Environ. Sci. Techno/. 1995, 29, 2031-2037
Environmental Behavior of 2. Stereo- and Enantioselective Degradation in Sewage Sludge and-Soil MARKUS D. MULLER* AND HANS-RUDOLF BUSER Swiss Federal Research Station, CH-8820 Wlidenswil, Switzerland
~~
~
The degradation of five acetamide pesticides (alachlor, acetochlor, metalaxyl, metolachlor and dimethenamid) with different types of stereoisomerism (axialand/or C-chirality) was studied in soil and sewage sludge using chiral high-resolution gas chromatography/ mass spectrometry (HRGC/MS). Overall degradation rates in both media were in the order of alachlor > acetochlor > dimethenamid > metolachlor > metalaxyl. The results indicated the degradation of metalaxyl, metolachlor, and dimethenamid to be stereo- and/ or enantioselective, particularily with respect to Cchirality. For acetochlor (axial-chiral), the results were inconclusive due to interference in the detection of this compound. Metalaxyl showed different enantioselectivity in the two media with the 1’s-(+)-enantiomer faster degraded in sewage sludge and the fungicidal more active 1’R-(-)-enantiomer faster degraded in soil. a-Hexachlorocyclohexane ( a HCH) and its enantioselective degradation were used as an internal control to confirm biotic action in the sewage sludge. Surface water (lakes, drainage canal) and rain contained detectable quantities (2-120 ng/L) of some of these pesticides. The peak area ratios in all but one sample (drainage canal) were close to those of the technical products, indicating little if any biological degradation.
Introduction Acetamide pesticides (some also known as chloroacetamides) are an important group of compounds used as selective herbicides in agriculture, in industrial weed control, and as systemic fungicides in crop protection (1). Two of these herbicides, alachlor and metolachlor, are amongthe most popular pesticides used in the United States (2). These pesticides are to some degree mobile in the environment (3). They are often applied in pre-emergent or early post-emergent modes and may then leach into groundwater and surface water systems. Not surprisingly herbicide residues have been detected in various waters, and alachlor and metolachlor are among the most frequently detected herbicides in surface waters (4- 7).
0013-936X/95/0929-2031$09.00/0
D 1995 American Chemical Society
Some pesticides are chiral and exist in different enantiomeric forms that may differ in biological properties (8). In some cases, the biological activity of a pesticide may be attributed to one enantiomer, while the other enantiomer has little or no activity (9-13). Furthermore, processes such as uptake and metabolism in plants and microbial degradation in soil are often enantioselective. Beside a knowledge on the biological activity, the environmental behavior of individual enantiomers should be known. Although some of the acetamide pesticides are chiral and exist as two or more stereoisomers (enantiomers, diastereomeric pairs of enantiomers), little is known with respect to differencesin the environmental behavior of these stereoisomers. In the preceeding paper (141, we described the use of chiral high-resolution gas chromatography (HRGC) and chiral high-performance chromatography (HPLC)for the analysis of some acetamide pesticides. In this paper, we now describe the first application of chiral HRGClmass spectrometry (HRGCIMS) toward the stereo- and enantioselective determination of these pesticides in environmental samples. The compounds investigated were alachlor, acetochlor, metalaxyl, metolachlor, and dimethenamid (for the structures and the IUPAC names, see ref 14). Whereas alachlor is achiral,the other four compounds are C- and/or axial-chiral and consist of two (acetochlor, metalaxyl) or four stereoisomers (metolachlor, dimethenamid). The compounds were incubated with sewage sludge andwith soil,two environmentallyimportant media. The results document the applicabilityof the new analytical techniques to environmental samples and show the first evidence for the stereo- and/or enantioselectivedegradation of these pesticides.
Experimental Section Acetamides Analyzed. Reference Compounds. The five acetamides investigated were alachlor, acetochlor, metalaxyl, metolachlor, and dimethenamid; their sources are listed in ref 14. In the case of metolachlor, the 1975-dated technical materialwas used (14). Stock solutionswere made up at concentrations of 1-1.25 mglmL in methanol and at 4-5 mg/mL in ethyl acetate, which were then diluted as required. a-Hexachlorocyclohexane (a-HCH; Riedel-deHaen, Seelze, Germany) was used as an internal control compound in the sewage sludge experiments. Pentadeuterioatrazine (atrazine-&;CambridgeIsotope Laboratories, Cambridge, MA) was used as an internal standard in the water analyses. Soil and Sewage Sludge Used in the Study. Garden soil (sandy loam; 1.6% organic carbon; pH value 7.0) from a location in Wadenswil was carefully air-dried (2 d final water content, ~ 1 8 % )and sieved (5 mm). The sewage sludge was from the anaerobic stabilizer of a sewage sludge treatment plant. More details on the installation and the sludges are given in ref 15. Description of the Lakes Sampled. Other Water Samples Analyzed. Surface water was collected with standard equipment in Lake Greifen, Lake Baldegg, and Lake Murten at a depth of 0.5-1 m. The lakes are situated in the central and western midland region of Switzerland. They have intensive agricultural activities nearby, and all
VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2031
are eutrophic (16).Also sampled was a drainage canal (Grosses Moos) within an agricultural area close to Lake Murten. The samples were filled on-site into methanolrinsed 1-Lmineral water bottles. Rainwater from a single event (July18-20,1994; 60 mm of rainfall) was collectedat Wiidenswilinto a2-L glass bottle using a30-cm glass funnel. A fossil groundwater ("zero-contaminant water"; ref 16) was analyzed periodically for control purposes. Incubation of Acetamides with Soil and with Sewage Sludge. Incubation in soil was done with portions of 200 g of soil placed in Petri dishes (20 cm diameter), fortified with 400 pL of a solution containing 400-500 pg of each acetamide in methanol (fortificationlevel,2-2.5 ppm).After adding 30 g of water (total humidity, =33%), the soil was carefully homogenized and incubated at 20-25 "C with normal daylight for up to 64 d while covered with a glass lid. Two experiments were carried out in parallel and with individual spiking. Occasionally, the weights were controlled, and distilled was water added to compensate for loss of water. Periodically, samples (-10 g) were removed for analysis; the first one immediately after fortification and mixing and then after 5, 17,28, and 64 d (experiment 1) and after 0, 17, and 64 d (experiment 21, respectively. Blank determinations of the soil prior to fortification revealed no acetamides present (detection limit, (0.01 PPm). Incubation in sewage sludge was done with -250 g of sludge placed into a 300-mL serum bottles, fortified with 100 pL of ethyl acetate solution containing 400-500 pg of each acetamide and 500 pg of a-HCH (fortificationlevel, 1.6-2 ppm). After a thorough mixing, 1 g of soluble starch and 2.4 g of bakers yeast in 10 mL distilled water were added as further nutrients. Two bottles were prepared in this way; a third bottle fortified with a-HCH only served as a control. The bottles were tightly capped and incubated on a horizontal shaker at 25 f 1"C in the dark. Periodically, samples (e20 g) were removed for analysis; the first one after vigorous shaking immediately after all the additions were made and then after 24, 48, 72, and 168 h. Blank determinations of sewage sludge prior to fortification revealed no acetamides present (detection limit, (0.01 PPm)* Extraction and Cleanup of Incubated Soil and Sewage Sludge Samples. In the case of soil, portions of ~ 1gwere 0 placed in a 20-mL vial and extracted with two 10-mL portions of methanol. After centrifugation, the clear supernatants were combined and reduced under vacuum to a few milliliters. After 10mL of distilled water was added, the analytes were reextracted with 10 mL of ethyl acetate. The organic layer was removed, reduced in volume to about 1 mL, and passed through a silica minicolumn (0.7 g of silica gel 60; Merck, Darmstadt, Germany; deactivated with 5%water; topped with 10 mm of sodium sulfate; 5 mm i.d. Pasteur pipet). The acetamides were eluted with 6 rnL of ethyl acetate, and the eluate was adjusted to 10 mL. A 1-pL aliquot was then analyzed by chiral HRGCIMS. In the case of sewage sludge, the procedure outlined in ref 15 was followed. Briefly, portions of -20 g of sewage sludge were collected in 50-mL Sovirel bottles. Extraction was done with two 10-mL portions of ethyl acetate and then followed as above for soil. The sample volume was finally adjusted to 10 mL, and 1pL of a further 1:10 dilution was analyzed by chiral HRGCIMS. Extraction and Cleanup of Water Samples. Water samples were immediately spiked in the laboratorywith 10 2032 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8,1995
pL of a 1ng/pL atrazine-d5in toluene solution (spikelevel, 10ngIL) (16). Sampleextractionwaseffectedbypercolation through a reusable small Bio-RadS-X2 macroreticular resin column (Bio-Rad,Richmond, CA,10 mL in a 150 x 10 mm i.d. glass column) covered with a small plug of glass wool. The fresh resin bed was conditioned by successivewashings with 20-mL portions of methanol, dichloromethane, and distilled water. Water samples were percolated through the resin at a rate of e15 mLImin by gravity. After passage, residual water in the column was removed by a stream of air. The analytes were then eluted with 5 mL of methanol, followed by 15mL of dichloromethane. The eluate formed two layers in a test tube, and the lower layer was removed, concentrated using a stream of nitrogen, and diluted to 200 pL. A 1-2-pL aliquot was then analyzed by chiral HRGC/MS. The procedure gave recoveries of ~ 8 0 % for acetamides and atrazine-& and relatively clean extracts. The data reported are uncorrected for recoveries. Chiral HRGC/MS Analyses. All samples were analyzed using instrument and conditions as described in the preceedingpaper (14). In particular, a chiral20-m OV1701BSCD fused silica column (0.25 mm i.d.1 (BSCD = tertbutyldimethylsilyl-/3-cyclodex&rin;relative amount, 50%), and selected ion monitoring (SIM)was used. The column was temperature programmed as follows: 70 "C, 2 min isothermal, 20 "C/min to 120 "C, then at 3 "CImin to 230 "C. This column also resolved a-HCH enantiomerically with (-1 -a-HCH earlier-elutedand (+) -a-HCHlater-eluted. All samples were analyzed by electron ionization (EI) using selected ion monitoring (SIM) and the ions listed in the preceeding paper (14). Some samples were reanalyzed using full-scan E1 MS (mlz 35-435, 1.16 slscan; resolution MI AM= 500). The amounts of an analytewere determined from peak areas (SIM data), and in the case of the water samples from peak area ratios relative to the internal standard, and corrected for sample size. The analytical precision was ~ t 5 % (relative standard deviation, RSD) for the external standard method (soil, sewage sludge) and kl%RSD for the internal standard method (surfacewater, rain) as determined from repeated injections. In the degradation studies, the concentrations were then expressed in percent relative to the inital concentrations at t = 0. For acetamides, peak areas of stereoisomers were summed to obtain overall (or total) concentrations (ctot). Overall degradation rate constants (ktot)were determined from normalized concentrations (ctot/c0)using regression plots of ln(ctot/c,,)versus time (t)and a no-intercept model and assuming first-orderkinetics. Enantiomeric ratios (ER) were defined as ER = p1/p2,where pl and p2 are the peak areas of the earlier- and later-eluted enantiomer, respectively (17). For a-HCH, ER is defined as (+)-a-HCH/(-)a-HCH. ER values were determined from SIM data with a precision of fl% (RSD).
Results and Discussion Degradation of Acetamides in Sewage Sludge. AU five acetamides degraded in both sewage sludge experiments to levels < 1-24% after 7 d of incubation. Changed peak area ratios of stereoisomers indicated a certain degree of stereo- and/or enantioselectivity. The degradation of a-HCH, added as an internal control, was highly enantioselective with the (+)-enantiomermuch faster degraded (see chromatograms in Figure 11, in agreement with previous data (19.The ER values changed from initially ~ 1 . to 0 0.17-0.21 after 48 h of incubation, irrespective of
100
+
/
10
2oi
10 1 03 , , , , I \ 14100 16100 100%
:"1
20
10
04
, , ,
14100 100%
20! 10
,,,e I
,
-
,
,
,
TI&
lei00
t
r t
,J ! , ;
16;OO
,e. ,
l8!00
, ,
,d
TIME
lL+
FIGURE 1. El SIM chromatograms (mhZl9)ofsewage sludge extracts showing elution of a-HCH, using the chiral OV1701-BSCD HRGC column and after (a) 0 h (ER = l.W), (b) 24 h (ER = 0.42). and (c) 48 h of incubation (ER = 0.19), respectively. Note the resolution of the a-HCH enantiomers and the more rapid degradation of (+)-a-HCH in sewage sludge.
the presence of acetamides. Degradation of both a-HCH enantiomers followed first-order kinetics. The rate constants for (+)-a-HCH and (-)-a-HCH were 0.0648 and 0.0298h-l, respectively,somewhat higher than in previous sludge experiments (15). As a measure of enantioselectivity (or stereoselectivity; ES), we define the excess of the rate of the faster over the slower degraded enantiomer (stereoisomer)in a particular medium as
whereby kl and k2 are the rate constants of the faster and the slower degraded enantiomers (stereoisomers),respectively. Equation 1 is analog to the equationused to calculate enantiomeric excess (EE = [R- S]/[R+ SI) (18). The ES
1
i
AL
\
0.1
0
50
100
150
incubation time (h) FIGURE 2. Degradation of five acetamide pesticides (metalaxyl, MX; metolachlor, ME dimethenamid, DM; acetochlor, A C alachlor, AL) in active sewage sludge (2!i "C). Normalized summed concentrations ( l W CmJc,logarithmic scale) ere plotted versus incubation time (h).
values thus defined range from 0 (kl = k2; nonenantioselective) to 1 (k2 = 0;fully enantioselective). The ES value for a-HCHcalculated from the data above is 0.37,somewhat lower than in sludge of an earlier study (15). In order to compare the five acetamides, respective degradation rate constants were determined by assuming pseudo-first-orderkinetics. In Figure 2,we plotted the data from both experimentsas normalized concentrations (ctot/ co) versus time whereby ctotand c, are now the total (or summed)concentrations of all stereoisomers. The reaction rate constants thus calculated (k,,,, h-9 reflect overall degradation, as measured when using achiral analyses.The data in Figure 2 show acceptable linearity (correlation coefficients, r = 0.97-0.99) but with a trend toward convexedness. This trend suggested a slowed overall degradation with continuing incubation possibly caused by some loss of activityof the sludge. For chiral compounds, the same trend could also be the result of an enantioselective degradation, as discussed in an earlier paper (15). The ktOt values are listed in Table 1. They were in the order alachlor > acetochlor > dimethenamid > metolachlor > metalaxyl and correspondedto half-lives (t)of 17-85 h. Stereo-and/ or enantioselectivity for some of the compounds was indicated by changed peak area (or enantiomer)ratios (see below). The degradation of the acetamides in sewage sludge is documented with some chromatogramsin Figures 3 and 4. In Figure 3a,c,e E1 SIM chromatograms (mlz 223) show the elution of acetochlor in three of the sewage samples. Incubation after 48 and 168h showed much reduced levels, but the marginal resolution of the two atropisomers and the interference at low levels with dibutyl phthalate left it VOL. 29, NO. 8.1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2033
TABLE 1
Overall Degradation Rate Constants kwt(and Half-lives) of Acetamide Pesticides and a-HCH in Sewage Sludge and Soil sewage sludge ktot h-' (z, hl
compound
alachlor acetochlor metalaxyl metolachlor dimethenamid
0.0403 0.0258 0.0081 0.0154 0.0234 0.0648 0.0298
(+)-a-HCH (-)-a-HCH a
(17.2) (26.9) (85.5) (45.0) (29.6) (10.7) (23.3)
soil.
km d-' (z, d) 0.1601 0.1010 0.0217 0.0460 0.0890
(4.3) (6.9) (31.9) (15.1) (7.8)
1
na na
Not analyzed = na
100
40
30 20 10
100%
:d c
20 10 20:oo ~~
22:oo
FlGURE4. El SIM chromatograms (M238 left ,panels; mhm,right panels) showing elution of metolachlor (ME)and metalaxyl(MX)in active sewage sludge after (a and bl0, (c and d) 48, and (e and f) 168 h of incubation. Note the stereo- and anentiosalectivedegradation of both compounds (1'S-metalaxyl faster degraded in sawage sludge). Peak denotationspl-hfor metolachlor, sea text Note the presence of signals for alachlor (AL) in left chromatograms.
21:oo
100%
expressed as
::k 0
20:oo
22:oo
24:ol
FIGURE 3. El SI# chromatograms (mh223, left panels; mh230,right panels) showing elution of acetochlor (AC) and dimethenamid (DM) in active sewage sludge after (a and b) 0, (c and d) 48, and (e and f) 168 h of incubation. Note the stereoselective degradation of dimethenamid. For acetochlor, enantiomer resolution is insufficient and detection at low levels is interfered by dibutyl phthalate (DBP).
doubtfulwhether degradationwas in fact enantioselective. In Figure 3b,d,f E1 SIM chromatograms (m/z230) show the elution of dimethenamid in the same samples. The two peaks represent the 1'R- and the 1's-stereoisomers (elution order not known, see ref 14). We presume that the atropisomers of dimethenamid are conformationally unstable at the column temperature used (70-170 "C). The peak area ratio changed from initially -1.0 to 1.33 after 72 h of incubation, indicating some degree of enantioselectivity. Enantio- and stereoselectivity as expressed by the ES values are related to the ER and ktotvalues. By assuming pseudo-fist-order kinetics and ktotto be the average of the individual rates [ktot (kl k2)/2,valid for small differing ki values] and neglecting enantiomerization, ES can be
+
2034
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8,1995
whereby ER is the enantiomer or peak area ratio at time t. In eq 2, the expression ktot t may be replaced by ln(c/co), the natural logarithm of the relative concentration remaining after incubation time t. For dimethenamid (ER = 1.33; t = 72 h; ktot = 0.0234 h-l, see Table l),the ES value thus estimated is 0.08 and indicates low to moderate enantioselectivity. In Figures 4a,c,e,E1 SIM chromatograms(mlz238) show the elution of metolachlor in the sewage samples after 0, 48, and 168 h of incubation. The total amount of metolachlor decreased during this period to -8%. The four stereoisomers are not fully resolved (see ref 14). Nevertheless, it is apparent that the peak ratios changed remarkably in that peaks 2 and 3 decrease faster than peak 1, indicating some stereoselectivity. In particular, the peak area ratio pl/p3 changed from an initial value of -1.0 to a value of -1.6. Since the stereoisomers in peaks 1 and 3 have an enantiomeric relationship (141, this result indicates some enantioselectivity. The ES value thus calculated (ER % 1.6; t = 168 h; ktot= 0.0154 h-l, see Table 1) according to eq 2 is -0.10 and indicates moderate enantioselectivity. Since peak 2 also decreased and the components of this peak have an atropisomeric relationship to those of peaks 1 and 3 (141, this degradation is also atropisomer selective. The
TABLE 2
Acetamide Pesticides in Swiss Lakes and in Rain (Single Event) concentration,’ ngA lake, location
collection date
alachor
metalaxyl
metolachlor
dimethenamid
Lake Greifen (inlet) Lake Greifen (outlet) Lake Baldegg (outlet) Lake Murten (center) Lake Murten (outlet) drainage canal rain
June 24,1994 June 24,1994 July 1, 1994 July 15, 1992 July 1, 1994 July 1, 1994 July 18-20, 1994
10 15 5 12 10 15 20
5 dimethenamid > metolachlor > metalaxyl (see Table 1) and thus in the same order as in sewage sludge. The half-lives (t)corresponded to 4-32 d. This is of the same magnitude as reported half-lives (15-90 d) from field data for alachlor, metalaxyl and metolachlor (19). In Figure 6a-f, E1 SIM chromatograms (m/z 238 and 206) show the elution of metolachlor and metalaxyl in the soil samples after 0, 28, and 64 d of incubation with the total amounts decreased to -5 and 22%,respectively. For metolachlor, peaks 2 and 3 decreased faster than peak 1, as was observed in sewage sludge. The peak area ratio changed initially from 1:l:lto about 1:0.5:0.56. The change in the peak area ratio p1Ips indicates some enantioselectivity in the same sense as was observed in sewage sludge. The VOL. 29, NO. 8 , 1 9 9 5 I ENVIRONMENTAL SCIENCE &TECHNOLOGY 12031
ES values calculated(ER = 2 and 1.8;t= 64 d; k,,,= 0.046d-I, see Table 1) according to eq 2 are 0.11 and 0.10 and thus indicate low to moderate stereo- and/or enantioselectivity. Since the relative intensity of peak 2 also decreased, the degradation of metolachlor is also atropisomer selective. The chromatograms also document the rapid degradation of alachlor. For metalaxyl, the ER values changed from an initial value of x1.0 to a value of x2.8 after 64 d of incubation. Whereas in sewage sludge the first-eluted 1’s-(+)-enantiomer was faster degraded (ER < 11, now the later-eluted 1’R-(-)-enantiomer was faster degraded. The ES value estimated (ER = 2.8; c = 64 d; k,,, = 0.0217 d-], see Table 1) according to eq 2 is 0.37 and indicates high enantioselectivity. In the case of dimethenamid, the total amount decreased to less than 1% after 64 d. The peak area ratio changed initially from xl.0 to 1.28after 28 d (data not shown). The ES value thus estimated (ER = 1.28; t = 28 d; k,,, = 0.089 d-I, see Table 1) according eq 2 is 0.05 and indicates low enantioselectivity,as was observed in sewage sludge. Acetamide Herbicides in Surface Waters and in Rain. A small number of surface waters (lakes,a drainage canal) and a sample of rain were analyzed. The samples contained detectable quantities (2-120 ng/L) of some acetamides, as reported in Table 2 and documented with E1 SIM chromatograms in Figure 7a-c. The samples also contained atrazine at concentrations ranging from 4 ng/L (rain) to 250 ng/L (Lake Greifen). Acetochlor was not quantified due to the interference by dibutyl phthalate under the analytical conditions used. Metolachlor was the most frequent acetamide found. It was present in approximate 1:l.Z:l peak area ratios in all the samples (see Figure 7a), These ratios are comparable to the ratio found for more recent metolachlor products (see ref 14). The peak area ratio pl/p3 of xl.0 indicated that the enantiomers were present as racemates. Dimethenamid was detected at low concentrations in all samples except in an older sample of L. Murten (1992)now analyzed; peak area ratios of ~ 1 . suggested 0 this compound to be present as a racemate. Metalaxyl was generally present at low concentrations except in the agriculturaldrainage canal (Grosses Moos) where larger concentrations were found (120 ng/L), likely originating from fungicide applications in the area. In the drainage canal, the ER value of x0.8 for metalaxyl indicatesthe 1’s-(+)-enantiomerdegraded faster, possibly due to anaerobic conditions during degradation of this compound. In the other water samples, the concentrations of metalaxyl were too low to allow precise ER measurements.
Conclusions All five pesticidesshowed significant degradation in sewage
sludge and in soil with varying degrees of stereo- and/or enantioselectivity. The overall degradation rates were in the order of alachlor > acetochlor > dimethenamid > metolachlor > metalaxyl in both media with the degradation curves showing some trend toward concavedness(increased rates with increasing exposure) in soil and some trend to convexedness (decreasingrates with increasing exposure) in sewage sludge. The former effect could be caused from an initial lag phase, the latter possibly by some loss of sewage sludge activity during the course of the experiments. Degradation in soil and in sewage sludge proceeded under 2036 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8,1995
:I a
100%
I
70
40
30
100%
100%
,
FIGURE 7. El SIM chromatograms showing the presence of acetamides in surface waters, using the 20-m OV1701-BSCD HRGC column: (a) Metolachlor (ME) and alachlor (AL; m/z 238) in Lake Greifen (inlet),(b) dimethenamid (DM,mh230)in Lake Greifen (inlet), and (c) metalaxyl (MX, mh 206) in the drainage canal (Grosses Moos).
aerobic and anaerobic conditions, respectively, with different microbiological populations prevailing in the two media. a-HCHwas added as an internal control in the sewage sludge experiments to confirm biotic action via its enantioselective degradation. The degradation rates and the resulting ER values were similar in all experiments and suggested a negligible influence from the presence of acetamides to overall sludge activity. As a measure of stereo- and enantioselectivityES values were defined according to eq 1. This definition of ES was preferred over a definition as S = kl/kz (stereoselectivity factor, see ref 18)because the ES values will range between 0 (nonenantioselective)and 1 (enantioselective)whereas these limitsare reversed with the Sdefinition. The ES values found in this study indicate low to moderate stereo- and/ or enantioselectivityfor dimethenamide and metolachlor in soil and sewage sludge (ES ~ 0 . 1 but ) high enantioselectivity for metalaxyl in both media (ES = 0.37). For acetochlor, the situation remained unclear because of insufficient enantiomer resolution and interference in its detection. Interestingly, metalaxyl was the slowest degrading compound in both media, but the one showing highest
enantioselectivity. The study showed 1’8-(+)-metalaxyl degraded faster in sewage sludge, and the fungicidal moreactive 1’R-(-)-enantiomer degraded faster in soil.The ER value of 2.8 (64-d soil incubation) indicates 73% of the metalaxyl residues present as the inactive l’S-(+)-enantiomer, and only 27% present as the fungicidal active 1’R(-)-enantiomer. Assuming a similar behavior under field conditions this would indicate that residue levels of metalaxyl are significantly lowered by using the pure 1’R(-)-enantiomer. Major modes of dissipation of herbicides from treated areas include microbial degradation, volatilization, and leaching into surface and groundwaters. Leaching through soil likely is accompagnied by more biotic action than surface runoff. Lower ER values (peak area ratios) should reflect longer exposure to biotic processes. The slow hydrolysis of the acetamides in water (metolachlor,metalaxyl, t > 200 d) is nonenantioselective and therefore will not change ER values. A determination of ER values in environmentalsamplesshould thus allow some distinction among contamination pathways. The study revealed several acetamidepesticides present at low but detectable concentrations (2- 120ngIL) in lakes, in a drainage canal, and in rain. The finding of these compounds in these aquatic systems is not unexpected since the samples were taken within a relativelyshort period after field application. The residues likely result from surface runoff (erosion) as was observed for alachlor, metolachlor, and other compounds in similar situations (6, 7,16,20). With the possible exception of metalaxyl, in the drainage canal the ER values of metalaxyl, metolachlor and dimethenamid did not differ significantly from those of the technical products, suggesting little biological degradation and pointing to more recent contaminations. Except metalaxyl, the exact isomerism and the absolute configurations of the stereoisomers remained unknown. Nevertheless, this study documents the applicability of chiral HRGC to aid in determining the environmentalfate of these compounds. The study shows the first evidence of a stereo- andlor enantioselective degradation of these compounds in two environmental media. So far we have not considered enantiomerization of these compounds in the media selected, and we have not investigated the likely enantioselective formationof metabolites. Further studies will have to address these issues. There must be continuing efforts to reduce the amounts of pesticides released into the environment. The use of enantiopure materials in place of racemic products not only is a convenientway to lower application rates but also prevents deployment of nontarget active stereoisomers in the biosphere, as was pointed out earlier (21). For chiral pesticides,clearlythe use of enantioenriched or enantiopure materials should be encouraged.
Acknowledgments We acknowledge detailed discussionswith and comments from H. Egli (Ciba, Basel, Switzerland) and H. P. Schelling and H. Sauer (Sandoz Agro, Basel, Switzerland). Furthermore, we thank H. P. Kohler of the Swiss Federal Institute of Water Resources and .Water Pollution Control (ETHI EAWAG, Diibendorfand Zurich) for the sewage sludge and A. Zurcher for carrying out the sampling of the lakes. We also thank W. Heller (Swiss Federal Research Station, Wadenswil, Switzerland)for characterizationof the soil used in this study.
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Received for review November 18, 1994. Revised manuscript received April 4, 1995. AcceptedApril 13, 1995.* ES940712N @
Abstract published in Advance ACS Abstracts, June 1, 1995.
VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12037
