil air samples are designated by the same name as the corresponding soil sample from Aigner et al. (i.e. PA1, OH5 (1)). A follow-up study was done at one of the above sites (OH13) to determine differences in concentration and EF with height. These air samples were collected at four heights above the soil (10, 50, 105, and 175 cm) using 3.5 cm diameter × 3.5 cm thick PUF plugs contained in PTFE filter holders. Glass wool was placed in front of the PUF plug in the filter holder to remove particulate matter. Samples were collected for 12 h periods (day/night) over 5 days with a low volume pump (flow rate ) 35 L/min). Thirteen ambient air samples were collected for 20-48 h in PA, OH, and MO (one sample) using the high volume sampler described above for the above-soil samples (flow rate ) 500 L/min). Ambient air samples, however, were collected ∼80 cm above the ground in rural but nonagricultural locations. These samples were analyzed for both enantiomer fractions and concentrations of chiral compounds. Ambient air samples are designated by the suffix -AA. (The sample designations for ambient air do not correspond to the same location as similarly designated samples from the above-soil or soil samples.) Immediately following sampling, all sample PUF were placed individually 10.1021/es010992o CCC: $20.00
2001 American Chemical Society Published on Web 11/02/2001
TABLE 1. Comparison of EFsa for Matching Soil and Air-Above Soil Samples TC soil PA2 OH5 OH13 IN5 IN6 IL5 a
0.375 0.398 0.441 0.363 0.459 0.398
EF ) A+/(A+ + A-).
CC air 0.371 0.471 0.412 0.448 0.419 0.419
b
soil 0.543 0.567 0.524 0.561 0.497 0.583
HEPX
o,p′-DDT
OXY
air
soil
air
0.554 0.517 0.537 0.515 0.507 0.526
NDb
NDb
0.781 0.615 0.810 NDb 0.785
0.766 0.624 0.678 NDb 0.729
soil 0.634 0.495 0.471 0.444 0.573 0.401
air
soil
air
0.608 0.541 0.502 0.465 0.533 0.425
NDb
NDb NDb 0.457 NDb NDb 0.495
NDb 0.448 NDb NDb 0.519
ND ) not determined, below detection.
into precleaned glass jars with Teflon lined lids and stored at 4 °C until further workup. Extraction and Cleanup. Sample PUF plugs were Soxhlet extracted with petroleum ether for 18-24 h. Extracts were reduced into 20 mL of hexane by rotary evaporation and further reduced into 1 mL of isooctane with a gentle stream of nitrogen. Extracts were cleaned up and fractionated on a column of silicic acid overlaid with neutral alumina and capped with sodium sulfate as previously described (5). The sample was applied in 1-2 mL of isooctane and eluted in two fractions. Fraction 1 (30 mL petroleum ether) contained the polychlorinated biphenyls and heptachlor. Fraction 2 (30 mL dichloromethane) contained the rest of the pesticides of interest. Both fractions were reduced into 1-2 mL of isooctane with nitrogen. A portion (10%) of each sample was removed and analyzed for HEPX, OXY, and dieldrin. The remainder of the sample was further treated by shaking with 0.5 mL of 18 M H2SO4 and adjusted to a suitable volume for analysis. Mirex, an OC pesticide not found during sample prescreening, was added as an internal standard for instrument calibration. Analysis. Quantitative analysis of samples was carried out with a Hewlett-Packard 5890 gas chromatograph equipped with an electron capture detector (GC-ECD) using a DB-5 column (60 m, 0.25 mm i.d., 0.25 µm film thickness; J&W Scientific). Samples were injected splitless (split opened after 1.0 min) at an initial temperature of 90 °C. After a 1-min hold, the oven was ramped at 10 °C min-1 to 160 °C, 2 °C min-1 to 240 °C, and 20 °C min-1 to 270 °C and held for 10 min. Injector and detector temperatures were 250 °C and 300 °C, respectively. The carrier gas was hydrogen at 60 cm s-1. Samples were quantified versus 4-8 standards that spanned a 1000-fold concentration range. Chromatographic data was collected and processed using HP Chemstation software. Enantiomeric composition was determined with a Hewlett-Packard 5890 GC-5989B MS Engine mass spectrometer (GC-MS) operated in the negative ion mode (NIMS). Separations were carried out using either a Betadex-120 column (20% permethylated β-cyclodextrin in SPB-35, 30 m, 0.25 mm i.d., 0.25 µm film thickness; Supelco Corp.) or a BSCD column (20% tert-butyldimethylsilylated β-cyclodextrin in OV-1701, 30 m, 0.25 mm i.d., 0.25 µm film thickness; BGB Analytik AG, Lettenstrasse 97, CH8134 Adliswil, Switzerland, column designation BGB-172). Samples (2 µL) were injected splitless (split opened after 1.0 min) at an oven temperature of 90 °C. After a 1-min hold, the following oven programs were used for the two columns: Betadex, 15 °C min-1 to 140 °C, 1 °C min-1 to 190 °C, hold 10 min, 20 °C min-1 to 230 °C, hold 10 min; BSCD, 15 °C min-1 to 140 °C, 2 °C min-1 to 210 °C, hold 10 min, 20 °C min-1 to 240 °C, hold 15 min. Carrier gas was helium at 50 cm s-1 (constant flow); injector and transfer line temperatures were 250 °C. The ion source and quadrupole temperatures were 150 °C and 100 °C, respectively. Methane pressure for chemical ionization was 1.0 Torr. The instrument was operated in the selected ion monitoring mode using the following ions: o,p′-DDT (m/z 246, 248),
R-HCH (m/z 255, 257), chlordanes (m/z 410, 412), HEPT (m/z 300, 302), HEPX (m/z 386, 388), OXY (m/z 420, 422), and endosulfans (m/z 404). Elution order for enantiomers was confirmed for HEPX, OXY, TC, CC, and R-HCH in this work with standards of the single-enantiomer pesticides (AXACT Standards, 203 Commack Rd., Suite 78, Commack, NY). The elution order for o,p′-DDT was determined from previously published work using the same column type. Endosulfan I was not quantified in this work but was monitored by GCMS due to its interference with the (-)-CC enantiomer on the Betadex column. If endosulfan I was present, the sample was further treated with 15% fuming sulfuric acid and reanalyzed. Quality Control. Blanks were processed by extracting and analyzing clean PUF plugs using the same procedure as for samples. Samples were not blank corrected as levels were below detection in blanks for both size PUF. Unfortunately, a very large interference with the p,p′-DDT peak resulted in none of the samples having p,p′-DDT concentrations above the limit of detection. Five spike recovery experiments were done for each size PUF. Clean PUF plugs were spiked with the components of interest, extracted, and analyzed following the same procedure as for samples. Recoveries ranged from 75 to 132% for all compounds analyzed for both PUF sizes, with the exception of the HCHs. HCH recoveries from spiked experiments were consistently below 50% for ambient air samples; therefore, HCH concentration data is not reported for these samples. Concentration data for the remaining 11 compounds required no recovery corrections in either size PUF. The enantiomer fraction (EF) is defined as A+/(A+ + A-) where A+ and A- correspond to the peak areas of the (+) and (-) enantiomers. If the identity of the (+) and (-) enantiomers is unknown (e.g. MC-5, a minor chiral component of chlordane), EFx is defined as A1/(A1 + A2), where A1 and A2 are the peak areas of the first and second eluting enantiomers on chiral column x. Thus, a racemic composition of a compound corresponds to EF ) 0.500. The relationship between EF and enantiomer ratio, ER (the more commonly used term to date) is EF ) ER/(ER + 1). Replicate injections of analytical standards reflected racemic compositions with a standard deviation of (0.002 or less for all compounds, demonstrating that chiral-phase GC-MS is capable of highly precise enantioselective analysis. As a quality control protocol, the following limits for acceptable EF values were set: (a) agreement of EF values at each of the two monitored ions within (5% and (b) agreement of area ratios of the two monitored ions for samples and standards within (5%.
Results and Discussion Above-Soil Samples. Above-soil air samples were collected (for enantiomeric analysis only) during fall 1996 and spring 1997 above five agricultural soils and one garden soil which had been previously analyzed for five chiral OC pesticides (1). Table 1 shows a comparison of EFs for the matching soil and air samples for o,p′-DDT, TC, CC, OXY, and HEPX. The VOL. 35, NO. 23, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Plots of concentration versus height for each 12 h sampling period and the average of all sampling periods for each of the five compounds. Sampling periods designated by day (D) or night (N). enantiomeric signatures of pesticides in the air above the soil generally followed the same patterns as in the soil, both in direction of degradation and in relative magnitude. For most samples, more degradation was seen in the soil than in corresponding air above the soil, possibly due to dilution by mixing with bulk air. The preservation of the EF signature above each soil gives direct evidence of the soil as a source to the air directly above it. Interestingly, two Indiana sites, which were side by side agricultural fields, showed very different selectivity in the soil samples as well as in the corresponding air above the soil for CC and OXY. For OXY, the EF is actually reversed in the two samples. This reversal of selectivity has also been reported in seawater and forest soils (6, 7). One of the fields was the site of a former house, which had been heavily treated with chlordane for termite control for several years before the land was turned into an agricultural field. Differences in enantioselective degradation in the two fields could be due to different application procedures, different technical formulations of chlordane used, or different microbial populations present when the chlordane was originally applied. Soil Volatilization Samples. Concentrations. A followup study was done at one of the above sites (OH13) in the summer of 1998 to determine differences in concentration and EF with height. Soil volatilization samples were quantified for TC, CC, trans-nonachlor (TN), and γ- and R-HCH. Plots of concentration vs height are shown for each of the five 12 h sampling periods for the five compounds in Figure 1. 4594
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TABLE 2. Average Chlordane EFs ((SD) at Different Heights above the Soila TC CC a
soil
10 cm
50 cm
105 cm
175 cm
0.441 (0.004) 0.524 (0.004)
0.426 (0.002) 0.533 (0.002)
0.433 (0.013) 0.534 (0.003)
0.433 (0.012) 0.531 (0.003)
0.439 (0.015) 0.527 (0.012)
EF ) A+/(A+ + A-).
Samples were not collected on nights three or five (N3 and N5) due to equipment malfunctions. R-HCH had the lowest concentrations for this study (ranging from 9 to 280 pg/m3), while TN was found at the highest concentrations (ranging from 130 to 1400 pg/m3). For all compounds except R-HCH, the concentration decreased with increasing height (Figure 1) suggesting the soil as a source of these compounds to the air directly above it. In a similar study, concentrations of several OCs, including the five studied in this work, were also found to decrease with increasing height above the soil (8). For R-HCH in the current study, the concentration remained constant at all heights above the soil (no significant difference at P ) 0.05). Previous analysis of this soil found R-HCH to be nondetectable (1). The low concentrations of R-HCH compared to the other compounds and the lack of detectable residues in the soil suggest background air as the source of R-HCH to the air above this soil. Higher concentrations were in general found for night samples possibly
TABLE 3. Concentrations (pg/m3) of Organochlorine Pesticides in Ambient Aira sample
HEPT
HEPX+OXY
aldrin
dieldrin
TC
CC
TN
p,p′-DDE
p,p′-DDD
o,p′-DDT
PA1-AA PA2-AA PA3-AA PA4-AA OH1-AA OH2-AA OH3-AA OH4-AA OH5-AA OH6-AA OH7-AA OH8-AA MO1-AA MAX MIN GEOMEAN
7.4 2.0 20 4.1 21 NDb 22 7.8 9.8 13 10 17 4.3 22 2.0 9.3
15 19 13 6.3 110 NDb 6.9 8.0 46 19 25 12 12 110 6.3 17
4.5 NDb NDb 1.7 17 NDb NDb NDb 6.6 8.0 7.8 12 1.2 17 1.2 5.4
2.5 NDb NDb NDb NDb NDb NDb NDb 2.9 2.5 1.4 21 1.7 21 1.4 3.1
24 1.1 26 4.4 61 NDb 16 14 26 25 25 41 9.1 61 1.1 16
26 2.6 27 7.0 64 NDb 12 19 29 27 29 40 10 64 2.6 19
25 1.1 26 8.0 72 NDb 8.0 9.5 28 25 31 33 9.6 72 1.1 16
7.2 1.1 18 2.8 0.60 NDb 33 15 11 11 NDb 12 3.6 33 0.60 6.3
5.8 NDb 2.2 0.67 0.81 NDb 1.4 2.3 1.4 3.5 1.8 1.5 5.5 5.8 0.67 2.0
37 NDb 13 3.7 9.1 NDb 4.2 2.8 27 57 11 31 16 57 2.8 13
a
p,p′-DDT was below detection for all samples due to a chromatographic interference. b ND ) not determined; below detection.
TABLE 4. Enantiomer Fractions (EFs) for Ambient Air
a
sample
TCa
CCa
MC-5b
r-HCHa
standard PA1-AA PA2-AA PA3-AA PA4-AA OH1-AA OH2-AA OH3-AA OH4-AA OH5-AA OH6-AA OH7-AA OH8-AA MO1-AA mean
0.502 ( 0.002 0.484 0.487 0.488 0.484 0.485 0.474 0.487 0.479 0.478 0.475 0.473 0.485 0.480 0.481 ( 0.005
0.499 ( 0.002 0.505 0.505 0.507 0.512 0.515 0.517 0.502 0.515 0.506 0.511 0.511 0.502 0.507 0.509 ( 0.005
0.498 ( 0.002 0.487 0.479 0.487 0.484 0.485 0.476 0.490 0.485 0.482 0.484 0.483 0.484 0.478 0.483 ( 0.004
0.503 ( 0.001 0.498 0.507 0.508 0.502 0.485 0.492 0.500 0.505 0.499 0.500 0.505 0.501 0.501 0.500 ( 0.006
EF ) A+/(A+ + A-).
b
EF ) A1/(A1 + A2).
due to less air movement during nighttime hours allowing the concentration directly above the soil to increase. Enantiomer Fractions. EFs for the soil volatilization study are given in Table 2 for TC and CC. Levels of the chiral pesticide R-HCH were too low for enantiomeric analysis in these samples. The soil sample from this location (1) was collected 2 years before the air samples that are reported here. The average EF remained relatively constant in the soil and the air at all heights above the soil for TC and CC. Decreasing concentrations with increasing height and constant EFs for soil and air above the soil at all heights for both TC and CC suggest the soil as a source of these pesticides to air. MC-5, another chiral component of technical chlordane, was not measured in the original soil study, but the MC-5 EFs in air at the different heights also remained constant (EF range ) 0.462-0.468). Finizio et al. (8) sampled soil and corresponding air-above-soil (5-140 cm) at agricultural sites in the Fraser Valley of British Columbia. They found very similar enantiomeric compositions in the soil and the air above the soil at all heights for o,p′-DDT, HEPT, and HEPX. However, for R-HCH the authors noted a clear trend toward lower ERs with increasing height and suggested mixing of soil volatilized R-HCH with racemic R-HCH atmospherically transported from elsewhere. Ambient Air Samples. Concentrations. Vapor phase concentrations determined for the 13 ambient air samples are shown in Table 3. Ambient air samples were collected in rural but nonagricultural locations from fall 1997 to fall 1998 in western Pennsylvania (n ) 4), eastern Ohio (n ) 8), and
Missouri (n ) 1). Pesticide residues above detection limit for the GC-ECD were found in 12 out of 13 samples. On average, concentrations of all compounds determined were similar to concentrations reported for ambient air from the Great Lakes region (9-14) but at least 2 orders of magnitude lower than average indoor air concentrations reported in a concurrent study in the same geographical region (15). Due to the limited number of samples, no trends based on season, location, or temperature were found. Average air temperature during sample collection ranged from -1 °C (OH4-AA, winter sample) to 30 °C (OH7-AA, summer sample). Levels for Σ(o,p′-DDT + p,p′-DDE + p,p′-DDD) for samples above the detection limits ranged from 1.1 to 71 pg/m3 with a geometric mean (GM) of 20 pg/m3 (Table 3). p,p′-DDT, the main constituent of the technical mixture, was not quantified in any of the samples due to a chromatographic interference, thus DDT/DDE ratios could not be calculated. Geometrical means for technical chlordane, which consists primarily of trans-chlordane (TC), cis-chlordane (CC), and trans-nonachlor (TN), were 21, 19, and 16 pg/m3, respectively (Table 3). ΣChlordanes (TC + CC + TN) for this study ranged from 3.7 to 196 pg/m3 which is similar to the means for Σchlordanes reported around the Great Lakes (9, 10). The ratio of TC/CC for the ambient air samples in this study averaged 0.89 ( 0.12, while in the technical mixture the ratio is approximately 1.1 (16, 17). The lower ratio in ambient air samples is likely due to TC degrading faster photochemically than CC (18). VOL. 35, NO. 23, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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HEPT was found in all but one sample with concentrations ranging from 2.0 to 22 pg/m3 and a GM of 9.3 pg/m3 (Table 3). HEPT is transformed in the environment to heptachlor epoxide (HEPX), which degrades more slowly and, thus, is more persistent (19). In the present study, HEPX and oxychlordane (OXY), a breakdown product of chlordane, were quantified together because the two compounds coeluted on the GC column. ΣHEPX + OXY, which was quantifiable in all but one sample, ranged from 6.2 to 110 pg/m3 with a GM of 17 pg/m3. Aldrin was found in eight of the 13 samples ranging from 1.2 pg/m3 to 17 pg/m3 with a GM of 5.4 pg/m3. Once in the environment, aldrin is quickly transformed to dieldrin (20). Dieldrin was detected in six of the 13 samples ranging from 1.5 to 21 pg/m3 with a GM of 3.3 pg/m3 (Table 3). Enantiomer Fractions. All 13 of the ambient air samples were analyzed on the GC-MS for enantiomeric data and EF values for TC, CC, MC-5, and R-HCH are given in Table 4. Levels of other chiral compounds were too low for accurate enantiomeric analysis. The mean EF ( standard deviation for R-HCH in the samples was 0.500 ( 0.006 (n ) 13), which was not significantly different from the standard. Mu ¨ ller et al. (21) found similar results in Norway where EFs for R-HCH in ambient air were also found to be racemic. Falconer et al. (22) found racemic EFs for R-HCH in air over Resolute Bay, Northwest Territories, Canada, even though water samples from a small arctic lake and seawater from the Bay showed selective degradation. Air samples taken 10 m above Lake Ontario showed a seasonal variability for R-HCH with racemic values in spring and fall but EFs as low as 0.476 in summer (23). The authors determined that minimum summer EF values occurred during the period of net R-HCH volatilization, suggesting that the air above the lake contains a mixture of racemic R-HCH from bulk tropospheric air (EF ) 0.500) and nonracemic R-HCH outgassing from the lake (lake average EF ) 0.459). The mean EF values ( standard deviation for chlordanes in 13 ambient air samples (Table 4) were 0.481 ( 0.005 for TC, 0.509 ( 0.005 for CC, and 0.483 ( 0.004 for MC-5. Mean sample values for all three compounds were significantly different from the respective standards (p ) 0.05). TC EF values ranged from 0.473 to 0.488, CC values ranged from 0.502 to 0.517, and MC-5 values ranged from 0.476 to 0.490. Wiberg et al. (24) reported EF values of 0.495 (n ) 20) for TC and 0.502 (n ) 20) for CC in ambient air from Muscle Shoals, AL. They also reported EF values of 0.500 (n ) 7) for TC and 0.505 (n ) 7) for CC in ambient air from Columbia, SC. Ulrich and Hites (25) reported an average EF for ambient air collected over Lakes Erie, Michigan, Ontario, and Superior as 0.468 for TC and 0.512 for CC. Bidleman et al. (26) found similar average values for air samples from Lake Ontario (0.479 for TC, 0.507 for CC, and 0.471 for MC-5). Comparison of the data shows degradation of chlordane in Great Lakes air to be similar to that of rural ambient air from the cornbelt region found in the current study. The EFs for both cornbelt and Great Lakes ambient air fall between the EF values found in air-abovesoil (nonracemic, current study) and indoor air (racemic (15)), suggesting soils may be an important source of chlordane to ambient air in the Great Lakes region.
Acknowledgments This work was supported in part by a contract from the Meteorological Service of Canada and by the U.S. Environmental Protection Agency through its Office of Research and Development. Mention of trade names or commercial
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Received for review May 23, 2001. Revised manuscript received September 12, 2001. Accepted September 18, 2001. ES010992O
