Environ. Sci. Technol. 1994, 28, 600-605
Preferential Dealkylation Reactions of *Triazine Herbicides in the Unsaturated Zone Margaret S. Mllls' and E. Michael Thurman US. Geological Survey, Water Resources Division, 4821 Quail Crest Place, Lawrence, Kansas 66049
The preferential dealkylation pathways of the s-triazine herbicides, atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine), propazine [2-chloro-4,6-bis(isopropylaminol-s-triazine], and simazine [2-chloro-4,6-bis(ethylaminol-s-triazine] , and two monodealkylated triazine metabolites, deisopropylatrazine (DIA 2-amino-4-chloro6-ethylamino-s-triazine)and deethylatrazine (DEA 2-mino-4-chloro-6-isopropylamino-s-triazine) were investigated on two adjacent Eudora silt-loam plots growing corn (Zea mays LJ. Results from the shallow unsaturated zone and surface-water runoff showed preferential removal of an ethyl side chain from atrazine, simazine, and DIA relative to an isopropyl side chain from atrazine, propazine, and DEA. It is hypothesized that deethylation reactions may proceed at 2-3 times the rate of deisopropylation reactions. It is concluded that small concentrations of DIA reportedly associated with the degradation of atrazine may be due to a rapid turnover rate of the metabolite in the unsaturated zone, not to small production levels. Because of continued dealkylation of both monodealkylated metabolites, a strong argument is advanced for the presence of a didealkylated metabolite in the unsaturated zone.
Introduction The widespread use of s-triazine compounds as preemergent herbicides in the production of corn (Zea mays L.), sorghum (Glycine max La),and orchard fruits has necessitated an understanding of the fate and potential accumulation of these compounds in the environment. The detection of dealkylated metabolites of atrazine in groundwater and surface water also has raised questions as to the importance, mobility, and stability of the dealkylated degradation products (1-3). Furthermore, the use of metabolites as tracers of groundwater and surface-water interactions has prompted a better understanding of their sources and rates of production (1-3), Although degradation reactions and general rates of dissipation of the s-triazine compounds in soils have been well-documented in the literature (4-15), the relative rates of dealkylation of parent triazines and monodealkylated metabolites in the environment have not been reported. Previous field and laboratory studies have reported deethylatrazine (DEA) as the prominent dealkylated metabolite of atrazine, with deisopropylatrazine (DIA) present often only at trace concentrations ( I , 15-19). Consequently, DIA has been regarded as a metabolite of minor importance in the degradation reactions of atrazine (1). However, 14C studies have shown that DIA has a shorter dissipation rate than DEA ( I I ) , indicating that its small concentrations may be due to a potential for rapid turnover in the unsaturated zone, not to its nonproduction. This study was designed to follow the relative rates of dealkylation of three parent triazine herbicides-atrazine, propazine, simazine-and two monodealkylated triazine metabolites-DIA and DEA-in the unsaturated zone and 600
Envlron. Scl. Technol., Vol. 28, No. 4, 1994
in surface runoff. This choice of parent triazines was used as each dealkylated metabolite had two sources, each source requiring the removal of either an ethyl or an isopropyl side chain. Atrazine and simazine both dealkylate to DIA by removal of an ethyl and an isopropyl side chain, respectively. Similarly, atrazine and propazine degrade to DEA by deethylation and deisopropylation, respectively. Differences in the concentration of the dealkylated metabolite from the two different sources should indicate any preferential removal of ethyl versus isopropyl side chain. Furthermore, because monodealkylated DIA and DEA have different side chains remaining, their relative rate of removal should provide further information on the lability of an ethyl side chain versus an isopropyl side chain. Therefore, the objectives of the study were to: (a) investigate the relative rates of removal of an ethyl side chain compared to an isopropylside chain from both parent herbicides and monodealkylated metabolites, (b) ascertain the preferential degradation pathway for the triazine herbicides in the unsaturated zone based on their structure, and (c) determine the potential 'flux' of the monodealkylated metabolites in the unsaturated zone and conclude whether a didealkylated triazine metabolite may be an important metabolite in this environment.
Experimental Procedures Laboratory Procedures. Methanol (Burdick and Jackson, Muskegon, MI), ethyl acetate, and isooctane (Fisher Scientific, Springfield, NJ) were pesticide-grade solvents used during the analyses of both soil and water samples. Deionized water was charcoal filtered and glass distilled prior to use. Atrazine and terbuthylazine standards were obtained from Supelco (Bellefonte, PA), and the triazine metabolites, deethylatrazine, deisopropylatrazine, and didealkylatrazine were from Ciba Geigy (Greensboro, NC). The C-18 solid-phase extraction cartridges (Sep-Pak from Waters, Milford, MA) contained 360 mg of 40-pm C-18-bonded silica. Standard solutions were prepared in methanol, and phenanthrene-& (New England Nuclear, Boston, MA) was used as an internal GUMS quantitation standard. Deuterated atrazine (Cambridge ScientificIsotopes) was used as a surrogate standard to determine recovery in soil extractions. Commercial atrazine (AAtrex, 4L), simazine (Princep, 80W), and propazine (Milogard, 80W) (all manufactured by Ciba Geigy) were obtained from a commercial retailer for application to the experimental plots. Soil Extraction. Soils were extracted according to the method described by Mills and Thurman (20). Briefly, 20 g of soil (or its wet weight equivalent) were spiked with the surrogate standard (deuterated atrazine), equilibrated for 1h, and then extracted with a methanokwater mixture (15:5,v:v) at 75 "Cfor 30 min. The sample was centrifuged, and the clear supernatant was poured directly into a 40mL tube for evaporation. The extraction procedure was
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Published 1994 by the American Chemlcal Society
repeated on the soil sample, and the second supernatant was combined with the first. The combined extract was evaporated using a turbovap (Zymark, Palo Alto, CA) until only 10 mL of water remained. This was transferred to a test tube for automated solid-phase extraction. Solid-Phase Extraction and GC/MS Analysis. Solid-phase extraction of the analytes was accomplished by the procedure previously described (20). Briefly, a Waters Millilab workstation (Milford, MA) was used for solidphase extraction of the analytes using (2-18 Sep-Pak cartridges (Waters, Milford, MA). Samples were spiked with a surrogate recovery standard, terbuthylazine (2.4 ng/pL, 100 pL), and pumped through the preconditioned C-18 cartridge at a rate of 20 mL/min. Analytes were eluted with ethyl acetate and spiked robotically with phenanthrene-dlo (0.2 ng/pL, 500 pL). Soil extract eluates were passed through an anion-exchange cartridge (QMA Accell resin, Waters of Millipore) and preconditioned sequentially with 2 mL each of water, methanol, and ethyl acetate to remove co-eluted humic substances. Finally, the extract was evaporated automatically usinga turbovap (Zymark, Palo Alto, CA) at 45 "C under a nitrogen stream to 50 pL. Automated GUMS analyses with SIM of the eluates were performed on a Hewlett Packard Model 5890 GC (Palo Alto, CA) and a 5970A mass selective detector (MSD), with operating conditions identical to those described previously (20, 21). Field Dissipation Plots. A field dissipation study was conducted at the Kansas River Valley Experimental Field, near Topeka, KS. Two 65-m2Eudora silt-loam plots were used, with a particle size distribution of 63-44% silt, 2650% sand, and 5-21 % clay in the top 45 cm; a pH range of 6.8-7.8; and an organic carbon content of 0.99%,0.69%, and 0.22 % at 15,30, and 45 cm, respectively (1,22). Bulk density increased from 1.53 gjcms in the top 15 cm to 1.65 g/cm3at 45 cm, and permeability of the top 45 cm ranged from 0.5 to 8.25 cmjh, depending on the relative proportions of silt, sand, and clay (1,22). Each plot had a slope of less than 1% and was isolated from the other by perimeter berms and flashing. Commercial atrazine was applied at a rate of 2 kg/ha (active ingredient) to one plot, and propazine and simazine were applied at 2 kg/ha each to the adjacent plot by spraying on May 22, 1990. Herbicides were incorporated to approximately 5 cm using a rotor tiller, followed by the planting of corn. Bromide was applied at a rate of 80 g/m2using a calibrated spray cannister over a 50-m2region of lysimeters and incorporated into the soil to approximately 5 cm using a hand hoe. Natural precipitation maintained crop growth and was supplemented with sprinkler-applied irrigation during May and July. A total of 42 cm of precipitation including irrigation was recorded throughout the season, with precipitation occurring approximately every 14 days. Surface runoff was collected down slope in a bucket level with the field surface and then pumped into a 380-L storage tank using a sump pump. The volume of surface runoff collected was recorded, and a 4-L sample of water was collected from the storage tanks for analysis. The samples were filtered through a Buchner funnel (Whatman filter paper no. 3) and preserved in 125-mL bottles at 4 OC until analyzed. Suction-cup lysimeters were installed in duplicate on the two plots at nested depths of 30,60,90,120, and 150 cm for unsaturated zone transport investigations. Laboratory studies have shown previously that suction lysim-
15
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Flgure 1. Concentratlon (pglkg, plotted on log scale) of atrazlne, DEA, and DIA in sol1 cores from 15, 30, and 45 cm on the atrazine experimental plot, May-October 1990.
eters quantitatively sample the triazine herbicides and metabolites without loss (1). Samples were collected after each precipitation event, beginning before herbicide application and continuing into November 1990. Background atrazine, propazine, and simazine, and metabolite concentrations in soil water were recorded prior to herbicide application (C0.80pg/L atrazine and DEA only detected), Soil cores were collected in 15-cm intervals to a depth of 45 cm in triplicate from three random sites across each plot, before and directly after application, and several times during the summer. Core samples were frozen until extracted. The soil core concentrations reported are averaged values from the triplicate cores from the three sampling locations ( n = 9) with a coefficient of variation of DEA > DIA in all soil cores at this time. By late season (October), DEA concentrations were equal to or exceeded atrazine concentrations in soil at all depths sampled except 15-cm cores while DIA remained at trace levels. The occurrence of DEA as the major dealkylated metabolite of atrazine has been reported in several atrazine dissipation studies, with the ratio of DEA to atrazine (deethy1atrazine:atrazine ratio, DAR) often exceeding 1 by late season (1, 2, 7, 14-16, 19). The metabolite DIA is also commonly reported as the minor metabolite, present only at trace levels in soil (1, 2, 7, 14-16,19). This pattern may reflect two reactions in the unsaturated zone: first, that atrazine may preferentially dealkylate to DEA and, second, that DIA may be rapidly degraded to another metabolite, perhaps didealkylatrazine (DDA). For example, the structure of atrazine and degradative pathways for the formation of DIA, DEA, and DDA are Envhon. Sei. Technol., Vol. 28, No. 4, 1994 801
CI
9
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Flgure 2. Dealkylatlon reactions of atrazine, simazine, and propazine to deethylatrazine (DEA), deisopropylatrazine (DIA), and didealkylatrazine (DDA).
shown in Figure 2. Atrazine degrades to DEA by the removal of an ethyl group and to DIA by the removal of an isopropyl group. DEA is further degraded by the removal of an isopropyl group and DIA by the removal of an ethyl group to DDA. Thus, both the production of DEA and the degradation of DIA proceed via removal of an ethyl group (Figure 2). Therefore, the higher levels of DEA and the lower levels of DIA during atrazine dealkylation may be a reflection of the greater ease of deethylation versus deisopropylation. Preferential removal of an ethyl versus an isopropyl moiety is a concept that has previously been suggested in the literature (7). To further investigate this hypothesis in a field dissipation study, two additional triazines, simazine and propazine, were applied to a separate experimental plot. Figure 2 also shows the structures of simazine and propazine and their respective dealkylation reactions. These two triazines were chosen because DIA is now produced by the removal of an ethyl group (from simazine) and DEA by the removal of an isopropyl group (from propazine), the opposite dealkylation pathways for atrazine. Preferential degradation of an ethyl moiety would therefore result in large concentrations of DIA compared to DEA, the opposite from the atrazine plot. Figure 3 shows the concentrations of simazine, propazine, and dealkylated metabolites in soil cores from three depths (15,30, and 45cm) through the season. Maximum concentrations of all compounds were observed early in the season in 15-cmcores,in the order propazine > simazine >> DIA > DEA. By late season (October), this order had changed in 30- and 45-cm cores to propazine > simazine > DEA > DIA. Although DIA concentrations exceeded DEA in early season, DIA had reduced to trace levels by late season and DEA became the major dealkylated metabolite on the propazine/simazine plot. These data show that propazine and DEA are the most resistant to degradation, both containing only isopropyl moieties. Simazine and DIA degrade more rapidly, both containing only ethyl moieties. This preferential dealkylation is most evident from the changing concentrations of metabolites. As a useful comparison of the relative concentrations of metabolites, the ratio of DIA to DEA is used, called the D2R (deisopropylatrazine to deethylatrazine ratio). A D2R of less than 1 indicates that DEA concentrations exceeded DIA. Figure 4 summarizes how the D2R changes through time 602
Envlron. Scl. Technol., Vol. 28, No. 4, 1994
-
-2
3
Concentration ( p g g )
Flgure 3. Concentrations (pg/kg, plotted on log scale) of simazlne, propazine, DEA, and DIA in soil cores from 15,30,and 45 cm on the propazine/simazine experimental plot, from May-October 1990.
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Figure 4. Ratio of deisopropylatrazlne to deethylatrazine (D2R)in soil cores from 15,30, and 45 cm on the atrazine and propazlne/slmazlne experimental plots, May-October 1990.
in the unsaturated zone on both the atrazine and propazine/simazine plots. The D2R remained constant in shallow cores from both plots throughout the season (0.47 atrazine plot, 3.01 propazine/simazine plot). In shallow soil the parent source term is large and metabolite production is large, overshadowingcontinued degradation of the metabolites. In all soil cores taken below this depth, however, the D2R declines to less than 1 through time regardless of the initial size of the ratio. At these depths the metabolites have moved chromatographically ahead of the parent triazines due to a large increase in solubility (19). Now degradation becomes the dominant reaction, and the ratio declines as DIA is preferentially decomposed over the more resistant DEA. The dealkylation rate of each parent triazine is less obvious due to dealkylation reactions contributing less than 10% ofthe overalldissipation process (15). However, the relative size of deethylation versus deisopropylation on the atrazine plot appears comparable to the propazine/ simazine plot. The ratio of deisopropylated atrazine: deethylated atrazine is equal to 0.47, and the ratio of
30
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Number of Days After Application of Herbicide
Figure 5. Concentrations (pg/L) of (A) atrazine, DEA and DIA from the atrazine plot and of (8)Simazine, propazine, DEA, and DIA from the propazlnelslmazlneplot in soil pore water from 30-cm lyslmeters from May to November 1990.
deisopropylated pr0pazine:deethylated simazine is equal to 0.3. Thus, the deethylation rates of atrazine and simazine appear similar and approximately two to three times more rapid than the rates of deisopropylation of atrazine and propazine. Furthermore, this indicates that removal of an ethyl side chain is preferential over an isopropyl side chain regardless of parent triazine. Metabolites in Unsaturated Zone Water. The concentrations of parent triazines and metabolites were followed in soil pore water collected from suction-cup lysimeters installed at 30-cm intervals from 30 to 180 cm in the unsaturated zone. Bromide was applied to the soil as a reference conservative tracer because of its large solubility, slight toxicity to plants, and small background concentrations (23). The leading edge of bromide was transported at an average rate of 1cm/day while the center of mass of bromide was transported at an average rate of between 0.75 and 0.5 cmfday. Detection of bromide occurred in 30-cm lysimeters within 21 days of application and was detected in 120-cm lysimeters after 168 days (October) (19). Parent herbicides and metabolites were not detected in 60-cm lysimeters, correlating to the small concentrations present in soil cores from 45 to 60 cm. This retarded transport presumably is due to sorption of the herbicides on organic matter and the clay fraction of the Eudora silt-loam, as bromide transport rates gave no indication of reduced hydraulic conductivity in the soil profile down to 120 cm. Figure 5A shows the concentration of atrazine, DEA, and DIA in the 30-cm lysimeter. Atrazine concentrations remained small throughout the season, DEA concentrations steadily rose to exceed atrazine by midseason with the DAR as large as 38 by late season, and DIA concentrations remained at trace levels. This pattern
of DEA > atrazine >> DIA correlates with the relative concentrations seen in soil cores from 30 to 45 cm by late season. Figure 5B shows that on the propazine/simazine plot, propazine and simazine remained at trace levels until propazine was detected in late season. Concentrations of DEA exceeded DIA throughout the season, with the D2R remaining below 1, consistent with the D2R ratios and residual propazine concentrations in soil cores from 30 to 45 cm by late season. The significantly larger DAR values in lysimeters compared to soil core concentrations again reflect the enhanced solubility of metabolites versus parents (19).AS lysimeters sample the water phase only, metabolites are preferentially sampled as they sorb less to soil. This emphasizes the need for the use of both soil cores and pore-water samples in field studies to understand transport and degradation. Soil cores provide the relative masses of each compound present while pore-water samples expose what is transitory and moving through the unsaturated zone to groundwater. Both soil core and lysimeter data described above provide strong evidence for the preferential removal of an ethyl moiety. Nevertheless, degradation of the isopropyl moiety may still be a significant dealkylation reaction in the unsaturated zone and an important dissipation route. The large concentrations of DEA produced by the removal of an isopropyl group from propazine in both soil cores and 30-cm soil pore water is evidence that deisopropylation occurs to a significant degree. Therefore, the commonly reported trace levels of DIA from atrazine deisopropylation may not be due tononproduction of the metabolite. Rather DIA is produced in significant concentrations, but it too is rapidly degraded, preventing accumulation. If this is the case, a didealkylated metabolite should be present in the unsaturated zone. This metabolite has previously been identified at small concentrations in a study of the dissipation of atrazine, simazine, and cyanazine (24) and 14Csoil column studies (25, 26). DDA has also been shown to be nonphytotoxic in oat bioassays (27) and has a rapid mineralization rate in soil (11). Furthermore, preliminary melting point determinations of the DDAmetabolite in our laboratory indicate that it is greater than 350 "C, with an aqueous solubility less than that of atrazine (33 mg/L). Despite its low aqueous solubility, however, adsorption of DDA onto nonpolar C-18 solidphase resins is limited (seen as rapid breakthrough) (21) due to a lack of nonpolar functional groups on DDA. Thus, soil organic matter may not effectively adsorb DDA in the soil profile, and retardation may primarily occur via hydrogen bonding and ion-exchange interactions between the primary amine moieties on DDA and cation-exchange sites on clay minerals such as smectite (28). Currently, however, DEA is the metabolite of most concern in the dissipation of atrazine because of its increased solubility (19),resistance to continued breakdown, and remaining herbicidal activity (29). Metabolites in Surface Runoff. Metabolites and parent herbicides were detected in surface runoff from both experimental plots, and the D2R ratios provide a snap-shot view of the dealkylation processes at work on the soil surface where the parent source term is largest and a large microbial population is present (30-32). Figure 6 shows the D2R in surface-water runoff from both experimental plots through time. The D2R remained essentially constant and less than 1 on the atrazine plot Environ. Sci. Technol., Vol. 28, No. 4, 1994
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of atrazine based on the same preferential dealkylation assumptions. Atrazine will undergo preferential deethylation to produce DEA (fast) and slower deisopropylation to form DIA (slow). Because continued DIA dealkylation is fast compared to DEA, it remains at trace concentrations, DEA is the major metabolite, and the D2R remains less than 1.
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9
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1990
Flgure 6. Ratio of deisopropylatrazineto deethylatrazine (D2R)values for surface-waterrunoff on (A) atrazine and (B) propazine/simazine plots. Surface-waterrunoff took place between May and August 1990.
A
XNAH 9” \ETHYL
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Figure 7. Preferential dealkylation pathways for atrazine, simazine, propazine, DEA, and DIA in the unsaturated zone.
(mean = 0.47, n = 10, coefficient variation = 1 7 % ) and greater than 1 on the propazine/simazine plot (mean = 1.3, n = 10, coefficient variation = 24%))consistent with D2R values in shallow soil cores on the atrazine plot, but a lower value than cores from the propazine/simazine plot. The D2R ratios at the surface soil represent a fine balance between production and continued dealkylation of the metabolites due to the larger microbial activity in shallow soil (30-32). A kinetic scheme to describe this production/ removal balance is shown in Figure 7, based on the evidence previously presented of preferential deethylation versus deisopropylation. Figure 7 shows that simazine dealkylation to DIA will be fast, but continued dealkylation of DIA also will be fast, and actual concentrations of the metabolite will remain small. Propazine deisopropylation to DEA is slow, so initial DEA concentrations will be smaller than DIA from simazine. However, the removal of DEA is also slow, resulting in its concentration. The result is almost equal concentrations of DIA and DEA in surface water on the propazine/simazine plot, hence the D2R remains close to 1. The ratio of propazine-to-simazine concentrations in surface-water runoff did not change perceptibly during the runoff period, again indicating that dealkylation constitutes only a small percentage of the dissipation term. Figure 7 also shows a kinetic scheme for the dealkylation 804
Environ. Sci. Technol., Vol. 28, No. 4, 1994
Under field conditions, the preferential removal of an ethyl side chain from atrazine, simazine, and DIA is observed compared to the removal of an isopropyl side chain from atrazine, propazine and DEA. The large D2R ratio in shallow soil on the propazine/simazine experimental plot is evidence that simazine is degrading more rapidly to DIA than is propazine to DEA. Furthermore, deethylation rates of atrazine and simazine are comparable and approximately 2-3 times more rapid than the rates of deisopropylation from atrazine and propazine. This indicates that the removal of an ethyl side chain is preferential over an isopropyl side chain regardless of parent triazine. Continued dealkylation of the monodealkylated metabolites at depth in the unsaturated zone, where the parent source term is removed, also shows a preferential removal of an ethyl side chains over isopropyl side chains, as DIA concentrations decrease relative to DEA. The production of DEA from propazine indicates, however, that deisopropylation can occur to a significant degree in the unsaturated zone. Therefore, the small concentrations of deisopropylated atrazine (DIA) commonly reported in the environment do not result purely from a smaller production of the metabolite but to a rapid removal once produced. This substantial turnover rate or ‘flux’ of DIA in the environment is evidence for the presence of a didealkylated metabolite in the unsaturated zone.
Acknowledgments The authors thank Philip L. Barnes, Kansas State University, for his valuable assistance in the field. The authors also thank Gail Mallard, Surface and Ground Water Toxic Program, US.Geological Survey, for financial support of this research.
Literature Cited (1) Adams, C. D.;Thurman, E. M. J . Enuiron. Qual. 1990,20, 540. (2) Squillace, P. J.; Thurman, E. M.; Furlong, E. T. Water Resour. Res. 1993,29,1719. (3) Thurman, E.M.; Goolsby, D. A.; Meyer, M. T.; Mills, M. S.;Pomes,M.L.;Kolpin,D.W. Enuiron. Sci. Technol. 1992, 26, 2440. (4) Armstrong, D.E.;Chesters, 6.;Harris, R. F. Soil Sci. SOC. Am. Proc. 1967,31,61. (5) Kaufman, D. D.; Kearney, P. C. Residue Reu. 1970,32,235. (6) Goswami, K.‘P.;Green, R. E. Enuiron. Sci. Technol. 1971, 5,426. (7) Skipper, H. K.; Volk, V. V. Weed Sci. 1972,20, 344. (8) Giardina, M. C.; Giardi, M. T.; Filacchioni, G. Agric. Biol. Chem. 1980,44,2067. (9) Gamble, D.S.;Khan, S. U. J. Agric. Food. Chem. 1990,38, 297. (10) Muir, D.C.;Baker, B. E. J. Agric. Food Chem. 1976,24, 122. (11) Winkelmann, D.A.;Klaine, S. J. Enuiron. Toxicol. Chem. 1991,10, 347.
(12) Hall, J. K.; Murray, M. R.; Hartwig, N. L. J.Environ. Qual. 1989,18,439. (13) Bowman, B. T. Environ. Toxicol. Chem. 1991,IO, 573. (14) Frank, R., Clegg,B. S.,Patni, N. K. Arch. Environ. Contam. Toxicol. 1991,21 , 41. (15) Sirons, G. J.; Frank, R.; Sawyer, T. J. Agric. Food. Chem. 1973,21,1016. (16) Muir, D. C. G.; Baker, B. E. Weed Res. 1978,18, 111. (17) Kaufman, D. D.; Blake, J. Soil Biol. Biochem. 1973,2,297. (18) Wolf, D. C.; Martin, J. P. J.Environ. Qual. 1975,4 , 134. (19) Mills, M. S.; Thurman, E. M. Environ. Sci. Technol. 1994, 28,73. (20) Mills, M. S.; Thurman, E. M. Anal. Chem. 1992,64,1985. (21) Thurman, E. M.; Meyer, M. T.; Pomes, M.; Perry, C. A.; Schwab, A. P. Anal. Chem. 1990,62,2043. (22) Perry,C. A. Water-Resour. Invest. (U.S. Geol. Suru.) 1991, NO.91-4017. (23) Jabro, J. D.; Lotse, E. G.; Simmons, K. E.; Baker, D. E. J. Soil Water Conserv. 1991,46,376. (24) Beynon, K. I.; Stoydin, G.; Wright, A. N. Pest. Biochem. Physiol. 1972,2, 153.
(25) Kruger, E. L; Somasundaram, L.; Kanwar, R. S.; Coats, J. R. Environ. Toxicol. Chem. 1993,12, 1959. (26) Baluch, H. U.; Somasundaram, L.; Kanwar, J. R.; Coats, J. R. J.Environ. Sci. Health 1993,B28 (2), 127. (27) Shimabukuro, R. H., Walsh, W. C., Lamoureux, G. C., Stafford, L. E. J. Agric. Food Chem. 1973,21,1031. (28) Laird, D. A.; Barriuso, E.; Dowdy, R. H.; Koskinen, W. C. Soil Sci. SOC.Am. J. 1992,56, 62. (29) Frank, R.; Sirons, G. J.; Anderson, G. W. Can. J.Soil Sci. 1983,63,315. (30) Alexander, M. Introduction to Soil Microbiology; John Wiley & Sons, Inc.: New York, 1961; pp 1-10, (31) Pau1,E. A.; Clark, F. E. Soil Microbiology andBiochemistry; Academic Press: New York, 1989; pp 74-76. (32) Nicholls, P. H. Pestic. Sci. 1988,22, 123.
Received for review May 18,1993.Revised manuscript received November 29, 1993. Accepted December 15, 1993.' @
Abstract published in Advance ACS Abstracts, February 1,1994.
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