Persistence of Temephos and Its Transformation Products in Rice

in the rice crop field waters was between 41 and 125 μg/L. The evolution of temephos and its transforma- tion products was recorded during a period o...
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Environ. Sci. Technol. 1996, 30, 917-923

Persistence of Temephos and Its Transformation Products in Rice Crop Field Waters SILVIA LACORTE, NADIA EHRESMANN, AND D A M I AÅ B A R C E L O Ä * Department of Environmental Chemistry, CID-CSIC, c/o Jordi Girona, 18-26, 08034 Barcelona, Spain

The persistence of temephos and its degradation products (temephos sulfoxide, temephos isomer, and temephos sulfoxide isomer) under real environmental conditions was studied. Other breakdown products identified were temephos oxon and temephos oxon isomer. Temephos was applied in the rice crop field of the Ebre Delta (Tarragona, Spain) during July and August 1994 by aircraft spraying ABATE 50E at a rate of 250 mL/Ha; the calculated concentration range in the rice crop field waters was between 41 and 125 µg/L. The evolution of temephos and its transformation products was recorded during a period of 72 h with sampling intervals of 5 and 10 h. The concentration of the compounds formed was measured inside and in the outlet of two rice crop field waters and varied between 0.01 and 2.27 µg/L. Only a maximum of 5.5% of the applied temephos was detected immediately following application. The analytical determinations were performed by using solid-phase extraction with C18 Empore extraction disks followed by liquid chromatography/thermospray/mass spectrometry (LC/TSP/MS) in the positive ion mode.

Introduction Temephos [O,O,O′,O′-tetramethyl O,O′-(thiodi-p-phenylene)phosphorothioate] is an organophosphorus pesticide recommended to combat the pests originated by mosquitoes. In the Ebre Delta (Tarragona, Catalonia), a large estuarine area where the rice crop is the main agricultural activity (18.000 Ha), ABATE 50E, which contains 50% temephos, is applied in large amounts during the summer by aerial spraying. After application, ABATE is deposited as a thin film over the water surface where its low solubility results in high concentration at the water surface where larvae exists. It kills mosquitoes with the inhibition of acetylcholinesterase by disrupting normal neurotransmission. This combination of physical properties and mode of action is an advantage of ABATE since it is selective toward mosquito larvae and lacks toxicity toward nontarget organisms. Another important factor to take into account is its * Author to whom correspondence should be addressed; fax: 343-204-59-04.

0013-936X/96/0930-0917$12.00/0

 1996 American Chemical Society

persistence in natural waters. Greater persistence may lead to long-term exposure, accumulation, and possibly increased toxicity toward nontarget organisms. In this sense, it becomes necessary to monitor the presence and persistence of organophosphorus pesticides (OP) in the aqueous environment. Degradation studies of fenitrothion (1-3), fenthion (4, 5), chlorpyrifos (6, 7) and temephos (8, 9) were performed, estimating in some cases the half-lives of these pesticides under either laboratory or natural environmental conditions. The persistence of pesticides in the aquatic environment is a combination of hydrolysis, photolysis, redox reactions, microbial degradation, volatilization, and adsorption (10). A lot of work on degradation studies was carried out under laboratory conditions using various types of water, but there is certainly a lack of field studies. Under laboratory studies, the early works of Eichleberger and Lichtenberg (11) showed the persistence of a variety of organophosphorus pesticides in river water by spiking the compounds at the 10 µg/L level and using liquid-liquid extraction afterward. But generally most of the studies were performed at higher concentrations, usually in the range of 5-0.5 mg/L, such as the studies of Freed (7) and Wang (12) where the influence of pH, temperature, and photolysis in the degradation of OP was examined. However, only two previous studies, one of them from our group (9), were conduced using controlled natural estuarine water stored in glass vessels and studying the half-lives and degradation products formed of various OP pesticides, temephos among them. Another study (13) investigated the persistence of temephos in artificial ponds after applications at 10 µg/L, and the half-life varied between 6 and 8 h. When comparing the data obtained under controlled laboratory conditions (9) and field studies, it can be noticed that after larvicide application by helicopter the half-life of temephos was shorter (8), mainly because the closed glass reservoirs avoided volatilization (9). Furthermore, data concerning the formation and evolution of transformation products (TPs) of OP pesticides under natural conditions are scarce. In a previous paper (1), it was shown that photolysis was the main degradation route of fenitrothion, with the formation of oxo metabolites, fenitrothion isomer, and 3-methyl-4-nitrophenol (1). The control of TPs has become a serious topic since the oxidation products of the pesticides (oxygen analogues, sulfoxides, sulfones) are commonly more toxic than the parent compound (14). Consequently, the National Pesticide Survey (NPS) has included some TPs in their list of the compounds to be monitored in natural waters (15). However, information on the persistence of OP pesticide TPs under environmental conditions is poor, despite the extensive use of OP pesticides in crop fields and salt marshes. Few studies (16-19) were reported in flooded rice culture with various pesticides, mainly molinate, thiobencarb, and carbofuran, which are typical pesticides used in rice fields, and only one study (20) about the OP methyl parathion used on rice in the Sacramento valley was reported. Previous studies carried out in our laboratory on the degradation of temephos under estuarine waters spiked and exposed to controlled natural sunlight conditions indicated the formation of temephos oxon (9).

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Nevertheless, the results obtained cannot be fully extrapolated to the field, and more information is needed on the formation of TPs of temephos and pathways of degradation followed to assess the degradation of this pesticide after application under natural waters. This will allow us to evaluate the impact on aquatic fauna. This paper reports results obtained from analyzing temephos from two different rice crop field water samples after aerial spraying application. The main objective of this study was to characterize the dissipation and persistence of temephos and their TPs in rice crop field waters by (i) measuring the amount of temephos reaching the water after aircraft spraying, (ii) studying the transfer of temephos through a rice field by measuring the levels of this pesticide in the rice field and in the outlet, and (iii) monitoring possible TPs formed during all the sampling period. To achieve such purposes, off-line SPE with C18 Empore extraction disks followed with LC/DAD and LC/themospray (TSP)/MS detection were carried out. The main advantage of using this technique is the possibility to determine temephos, not GC-amenable, but also the possibility to determine its more polar TPs.

Experimental Section Chemicals. HPLC-grade water, acetonitrile, and methanol (Baker, Deventer, The Netherlands) were filtered through a 0.45-µm filter before use. Ammonium formate was purchased from Fluka (Buchs, Switzerland). Pure analytical standards of temephos and temephos sulfoxide were obtained from Promochem (Wesel, Germany). Pesticide Application. ABATE 50E was applied by aircraft spraying the formulation over the rice fields of the area of the Ebre Delta (Tarragona, Catalonia) at a rate of 250 mL/Ha. The amount of active ingredient reaching the water surface was estimated, assuming that the flood depth in the rice plots was maintained at 25 ( 10 cm for the days following the application. Calculated residue concentrations of temephos for this applied concentration were of 125 µg/L for water that is 10 cm deep to 41 µg/L for water that is 30 cm deep. Two treatments were carried out over the same area. The first one, performed during the first week of (July 6, 1994), was done in order to eliminate the larvae of mosquitoes, which abound in this area specially during the summer months when the rice fields are flooded. The second treatment was done during the first week of (August 4 1994) to eradicate the larvae that might have appeared during July. The amount sprayed over the crop fields was the same during both treatments. Sampling. Two different rice fields, named Poble Nou and Eucaliptus, were chosen. In the former field, there was an irrigation ditch that provided freshwater in the rice field. In Eucaliptus, the freshwater entrance was coming from another rice field, and the water exit was 50 m diagonal from where the samples were collected. The sampling strategy adopted in each site was different. Samples from Poble Nou were taken 1, 5, 11, 24, 34, 48, 60, and 72 h after spraying, plus a blank was collected just before the treatment started. During the treatment performed in Eucaliptus in August 1994, a different sampling approach was undertaken, which entailed a study of the mass transfer of temephos through the rice field. This was performed not only to evaluate the initial amount that reached the water surface but also to appraise the quantity eliminated by runoff and for a better

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estimation of the TPs formed. Accordingly, samples were taken from the rice field and at the outlet water of the field at time 0 (immediately after spraying), 1, 2, 5, and 7 h after spraying. Three samplings were performed: one in Poble Nou and two in the Eucaliptus rice field and in the outlet waters. In each case, the collection of water samples consisted of composites of 20 grab samples separated 1 m from each other, and the samples were pooled before getting a final volume of 2 L. Water samples were collected in jars below the water surface layer and at water depth varying between 15 and 30 cm without field spillage, as similarly reported for molinate in a rice field (17). Water sample pH varied between 7.8 and 8.8, and the rice water temperature varied between 26 (early morning) and 33 °C during the day. Paddy water pH followed a diurnal pattern due to photosynthesis and respiration of algae, and pH range follows expectations (18). The spectral solar irradiance taken over the days of the experiments was 0.2-0.3 W/m-2 nm-1 and was achieved using a precision spectroradiometer LICOR 1800. Sample Preparation. Water samples were filtered through filter paper (0.7-1 µm) to remove the gross material, and afterward they were filtered through 0.45-µm glass fiber filters (Millipore, Bedford, MA) to remove suspended particles. The water samples were extracted by SPE immediately after collection. A total of 1 L of each sample was extracted with C18 Empore extraction disks. The disks were conditioned with 10 mL of methanol and 10 mL of HPLC water prior to percolation of 1 L of water. The disks were then stored at -20 °C until analysis. Elution of the disks was carried out twice with 10 mL of acetonitrile. The eluate was rotaevaporated just to dryness, and the extract was finally dried under a gentle stream of nitrogen. The final extract was diluted in 100 µL of acetonitrile. The extracts corresponding to the samples collected during the first treatment were injected by both LC/DAD and LC/ TSP/MS. Extracts from the samples of the second treatment were only analyzed by LC/TSP/MS. Sample Analysis. LC/DAD. The LC analyses were performed with a Waters 600-MS solvent delivery unit with a 20-µL injection loop and equipped with a Waters 996 Photodiode Array Detection (Waters, Milford, MA). A Carbamate cartridge column (150 × 3.9 mm i.d.) packed with 5 µm of C18 (Waters, Milford) was used to analyze the water samples. The gradient elution was from 30% acetonitrile and 70% water to 50% acetonitrile and 50% of water in 7 min, to 100% of acetonitrile in 10 min, and kept isocratic for 3 min, at a flow rate of 1 mL/min. Total run time was 20 min. It went back to initial conditions in 5 min. LC/TSP/MS. The same column as for LC/DAD analysis was used. The LC eluents were acetonitrile-water with 0.05 M ammonium formate with the following gradient: from 30% of acetonitrile and 70% of water to 50% acetonitrile and 50% of water in 7 min, and from these conditions to 90% of acetonitrile and 10% of water in 11 min, which was kept isocratic for 5 min. It went back to initial conditions in 5 min. Samples of 20 µL of the extract were injected each time. A Hewlett-Packard (Palo Alto, CA) Model 5988A thermospray LC/MS quadrupole mass spectrometer and a Hewlett-Packard Model 35741B instrument for data acquisition and processing were employed. The thermospray temperatures were 100 °C (stem), and the tip was maintained at 220 °C at the beginning and end of the gradient.

TABLE 1

Main Ions and Their Percent Abundance of Temephos and Its Transformation Products Using LC/TSP/MSa MW

a

compd

Rt (min)

ion (% abundance) [structure]

484 (100) [M + NH4]+, 523 (4) [M + 57]+ 468 (100) [M + NH4]+ 482 (100) [M]•+, 483 (18) [M + H]+, 500 (4) [M + NH4]+, 523 (7) [M + CH3CN)+ 484 (100) [M + NH4]+ 451 (29) [M + H]+, 468 (100) [M + NH4]+, 498 (21) 482 (14) [M]•+, 483 (100) [M + H]+, 523 (4) [M + CH3CN]+

466 450 482

temephos temephos oxon temephos sulfoxide

15.98 16.73 11.28

466 450 482

temephos isomer temephos oxon isomer temephos sulfoxide isomer

13.15 12.02 6.79

The ions used for quantification are italicized.

FIGURE 1. UV spectra of temephos obtained after injection of the extract corresponding to 1 h after treatment. Analytical conditions are described in the text.

The ion source was set at 250 °C. The filament-onionization mode was used with positive ion chemical ionization. Quantitation was performed with external standard calibration method, using selected-ion monitoring (SIM) (Table 1). Calibration equations for temephos were reported in a previous paper (21).

Results and Discussion Quantitative Information. LC/DAD. Calibration graphs were performed with direct injection of standards from 0.5 to 40 µg/mL. Linearity was observed within this range, with a correlation coefficient of 0.9997. The LOD was 50 ng/L. Recovery values were over 90% with a coefficient of variation of 8% (n ) 5). With that technique, temephos was identified in all samples. As previously reported (22), the LODs are increased when using natural waters due to the presence of interferences. Thus, quantification with LC/DAD could not be performed at this low level since interferences were abundant due to the elution of temephos at the end of the LC traces. Figure 1 shows the UV spectrum of temephos obtained after direct injection of an extract corresponding to a water sample collected immediately after spraying. Under these conditions, 100% of acetonitrile is used, and consequently most of the common interferences present in the water matrix that have been trapped into the column are eluted. These interferences make the quantitation of this compound difficult at low concentration levels in this region, and for this reason, LC/TSP/MS is an ideal technique that overcomes these problems. LC/TSP/MS. Calibration curves were performed by direct injection of 20 µL of temephos and its sulfoxide over

a concentration range of 10-10000 ng (0.5-500 µg/mL after SPE), being linear over the whole range. The LODs of the compounds under study were of 1-2 ng with LC/TSP/MS with a S/N of 3. The mean recovery rates of temephos was above 80% with a coefficient of variation of 12% (n ) 5) (9, 21). Quantification of the environmental samples was performed using the most abundant ion (underlined in Table 1). The quantification of temephos isomer and temephos sulfoxide isomer was performed in relation to their parental compounds, respectively. Temephos oxon and its isomer were not quantified since the standards of these two compounds are not commercially available. Qualitative Information. Using off-line SPE with LC/ TSP/MS, it was possible to identify for the first time temephos and five of its TPs: temephos oxon, temephos sulfoxide, temephos isomer, temephos oxon isomer, and temephos sulfoxide isomer. Other works (23) encountered temephos sulfoxide and oxon on bean leaves, but to our knowledge, the above-mentioned list of temephos TPs was never identified before in natural waters. The typical ions of all the compounds identified are reported in Table 1. Since TSP/MS in PI mode is a soft ionization technique, it provides poor structural information, so identification of the compounds at three different ions was not always possible. The SIM mode was selected in order to improve in sensitivity, although SIM lead in a loss of structural information. Previous works (21) on the characterization of temephos and its sulfoxide indicated the formation of [M + NH4]+ and [M]•+ as base peaks at m/z 484 and 482, respectively. Following this information, the ions 451, 468, 482, 483, 484, 498, 499, 500, 516, and 523 were selected, which covered all the TPs that were supposed to be found in the water sample, e.g., temephos, its oxons, the sulfoxide and sulfone, and all their isomers. The rule followed for the identification of the compounds was through spectral and retention time comparison in the case of temephos and its sulfoxide (the only two standards available) and by spectral identification in the case of temephos oxon and all the isomers. As indicated in Table 1, temephos oxon has the same adduct formation as its parental pesticide, with the formation of [M + NH4]+ giving m/z 468 as the main peak. No further fragmentation was observed for this compound. Temephos oxon elutes slightly after its parental pesticide, as reported previously (9). Temephos sulfoxide exhibits [M]•+ at m/z 482 as the base peak, behaving differently from all the other forms. Since the isomers have the same molecular weight as their parental forms, it is expected that they follow the same fragmentation pattern as their parents. This was the case for temephos isomer, which had exactly the same fragmentation as temephos, and together with its faster elution

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FIGURE 2. Degradation paths followed by temephos. Tank and field indicate the formation of TPs in the tank during dilution and spraying or in the field, respectively. TABLE 2

Concentration of Temephos and Some of Their TPs (µg/L) in Rice Field (R) and in Outlet Waters (O) after 0, 1, 2, 5, and 7 h of Treatment in the Eucaliptius Rice Fielda sample type R0 R1 R2 R5 R7 O0 O1 O5

temephos

temephos sulfoxide

temephos isomer

temephos sulfoxide isomer

2.27 0.23 0.17 0.17 0.21 2.23 0.23 0.16

1.8 0.96 0.38 0.57 0.09 0.99 1.21 1.62

0.13 0.05 0.06 0.04 0.05 0.69 0.05 0.04

0 0.01 0.02 0.07 0.05 0 0.16 0.08

a Analysis and quantification were performed using off-line SPE with C18 Empore extraction disks and LC/TSP/MS detection using SIM at m/z ions used for quantitation, see Table 1.

led us to conclude the presence of the isomer in the water sample. The same occurred with temephos oxon and its isomer. This was possible since the molecule has two dimethoxy groups that can undergo the processes of isomerization. As indicated in Figure 2, isomerization only substitutes the S group for one oxygen present in the methoxy group; thus, the molecular weight of the isomeric species does not change in relation to its parental form. In the case of temephos sulfoxide and its isomer, the two major ions correspond to [M]•+ and [M + H]+, where the abundance of each of these ions is different (see Table 1). This is caused by an increased proton affinity of the isomer due to the presence of the PdO instead of PdS. In all cases, the isomer eluted earlier than its parental compound, as expected from literature (21), probably due to the fact that to PO(SCH3)(OCH3) is a more polar structure than the PS(OCH3)2 group. Seeking additional structural evidence for the unequivocal identification of the isomers, it was found that temephos oxon isomer presented an additional fragment at m/z 451, corresponding to [M + H]+, and that temephos sulfoxide isomer exhibits ions 482, 483, and 523, similar to the parental compound. Environmental Levels. Poble Nou, July 1994. The concentration of temephos was 0.49 and 0.07 µg/L 1 and 5 h after spraying, respectively. The low concentration of

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temephos 1 h after spraying can be due to the facts that the site where the samples were collected was perturbed by a freshwater entrance from a channel and that a high dilution of temephos already occurred within the first few hours of spraying. Temephos was not detected 11 h after spraying. Moreover, temephos sulfoxide was formed at concentrations varying from 0.06 to 0.1 µg/L over the 72 h it was monitored. Eucaliptus, August 1994. The transfer of temephos through the rice field was evaluated. As seen before, the amount of temephos reaching the water surface was very low (µg/L). Similarly, when temephos was determined after spraying in stagnant ponds, a sharp decrease in its concentration was observed under real environmental conditions with a half-life that varied between 6 and 8 h (8). From previous data (9), it was observed that the halflife of temephos was 5 days when using spiked estuarine waters submitted to sunlight degradation in closed glass vessels during wintertime. In this experiment, the water was kept in a closed glass reservoir, avoiding any losses by vaporization and circulation of the water, contrary to what happens in the rice crop fields under real natural conditions where a mixture of volatilization, dilution, and degradation processes occur. Since dissipation occurs faster at higher temperatures and especially in the upper water layer of the rice field, it is of interest to carry out a short-term study on the persistence of temephos under such conditions. The concentrations of temephos in the inside waters and the outlet water of a specific rice field and the monitoring and disappearance of their TPs was undertaken. Figure 2 shows the main degradation routes followed by temephos in natural waters, which involve processes of oxidation and isomerization. Table 2 shows the concentration (in µg/L) of temephos and some of its TPs in water monitored in Eucaliptus. Dissipation Process. General Remarks. Regarding temephos, it can be observed, first, that the amount reaching the water was 2.27 and 2.23 µg/L in the rice field and in the outlet water, respectively (Table 2). This amount represents only 2-5.5% of what was estimated to reach the water considering the different depths. The values found can be related to (i) the formulated compound as an emulsionable liquid that is dissolved in water before it is applied by aircraft spraying. An important problem that experts have encountered is the deposition of the compound upon the rice leaves after application. (ii) The water sampling strategy adopted was collecting a water volume from a water depth

of 15-30 cm. (iii) The water temperature varied between 26 and 33 °C, thus enhancing the dissipation of temephos. By selecting a thinner layer of less than 5 cm in the water surface, given that temephos is a compound that could form such thin layer in the surface due to its poor water solubility, the values of temephos encountered would be higher, and so lower losses could be expected. The higher temperature favors a fast dissipation of temephos, since it is a compound that can be partly degraded by such high water temperatures and in general higher temperatures accelerate other dissipation processes such as volatilization. An additional factor to take into account in the dissipation of pesticides in natural waters is the Koc value. The Koc of temephos varies between 200 and 500, depending on the soil, which implies a partitioning of temephos in the organic matter of the water sample. However, it was proved that chlorpyrifos, with a similar Koc value of 498, does not adsorb on the organic matter since the contact time of 72 h and the concentration of the pesticide applied is too low (24). In addition to that, we should consider other factors: the concentration of temephos was low as was the particulate matter in the rice field water. This will be different if temephos is present in the sediment at a high concentration and is being released to the water or if there is a continuos spill. In such cases, probably the binding of temephos to soil particles will be of importance. In this respect, it has been reported (25) that at least 5 days are needed for equilibration between the highly lipophilic compound, e.g., mirex, and the dissolved organic matter (DOM). This time of equilibration is of importance for the association kinetics between the compounds of interest and the DOM. In the case of temephos, this time was too short; however, we did not directly measure temephos residues in sediment so that arguments regarding the role of sediment in dissipation must be regarded as hypothetical. As can be observed from Table 2, the disappearance of temephos in the rice field and in the outlet maintain a relationship with 7.48 and 7.17% of the initial value, respectively, after 5 h of treatment, indicating on one hand that the concentration of temephos is uniform throughout the rice field and on the other hand that a stronger dilution of temephos does not occur in the rice field in comparison to the outlet as time passes. Henry’s Law Constants. One of the key issues in the present study is the fast dissipation of temephos following application, which implies that most of the active ingredient is lost during treatment probably due to drift and volatilization during spraying. Several considerations need to be made regarding the amount of temephos present after aircraft application. In a previous study (1), it was also noticed that only 6% of the applied fenitrothion was detected after 2 h of treatment. In another study where fenthion was applied by aircraft (5), the maximum amount of fenthion deposited in the water was only 5.5% of the application rate. The highest peak of fenthion appeared after 45 min of spraying, and it was only 1.69 µg/L in the water. It was concluded that fenthion underwent volatilization or degradation during aerial spraying. If we compare the physicochemical data of both pesticides, the water solubilities (WS) of fenthion and temephos are 4.2 mg/L and 0.001 or 0.27 mg/L and the vapor pressures are 2.8 × 10-6 and 8.6 × 10-10 mmHg, respectively (26-28). Although temephos has a much lower vapor pressure (VP) as compared to fenthion, the parameter to take into consideration in the experiments involving air-water interfaces

is the Henry’s law constant (HLC), which can be used to estimate the evaporation rates of chemicals from water if one assumes that all the chemical is dissolved in water. The ratio between VP/WS is an excellent estimation method for the HLC, and it can be used to evaluate the possibility of the evaporation rate from a water solution in a more accurate way than VP (26). A second problem that can be pointed out for temephos is the disagreement in the WS values being 0.27 (27), 0.001 (28), and 0.03 (29) mg/L, which is an additional difficulty in estimating the HLC. When making the appropriate calculations, and due to the fact that temephos exhibits a very low WS and is different from the literature, the HLC of fenthion is 0.022 Pa m3/mol whereas for temephos it can be 0.54 (when solubility is 0.001 mg/L), 0.002 (when solubility is 0.27 mg/L), or 0.022 (when the solubility is 0.03 mg/L). For comparison with other OP pesticides, dichlorvos (a very volatile OP) has a HLC value of 0.19, and fenitrohion, which also suffers important losses in the field application (1), has a HLC value of 0.0036. As a consequence, the HLC for temephos indicates that this compound will have a tendency to volatilize from a water solution following aerial application. However, we did not directly measure voaltilizated residues to obtain experimental evidence on this point. Also, as pointed out above, the extremely low water solubility of temephos may result in formation of a surface film of undissolved material. In this case, HLC is not a predictor of volatilization rate, and vapor pressure is the more relevant parameter. The low vapor pressure of temephos argues against volatilization as the primary course for the low recovery of temephos from those paddy waters. Photolysis. The fast disappearance of temephos (1.9 h) can also be attributed to its chemical structure. First of all, due to the presence of two aromatic rings, this compound will have a tendency to exhibit high quantum yields (30). However this is not the only aspect to consider in order to exhibit high rate photolysis constants. If we look at the UV spectrum of Figure 1, we can observe that temephos contains suitable chromophore for absorption at a wavelength longer than 295 nm, and consequently the observed fate for this compound under natural conditions can be explained by these two combined effects. In addition to that and due to the fact that running water is entering continuously to the rice field, the pH of the water under environmental conditions can also suffer slight changes (usually the water pH varies between 7.8 and 9). Basic pH has been reported to promote the degradation of OP pesticides in water samples by stabilizing more phenolic structures, which can in addition have a tendency to absorb more sunlight, and thus increasing the degradation rate constants (30). Factors Affecting the Formation of TP Products of Temephos, after Spraying. Blank analyses were performed before the treatment started. Those blanks did not contain traces of either temephos or its TPs. However, temephos TPs were found immediately after spraying both in the rice field water and in the outlet water. The TPs identified by LC/TSP/MS and the levels encountered are shown in Table 2. Figure 3 represents a chromatogram obtained by LC/ TSP from rice field water sample collected immediately after spraying (A) and after 5 h of treatment (B), indicating the TPs formed. As indicated (10), oxidation is one of the major photolytic routes of pesticide degradation. In this study, oxidation was found to be the first degradative mechanism of temephos. Through oxidation, temephos is

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FIGURE 3. (A) Chromatograms obtained at selected ions at m/z 484, 482, and 468 and total ion current (TIC), respectively, with LC/TSP/ MS corresponding to a water sample collected at the outlet of the rice field immediately after treatment. The TPs identified are as follows: (1) temephos, (2) temephos oxon, (3) temephos sulfoxide, (4) temephos isomer, and (5) temephos oxon isomer. (B) Chromatograms obtained at selected ions at m/z 483, 482, and 468 and TIC, respectively, with LC/TSP/MS corresponding to a water sample collected at the outlet of the rice field 5 h after treatment. The TPs identified are as follows: (2) temephos oxon, (3) temephos sulfoxide, (5) temephos oxon isomer, and (6) temephos sulfoxide isomer.

converted to its oxon and sulfoxide, which are the activated forms of the pesticide since they have a much higher inhibitory activity toward the acetylcholine esterase. Therefore, it is necessary to measure not only the parental compound but also the oxo derivatives of the pesticides during treatments against pests, since the environmental damage that they can perform is much higher than that of the parental pesticide. From these results, it can be noted that the oxidation of the parental compound occurs rapidly, e.g., oxidation can take place during the operation of dissolution of the formulation in the tanks, since temephos sulfoxide and temephos isomer are already formed immediately after spraying. Oxidation can also occur while temephos is being sprayed over the fields before reaching the water surface. The formation of temephos sulfoxide is mainly due to the process of photooxidation, attributed to the quantum yields of temephos. Temephos oxon is also formed immediately after spraying. This compound could not be accurately quantified since the standard was not available, but the amount encountered in the outlet water is two times higher than in the rice field, probably caused by a concentration of the TP (in the outlet there is a concentration of many of the compounds formed as compared with the data in the rice filed, see Table 2). The polarity of temephos, temephos oxon, and temephos oxon isomer is not so different (see retention times in Table 1),

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and therefore the recovery would be similar to that of temephos (22). The S-methyl isomers are produced by thermally induced isomerization generally during synthesis and/or storage (1) but can also take place during exposure to sunlight (31). Temephos isomer was found immediately after spraying, similar to (1) the case of fenitrothion. The isomer of temephos was present five times more concentrated in the outlet water than in the rice field just after treatment. Its concentration 1 h after treatment is 38% and 7.2% of the initial concentration in the rice field and outlet water, respectively, and is maintained constant at levels of 0.05 µg/L until after 5 h of spraying. The detection of S-methyl isomers is important in environmental studies since such compounds have a very different biological activity as compared to their O-methyl isomer (parent compounds) (32). Similarly, temephos oxon isomer is also found immediately after spraying, because the levels are alike in the rice field and in the outlet water and decrease with time. The drop of these two compounds indicates that the formation of temephos isomer and temephos oxon isomer are not important routes of temephos dissipation. Inside the Rice Field. The initial concentration of temephos sulfoxide is higher in the rice field than in the outlet water. However, its concentration decreases in time in the rice field but increases in the outlet water. The large number of degradation products for temephos formed inside the rice field can be attributed to (i) the large surface area of the rice fields, with temephos being distributed along the upper water layer; (ii) the high water temperatures of the water during the days of application (30 °C), which will enhance thermal degradation and hydrolysis and will increase the microbial activity; (iii) the content of organic matter of the flooded soils is quite high (3.3%) caused by the fact that rice straw is mixed up with the soil samples, which also enhances the biodegradation of pesticides into the rice field. In this sense, oxidation has been reported as a typical behavior of organic rich soils under flooded conditions with the rapid formation of TPs (33). It appears evident that the sulfoxide is formed in the water matrix at increasing concentration levels, and consequently, it can persist in the water sample for a longer period than the parent compound. This has also been noticed in previous experiments from our group for disulfoton sulfoxide and fenthion sulfoxide (9). Different from the formation of the previous isomers, the isomer of temephos sulfoxide was not found immediately after treatment but 1 h after at levels of 0.01 and 0.16 µg/L level in the rice field and outlet water, respectively. The higher levels found in the outlet water match the higher levels of temephos sulfoxide encountered in this area. The levels of temephos sulfoxide isomer in the rice field follow a slight increase during the 7 h of monitoring. The levels of temephos sulfoxide decrease, thus indicating that it can undergo an isomerization process in the water that enhances its dissipation.

Conclusions ABATE 50E, containing 50% temephos, was applied in the Ebre Delta by aircraft spraying at a concentration of 250 mL/Ha. Temephos was monitored directly in the rice crop field waters, its initial concentration being 2.27 µg/L. The persistence of temephos under real environmental conditions has led to the detection, for the first time, of various transformation products in water: temephos isomer and

temephos sulfoxide were encountered immediately after spraying, indicating their presence in the formulation. Temephos sulfoxide isomer was the last compound to be formed in the water after spraying. Temephos oxon and oxon isomer were also found immediately after spraying and exhibit a clear decay in time. All those compounds were surveyed for a period up to 72 h, and the concentration levels of the different TPs were in the range of 0.04-1.6 µg/L at the end of the survey. Rapid photodegradation was one of the main routes of degradation of this compound due to its absorption above 295 nm and of high quantum yield due to its double aromatic structure. For the identification of temephos and its transformation products, the use of LC/thermospray/MS was needed due to the problems encountered with diode array detection. Probably this is one of the reasons that previous studies were unable to detect the amount of degradation products of temephos. More studies in this area are needed to investigate the presence of different pesticide transformation products under real environmental conditions.

Acknowledgments R. Alonso is thanked for technical assistance. This work was supported by the Commission of the European Communities (Contract EV5V-CT92-0105) and by CICYT (AMB94-1507-CE). Research Agreement 7978/CF from the International Atomic Energy Agency (IAEA) is also acknowledged. S.L. and N.E. gratefully acknowledge financial support from CICYT (Grant AMB 92-0218) and ACTIVECOMETT. One of the reviewers is acknowledged for helpful discussions to improve the quality of the paper.

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Received for review May 25, 1995. Revised manuscript received October 10, 1995. Accepted October 11, 1995.X ES9503589 X

Abstract published in Advance ACS Abstracts, December 15, 1995.

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