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Fate of aldicarb in the vadose zone beneath a cotton field
Journal of Contaminant Hydrology, 14 (1993) 129-142 Elsevier Science Publishers B.V., Amsterdam
129
Fate o f aldicarb in the vadose zone beneath a cotton field Cai Daoji, Xiang Feng, Jiang Xinming, Zhu Zhonglin*, Hua Xiaomei and Dai Zhenke
Nanjing Research Institute of Environmental Sciences, National Environmental Protection Agency ( NEPA ), P.O. BOX 4202, Nanjing 210042, People's Republic of China (Received January 9, 1991; revised and accepted March 30, 1993)
ABSTRACT Cai, D., Xiang, F., Jiang, X., Zhu, Z., Hua X. and Dai, Z., 1993. Fate of aldicarb in the vadose zone beneath a cotton field. J. Contam. Hydrol., 14: 129-142. A 1.0-ha cotton field in Nantong city (Jiangsu province, P.R.C.) was selected to monitor the movement and fate of aldicarb residues in soil and to check whether it will pollute the groundwater. The field measurement results showed that > 90% of the total aldicarb applied had been dissipated within 60 days and no detectable level of aldicarb residues was found in any of the soil profiles 120 days after the aldicarb was applied. The maximum leaching depth of aldicarb in soil was 0.60 m. None of the 20 observation wells in and around the study site had any detectable aldicarb residues during the entire experimental period. The PRZM model was used to simulate the movement of aldicarb in soil using the input data including soil properties, meteorological data and agriculture practices at the study site. Results predicted by the PRZM model agreed fairly well with those of the field measurements. Under the conditions of this experiment, the application of aldicarb to the cotton fields did not result in groundwater contamination. The results suggest that the PRZM model can be used to estimate the possibility of aldicarb leaching into groundwater and its effects on groundwater in other areas of China that have similar soil properties.
INTRODUCTION A l d i c a r b is an effective, systematic a n d b r o a d - s p e c t r u m n e m a t i c i d e a n d insecticide. It has been widely used on s o m e fifty types o f plants such as c o t t o n , p e a n u t , p o t a t o , s u g a r beet, o r n a m e n t a l plants, s u g a r cane, citrus, c o r n , maize, o r a n g e , s o y b e a n , t o b a c c o , etc. for c o n t r o l l i n g aphids, thrips, m e a l y b u g s , p l a n t bugs, leaf miners, whiteflies, mites, foliar, bulb a n d r o o t * To whom correspondence may be addressed.
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nematodes, etc., in over seventy countries since it was developed by the Union Carbide Corporation in 1962. In China, Temik '~' (its trade name) was registered for use on peanut, cotton and tobacco separately in 1986 and 1987. Because traces of aldicarb residues* have appeared in drinking water in Long Island, New York, U.S.A. (Zaki et al., 1982), pesticide groundwater contamination surveys have been carried out by the U.S. Environmental Protection Agency (USEPA), and have also discovered aldicarb groundwater contamination in other parts of the U.S.A. and Canada (Holden, 1986; Jackson et al., 1990). Many research programs including laboratory, field and computer modeling have been undertaken in the last 10 years (Coppedge et al., 1977; Hansen and Spiegel, 1983; Toson et al., 1984; Donigian and Rao, 1986, 1990; Jones, 1986, 1987; Jones et al., 1986a, b, 1987a, b, 1988; Lightfoot et al., 1987; Hegg et al., 1988; Ou et al., 1988; Hornsby and Rao, 1990; Pennell et al., 1990; Wagenet and Rao, 1990) in order to better understand the degradation and movement of aldicarb and its two biological metabolites, aldicarb sulfoxide and aldicarb sulfone, in various fields and other environmental conditions. Since the movement of aldicarb residues from soil into groundwater is strongly affected by local conditions, evaluations made in other countries will not always be applicable to the particular circumstances in China. The objectives of this study were: (1) to study the degradation, movement and fate of aldicarb in a cotton-field soil profile; and (2) to study the factors determining the movement of aldicarb in soils and investigate the potential for aldicarb contamination of groundwater in the area. The research consisted of two parts: a field experiment and a computer simulation. EXPERIMENT
Site and application description The study site was a 1.0-ha cotton field (Fig. 1.) located at the town of Libao, HaiAn county, Nantong city, Jiangsu province, which lies at the southeastern coast of China in the coastal alluvial plain at the northern boundary of the northern subtropical zone, and which represents a typical cotton growing area in the Jiangsu province. The mean annual precipitation is 1021.9 mm. The rainy season contributes > 50% of the annual precipitation and ranges from July to September. The mean annual temperature in this area is 14.6°C, the highest monthly mean temperature is 27.2°C and occurs in July and/or August. The crop rotation at the site was wheat in winter and cotton in * Aldicarb residues include the aldicarb parent compound and its two biological metabolites, aldicarb sulfoxide and aldicarb sulfone.
FATE OF A L D I C A R B IN THE VADOSE ZONE BENEATH A COTTON FIELD
131
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summer. Before the experiment, no aldicarb had ever been applied to the site. During the study, six well clusters (total of 20 wells) were installed, two months before the harvesting of the winter wheat, at various locations in and around the study site (Fig. 1). Two of the well clusters are located in the experimental field, and each consists of four wells with depths of 1.8, 2.6, 4.0 and 5.3 m for monitoring the vertical movement of aldicarb. The other four well clusters are located 10 m from the boundary of the experimental field, each consisting of three wells with depths of 1.8, 2.6 and 4.0 m, for monitoring the horizonal movement of aldicarb. All the wells consist of 3.8cm-diameter polyvinylchloride (PVC) pipes connected to a 0.3-m-long PVC screen with 0.15-mm-wide slots, which permits water to permeate into the pipe. Bentonite (r~ was used to seal all the wells located in the experimental field. All materials used were provided by the Rh6ne-Poulenc Agriculture Company of France. Aldicarb was applied to the experimental field on June 8, 1988 as a 15 wt% granular formulation (Temik ~ 15G) at a rate of 0.9 kg active ingredient ha -l after the cotton seedlings were transplanted into the experimental field. The aldicarb granular material was incorporated into the top 5 cm of the soil at points halfway between two cotton seedlings of each row. The row width is 0.5 m while the seedlings are 0.3 m apart.
Soil and well-water sampling Soil and well-water samples were collected six times at 30, 60, 90, 120, 165 and 210 days after aldicarb was applied. Soil samples were collected from soil. profiles at sixteen of the aldicarb application points in the experimental field using a 8.3-cm-diameter bucket auger on each of the six sampling dates. Each
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profile was divided into five layers: 0-0.3, 0.3-0.6, 0.6-1.2, 1.2-1.8 and 1.82.4 m. Between each sampling layer the bucket auger was washed with water, scrubbed with a bristle brush and rinsed with water. The top 5 cm of soil in each auger were discarded (except for the first auger of each core) to avoid introducing any residues from the upper layer. Each auger sample of soil from a layer was placed into a plastic bag, and the samples were mixed thoroughly by shaking and kneading the bag after all the soil from a particular layer had been obtained. Then the samples were labelled and kept cool before they were brought to the laboratory within two days. All samples were maintained frozen in an icebox until analyzed. Well-water samples were collected by means of peristaltic pumps. All materials coming in contact with the water had been checked in laboratory experiments to ensure that their use would not result in sorption or degradation of aldicarb residues. A volume of water equal to five times the original volume standing in the well was pumped out and discarded before collecting each well sample. In the case of wells with flow rates too slow to permit pumping of five well volumes of water, their standing water was pumped out after which the water entering these wells was pumped for an additional 5 min before the samples were collected. The sample bottle and cap was rinsed twice with well water before filling. The well-water samples were labelled and transferred to an icebox to be kept frozen and brought to the laboratory within two days. All water samples were maintained frozen until analyzed.
Sample analyses Aldicarb residues in the soil and water samples were analyzed by highperformance liquid chromatography (HPLC) equipped with a post-column derivatization/fluorescence device (Krause, 1979; Chaput, 1986). Aldicarb and its two toxic metabolites, aldicarb sulfoxide and aldicarb sulfone, can be detected simultaneously. The detection limits for all three forms of aldicarb residues are 2 #g kg 1. Well-water samples were analyzed directly by this procedure without any pre-treatment. Soil samples were extracted with water. The average recovery of total aldicarb (aldicarb + residues) was 92.0% (87.9-96.2%).
Determination of aldicarb degradation The aldicarb degradation at various soil depths (0-0.3, 0.3-0.6, 0.6-1.2, 1.2-1.8 and 1.8-2.4 m) was measured in the laboratory at different temperatures corresponding to the actual field conditions. The soil of 0-0.3-m depth was incubated at 20% moisture content and 0.2 #g aldicarb/g soil at 25°C. The incubated soils were sampled and all three forms of aldicarb residues
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were measured using H P L C equipped with a post-column derivatization/ fluorescence device at 1, 2, 5, 9, 14, 21 and 30 days after incubation. Soil samples from depths of > 0.3 m were prepared and measured as above, but incubated at day 15 and sampled at 2, 5, 7, 10, 15, 19, 26, 36 and 50 days after incubation. Vadose zone simulation
M o v e m e n t of aldicarb residues in the vadose zone was simulated using the pesticide root zone model ( P R Z M ; Carsel et al., 1984, 1985), a vadose zone model developed by the USEPA. The major input parameters of the model include soil physical-chemical properties, pesticide properties and information on management practices as shown in Table 1. The field soil physical and chemical properties were analyzed (Table 2) in the laboratory using standard methods (Nanjing Institute of Soil Science, 1978). Meteorological data (Table 3) were provided by the local meteorological station, and management practices by the local agriculture department. RESULTS AND DISCUSSION This section summarizes the results of the field and laboratory experiments and computer simulations. Data on individual sample analyses, soil properties and meteorological parameters are available upon request from the senior author.
TABLE 1 Main input parameters used in PRZM model simulation Weather data:
1988 daily rainfall and pan evaporation at the town of Libao, HaiAn county, Jiangsu province Root zone depth: 60 cm Aldicarb application: 0.9 kg ha -I on June 8, 1988 Depth (cm)
Sand (%)
Clay (%)
Organic carbon (%)
Pesticide half-life* (days)
0 30 30 60 60 120 120-180 180--240
61.2 66.0 68.9 61.0 62.2
13.8 10.1 8.3 9.7 10.4
0.81 0.26 0.16 0.12 0.20
12.3 50 65 65 65
* Results of laboratory degradation experiments.
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TABLE 2 Soil properties at the town of Libao, HaiAn county, Jiangsu province Depth (cm)
Bulk Total density pore (g cm 3) space (%)
Noncapillary space* (%)
Field capacity ( p F = 2, %)
pH OM (%)
CEC (meq/100 g)
Wilting Texture (%) point (vol%) sand clay
0-30 30-60 60-120 120-180 180-240
1.25 1.46 1.49 1.50 1.50
14.7 11.6 12.6 10.7 11.7
30.4 27.1 30.2 31.9 31.9
8.5 9.0 9.1 9.2 9.0
8.19 6.30 5.89 6.84 6.70
6.66 7.38 4.12 5.09 6.22
52.7 44.8 44.8 44.5 44.5
1.40 0.44 0.27 0.21 0.34
61.2 66.0 68.9 61.0 62.2
13.8 10.1 8.3 9.7 10.4
p F - - soil attraction, which is defined as the negative logarithm of the soil water free-energy, and which equals to the negative logarithm of the soil water weight potential (cm); O M = organic matter; C E C = cation-exchange capacity. * The soil non-capillary space is a measurement of soil aeration which is equal to the difference of the total soil pore space and the soil capillary porosity.
Soil properties The soil properties at the town of Libao are summarized in Table 2. The soil was sandy loam in texture at all depths, and was highly alkaline with a pHvalue of ~ 8.5 in the surface layer (0-0.3 m) and increased in alkalinity with depth to a pH-value of 9.2 in the 1.2-1.8-m-depth soil layer. The soil bulk TABLE 3 Summary of weather data for the study site Month
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Rainfall (mm)
Pan evaporation (mm)
1988
average*
1988
average*
8.3 48.4 53.2 31.9 175.5 119.7 224.5 75.5 118.6 22.9 0.2 3.4
27.1 40.8 54.9 71.9 105.8 114.9 202.4 126.9 89.7 77.0 57,2 40.0
57.8 52.9 77.7 134.4 143.3 135.0 190.6 167.4 121.5 116.8 111.5 66.6
32.3 36.9 62.9 84.0 91.8 157.4 116.7 142.7 104.9 66.5 46.8 24.4
* 20-year average for 1969-1988.
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density increased slightly with depth from 1.25 g cm 3 in the surface layer to 1.50 g cm -3 below 1.2 m. The organic matter content of the soil decreased with depth from 1.4% in the surface layer to < 0 5 % below this layer. The initial field capacity of the entire soil profile was ~30%. The soil properties were similar to those at Maricopa, Arizona, U.S.A. (Jones et al., 1986b) except that the organic matter content of this experimental field was greater than that of Maricopa. Aldicarb residues
Mean aldicarb residue concentrations at different times and soil depths are presented in Table 4. The values shown represent the average aldicarb residues at the Temik :R~application point and are not the field-average concentrations. Soil sample analyses indicated that aldicarb residues were mainly confined within the upper 0.6 m, only ~ 0.8% of the total aldicarb applied leached to below the 0.6-m soil layer. The dissipation rate of total aldicarb (aldicarb + residues) in the field was ~ 0.5 month. This is similar to results obtained from the Maricopa field experiments by Jones et al. (1986b). Four months after application, the concentration of aldicarb residues in all soil layers was below the detection limit. Aldicarb residue concentrations measured in the field soils were moderately variable (Table 4). The coefficients of variation for total aldicarb (aldicarb + TABLE 4
Total aldicard residues in soils Sampling
Number
Depth
time
of
(cm)
(days)
samples
30
60
16 16 16 16 16 16 16 16 16 16
0-30 30-60 60 120 120-180 180-240 0-30 30-60 60 120 120 180 180 240
Number of non-zero samples
16 15 2 0 0 8 13 3 0 0
not detected at an analytical sensitivity of 2/~g L t. ,i Calculations include all samples (including non-detectable). ,2 Calculations using only non-zero samples. n.d.
Total aldicarb residues (#g/kg soil) mean *~
C.V. .2
114 16 n.d. n.d. n.d. 10 6 2 n.d. n.d.
0.594 0.772
0.499 1.000 0.616
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residues) concentrations in soils at different layers and sampling time ranged from 49.9% to 100%, and the variation in the surface layer was less than that in the deeper layers. The variability probably results from the nonuniform application of the aldicarb, the nonuniformity of the soil structure and soil physicochemical and biological properties as well as the nonuniform infiltration of water in soil. Jones et al. (1988) stated that "the non-uniform application of aldicarb may be the dominant cause of the variability in the total carbamate residue levels".
But in this study, where the sampling position was at the application position, the variability may be mainly due to inherent variations in soil properties or the so-called intrinsic variability used by Rao et al. (1986). Smith et al. (1987) noted that the coefficient of variation for field measurements of pesticide concentrations may range from 40% to 450%. The results of our field experiments and a number of other field studies (Jones, 1986; Jones et al., 1988; Hornsby and Rao, 1990) all fall within this range. Smith et al. (1987) indicated that spatial variations in pesticide residue concentrations in soil and their effects on pesticide movement have important implications in assessing groundwater pesticide contamination. In order to obtain average values of the required precision (e.g., the level of confidence, tolerable error and variability were chosen at 90%, 50% and 100%, respectively) and to arrive at statistically valid conclusions, a sufficiently large number of soil samples (e.g., 13) must be collected. The maximum leaching depth* of aldicarb residues in soil was 0.6 m, and was lower than that found at Maricopa by Jones et al. (1986b). This may be contributed to a lower aldicarb application, lower rainfall and irrigation and a higher soil organic matter content as compared to Maricopa. Many factors can influence the movement of pesticide residues in soil, such as the pesticide application rate, soil properties, the degradation and adsorption of the pesticide in the soil, and the amount of rainfall and irrigation, etc. The literature is replete with reports on the influence of numerous factors on the movement of pesticide in soil (Cheng and Koskinen, 1986; Helling and Gish, 1986; Jones, 1986). Of these, the movement of water in the soil is the dynamic factor responsible for pesticide movement. The depth to which water moves in a soil is mainly determined by precipitation, evaporation and the soil field capacity. Soils in this study area have a sandy loam texture. Normally the highest temperatures occur from June to August. The evapotranspiration rate during this period is higher than at any other time of the year. During the first month of the field experiment after aldicarb application, the total precipi-
* The maximum leaching depth is defined as the depth below which the average concentration does not exceed 5 #g/kg soil.
EATE O F A t . D I C A R B IN T H E V A D O S E Z O N E B E N E A T H A C O T T O N F I E L D
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tation was 119.7 ram, while the total amount of evapotranspiration reached 108.0 mm (0.8 times the amount of pan evaporation), so that the amount of percolation water was very limited. Based on this, it can be concluded that most of the aldicarb residues would be confined within the surface layer, with little of the aldicarb residues leaching into the lower layers. Although there was a heavy rainfall event of 108.9 mm 56 days after the aldicarb application, the amount of aldicarb which could be leached to lower layers was very limited, because most of the residual aldicarb had already been degraded during the intervening period. The results of groundwater analyses support the above conclusion. During the entire duration of the experiment, no aldicarb, or its metabolites, was detected in any of the wells, neither inside nor outside the study area. The measured depth to the groundwater table was found to fluctuate as shown in Fig. 2. During the rainy season the groundwater table was ~ 1.2 m below the surface, but in the dry season the groundwater table fell to ~ 2 m. From the above, it can be concluded that the aldicarb applied had almost entirely degraded in the unsaturated root zone before it reached the groundwater and that the application of aldicarb did not result in groundwater contamination in our experimental area.
Degradation study The results of the laboratory degradation studies showed that the degradation half-life of aldicarb in soil increased with depth under the experimental conditions (Fig. 3). This may be the result of the high temperature and high biota activity in the surface layer. The aldicarb parent compound degraded more rapidly than its metabolites. In the initial periods of the incubation, the amount of aldicarb daughter products increased rapidly while the amount of 1
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parent c o m p o u n d decreased. The concentration of the metabolites in soil reached a maximum on the 8th day and then decreased gradually. The half-life of the aldicarb parent compound was 2 days in the surface layer and on average 18 days in the lower layers (0.6-1.2, 12-1.8 and 1.8-2.4 m). The degradation half-life of total aldicarb (aldicarb + residues) was 12.3 days in the surface layer and > 65 days in the lower layers. Model simulation
Many simulation models for predicting pesticide behavior in the vadose zone have been developed (Carsel et al., 1984, 1985; Nofziger and Hornsby, 1987; Steenhuis et al., 1987; Wagenet and Huston, 1987; Knisel et al., 1989), and their validation and use for predicting pesticide behavior in the vadose zone have received considerable attention over the past decade. The virtues and defects of the models in predicting the fate and transport of pesticides in soils and groundwater have been critically reviewed by Pennell et al. (1990), Donigian and Rao (1986, 1990), and Wagenet and Rao (1990). The P R Z M model was selected to simulate the fate of aldicarb in soil in the present study mainly because it has been tested extensively (e.g., Jones et al., 1986a; Pennell et al., 1990) and is easy to use. The predicted and measured aldicarb residue concentrations in the soil layers of the cotton field are shown in Fig. 4 as a function of time. The results of the P R Z M model simulation indicate that the initial mean concentration of aldicarb residues in the surface layer was 248 #g kg-1 and that this would be reduced to 60 #g kg i 30 days later. Only ~ 2% of the total aldicarb applied
FATE OF ALDICARB
IN THE VADOSE ZONE
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was leached to below 0.3-m depth while > 90% of all aldicarb residues would be distributed in the surface layer. On the 60th day, the amount of aldicarb residues in the entire profile was ~ 5% of the total aldicarb applied; > 80% of this was confined to the upper two layers (0-0.3- and 0.3-0.6- m depths) and only ~ 1% of the total aldicarb applied was leached to below 0.6 m. After 120 days, almost all the aldicarb would have been dissipated from the soil layers. The maximum leaching depth predicted by model simulation was 0.6 m. This figure was in fairly good agreement with the result measured in the field. Measured aldicarb residues seemed to persist for a longer time than those predicted by the P R Z M model simulation. The phenomenon was especially significant in the surface layer during the initial periods after aldicarb application. This may be due to the fact that the field soil sampling core was collected immediately beneath the aldicarb application point, so that the aldicarb residue concentrations cannot represent the field-average concentrations. If converted to "field-average" concentrations, assuming that no lateral dispersion occurred [(measured values)× (area of total application points)/(area of the study field)], the predicted results would be close to the measured concentrations in the field. With increasing sample depth and time, the two results tend to be more consistent. This is due to aldicarb diffusing and moving downward in the form of a pyramid beneath the application point with lateral dispersion producing a more even aldicarb distribution in the soil profile. Hornsby and Rao (1990) arrived at the same conclusion after summarizing their field experiment data. From a macroscopic point of view, the distribution of aldicarb in the soil of the field can be seen as even since, though it was applied in individual holes, these were very closely spaced (between individual cotton seedlings). The comparison between the measured and predicted results indicates that the P R Z M model is a good tool for predicting pesticide transport in the soil if
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it is used properly, although it has some limitations, i.e. it only simulates a single solute, underestimates solute movement when an intense rainfall event occurs, and cannot accurately predict the pesticide behavior in soils with a more complicated structure where a preferential flow o f water exists. Jones et al. (1986a) arrived at conclusions similar to ours after they compared the P R Z M model predictions with some unsaturated-zone field data. China covers a vast area with great variations in natural conditions. In order to assess the potential for aldicarb contamination of groundwater, we can use the P R Z M model to define areas which are suitable for aldicarb use according to the specific climatic conditions and soil characteristics. This work will also provide a scientific basis for guiding the safe application o f aldicarb in China. CONCLUSIONS The field monitoring data indicate that almost all aldicarb had been degraded within 60 days of application and that aldicarb residues did not leach below a depth o f 0.6 m in the soil. The aldicarb residue concentrations in soil varied moderately, with coefficients o f variation between 49.9% and 100%. No aldicarb residues were found in groundwater samples, suggesting that the application of aldicarb to this cotton field is safe and should not threaten groundwater quality. The P R Z M model offers good potential for predicting the fate of aldicarb in soil and could be an effective tool in assessing pesticide residue movement in soils if it is properly used. ACKNOWLEDGEMENTS The authors would like to acknowledge Dr. Russell L. Jones (RhOnePoulenc Agriculture Company, Research Triangle Park, North Carolina, U.S.A.) for his helpful suggestions and his assistance in installing wells. We do thank Qian Jinsheng, Miao Changping and Miao Baogen who participated in the collection of soil and well-water samples; Mr. Xiao Xingji for preparing the figures and Mr. Tang Guocai for useful critical comments on the manuscript. REFERENCES Bonazountas, M. and Wagner, J., 1981. SESOIL: A seasonal soil compartment model. Off. Toxic Substances, U.S. Eviron. Prot. Agency, Washington, DC. Bromilow, R.H. and Leistra, M., 1980. Measured and simulated behavior of aldicarb and its oxidation products in fallow soils. Pestic. Sci., 11: 389-395. Bromilow, R.H., Baker, R.J., Freeman, M.A.H. and G6r6g, K., 1980. The degradation of aldicarb and oxamyl in soil. Pestic. Sci., 11: 371-378.
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Carsel, R.F., Smith, C.N., Mulkey, L.A., Dean, J.D. and Jowise, P., 1984. User's manual for the Pesticide Root Zone Model (PRZM). U.S. Environ. Prot. Agency, Athens, GA, Release 1, EPA 600/3-84-109, 216 pp. Carsel, R.F., Mulkey, L.A., Lorber, M.N. and Basking, L.B., 1985. The Pesticide Root Zone Model (PRZM): A procedure for evaluating pesticide leaching threat to ground water. Ecol. Modelling, 30: 49-69. Chaput, D., 1986. On-line trace enrichment for determination of aldicarb species in water, using liquid chromatography with post-column derivatization. J. Assoc. Off. Anal. Chem., 69:985 989. Cheng, H.H. and Koskinen, W.C., 1986. Processes and factors affecting transport of pesticides to ground water. In: W.Y. Garner, R.T. Honeycutt and H.N. Nigg (Editors), Evaluation of Pesticide in Groundwater. Am. Chem. Soc., Syrup. Ser., 315: 2-13. Coppedge, J.R., Bull, D.L. and Ridgway, R.L., 1977. Movement and persistence of aldicarb in certain soils. Arch. Environ. Contam. Toxicol., 5: 129-141. Donigian, Jr., A.S. and Rao, P.S.C., 1986. Example model testing studies, In: S.C. Hem and S.M. Melancon (Editors), Vadose Zone Modeling of Organic Pollutants. Lewis, Chelsea, MI, pp. 103 132. Donigian, Jr., A.S. and Rao, P.S.C., 1990. Selection, application and validation of environmental fate models. In: D. Decorsey (Editor), Processings of the International Symposium on Water Quality Modeling of Agricultural Non-Point Sources, Part II. U.S. Dep. Agric.. Washington, DC, pp. 557-600. Hansen, J.L. and Spiegel, M.H., 1983. Hydrolysis studies of aldicarb, aldicarb sulfoxide and aldicarb sulfone. Environ. Toxicol. Chem., 2:147 153. Hegg, R.O., Shelley, W.H., Jones, R.L. and Romine, R.R., 1988. Movement and degradation of aldicarb residues in South Carolina loamy sand soil. Agric. Ecosystems Environ., 20: 303 315. Helling, C.S. and Gish, T.J., 1986. Soil characteristics affecting pesticide movement into ground water. In: W.Y. Garner, R.T. Honeycutt and H.N. Nigg (Editors), Evaluation of Pesticide in Groundwater. Am. Chem. Soc., Symp. Ser., 315:14 38. Holden, P.W., 1986. Pesticides and Groundwater Quality - - Issues and Problems in Four States. National Academy Press, Washington, D.C., 124 pp. Hornsby, A.G. and Rao, P.S.C., 1990. Fate of aldicarb in the unsaturated zone beneath a citrus grove. Water Resour. Res., 26:2287 2302. Jackson, R.E., Mutch, J.P. and Priddle, M.W., 1990. Persistence of aldicarb residues in the sandstone aquifer of Prince Edward Island, Canada. J. Environ. Hydrol., 6:21 35. Jones, R.L., 1986. Field, laboratory and modeling studies on the degradation and transport of aldicarb residues in soil and ground water. In: W.Y. Garner, R.T. Honeycutt and H.N. Nigg (Editors), Evaluation of Pesticide in Groundwater. Am. Chem. Soc., Symp. Ser., 315:197 218. Jones, R.L., 1987. Central California studies on the degradation and movement of aldicarb residues. J. Contain. Hydrol., 1: 287-298. Jones, R.L., Black, G.W. and Estes, T.L., 1986a. Comparision of computer model predictions with unsaturated zone field data for aldicarb and aldoxycarb. Environ. Toxicol. Chem., 5: 1027 1037. Jones, R.L., Hansen, J.L., Romine, R.R. and Marquardt, T.E., 1986b. Unsaturated zone studies of the degradation and movement of aldicarb and aldoxycarb residues. Environ. Toxicol. Chem., 5:361 372. Jones, R.L., Hornsby, A.G., Rao, P.S.C. and Anderson, H.P., 1987a. Movement and degradation of aldicarb residues in the saturated zone under citrus groves on the Florida ridge. J. Contain. Hydrol., 1: 265-286.
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Jones, R.L., Kirkland, S.D. and Chancey, E.L., 1987b. Measurement of environmental fate of aldicarb residues in a Nebraska Sand Hills soil. Appl. Agric. Res., 2:177 182. Jones, R.L., Hornsby, A.G. and Rao, P.S.C., 1988. Degradation and movement of aldicarb residues in Florida citrus soils. Pestic. Sci., 23:307 325. Knisel, W.G., Leonard, R.A. and Davis, F.M., 1989. GLEAMS User Manual. Southe. Watershed Res. Lab., Tiffon, GA, 25 pp. Krause, R.T., 1979. Resolution, sensitivity and selectivity of a high-performance liguid chromatographic post-column fluorometric labeling technique for determination of carbamate insecticides. J. Chromatogr., 185:615 624. Lightfoot, E.N., Thorne, P.S., Jones, R.L., Hansen, J.L. and Romine, R.R., 1987. Laboratory studies on mechanisms for the degradation of aldicarb, aldicarb sulfoxide and aldicarb sulfone. Environ. Toxicol. Chem., 6:377 394. Nanjing Institute of Soil Science, Chinese Academy of Science, 1978. Soil Physical and Chemical Analysis. Shanghai Scientific Press, Shanghai, 593 pp. (in Chinese). Nofziger, D.L. and Hornsby, A.G., 1987. CMLS: Interactive simulation of chemical movement in layered soils. In: User's Manual. Inst. Food Agric. Sci., Univ. of Florida, Gainesville, FL, Circ. 780., 44 pp. Ou, L.T., Rao, P.S.C., Edvardsson, K.S.V., Jessup, R.E., Hornsby, A.G. and Jones, R.L., 1988. Kinetics of aldicarb degradation in soils. Pestic. Sci., 23: 1-12. Pennell, K.D., Hornsby, A.G., Jessup, R.E. and Rao, P.S.C., 1990. Evaluation of five simulation models for predicting aldicarb and bromide behavior under field conditions. Water Resour. Res., 26: 2679-2693. Rao, P.S.C., Edvardsson, K.S.V., Ou, L.T., Jessup, R.E., Nkedi-Kizza, P. and Hornsby, A.G., 1986. Spatial variability of pesticide sorption and degradation parameters. In: W.Y. Garner, R.T. Honeycutt and H.N. Nigg (Editors), Evaluation of Pesticide in Groundwater. Am. Chem. Soc., Symp. Ser., 315: 100-135. Smith, C.N., Parrish, R.S. and Carsel, R.F., 1987. Estimating sample requirements for field evaluations of pesticide leaching. Environ. Toxicol. Chem., 6: 343-357. Steenhuis, T.S., Pacenka, S. and Porter, K.S., 1987. MOUSE: A management model for evaluating groundwater contamination from diffuse surface sources aided by computer graphics. Appl. Agric. Res., 2:277 289. Toson, M.A., Wendell, W.K. and Wary, W., 1984. Movement of aldicarb in different soil types. Bull. Environ. Contam. Toxicol., 23: 377-382. Wagenet, R.J. and Huston, J.L., 1987. LEACHM: A finite-difference model for simulating water, salt and pesticide movement in the plant root zone - - Continuum 2. N.Y. State Resour. Inst., Cornell Univ., Ithaca, NY. Wagenet, R.J. and Rao, P.S.C., 1990. Modeling pesticide fate in soils. In: H.H. Cheng (Editor), Pesticides in the Environment: Processes, Impacts and Modeling. Soil Sci. Soc. Am., Madison, WI, Soil Ser. Sci. Bk. Ser., 2:351 399.. Zaki, M.H., Moran, D. and Harris, D., 1982. Pesticides in groundwater: The aldicarb story in Suffolk County, N.Y. Am. J. Public Health, 72:1391 1395.
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Fate o f aldicarb in the vadose zone beneath a cotton field Cai Daoji, Xiang Feng, Jiang Xinming, Zhu Zhonglin*, Hua Xiaomei and Dai Zhenke
Nanjing Research Institute of Environmental Sciences, National Environmental Protection Agency ( NEPA ), P.O. BOX 4202, Nanjing 210042, People's Republic of China (Received January 9, 1991; revised and accepted March 30, 1993)
ABSTRACT Cai, D., Xiang, F., Jiang, X., Zhu, Z., Hua X. and Dai, Z., 1993. Fate of aldicarb in the vadose zone beneath a cotton field. J. Contam. Hydrol., 14: 129-142. A 1.0-ha cotton field in Nantong city (Jiangsu province, P.R.C.) was selected to monitor the movement and fate of aldicarb residues in soil and to check whether it will pollute the groundwater. The field measurement results showed that > 90% of the total aldicarb applied had been dissipated within 60 days and no detectable level of aldicarb residues was found in any of the soil profiles 120 days after the aldicarb was applied. The maximum leaching depth of aldicarb in soil was 0.60 m. None of the 20 observation wells in and around the study site had any detectable aldicarb residues during the entire experimental period. The PRZM model was used to simulate the movement of aldicarb in soil using the input data including soil properties, meteorological data and agriculture practices at the study site. Results predicted by the PRZM model agreed fairly well with those of the field measurements. Under the conditions of this experiment, the application of aldicarb to the cotton fields did not result in groundwater contamination. The results suggest that the PRZM model can be used to estimate the possibility of aldicarb leaching into groundwater and its effects on groundwater in other areas of China that have similar soil properties.
INTRODUCTION A l d i c a r b is an effective, systematic a n d b r o a d - s p e c t r u m n e m a t i c i d e a n d insecticide. It has been widely used on s o m e fifty types o f plants such as c o t t o n , p e a n u t , p o t a t o , s u g a r beet, o r n a m e n t a l plants, s u g a r cane, citrus, c o r n , maize, o r a n g e , s o y b e a n , t o b a c c o , etc. for c o n t r o l l i n g aphids, thrips, m e a l y b u g s , p l a n t bugs, leaf miners, whiteflies, mites, foliar, bulb a n d r o o t * To whom correspondence may be addressed.
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nematodes, etc., in over seventy countries since it was developed by the Union Carbide Corporation in 1962. In China, Temik '~' (its trade name) was registered for use on peanut, cotton and tobacco separately in 1986 and 1987. Because traces of aldicarb residues* have appeared in drinking water in Long Island, New York, U.S.A. (Zaki et al., 1982), pesticide groundwater contamination surveys have been carried out by the U.S. Environmental Protection Agency (USEPA), and have also discovered aldicarb groundwater contamination in other parts of the U.S.A. and Canada (Holden, 1986; Jackson et al., 1990). Many research programs including laboratory, field and computer modeling have been undertaken in the last 10 years (Coppedge et al., 1977; Hansen and Spiegel, 1983; Toson et al., 1984; Donigian and Rao, 1986, 1990; Jones, 1986, 1987; Jones et al., 1986a, b, 1987a, b, 1988; Lightfoot et al., 1987; Hegg et al., 1988; Ou et al., 1988; Hornsby and Rao, 1990; Pennell et al., 1990; Wagenet and Rao, 1990) in order to better understand the degradation and movement of aldicarb and its two biological metabolites, aldicarb sulfoxide and aldicarb sulfone, in various fields and other environmental conditions. Since the movement of aldicarb residues from soil into groundwater is strongly affected by local conditions, evaluations made in other countries will not always be applicable to the particular circumstances in China. The objectives of this study were: (1) to study the degradation, movement and fate of aldicarb in a cotton-field soil profile; and (2) to study the factors determining the movement of aldicarb in soils and investigate the potential for aldicarb contamination of groundwater in the area. The research consisted of two parts: a field experiment and a computer simulation. EXPERIMENT
Site and application description The study site was a 1.0-ha cotton field (Fig. 1.) located at the town of Libao, HaiAn county, Nantong city, Jiangsu province, which lies at the southeastern coast of China in the coastal alluvial plain at the northern boundary of the northern subtropical zone, and which represents a typical cotton growing area in the Jiangsu province. The mean annual precipitation is 1021.9 mm. The rainy season contributes > 50% of the annual precipitation and ranges from July to September. The mean annual temperature in this area is 14.6°C, the highest monthly mean temperature is 27.2°C and occurs in July and/or August. The crop rotation at the site was wheat in winter and cotton in * Aldicarb residues include the aldicarb parent compound and its two biological metabolites, aldicarb sulfoxide and aldicarb sulfone.
FATE OF A L D I C A R B IN THE VADOSE ZONE BENEATH A COTTON FIELD
131
OOO
X
X
X
X
X
X
OO(3O
X
X
X
X
X
X
X
X
0
X
X
100 N -
0 0
0000
-
ooo~lO I~ Fig. 1. Soil sampling plot and well-group distribution in field experiment (× = soil samples; © = wellgroup cluster).
summer. Before the experiment, no aldicarb had ever been applied to the site. During the study, six well clusters (total of 20 wells) were installed, two months before the harvesting of the winter wheat, at various locations in and around the study site (Fig. 1). Two of the well clusters are located in the experimental field, and each consists of four wells with depths of 1.8, 2.6, 4.0 and 5.3 m for monitoring the vertical movement of aldicarb. The other four well clusters are located 10 m from the boundary of the experimental field, each consisting of three wells with depths of 1.8, 2.6 and 4.0 m, for monitoring the horizonal movement of aldicarb. All the wells consist of 3.8cm-diameter polyvinylchloride (PVC) pipes connected to a 0.3-m-long PVC screen with 0.15-mm-wide slots, which permits water to permeate into the pipe. Bentonite (r~ was used to seal all the wells located in the experimental field. All materials used were provided by the Rh6ne-Poulenc Agriculture Company of France. Aldicarb was applied to the experimental field on June 8, 1988 as a 15 wt% granular formulation (Temik ~ 15G) at a rate of 0.9 kg active ingredient ha -l after the cotton seedlings were transplanted into the experimental field. The aldicarb granular material was incorporated into the top 5 cm of the soil at points halfway between two cotton seedlings of each row. The row width is 0.5 m while the seedlings are 0.3 m apart.
Soil and well-water sampling Soil and well-water samples were collected six times at 30, 60, 90, 120, 165 and 210 days after aldicarb was applied. Soil samples were collected from soil. profiles at sixteen of the aldicarb application points in the experimental field using a 8.3-cm-diameter bucket auger on each of the six sampling dates. Each
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profile was divided into five layers: 0-0.3, 0.3-0.6, 0.6-1.2, 1.2-1.8 and 1.82.4 m. Between each sampling layer the bucket auger was washed with water, scrubbed with a bristle brush and rinsed with water. The top 5 cm of soil in each auger were discarded (except for the first auger of each core) to avoid introducing any residues from the upper layer. Each auger sample of soil from a layer was placed into a plastic bag, and the samples were mixed thoroughly by shaking and kneading the bag after all the soil from a particular layer had been obtained. Then the samples were labelled and kept cool before they were brought to the laboratory within two days. All samples were maintained frozen in an icebox until analyzed. Well-water samples were collected by means of peristaltic pumps. All materials coming in contact with the water had been checked in laboratory experiments to ensure that their use would not result in sorption or degradation of aldicarb residues. A volume of water equal to five times the original volume standing in the well was pumped out and discarded before collecting each well sample. In the case of wells with flow rates too slow to permit pumping of five well volumes of water, their standing water was pumped out after which the water entering these wells was pumped for an additional 5 min before the samples were collected. The sample bottle and cap was rinsed twice with well water before filling. The well-water samples were labelled and transferred to an icebox to be kept frozen and brought to the laboratory within two days. All water samples were maintained frozen until analyzed.
Sample analyses Aldicarb residues in the soil and water samples were analyzed by highperformance liquid chromatography (HPLC) equipped with a post-column derivatization/fluorescence device (Krause, 1979; Chaput, 1986). Aldicarb and its two toxic metabolites, aldicarb sulfoxide and aldicarb sulfone, can be detected simultaneously. The detection limits for all three forms of aldicarb residues are 2 #g kg 1. Well-water samples were analyzed directly by this procedure without any pre-treatment. Soil samples were extracted with water. The average recovery of total aldicarb (aldicarb + residues) was 92.0% (87.9-96.2%).
Determination of aldicarb degradation The aldicarb degradation at various soil depths (0-0.3, 0.3-0.6, 0.6-1.2, 1.2-1.8 and 1.8-2.4 m) was measured in the laboratory at different temperatures corresponding to the actual field conditions. The soil of 0-0.3-m depth was incubated at 20% moisture content and 0.2 #g aldicarb/g soil at 25°C. The incubated soils were sampled and all three forms of aldicarb residues
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were measured using H P L C equipped with a post-column derivatization/ fluorescence device at 1, 2, 5, 9, 14, 21 and 30 days after incubation. Soil samples from depths of > 0.3 m were prepared and measured as above, but incubated at day 15 and sampled at 2, 5, 7, 10, 15, 19, 26, 36 and 50 days after incubation. Vadose zone simulation
M o v e m e n t of aldicarb residues in the vadose zone was simulated using the pesticide root zone model ( P R Z M ; Carsel et al., 1984, 1985), a vadose zone model developed by the USEPA. The major input parameters of the model include soil physical-chemical properties, pesticide properties and information on management practices as shown in Table 1. The field soil physical and chemical properties were analyzed (Table 2) in the laboratory using standard methods (Nanjing Institute of Soil Science, 1978). Meteorological data (Table 3) were provided by the local meteorological station, and management practices by the local agriculture department. RESULTS AND DISCUSSION This section summarizes the results of the field and laboratory experiments and computer simulations. Data on individual sample analyses, soil properties and meteorological parameters are available upon request from the senior author.
TABLE 1 Main input parameters used in PRZM model simulation Weather data:
1988 daily rainfall and pan evaporation at the town of Libao, HaiAn county, Jiangsu province Root zone depth: 60 cm Aldicarb application: 0.9 kg ha -I on June 8, 1988 Depth (cm)
Sand (%)
Clay (%)
Organic carbon (%)
Pesticide half-life* (days)
0 30 30 60 60 120 120-180 180--240
61.2 66.0 68.9 61.0 62.2
13.8 10.1 8.3 9.7 10.4
0.81 0.26 0.16 0.12 0.20
12.3 50 65 65 65
* Results of laboratory degradation experiments.
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D. CAI ET AL,
TABLE 2 Soil properties at the town of Libao, HaiAn county, Jiangsu province Depth (cm)
Bulk Total density pore (g cm 3) space (%)
Noncapillary space* (%)
Field capacity ( p F = 2, %)
pH OM (%)
CEC (meq/100 g)
Wilting Texture (%) point (vol%) sand clay
0-30 30-60 60-120 120-180 180-240
1.25 1.46 1.49 1.50 1.50
14.7 11.6 12.6 10.7 11.7
30.4 27.1 30.2 31.9 31.9
8.5 9.0 9.1 9.2 9.0
8.19 6.30 5.89 6.84 6.70
6.66 7.38 4.12 5.09 6.22
52.7 44.8 44.8 44.5 44.5
1.40 0.44 0.27 0.21 0.34
61.2 66.0 68.9 61.0 62.2
13.8 10.1 8.3 9.7 10.4
p F - - soil attraction, which is defined as the negative logarithm of the soil water free-energy, and which equals to the negative logarithm of the soil water weight potential (cm); O M = organic matter; C E C = cation-exchange capacity. * The soil non-capillary space is a measurement of soil aeration which is equal to the difference of the total soil pore space and the soil capillary porosity.
Soil properties The soil properties at the town of Libao are summarized in Table 2. The soil was sandy loam in texture at all depths, and was highly alkaline with a pHvalue of ~ 8.5 in the surface layer (0-0.3 m) and increased in alkalinity with depth to a pH-value of 9.2 in the 1.2-1.8-m-depth soil layer. The soil bulk TABLE 3 Summary of weather data for the study site Month
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Rainfall (mm)
Pan evaporation (mm)
1988
average*
1988
average*
8.3 48.4 53.2 31.9 175.5 119.7 224.5 75.5 118.6 22.9 0.2 3.4
27.1 40.8 54.9 71.9 105.8 114.9 202.4 126.9 89.7 77.0 57,2 40.0
57.8 52.9 77.7 134.4 143.3 135.0 190.6 167.4 121.5 116.8 111.5 66.6
32.3 36.9 62.9 84.0 91.8 157.4 116.7 142.7 104.9 66.5 46.8 24.4
* 20-year average for 1969-1988.
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density increased slightly with depth from 1.25 g cm 3 in the surface layer to 1.50 g cm -3 below 1.2 m. The organic matter content of the soil decreased with depth from 1.4% in the surface layer to < 0 5 % below this layer. The initial field capacity of the entire soil profile was ~30%. The soil properties were similar to those at Maricopa, Arizona, U.S.A. (Jones et al., 1986b) except that the organic matter content of this experimental field was greater than that of Maricopa. Aldicarb residues
Mean aldicarb residue concentrations at different times and soil depths are presented in Table 4. The values shown represent the average aldicarb residues at the Temik :R~application point and are not the field-average concentrations. Soil sample analyses indicated that aldicarb residues were mainly confined within the upper 0.6 m, only ~ 0.8% of the total aldicarb applied leached to below the 0.6-m soil layer. The dissipation rate of total aldicarb (aldicarb + residues) in the field was ~ 0.5 month. This is similar to results obtained from the Maricopa field experiments by Jones et al. (1986b). Four months after application, the concentration of aldicarb residues in all soil layers was below the detection limit. Aldicarb residue concentrations measured in the field soils were moderately variable (Table 4). The coefficients of variation for total aldicarb (aldicarb + TABLE 4
Total aldicard residues in soils Sampling
Number
Depth
time
of
(cm)
(days)
samples
30
60
16 16 16 16 16 16 16 16 16 16
0-30 30-60 60 120 120-180 180-240 0-30 30-60 60 120 120 180 180 240
Number of non-zero samples
16 15 2 0 0 8 13 3 0 0
not detected at an analytical sensitivity of 2/~g L t. ,i Calculations include all samples (including non-detectable). ,2 Calculations using only non-zero samples. n.d.
Total aldicarb residues (#g/kg soil) mean *~
C.V. .2
114 16 n.d. n.d. n.d. 10 6 2 n.d. n.d.
0.594 0.772
0.499 1.000 0.616
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D. CAI EIAL
residues) concentrations in soils at different layers and sampling time ranged from 49.9% to 100%, and the variation in the surface layer was less than that in the deeper layers. The variability probably results from the nonuniform application of the aldicarb, the nonuniformity of the soil structure and soil physicochemical and biological properties as well as the nonuniform infiltration of water in soil. Jones et al. (1988) stated that "the non-uniform application of aldicarb may be the dominant cause of the variability in the total carbamate residue levels".
But in this study, where the sampling position was at the application position, the variability may be mainly due to inherent variations in soil properties or the so-called intrinsic variability used by Rao et al. (1986). Smith et al. (1987) noted that the coefficient of variation for field measurements of pesticide concentrations may range from 40% to 450%. The results of our field experiments and a number of other field studies (Jones, 1986; Jones et al., 1988; Hornsby and Rao, 1990) all fall within this range. Smith et al. (1987) indicated that spatial variations in pesticide residue concentrations in soil and their effects on pesticide movement have important implications in assessing groundwater pesticide contamination. In order to obtain average values of the required precision (e.g., the level of confidence, tolerable error and variability were chosen at 90%, 50% and 100%, respectively) and to arrive at statistically valid conclusions, a sufficiently large number of soil samples (e.g., 13) must be collected. The maximum leaching depth* of aldicarb residues in soil was 0.6 m, and was lower than that found at Maricopa by Jones et al. (1986b). This may be contributed to a lower aldicarb application, lower rainfall and irrigation and a higher soil organic matter content as compared to Maricopa. Many factors can influence the movement of pesticide residues in soil, such as the pesticide application rate, soil properties, the degradation and adsorption of the pesticide in the soil, and the amount of rainfall and irrigation, etc. The literature is replete with reports on the influence of numerous factors on the movement of pesticide in soil (Cheng and Koskinen, 1986; Helling and Gish, 1986; Jones, 1986). Of these, the movement of water in the soil is the dynamic factor responsible for pesticide movement. The depth to which water moves in a soil is mainly determined by precipitation, evaporation and the soil field capacity. Soils in this study area have a sandy loam texture. Normally the highest temperatures occur from June to August. The evapotranspiration rate during this period is higher than at any other time of the year. During the first month of the field experiment after aldicarb application, the total precipi-
* The maximum leaching depth is defined as the depth below which the average concentration does not exceed 5 #g/kg soil.
EATE O F A t . D I C A R B IN T H E V A D O S E Z O N E B E N E A T H A C O T T O N F I E L D
137
tation was 119.7 ram, while the total amount of evapotranspiration reached 108.0 mm (0.8 times the amount of pan evaporation), so that the amount of percolation water was very limited. Based on this, it can be concluded that most of the aldicarb residues would be confined within the surface layer, with little of the aldicarb residues leaching into the lower layers. Although there was a heavy rainfall event of 108.9 mm 56 days after the aldicarb application, the amount of aldicarb which could be leached to lower layers was very limited, because most of the residual aldicarb had already been degraded during the intervening period. The results of groundwater analyses support the above conclusion. During the entire duration of the experiment, no aldicarb, or its metabolites, was detected in any of the wells, neither inside nor outside the study area. The measured depth to the groundwater table was found to fluctuate as shown in Fig. 2. During the rainy season the groundwater table was ~ 1.2 m below the surface, but in the dry season the groundwater table fell to ~ 2 m. From the above, it can be concluded that the aldicarb applied had almost entirely degraded in the unsaturated root zone before it reached the groundwater and that the application of aldicarb did not result in groundwater contamination in our experimental area.
Degradation study The results of the laboratory degradation studies showed that the degradation half-life of aldicarb in soil increased with depth under the experimental conditions (Fig. 3). This may be the result of the high temperature and high biota activity in the surface layer. The aldicarb parent compound degraded more rapidly than its metabolites. In the initial periods of the incubation, the amount of aldicarb daughter products increased rapidly while the amount of 1
2 I
3 t
4 I
5 ' I
6 I
"'
7 I
8 I
9 10 11 12 (Month) I
I
I
I
1.0
o
2.0
Fig. 2. Monthly changes of groundwater level [groundwater pH 7.3 (range 6.8 7.6) and temperature 17.7C (range 13-24°C)].
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D. CAI ET AL.
0
10
20 I
I
Half-life (days) 30 40 50 I
I
I
60
70
I
I
,
0.3
Iz
0.6 II 1.2
}2
1.8
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2.4
Fig. 3. Degradation half-life of aldicarb in soil (1 = aldicarb; 2 = total aldicarb).
parent c o m p o u n d decreased. The concentration of the metabolites in soil reached a maximum on the 8th day and then decreased gradually. The half-life of the aldicarb parent compound was 2 days in the surface layer and on average 18 days in the lower layers (0.6-1.2, 12-1.8 and 1.8-2.4 m). The degradation half-life of total aldicarb (aldicarb + residues) was 12.3 days in the surface layer and > 65 days in the lower layers. Model simulation
Many simulation models for predicting pesticide behavior in the vadose zone have been developed (Carsel et al., 1984, 1985; Nofziger and Hornsby, 1987; Steenhuis et al., 1987; Wagenet and Huston, 1987; Knisel et al., 1989), and their validation and use for predicting pesticide behavior in the vadose zone have received considerable attention over the past decade. The virtues and defects of the models in predicting the fate and transport of pesticides in soils and groundwater have been critically reviewed by Pennell et al. (1990), Donigian and Rao (1986, 1990), and Wagenet and Rao (1990). The P R Z M model was selected to simulate the fate of aldicarb in soil in the present study mainly because it has been tested extensively (e.g., Jones et al., 1986a; Pennell et al., 1990) and is easy to use. The predicted and measured aldicarb residue concentrations in the soil layers of the cotton field are shown in Fig. 4 as a function of time. The results of the P R Z M model simulation indicate that the initial mean concentration of aldicarb residues in the surface layer was 248 #g kg-1 and that this would be reduced to 60 #g kg i 30 days later. Only ~ 2% of the total aldicarb applied
FATE OF ALDICARB
IN THE VADOSE ZONE
BENEATH
A COTTON
FIELD
] 39
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bO :3.
10 ~ . . . . ,
120 ~ . . . . .
30 days
100 1
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bo
80
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:I
°~
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',
60 days
AL ~
predicted
5
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0.3 0.6 0.9
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was leached to below 0.3-m depth while > 90% of all aldicarb residues would be distributed in the surface layer. On the 60th day, the amount of aldicarb residues in the entire profile was ~ 5% of the total aldicarb applied; > 80% of this was confined to the upper two layers (0-0.3- and 0.3-0.6- m depths) and only ~ 1% of the total aldicarb applied was leached to below 0.6 m. After 120 days, almost all the aldicarb would have been dissipated from the soil layers. The maximum leaching depth predicted by model simulation was 0.6 m. This figure was in fairly good agreement with the result measured in the field. Measured aldicarb residues seemed to persist for a longer time than those predicted by the P R Z M model simulation. The phenomenon was especially significant in the surface layer during the initial periods after aldicarb application. This may be due to the fact that the field soil sampling core was collected immediately beneath the aldicarb application point, so that the aldicarb residue concentrations cannot represent the field-average concentrations. If converted to "field-average" concentrations, assuming that no lateral dispersion occurred [(measured values)× (area of total application points)/(area of the study field)], the predicted results would be close to the measured concentrations in the field. With increasing sample depth and time, the two results tend to be more consistent. This is due to aldicarb diffusing and moving downward in the form of a pyramid beneath the application point with lateral dispersion producing a more even aldicarb distribution in the soil profile. Hornsby and Rao (1990) arrived at the same conclusion after summarizing their field experiment data. From a macroscopic point of view, the distribution of aldicarb in the soil of the field can be seen as even since, though it was applied in individual holes, these were very closely spaced (between individual cotton seedlings). The comparison between the measured and predicted results indicates that the P R Z M model is a good tool for predicting pesticide transport in the soil if
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it is used properly, although it has some limitations, i.e. it only simulates a single solute, underestimates solute movement when an intense rainfall event occurs, and cannot accurately predict the pesticide behavior in soils with a more complicated structure where a preferential flow o f water exists. Jones et al. (1986a) arrived at conclusions similar to ours after they compared the P R Z M model predictions with some unsaturated-zone field data. China covers a vast area with great variations in natural conditions. In order to assess the potential for aldicarb contamination of groundwater, we can use the P R Z M model to define areas which are suitable for aldicarb use according to the specific climatic conditions and soil characteristics. This work will also provide a scientific basis for guiding the safe application o f aldicarb in China. CONCLUSIONS The field monitoring data indicate that almost all aldicarb had been degraded within 60 days of application and that aldicarb residues did not leach below a depth o f 0.6 m in the soil. The aldicarb residue concentrations in soil varied moderately, with coefficients o f variation between 49.9% and 100%. No aldicarb residues were found in groundwater samples, suggesting that the application of aldicarb to this cotton field is safe and should not threaten groundwater quality. The P R Z M model offers good potential for predicting the fate of aldicarb in soil and could be an effective tool in assessing pesticide residue movement in soils if it is properly used. ACKNOWLEDGEMENTS The authors would like to acknowledge Dr. Russell L. Jones (RhOnePoulenc Agriculture Company, Research Triangle Park, North Carolina, U.S.A.) for his helpful suggestions and his assistance in installing wells. We do thank Qian Jinsheng, Miao Changping and Miao Baogen who participated in the collection of soil and well-water samples; Mr. Xiao Xingji for preparing the figures and Mr. Tang Guocai for useful critical comments on the manuscript. REFERENCES Bonazountas, M. and Wagner, J., 1981. SESOIL: A seasonal soil compartment model. Off. Toxic Substances, U.S. Eviron. Prot. Agency, Washington, DC. Bromilow, R.H. and Leistra, M., 1980. Measured and simulated behavior of aldicarb and its oxidation products in fallow soils. Pestic. Sci., 11: 389-395. Bromilow, R.H., Baker, R.J., Freeman, M.A.H. and G6r6g, K., 1980. The degradation of aldicarb and oxamyl in soil. Pestic. Sci., 11: 371-378.
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