Environ. Sci. Technol. 2005, 39, 7005-7011
Toxaphene Deposition to Lake Ontario via Precipitation, 1994-1998 DEBORAH A. BURNISTON,* WILLIAM M. J. STRACHAN, AND ROBERT J. WILKINSON National Water Research Institute, Canada Centre for Inland Waters, 867 Lakeshore Rd, Burlington, Ontario, L7R 4A6, Canada
Precipitation samples collected continuously at Point Petre on Lake Ontario from November 1994 through December 1998 were analyzed for total toxaphene ()sum of hexa-, hepta-, octa-, and nonachloro bornanes) and chlorobornane congeners (1997-98 only). Composite triplicate samples were collected during 4-week intervals throughout the 4-year study using heated wet-only samplers. These results represent the first detailed data for toxaphene in Great Lakes precipitation. Seasonal volume-weighted mean concentrations for total toxaphene in precipitation ranged from 0.25 to 1.5 ng/L. Highest concentrations were found during the four spring (March-May) periods at roughly twice the annual means. The pattern for hexathrough nona-homologues over the 4 years did not vary appreciably with average ratios (relative to hepta-) of 0.08: 1.0:1.3:0.2. The volume-weighted mean concentrations for individual chlorobornane congeners were consistent in their season pattern with maximums seen in the spring. The major chlorobornane in precipitation, B8-2229 (Parlar 44), which was present at concentrations ranging from 0.016 to 0.079 ng/L, constituted 28 and 29% of the congener sum for 1997 and 1998, respectively. Lakewide loadings of toxaphene for Lake Ontario via precipitation were estimated to be 12, 17, 12, and 13 kg/year for 1995-1998, respectively. Previous toxaphene loading estimates were calculated for the individual Great Lakes on the basis of the only concentration data available, a single precipitation estimate of 0.2 ng/L from early work in northwestern Ontario. The loading estimates in this study indicate that precipitation inputs of toxaphene are 3-4 times higher than previously reported for Lake Ontario. The 1998 estimates of Lake Ontario wet deposition flux are 50% of the estimated gas deposition flux. However, wet flux values from this study exceed the net gas-phase mass transfer of toxaphene across the air-water interface.
Introduction Toxaphene (PCC) is the common name used for the pesticide consisting primarily of chlorinated bornane congeners. The product is synthesized by chlorination of alpha-pinene to a chlorine content of 67-69% which produces largely polychlorinated bornanes (1). The mixture has been found to contain more than 1000 components by two-dimensional * Corresponding author phone: (905)336-4703; fax: (905)336-4609; e-mail:
[email protected]. 10.1021/es050167y CCC: $30.25 Published on Web 08/09/2005
Published 2005 by the Am. Chem. Soc.
gas chromatography (2). It was first introduced by the Hercules Chemical Company in 1947 as a pesticide for use on cotton, replacing DDT. Total toxaphene use in North America has been estimated at 534 kt of which 490 kt was used in the United States, mainly in the Southeast, Delta States, southern plains, and Appalachian agricultural regions (3, 4). Toxaphene use was severely restricted, then banned, in Canada and the United States in 1982, although existing stocks were allowed to be applied for specific purposes through 1986, but use continued in Mexico and central America until about 1993 (4). Despite being banned for almost 20 years, toxaphene is still an environmental concern because it is persistent, bioaccumulative, and extremely toxic to aquatic organisms (5) as well as carcinogenic (6) and mutagenic (7). It is observed in water, fish, marine mammals, and lake sediments throughout the northern hemisphere (8, 9, 4). The earliest Great Lakes observations of toxaphene in atmospheric samples were from Lake Michigan where a maximum level in air at the northern part of the lake of 280 pg/m3 in 1981 was observed (10); higher levels were found south of the lake, closer to the corn belt area, in Greenville, MS, averaging 7 ng/m3. Seasonal averages reported by Hoff et al. (11) ranged from 3 to 71 pg/m3 for 1989-1990 samples from Egbert, Ontario, peaking in July-August. A workshop specifically focused on toxaphene in the Great Lakes (12) reported a new consensus value of 18 pg/m3 for 1992-1996, down from the 30-50 pg/m3 of the 1989-1990 period. Toxaphene has been measured in the air in the Lake Ontario region. Air concentrations over Lake Ontario in July 1998 of 19 ( 4 pg m-3 and 25 ( 20 pg m-3 in June 2000 were found by Jantunen and Bidleman, unpublished data cited in Muir et al. (4). Shoeib et al. (13) using different methodology found somewhat lower values of 1-18 pg/m3 (mean 3.8 pg/m3) for the period 1995-97. There has been far less study of toxaphene in precipitation than in air. In an early 1982 report, Swain, cited in Rice and Evans (10), stated that 1980-1981 samples from Lake Huron ranged between 7 and 108 ng/L and Rice and Evans (10) reported a concentration of 9 ng/L for southern Lake Michigan for the same period. A 1992 workshop on persistent organochlorines in Great Lakes air (14) reported an uncertain, basin-wide consensus concentration of 0.2 ng/L for the period 1989-1991 on the basis of measurements, 0.1-1.2 ng/L, in northwestern Ontario (14, 15). There are no further reports on toxaphene in precipitation in the Great Lakes at all or indeed worldwide for samples taken after 1982. The input of toxic chemicals, particularly the persistent organochlorines (OCs), to the Great Lakes via the atmosphere was addressed binationally in Annex 15 of the Great Lakes Water Quality Agreement. The Integrated Atmospheric Deposition Network (IADN) was established in 1990 to determine the atmospheric loadings of toxic chemicals to the Great Lakes to partially fulfill the obligations of this Annex. Master stations as shown in Figure 1 have been established on each of the Great Lakes, and air and precipitation samples were analyzed for several organochlorine pesticides and trace metals; the media included gaseous phase, air particulate, and precipitation (rain and snow). Sampling for this study was done at the Lake Ontario master station, located at Point Petre on the north central shore of the lake (43°50′34"N, 77°9′13"W). The half-life of OC pesticides in precipitation to the Great Lakes on the basis of IADN measurements (in part extended data from this study) have been estimated by Simcik et al. (16) and show a significant decrease over the early 1990s but do not include toxaphene. Precipitation (28-day samples continuously throughout the year) was examined VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Master stations on the Great Lakes for the integrated atmospheric deposition network. for toxaphene starting in December 1994 until December 1998.
Methodology (i) Sample Collection. The precipitation samples were collected using three winterized MIC-B precipitation samplers operating as wet-only collectors and equipped with a 0.2 m2 stainless-steel funnel leading to a column of XAD-2 (polystyrene divinylbenzene) resin in water (17). Winter funnel temperatures were maintained at 5-10 °C using a heated, insulated compartment under the funnel. The collection efficiency of these samplers, compared with precipitation reported by the Meteorological Service of Canada (M.S.C.) using a nipher gauge, ranged from 29% to 101% over the 4-year sample period. The collections of rain (March-November) had a seasonal efficiency greater than 65% (av 85%), whereas the efficiency for snow collection (December-February) was lower (minimum 29%, av 54%). In these 4 years, the collection precision among three colocated samplers for rain ranged between 0 and 21% (av 5%) and for snow between 3 and 15% (av 10%). Toxaphene and other neutral OC compounds were adsorbed from precipitation onto approximately 30 g of settled, precleaned, XAD-2 resin in water in a 20-mm i.d. thick-walled Teflon column. The samples for this study were originally collected in triplicate from December 1994 through November 1998 on a 28-d basis and then were shipped in a cooler to our laboratory (Burlington, ON), were extracted, and were analyzed for OC pesticides and polychlorinated biphenyls (PCBs). Sample columns were received within 1-2 days after removal from the sampler and were transferred to precleaned glass jars and were refrigerated until further processed. Experience has shown that samples with less than 2 L volume give anomalously high concentrations values for the OC pesticides; when such samples were received, the three replicate samples were combined as a single sample. (ii) Laboratory Processing. Samples were transferred to a 19 × 420 mm glass column and were drained. The analytes were eluted using 200 mL methanol followed by 250 mL dichloromethane, and the mixed solvents were backextracted using 200 mL of 3% sodium chloride. The organic phase was separated; the aqueous phase was washed with a further 2 × 50 mL DCM and the combined organic solvent layer was dried with sodium sulfate before solvent exchanging 7006
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into 1 mL isooctane. Cleanup and fractionation of the samples were done on 12-mm i.d. glass columns filled to a height of 20 cm with 70/230 mesh fully activated silica gel (activated at 350 °C for 16 h), settled in hexane. Fraction A was eluted with 50 mL hexane and contained the chlorobenzenes, all of the PCBs, and a few OC pesticides. Fraction B, obtained with 50 mL 50:50 hexane/dichloromethane, contained the polyaromatic hydrocarbons (PAHs), most of the OC pesticides, and all of the toxaphene. Each fraction was exchanged into isooctane and was reduced to 1.0 mL. Determination of toxaphene was done by combining 0.6-0.8 mL from each of the three fraction B’s from a sample period, and the combined subsamples were then reduced to 0.1 mL. The 1994 replicates were not combined. (iii) Analysis and Data Development. The analysis of the sample extracts was performed on a Hewlett-Packard 6890 GC equipped with an HP 5973 mass selective detector, using electron capture negative ion mass spectrometry (ECNI-MS) (helium carrier). An HP5-ms (5% diphenyl-, 95% dimethylsiloxane), 30 m, 0.25-mm i.d. column with a 0.25-µm film was used at an initial oven temperature of 40 °C and was held for 1 min, was ramped at 10 °C/min to 200 °C, then at 1.5 C/min to 230 °C, followed by 10 °C/min to a final temperature of 280 °C where it was held for 75 min. Sample volumes of 1 µL were injected, pulsed splitless, the split was opened after 1.5 min, and the injector temperature was 250 °C. Total toxaphene, defined as the sum of the concentrations of the chlorobornane homologues hexa- through nona-, was quantified by comparison of sample chromatograms with those of technical toxaphene (Hercules Chemical Co.); a similar method has been described by Swackhamer (18), and this methodology is adapted from there. [13C8] mirex was added, as an internal standard, to the samples just prior to analysis. Ions monitored were (target/qualifier) Cl-6 307/ 309; Cl-7 343/345; Cl-8 377/379; and Cl-9 411/413. Mirex was monitored with ions 447/449. While it is recognized that toxaphene congeners can undergo multiple chlorine losses that can show up in the late portion of the lighter homologue windows, this method did not correct for this. The dechlorination of a congener occurs in the detector after chromatography and therefore will have the same retention time across relevant ion tracks. The difficulty in removing a suspect peak from a lower homologue group that has a common
FIGURE 2. Toxaphene and homologue concentrations in precipitation at Point Petre. retention time with a peak from the heavier ion track is the existing coelution of different congeners across the different homologue groups. This has been demonstrated by Shoeib et al. (19) most notably with B8-1413 (P26), a dominant octa congener in the technical standard that coelutes with eight hepta congeners. While the contribution of B8-1413 to the single peak in the technical standard was 35% of the ECD response, this percentage could certainly differ in environmental samples, and therefore any contribution from the dechlorination of B8-1413 to the hepta congeners could not be determined. The comparison of homologue groups in this study, however, appears to be appropriate as there are no peaks that have common retention times across the different ion traces including B8-1413 which was not detected in the samples. Ions were monitored for β-endosulfan and chlordanes, and peaks with corresponding retention times were omitted from the total area. The β-endosulfan interference occurred most noticeably in the spring periods. In the eight samples where this interference was found, the average area reduction was 14% of the octachloro-homologue area (5% of the total homologue area). Potential interferences by chlordane congeners were monitored but were not significantly realized in any of these samples. The 25 congener standard (Tox 482) was purchased from Promochem (Wesel, Germany). Isotopically labeled [13C8] mirex was purchased from Cambridge Isotope Laboratories. Relative response factors to the [13C8] mirex were calculated from the standard and were used to quantify peaks that met the criteria of retention time ((2 s from standard peak) and determined ion ratios ((30% as determined from the standard run). Recoveries of toxaphene were examined by spiking three XAD-2 columns with 1.0 mL of 0.1 ng/µL technical toxaphene in isooctane (100 ng each column). The different homologue groups were recovered at an average of 110% with an RSD range of 9-36%. The values for total toxaphene were similar at a mean of 108 ( 12%. The separation of PCBs and toxaphene in these samples (into Fractions A and B, respectively) was clean, and tests run with congener B81413 (Parlar 26), the most likely crossover component, showed that at least 95% of this toxaphene component was found in Fraction B. The laboratory also participated in a Northern Contaminants Program Interlaboratory study on toxaphene analysis which included 10 labs previously or currently involved in the analysis of toxaphene in samples from the Great Lakes (20). All of the laboratories did very well with a coefficient of variation for the congener specific analysis ranging from 7% to 25%. While this laboratory performed in the top half of the results for the total toxaphene analysis, the overall coefficient of variation for the two technical standards were, for all 10 laboratories, 48% and 43% with the resultant means and medians significantly lower than the target values. Data comparisons may also be used to assess analytical quality
and is demonstrated here with results from a 1996 and 1997 Lake Superior water survey which was collected simultaneously and was analyzed for toxaphene independently by Jantunen and Bidleman (21) and by this laboratory. The Jantunen and Bidleman results for 1996 and 1997 averaged 918 ( 218 pg/L, n ) 26, (21) while this lab found slightly lower values of 815 ( 230 pg/L, n ) 43. This 11% difference is remarkably small considering the sampling differences and analytical complexities of toxaphene. It is also very close to the 16% recovery correction which Jantunen and Bidleman applied to the samples. This lab does not apply any such corrections. Method detection limits (MDL) for toxaphene and toxaphene congeners were calculated using results for method (reagent) blanks and instrumental detection limits (22). Total toxaphene had an MDL of 0.25 ng/L while congener detection limits range from 0.001 to 0.08 ng/L on the basis of a 5-L sample and are based on estimated instrument detection limits with a signal-to-noise ratio of 3. Instrument responses in blank samples, when present, consisted mainly of low level peaks at m/z 377-379; however, because of the very low and inconsistent levels, the sum of peaks in blanks was never greater than 1% of the homologue total, and so no correction for blanks was necessary.
Results The individual concentrations and the volume-weighted seasonal and annual concentrations of toxaphene are available in the supplementary information where the dates for the samples are given as the Julian day for the midpoint in the sample period to facilitate plotting and comparison between the years. Figure 2 presents the concentrations of the four homologue groups and of total toxaphene plotted against the Julian date. The ranges of seasonal and annual volume-weighted concentrations for total toxaphene were also similar; values for 1995 total concentrations ranged from 0.5 to 1.5 ng/L (annual 0.71 ng/L), for 1996 the values were 0.38-1.5 ng/L (annual 0.85 ng/L), for 1997 the values were 0.27-1.0 ng/L (annual 0.68 ng/L), and for 1998 the values ranged from 0.25 to 1.4 ng/L (annual 0.77 ng/L). Generally, the spring level was roughly twice the annual value. An apparent outlier can be seen in Figure 2 for winter 1998. This sample date corresponds to the period immediately after a major ice storm in January 1998. The reasons for this elevated concentration are unclear. The homologue percentages of total toxaphene for each of the volume-weighted seasonal concentrations show only modest differences in the proportions of the homologues in the precipitation. The samples from winter 1995 were considerably elevated in the percentages of heptachlorobornanes although this was not the case for 1996-1998. All of the seasons in 1996, however, showed moderate elevations in the hepta homologue. With winter 1995 as the only exception, the “normal” pattern shows octa g hepta f nona VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Concentration (ng/L) and Flux Rates (ng/m2/Season) of Total Toxaphene on a Seasonal and Annual Basis at Point Petre 1995-1998 winter spring summer autumn annual precip. (mm)
1995
1996
1997
1998
0.56 (110) 1.5 (200) 0.73 (140) 0.50 (180) 0.71 (630) 883
0.88 (180) 1.5 (340) 0.87 (220) 0.38 (130) 0.85 (880) 1031
0.88 (240) 1.0 (210) 0.27 (56) 0.52 (140) 0.68 (650) 957
0.88 (220) 1.4 (280) 0.63 (180) 0.25 (40) 0.77 (690) 892
> hexa over the 4 years. This trend holds true even for the spring seasons when the concentrations were elevated. The nona- homologue group was much higher in technical toxaphene relative to the proportions observed for samples.
Discussion There are few published data available for toxaphene in precipitation in the Great Lakes, and these are generally single samples collected over a short summer period in different years (with total toxaphene determined) using GC-ECD rather than GC-ECNI-MS as used in this study. Consequently, meaningful comparisons and the development of a reliable trend rate is problematic. Concentrations ranged from 10 to 100 ng/L in the early 1980s in Lakes Michigan and Huron (23) and as low as 0.2 ng/L for ca. 1990 (14). By comparison, annual levels for total toxaphene reported in Table 1 range from 0.68 to 0.85 ng/L. It is apparent that the Point Petre data are below the data of the early 1980s when toxaphene usage was still practiced and are above the 0.2 ng/L consensus concentration although the latter was based on data from the Experimental Lakes Area (ELA), 250 km east of Winnipeg and 50 km east-southeast of Kenora, which may not be a representative location, using GC-ECD methodology. Also, the consensus annual value of 0.2 ng/L was estimated from a data set range of B8-789 > B8-1471 > B8-1414/ B8-1945. Shoeib et al. (13) found B8-1413 and B9-1679 to be enriched in the air samples taken at Point Petre (7.1% and 6.1%, respectively, of the total) in comparison with the technical mixture (0.49% and 1.5% of the total for the same congeners), but this was not the case for the precipitation
FIGURE 3. Volume-weighted mean concentrations for toxaphene congeners.
FIGURE 4. Seasonal loadings of toxaphene homologues to Lake Ontario. samples for the same time period. The precipitation samples were below detection for B8-1413 while B9-1679 was detected at 1.3% of the total. The congener pattern for annual volumeweighted mean concentrations shown in Figure 3 closely resembles those for the individual season profiles with congener B8-789 showing the most variation. While congener data is not available for Lake Ontario surface waters, the Lake Superior water (36) shows a similar congener profile with three prevalent congeners B8-2229, B8-789, and B81414/B8-1945 among the top five. The most notable difference in the profiles is the elevated B7-1001 concentration in the lake water. In water, its concentration is second only to B8-789 (36) which may result from a disproportionate uptake of B7-1001 from air, where its concentration is reported as the highest (13). Figure 4 shows a graphical presentation of the direct seasonal loadings of toxaphene via precipitation over the 4-year period (Lake Ontario surface area 19 000 km2) calculated using current IADN convention (37). Flux rates and loadings were calculated using the precipitation data supplied by IADN. The current IADN protocol obtains rates of precipitation obtained from the Great Lakes Environmental Research Laboratory and are averaged monthly. These estimates use multiple land-based measurements to interpolate area weighted over the lake precipitation amounts. Differences between on-site measurements and over the lake measurements differ from between (0.5 to 44%, average 16% seasonally, and between (5 and 12%, average 8.8% annually, over the 4-year study period. Fluxes are given in Table 1. Lake Ontario seasonal loadings (ng/m2) are presented graphically (Figure 4) assuming a concentration in precipita-
tion across all the lake which is represented by the data from Point Petre. Annual fluxes of total toxaphene at Point Petre ranged from 630 to 880 ng m-2 yr-1 for the period 19961998. Swackhamer et al. (38) have provided the most comprehensive assessment of the different pathways of toxaphene inputs to the Great Lakes and have used data of Eisenreich and Strachan (0.2 ng/L) (14) for precipitation inputs. They reported a net gas-phase input rate for Lake Ontario of 3400 ng m-2 y-1 (65.5 kg/yr) with most of this being delivered in the summer (maximum 2500 ng/m2/season). Precipitation and dryfall rates were estimated as a relatively small fraction of the summer gas-phase net rate, 180 ng m-2 yr-1 and 59 ng m-2 yr-1, respectively. These estimates for precipitation are 3- to 4-fold lower than our measured toxaphene fluxes. While it appears that the precipitation is a small fraction (4%) of the total, much of the data used to calculate the loadings were best estimates at the time with appropriate precipitation data almost nonexistent. Pearson et al. (33) reported sediment accumulation rates for toxaphene of 3900-6000 ng m-2 yr-1 for 1990-1991 surficial sediment while Howdeshell and Hites (32) using results from eight Lake Ontario cores found surface fluxes (1985-1990) averaging 8800 ng m-2 yr-1 for 1968-1982. A more recent core from the western basin showed a surface flux of 2900 ng m-2 yr-1 for the period 1995-1998 (15). Thus, precipitation fluxes may represent a significant fraction (about 20-25%) of sediment deposition inputs to Lake Ontario in the mid-1990s. Seasonal net air-water flux, calculated as reported in Swackhamer (38): VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Seasonal Net Flux and Loading with Parametersa for Calculating Gas Exchange of Toxaphene to Lake Ontario 1998 wind speed, ms-1 water temp., K H, Pa m3 mol-1 Cwater, pg/L Cair, pg/m3 Ka, m/s fluxgas deposition, ng m-2 loadingair, kg/season fluxwet deposition, ng m-2 loadingwet, kg/season gas deposition/wet deposition fluxnet air-water, ng m-2 loadingnet air-water, kg/season
summer
autumn
winter
spring
annual
3.9 292 0.27 88 19 0.0038 570 11 180 3.4 3.2 280 5.3
5.9 288 0.19 88 7.4 0.0052 300 5.7 40 0.74 7.5 17 0.32
6.3 276 0.063 88 4.6 0.0055 200 3.8 220 4.1 0.90 95 1.8
4.9 278 0.076 88 8.7 0.0045 310 5.9 280 5.3 1.1 210 4.0
1380 26 690 13 2.0 600 11
a H is the Henry’s law constant calculated according to Jantunen (40), T is absolute temperature, K is the air-sided mass transfer coefficient a calculated according to Galerneau (41). The ideal gas constant used was 8.31 Pa m3 deg-1. The molecular weight is 414 g mol-1, and sum of atomic diffusions volume is 331.8. Summer Cair and Cwater, Jantunen and Bidleman, unpublished data as cited in Muir et al. (4). Fall, winter, and spring Cair and Cwater were estimated using seasonal profiles from Shoeib (13) and Glassmeyer (39). Precipitation, water temperature, and wind speed obtained from NOAA National Weather Service.
TABLE 3. Annual Volume Weighted Mean Concentrations of Organochlorines in Precipitation (ng/L) 1995 1996 1997 1998
ΣPCB 2.3 1.4 1.2 0.89
R-HCH 1.1 1.7 1.3 0.88
γ-HCH 1.1 1.4 1.9 0.89
toxaphene 0.71 0.85 0.68 0.77
fluxnet air-water ) ka [Cair - Cwater (H/RT)]
(1)
is shown with relevant parameters in Table 2. For Lake Ontario in 1998, using similar methodology, an open lake summer water concentration of 88 ( 16 pg/L and an open lake air concentration of 19 ( 4 pg/m3 was reported by Jantunen and Bidleman, unpublished data cited in Muir et al. (4). Seasonal Great Lakes air concentrations can be estimated from the work by Shoeib (13) in Lake Ontario and Glassmeyer (39) in Lake Michigan. Relative seasonal concentrations from these reports show a similar seasonal pattern which averages to spring (1.9):summer (4.1):fall (1.6):winter (1.0). Assuming that the water concentration remains constant, then a reduced net load (from 65.5 kg/yr) (38) of 11 kg/yr net gas absorption can be calculated. The wet deposition load for 1998 on the basis of this study is 13 kg/yr which can be compared to the air deposition calculated using the formula
fluxgas deposition ) kaCa
(2)
This gives an annual gas deposition to Lake Ontario of 1380 ng m-2 or 26 kg/yr. While the annual ratio of gas deposition to wet deposition is 2, it is the seasonal ratios that reflect the changing relative importance of the wet deposition to gas deposition. Across the seasons, the ratios for summer: autumn:winter:spring in 1998 were 3.2:7.5:0.9:1.1. While the air toxaphene concentrations are at their maxima in the spring, the relative importance of gas to wet loading is greatest in autumn while the wet flux dominates in the winter. While seasonal water and air concentrations would certainly improve the air-water flux calculation, and noting that the calculation of ka can vary significantly depending on wind speed, it is clear that precipitation is a major contributor to the Lake Ontario toxaphene mass budget. Toxaphene was a major OC pesticide in precipitation at Point Petre during the mid-1990s ranking second (average 750 pg/L) behind HCH isomers (average conc ) 4390 pg/L for the period 1990-1997) as shown by our extended data set reported in Simcik et al. (16). While the HCH isomers and total PCBs remain the most significant organochlorines in 7010
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t-DDT 0.65 0.43 0.70 0.32
dieldrin 0.22 0.24 0.20 0.18
trans-chlordane 0.015 0.022 0.031 0.012
cis-chlordane 0.013 0.019 0.006 0.018
precipitation for the time of this study (Table 3), the relative importance of toxaphene has increased over time. When compared to PCBs (banned in 1977), it is significant that a yearly decline from 1995 through 1998 accounting for a 61% reduction is seen, while toxaphene remained the same. This may be caused by the different uses of the two banned compounds. While the exposure of PCBs to the environment has declined because of sedimentation, degradation, and government initiatives, toxaphene has large reservoirs in soils which is thought to maintain high atmospheric levels (42).
Acknowledgments Dr. D.C.G. Muir is thanked for his tremendous input, guidance, and support in the preparation of this paper. Ms. L. Brown is thanked for her preparation and extraction of all the sample materials and Daryl Smith for field operations at the master station at Point Petre. Camilla Teixeira is also thanked for her contribution in the congener analysis. Financial support from the Canada-Ontario Agreement program and the AES/MSC is also acknowledged.
Supporting Information Available Two tables with the complete toxaphene data set reported for homologue and total concentrations. The tables also report seasonal and annual concentrations and flux rates for homologues and total toxaphene. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 25, 2005. Revised manuscript received June 23, 2005. Accepted June 28, 2005. ES050167Y
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