ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 8, 540-550 (1984)

Persistence and Movement of Phorate at High Concentrations in Soil

GULAB SINGH AND ZILE SINGH

Department of Entomology, Haryana Agricultural University, Hissar-125004, India

Received October 22, I983

The entrance of a large number of chemicals into the agricultural market has caused concern to both cultivators and scientists because of their effectiveness at the target site, possibility of underground water contamination, and other undesirable effects such as phytotoxicity and effects on nontarget species. The persistence and movement of phorate-a systemic granular insecticide-were studied at high concentrations (4 and 8 kg a.i./ha) in field soil in two seasons, i.e., winter and summer. Periodic sampling for total phorate residues (TPR) at various vertical depths showed that it leached more efficiently at the higher dose (i.e., 8 kg a.i./ha) in both seasons. The horizontal mobility was also higher at this dose. Irrespective of the treatments, TPR leaching was significantly greater in winter with the higher dose, whereas its movement in the horizontal plane was comparatively greater in summer with this dose. The TPR concentration peaks remained stationary at 7.5 cm depth at ah the redistribution times until the pesticide was nearly dissipated under both treatments and in both seasons. TPR persistence was higher at the higher dose. Its persistence was comparatively greater in winter than in summer. The present findings show that there seems to be no threat of underground water contamination by this pesticide even at a rate as high as 8 kg a.i./ha. o 1984 Academic press, IIIC.

INTRODUCTION

The number of pesticides continues to increase annually despite their known and suspected environmental consequences. The information related to their persistence and mobility in soils is of greater significance from the point of view of their effectiveness at the target site, possibility of environmental contamination, and other undesirable side effects, e.g., phytotoxicity and effect on nontarget species. A thorough understanding of the various processes that influence the persistence, retention, and leaching of pesticides in soils is required to develop technology for the selection and management of pesticides to suit different crop management practices. The fate of pesticides in soils when applied at concentrations similar to those associated with agricultural practices has been well documented in several reviews (Bailey and White, 1970). However, the direct extrapolation of these data to systems containing high pesticide concentrations, such as those occurring at or below disposal sites, may not always be feasible (Davidson et al., 1976; Rao and Davidson, 1979). The movement of pesticides in soils at higher concentrations has been studied by several workers (Lichtenstein and Schulz, 1970; Lichtenstein et al., 197 1; Ou et al., 1978; Rao and Davidson, 1979). Although the movement and persistence of phorate in soils have been extensively studied under different conditions (Patterson and Rawlins, 1968; Schulz et al., 1973; Lichtenstein et al., 1974; Walter-Echoles and Lichtenstein, 1978; Singh et al., 1983a-c), practically no information is available on these aspects at high concentrations.

0147-6513184 $3.00 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.

540

PHORATE MOVEMENT AT HIGH CONCENTRATIONS 541

The research presented in this manuscript was designed to determine the persistence and movement behavior of phorate (Thimet: O,O-diethyl S-(ethyl- thio)methyl phosphorodithioate) in field microplots at high concentrations. This insecticide has an aqueous solubility of 50 ppm and a half-life of 30 days. The recommended dose is 1.5 kg a.i./ha.

MATERIALS AND METHODS

The present study was carried out on the Research Farm of Haryana Agricultural University, Hissar, India, in field microplots (2.44 X 1.22 m) during winter, 1979- 1980, and summer, 1980-198 1. These plots had galvanized iron sheets embedded to 45 cm depth around them to control lateral water movement. The soil was sandy loam (Table 1).

The insecticide (Phorate-LOG) granules were applied to the experimental plots through band placement at rates of 4 and 8 kg a.i./ha (2.5 and 5 times the recommended rate). The treatments were replicated thrice, and repeated in two seasons (winter and summer). In each plot (treatment) there were four bands (2.5 cm wide and 2.5 cm deep) placed 60 cm from each other with the help of a hand hoe.

The total amounts of water received by each plot were 25.8 and 35.8 cm in winter and summer seasons, respectively. Irrigation was first employed after the placement of phorate bands. The rate of application of irrigation water and the intervals between applications were based on the requirements of the crops (rec- ommended by the agronomists). The plots were kept exposed to nature. Weather data during the experimental periods are presented in Fig. 1.

Total phorate residues (TPR; including ail metabolites) and water profiles in soil were determined at intervals of 1, 7, 15, 30, 60, 90, and 120 days after first irrigation. Soil samples were taken with a tube auger up to 60 cm (added all horizontal samples up to 30 cm) in vertical directions and up to 30 cm (added all vertical samples up to 60 cm) in horizontal directions at 5-cm intervals. The sampling holes were refilled with untreated soil, and the sampling spots were tagged with wooden pegs. A portion of the fresh soil sample was used to determine its water content gravimetrically and the rest was processed for phorate estimation. A summary of experimental treatments is presented in Tables 2 and 3.

TABLE 1

Soil composition (%)

silt Soil Sand (0.02-

depth (2-0.02 0.002 (<?A2 (cm) mm) mm) mm)

Textural CliiS”

BUlk satu- EC Organic density ration pH (mmhof matter (p/cm’) (96) (1:2) cm) (%)

O-30 65.0 15.4 10.05 Sandy loam 30-60 51.4 21.1 20.8 Sandy loam 60-90 51.8 25.6 22.1 Loam

a According to USDA triangle-Typic Comborthids.

1.50 36.3 7.8 0.345 0.63 1.55 37.8 8.0 0.446 0.43 1.62 41.0 7.8 0.326 0.32

542 SINGH AND SINGH

,I -

c cvoporation 4

I = = = P

45 49 1 5 9 (3 17 21 25 29

ND” 1979 STANDARD WEEKS JULY 1990

FIG. 1, Weather conditions during the study period.

The single-step extraction and cleanup technique for soil samples used by Singh et al. (1983b) was followed with minor modifications. A 20-g representative soil sample was mixed with 0.5 ml ammonia solution (25%) in a 150-ml beaker and kept for an hour to saturate. To every sample, 0.5 g each of anhydrous sodium sulfate, florisil (60-100 mesh sieve), and activated charcoal were added and mixed well until the mixture became homogeneous and friable. The mixture was packed in a glass chromatogmphic column (30 cm X 20 mm i.d.) which had already been packed with 5 cm of an adsorbent mixture (Hyflosupersell, charcoal, sodium sulfate, and magnesium oxide in the ratio 1: 1: 1: 1). A l-cm layer of sodium sulfate was placed below and above the mixture to adsorb moisture from the samples. Extraction and cleanup were simultaneously achieved by eluting the columns with 100 ml redistilled dichloromethane.

For analysis of total phorate residues, the method developed by Getz and Watts (1964) and modified by Jain et al. (1974) was used. Phorate and all its metabolites, on reacting with 4-(pnitrobenzyl)-pyridine in a slightly basic solution at 175- 18O”C, gave rise to a red-purple colored complex which was measured at 540 nm. Recovery of phorate from samples fortified at 1 and 2 &g was 96%.

RESULTS AND DISCUSSION

The mobility of phorate in soil was determined by measuring the TPR concen- trations at various vertical and horizontal distances from the insecticide band (Figs. 2-5). Summaries of the results for leaching efficiency and lateral mobility of TPR are presented in Tables 2 and 3, respectively.

PHORATE MOVEMENT AT HIGH CONCENTRATIONS

TABLE 2

SUMMARY OF EXPERIMENTAL TREATMENTS AND TPR’ LEACHING RESULTS

543

Dose of Redistribution Concentration phorate time TPR peak TPR front at peak

Expt (kg a.i./ha) (&YS) depth (cm) (cm) baJg) % TPR

WI.1 w1.2 w1.3 WI.4 w1.5 W1.6 w1.7

w2.1 w2.2 W2.3 W2.4 w2.5 W2.6 W2.7

Sml.1 Sm1.2 Sm1.3 Sm1.4 Sml.5 Sm1.6

Sm2.1 Sm2.2 Sm2.3 Sm2.4 Sm2.5 Sm2.6

4 1 7.5 27.5 4 7 7.5 27.5 4 15 7.5 32.5 4 30 7.5 27.5 4 60 7.5 22.5 4 90 7.5 12.5 4 120 2.5 7.5

8 1 7.5 27.5 8 7 7.5 27.5 8 15 7.5 32.5 8 30 7.5 27.5 8 60 7.5 22.5 8 90 7.5 17.5 8 120 2.5 12.5

4 1 7.5 22.5 4 7 7.5 27.5 4 15 7.5 22.5 4 30 7.5 22.5 4 60 7.5 12.5 4 90 2.5 7.5

8 1 7.5 22.5 8 7 7.5 27.5 8 15 7.5 27.5 8 30 7.5 27.5 8 60 7.5 22.5 8 90 2.5 12.5

Winter experiments

Summer experiments

3.75 100 3.50 99 3.20 97 2.65 86 1.25 35 0.80 16 0.15 2

7.00 102 6.80 99 6.00 98 6.50 92 3.00 46 1.90 29 1 .oo 9

3.50 101 3.20 99 3.15 91 2.95 88 1.30 22 0.50 5

7.11 103 6.30 99 5.70 98 5.50 78 4.00 35 2.00 17

0 Total phorate residues (parent compound and all its metabolites).

Vertical Movement

The TPR values plotted at different depths in Fig. 2 show that just after infiltration of water through the first irrigation, TPR downward movement (leaching) was more efficient (more quantity at deeper depths) at the higher dose, i.e., 8 kg a.i./ha (Experiment W2) than at the lower one, i.e., 4 kg a.i./ha (Experiment Wl). In the winter, TPR concentrations at almost all the sampling depths were significantly higher in the high-concentration case. However, the concentration peaks of TPR were located at the same depth (i.e., 7.5 cm) in both treatments (Table 2 and Fig. 2). The leaching behavior of TPR continued to be the same during the subsequent redistribution, evaporation, and irrigation cycles (Fig. 2 and 3) until the pesticide was nearly dissipated from the soil. The TPR fronts under the two treatments continued to shift to deeper layers in the initial stages of the experiment (Experiments

544 SINGH AND SINGH

TABLE 3

SUMMARYOFREXJLTSFORTPRHORIZONTALMOVEMENT

Expt

Redistribution TPR concentration &g/g) at time TPR front

Ww) (cm) Band 5 cm 10 cm 15 cm 20 cm

WI.1 1 WI.2 7 WI.3 15 WI.4 30 WI.5 60 W1.6 90 w1.7 120

w2.1 1 w2.2 7 W2.3 15 W2.4 30 w2.5 60 W2.6 90 W2.7 120

Sml.1 1 Sm1.2 7 Sm1.3 15 Sm1.4 30 Sm1.5 60 Sm1.6 90

Sm2.1 1 Sm2.2 7 Sm2.3 15 Sm2.4 30 Sm2.5 60 Sm2.6 90

Winter experiments

10 3.75 2.75 15 3.25 2.40 20 3.00 1.40 15 2.80 1.60 15 1.38 0.60 10 0.50 0.35 5 0.15 0.10

10 7.80 6.50 20 5.75 4.00 20 5.00 4.60 15 4.65 3.80 15 3.50 2.25 15 2.25 0.25 10 1.00 0.15

Summer experiments

15 3.00 2.25 15 2.75 2.25 15 2.70 1.50 15 2.25 1.50 10 0.80 0.65 10 0.20 0.15

15 7.20 5.90 20 7.20 5.00 20 6.05 4.50 20 5.75 4.10 20 2.50 2.30 10 0.75 0.70

0.10 1.20 1.00 0.60 0.44 0.10

0.10 3.50 3.00 3.00 1.00 0.20 0.10

1.10 1.50 1.00 1.10 0.10 0.10

1.50 1.00 2.00 2.00 1.00 0.10

0.10 0.15 0.10 0.10 0.10

I .oo 0.10 1.50 0.10 0.80 0.20 0.10

0.10 0.10 0.15 0.10

0.10 0.75 0.15 0.80 0.20 1.00 0.15 0.65 0.10

W 1.1 to W1.3 and W2.1 to W2.3) but in the advanced stages they were found nearer to the soil surface with the lapse of time (Experiments W 1.4 to W 1.7 and W2.4 to W2.7) in the winter.

The same trend of leaching behavior was observed in the summer (Figs. 4 and 5). Unlike in winter, the TPR in summer did not penetrate beyond 30 cm (Table 2). The concentration peaks here too remained stationary at 7.5 cm depth. Figure 6 shows that TPR leached significantly deeper in the winter than in the summer under both treatments. In the summer the TPR fronts under both treatments continued to shift to the deeper layers in the initial stages of the experiment (Experiments Sml. 1 to Sml.2 and Sm2.1 to 2.4) but were found near the soil top layers in the advanced stages) (Experiments Sm1.3 to Sml.6 and Sm2.5 to Sm2.6).

These findings support earlier reports in which the leaching of pesticide was shown to be a function of the concentration of pesticide in soil (Lichtenstein and

PHORATE MOVEMENT AT HIGH CONCENTRATIONS

PHORATE CONTENT ( yq,q > WATER CONTENT t%7 _ . O-

I: EXPT. zoo- ;: -- v/ 1.1

i

---.--- w ( 3 - W2, ---A--- w 2.3

33

‘0 t

* 5 (0 15

,

2c

545

9.0

7.0

6.0 ^ ,” I 4

5.0 + 6

5 0

40 u

z

8 3.0 2

20

I.0

0

, HORIZONTAL DISTANCE tcm)

FIG. 2. Vertical and horizontal distribution profiles of TPR and water at application doses of 4 (W 1) and 8 (W2) kg a.i./ha after the first irrigation (W 1.1 and W2.1) and after allowing a redistribution period of 15 days (W 1.3 and W2.3) in winter.

Schulz, 1959; Lichtenstein et al., 1967; Bailey and White, 1970; Davidson et al., 1976; Walter-Echoles and Lichtenstein, 1978; Rao and Davidson, 1979). The higher mobility of phorate at the higher dose may be explained on the basis of variations in the adsorption behavior of soils with solution concentration. Adsorption always retards pesticide movement in soil. Previous reports (Rao and Davidson, 1979) also favored this phenomenon, that the retardation of pesticide mobility in soil due to adsorption varies with solute concentration, i.e., it decreases as the concentration in solution increased. Therefore, this factor might have increased the mobility of TPR at the higher dose. Rao and Davidson (1979) also observed similar trends in leaching behavior of 2,4-D amine, atrazine, terbacil, and methyl parathion in three soils. They found that in all cases pesticide mobility was significantly greater at high concentrations.

The TPR concentration peaks remained stationary at a particular depth, i.e., 7.5 cm. This situation might have occurred due to early and easy adsorption of phorate on soil particles. The phorate bands were placed 2.5 cm deep in the soil. Therefore, most of the phorate moved only 5 cm (7.5 cm from soil surface). The remaining amount, which must have dissolved in the soil solution with the subsequent irrigation cycles, might have been displaced deeper through the macropores (high velocity pathways) in the soil and formed the tailing (Sir& et al., 1983a). Based on bioassay results, Patterson and Rawlins (1968) reported that after the application of insecticide granules to soil, very little phorate was translocated through more than

SINGH AND SINGH 546

PHORATE CONTENT <Pqlq J

EXPT. -.- w11 ---.--- w ,.6

._. l . w 1.7 A- w 2.1 ---A--- w 2.6

. ~2.7

WATER CCNT ENT t% )

10 15 20 25 80

\ - 7.0

- 6.0

,"

- 4.0 T

5 ?

-4.0 g "

?

- 3.0 2 P a

- 2.0

- 3.0

-0 I

0 5 10 (5 20 HORI ZONTAL DISTANCE ‘cm 1

FIG. 3. Vertical and horizontal distribution profiles of TPR and water at application doses of 4 (Wl) and 8 (W2) kg a.i./ha after the first irrigation (W 1. I and W2.1) and after allowing redistribution periods of 90 days (W1.6 and W2.6) and 120 days (W1.7 and W1.7) in winter.

3 in. of soil. Schulz et al. (1973) also reported that phorate residues did not move beyond 6 in. vertically after band application of granular insecticide at the rates of 5 and 10 lb/acre under field conditions. TPR concentration peaks have also been reported at 7.5 cm depth under field conditions at different flow velocities and initial soil water content levels (Singh et al., 1983a), and under cropped and uncropped conditions (Singh, 1981). However, the TPR peaks were reported at different depths in laboratory columns at different redistribution times (Singh et al., 1983~) the reason being the change in chemical and physical properties of field soils, i.e., pore geometry and soil pore size sequence, under laboratory conditions (Babalola, 1978 and Cameron, 1978).

The lower mobility of the TPR front in summer under both treatments could well be explained by the variations in the environmental conditions (Fig. 1) in the two seasons. The higher atmospheric and soil temperatures, high rate of evaporation and sunshine, and lower atmospheric humidity in summer might have contributed to lesser mobility of TPR in soil (Spencer, et al., 1973 and Chopra, 1976). The movement of more phorate-containing water through capillaries to the surface layers might have restricted the leaching of TPR in the summer. Due to the high magnitudes of these climatic factors, the solubility of TPR in soil solution might have also increased, thereby causing more rapid losses with evaporating water (Biggar and Riggs, 1974) and decreasing the downward mobility of the pesticide.

PHORATE MOVEMENT AT HIGH CONCENTRATIONS 547

PHORATE CONTENT (uq/q, WATER CONTENT<*X)

i EXPT. -.- sm 1.1 ---.--- *In (1 -.- sm 2.1 ---A--- Srn 2.4

. - 70

\ - 6.0 L '1 A 5

ST - 5.0 "

5 c"

-4.0 6 "

?

-20 ;

:

- 20

- 1.0

'\ '\

10

HORIZONTAL DISTANCE (cm)

FIG. 4. Vertical and horizontal distribution profiles of TPR and water at application doses of 4 (Sml) and 8 (Sm2) kg a.i./ha after the first irrigation (Sml.1 and Sm2.1) and after allowing a redistribution period of 30 days (Sm I .4 and Sm2.4) in summer.

Irrespective of the seasons, under both treatments, the maximum TPR concen- tration remained confined to the surface layers (O-10 cm). It, therefore, indicated that there is no threat of the underground water being contaminated with these doses of pesticide and irrigation water. This point is further supported by (a) the displacement of TPR fronts toward the surface layers with the advancement of the experiment because of its rapid degradation and (b) the stationary position of maximum TPR concentration peaks.

Horizontal Movement

Values of TPR concentrations at different points in the horizontal plane plotted in Fig. 2 showed that just after the infiltration of water through first irrigation, TPR lateral movement was greater at the higher dose, i.e., 8 kg a.i./ha (Experiment W2), than at the lower one, i.e., 4 kg a.i./ha (Experiment WI), in the winter. Its concentrations at almost all sampling points were comparatively higher in the former case. This lateral mobility behavior of TPR in soil remained the same during the subsequent redistribution, evaporation, and irrigation cycles (Fig. 2 and 3) until the pesticide was nearly dissipated from the soil. The TPR fronts under both treatments continued to shift to the farther points in the initial stages of the experiment (Experiments W 1 to W 1.3 and ~2.1 to W2.3) but in the advanced stages they were found displaced nearer to the band placement site (Experiments W1.4 to W1.7 and W2.4 to W2.7).

SINGH AND SINGH

PWORATE CONTENT OJ~/¶ ) 0 2 4 6 8 I I 1

EXPT -.- sm 1.1 --a--- Snl 1.5

..__ l . . sm (,6

-A- sm 2.1 --A--- Sin 2.5

* Sm 2.6

WATER CONTENT (7.1 (0 15 20 25

.

\

A

. . . . \

\ --__

-A 0’.

\

A \ ‘\

‘\ . . .

..* ‘A, L---- ‘...

-a.;.. . .._ --__

-A

6 , ,\

’ +., -. ‘. \ . . ‘“-.. ._...__.... :* -.

1

HORIZONTAL DISTANCE b-n)

FIG. 5. Vertical and horizontal distribution profiles of TPR and water at application doses of 4 (Sml) and 8 (Sm2) kg a.i./ha after the first irrigation (Sml.1 and Sm2.1) and after allowing redistribution periods of 60 days (Sm1.5 and Sm2.5) and 90 days (Sm1.6 and Sm2.6) in summer.

Repetition showed that the lateral mobility of TPR was similar both just after the infiltration of water through first irrigation (Fig. 4 and Experiments Sm 1.1. and Sm2.1) and after allowing redistribution time up to 90 days (Fig. 4 and 5; (Experiments Sm1.2 to Sml.6 and Sm2.2 to Sm2.6). The TPR fronts under both the treatments here too went on shifting to the farther points in the initial stages (Experiment Sml. 1 to Sm1.4 and Sm2.1 to 2.5) but in the later stages they were displaced closer to the phorate band site (Experiment Sm 1.5 to Sm 1.6 and Sm2.6).

In both seasons, irrespective of the treatments, most of the TPR remained confined in the vicinity of the site of pesticide placement (O-10 cm). Irrespective of the treatments, the movement of TPR was comparatively greater in summer than in winter.

The reasons for the higher mobility of TPR at the higher concentration, i.e., 8 kg a.i./ha, have already been explained above. Confinement of most of the TPR to the band site showed that it had very little mobility in the horizontal plane. The tailing here too could have been due to the transverse diffusion. At the higher dose, it was expected that there would be more lateral diffusion and subsequently more movement of the pesticide. Patterson and Rawlins (1968) reported little horizontal movement of phorate in field soils. The higher horizontal mobility of TPR in summer, irrespective of the doses, might have been due to shallower penetration of the pesticide in soil, higher temperatures and evaporation rates, etc. (Spencer et al., 1973) in the summer (Fig. 6). The lower lateral movement of phorate, even at

PHORATE MOVEMENT AT HIGH CONCENTRATIONS 549

PHORATE CONTENT C /,q,q’

OO

30 -

35 -

Lo-

EXPT -.- w2.1 ---*--. wz* -A- sm 2.1 ---‘--- sm2.4

WATER CONTENT <U 90 15 20 25

, 5 10 15 20 HORIZONTAL DISTANCE tcml

FIG. 6. Vertical and horizontal distribution profiles of TPR and water in winter (W2) and summer (Sm2) after the first irrigation (W2.1 and Sm2.1) and after allowing a redistribution period of 30 days (W2.4 and Sm2.4) at an application dose of 8 kg a.i./ha.

higher doses, suggests that placing the pesticide near the crop plants is necessary to obtain the best insect control.

The soil water content profiles (Figs. 2 to 6) were significantly variable for the two treatments in both seasons at various redistribution times, and hence, might have not affected the pesticide movement behavior in soil.

Persistence

The data presented in Table 2 show that, irrespective of the seasons, TPR persisted longer with higher concentrations in the treatment where the dose was 8 kg a.i./ha (Experiments W2 and Sm2). This may be attributed to slower dissipation of phorate at higher rates (Lichtenstein and Schulz, 1959, 1970; Walter-Echoles and Lichtenstein, 1978). Irrespective of the treatments, TPR persisted to a greater extent in the winter. This may be due to the higher soil and atmospheric temperatures and evaporation rates from the soil matrix as well as the higher levels of microbial activity in the summer.

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550 SINGH AND SINGH

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BIGGER, J. W., AND RIGGS, R. L. (1974). Apparent solubility of otganochlotine insecticides in water at various temperatures. HiIgardia 42, 383-391.

CAMERON, D. R. (1978). Variability of soil water retention curves and predicted hydraulic conductivities on a small plot. Soil Ski. 126, 364-37 1.

CHOPRA, S. L. (1976). Adsorption of pesticides on minerals and soils. Indian J. Agric. Chem. 9, 125- 146.

DAVIDSON, J. M., Ou, L. T., AND RAO, P. S. C. (1976). Behaviour of high pesticide concentration in soil-water system. In Residual Management by Land Disposal. Proceedings of the Hazardous Wastes Research Symposium (W. H. Fuller, Ed.), pp. 206-212.

GETZ, M. E., AND WA-I-B, R. R. (1964). Application of 4(pnitrobenzyl) pyridine as a rapid quantitative reagent for organophosphate pesticides. J. Assoc. OR Anal. Chem. 47, 1094-1096.

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LICHTENSTEIN, E. P., AND SCHULZ, K. R. (1959). Persistence of soil chlorinated hydrocarbon insecticides as influenced by soil types, rate of application and temperature. J. Econ. Entomol. 52, 126-131.

LICHTENSTEIN, E. P., AND SCHULZ, K. R. (1970). Volatilization of insecticides from various substrates. J. Agric. Food Chem. l&814-8 18.

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LICHTENSTEIN, E. P., FUHREMANN, T. W., AND SCHULZ, K. R. (1974). Translocation and metabolism of (“C)phorate as affected by percolating water in a model soil-plant ecosystem. J. Agric. Food Chem. 22,99 l-996.

LICHTENSTEIN, E. P., FUHREMANN, T. W., SCHULZ, K. R., AND SKRENTNY, R. F. (1967). Effect of detergents and inorganic salts in water on the persistence and movement of insecticides in soils. J. Econ. Entomol. 60, 17 14-I 72 1.

Ou, L. T., ROTHWELL, D. F., WHEELER, W. B., AND DAVIDSON, J. W. (1978). The effect of high 2,4-D concentrations on degradation and COz evolution in soils. J. Environ. Qua/. 7, 241-246.

PATTERSON, R. S., AND RAWLINS, W. A. (1968). Loss of phorate from a granular formulation applied in soil. Fla. Entomol. 51, 141-150.

RAO, P. S. C., AND DAVIDSON, J. W. (1979). Adsorption and movement of selected pesticides at higher concentrations in soils. Water Res. 13, 375-380.

SCHIJU, K. R., LICHTENSTEIN, E. P., FUHREMANN, J. W., AND LIANG, T. T. (1973). Movement and metabolism of phorate under field conditions after granular band application. J. Econ. Entomol. 66, 873-875.

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SINGH, G., DAHIYA, I. S., AND SINGH, Z. (1983a). Water management practices affecting leaching of phorate in soil. Znt. J. Trop. Agric. 1, 8 l-90.

SINGH, G., DAHIYA, I. S., AND SINGH, Z. (1983b). Movement of phorate under constant and variable surface water flux in initially dry and moist soils. Water, Air, Soil Pollut. 21, 439-446.

SINGH, G., SINGH, Z., DAHIYA, I. S., AND MALIK, R. S. (1983~). Movement of surface-applied phorate and water through unsaturated soil. Agro-Ecosystems 8, 259-263.

SPENCER, W. F., FARMER, W. J., AND CLIATH, M. M. (1973). Pesticide volatilization. Residue Rev. 49, l-49.

WALTER-ECHOLES, G., AND LICHTENSTEIN, E. P. (1978). Movement and metabolism of (‘4C)phorate in a floded soil system. J. Agric. Food Chem. 26, 599-604.