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Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesb20
Loss of carbofuran from rice paddy water: Chemical and physical factors J. N. Seiber Barril
a b
c
, M. P. Catahan & C. R.
c
a
Pesticide Laboratory , The International Rice Research Institute , Los Baños, Laguna, Philippines b
Department of Environmental Toxicology , University of California , Davis, California, 95616 c
Department of Chemistry , The University of the Philippines , Los Baños, Laguna, Philippines Published online: 21 Nov 2008.
To cite this article: J. N. Seiber , M. P. Catahan & C. R. Barril (1978) Loss of carbofuran from rice paddy water: Chemical and physical factors, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 13:2, 131-148, DOI: 10.1080/03601237809372083
To link to this article: http://dx.doi.org/10.1080/03601237809372083
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PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/ terms-and-conditions
J. ENVIRON. SCI. HEALTH, B13(2), 131-148 (1978)
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LOSS OF CARBOFURAN FROM RICE PADDY WATER: CHEMICAL AND PHYSICAL FACTORS
KEY WORDS: Carbofuran, rice paddy water, pH-dependent hydrolysis J. N. Seiber1 Pesticide Laboratory The International Rice Research Institute Los Baños, Laguna, Philippines
M. P. Catahan and C. R. Barril Department of Chemistry The University of the Philippines Los Baños, Laguna, Philippines
ABSTRACT The loss of carbofuran was studied from rice paddy water treated with a granular formulation of the insecticide, and from ponds filled with drainage from the paddy.
The average
half-life (t 1/2 ) for carbofuran loss was 57 hr.
Controlled
experiments indicated that pH was the predominating factor governing carbofuran loss from water in the environment studied. The loss due to hydrolysis was over 700 times more rapid at 131 Copyright © 1978 by Marcel Dekker, Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
132
SEIBER, CATAHAN, AND BARRIL
pH 10 (t 1 / 2 = 1.2 hr.) than at pH 7 (t1/2 = 864 hr.) in buffered deionized water. The average pH of the rice paddy was 8, but diurnal fluctuations of 7 to 9.5 are common in
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similar environments.
Impurities in the water, sunlight, and
temperature influence the rate of carbofuran loss but not nearly so much as pH.
There was no evidence for significant loss due
to evaporation or oxidation.
The results have important
implications for the duration of the insecticide's activity and the effect on fish within or downstream from treated paddies.
INTRODUCTION Hand broadcast of systemic insecticides to the paddy water has become quite popular in rice culture in Asian countries. The method gives uniform coverage and fair residual control without 2 the need for complicated application equipment . When applied as granules systemic insecticides are released to the water and penetrate to the root zone for subsequent uptake by the plant and translocation to the foliage. Fumigation, systemic uptake through the stems, and capillary movement en plant surfaces are alternate mechanisms for movement of broadcast insecticides from 3 water to target foliage . Duration of insecticide action and environmental side-effects depend partly on the lifetime of the chemical in the water; too rapid dissipation would lessen the
LOSS OF CARBOFURAN FROM RICE PADDY WATER
133
chance for systemic uptake, but too slow dissipation would increase the chance for run-off and damage to non-target biota in the paddy vicinity.
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The persistence of pesticides in water are influenced by physical, chemical, and biochemical processes, the relative importance of which depend on the chemical and environment in question.
For example, pesticides of high vapor pressure and
low water solubility, such as llndane, volatilize readily particularly when environmental conditions favor a high rate of water evaporation
5,6
. On the other hand N-methylcarbamates and
organophosphates are much more prone to undergo degradation in water than organochlorine insecticides ; hydrolysis, a principal o
degradation route, may be influenced by pH and temperature , 9 10 sunlight', and catalytic impurities in the water
. For some
insecticides, notably the organophosphates, degrading microorganisms can accelerate hydrolysis which otherwise would be 11 12 quite slow in sterile systems Carbofuran is one of several N-methylcarbamates currently marketed in the Philippines for control of insects in rice It may be applied as a foliage spray or by hand broadcast of granules, the latter being preferred for control of leafhoppers, stem borers, and whorl maggot. As many as 4-5 applications of 1-2 kg/ha each may be required with susceptible rice varieties when heavy insect infestations are encountered.
This work was
134
SEIBER, CATAHAN, AND BARRIL
done to determine the rate of carbofuran loss from paddy water, and the factors which most influence it. An understanding of such factors could suggest application methods which increase
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the efficiency of this relatively expensive insecticide, and minimize the chance of harmful side-effects to fish within or downstream from the paddies. MATERIALS AND METHODS Field Samples A 1 ha rice paddy located at the International Rice Research Institute, Los Baños, Laguna, Philippines, was treated with Furadan 2G at 2 kg/ha (a.i.) on February 24, 1977, approximately 60 days after transplanting.
Water was sampled at intervals
after treatment by combining 50 ml dip samples taken at random from 1/3 sections of the field. Two composite samples for each 1/3 ha section were analyzed, and the results for duplicates of all 3 sections were averaged to arrive at the concentration in the entire paddy. At 1 day after treatment water from the paddy was pumped into a drained 7 x 4 m pond (A) located adjacent to the paddy until a depth of 1 m was attained.
Duplicate composite water
samples were taken at intervals after treatment. At 5 days after treatment a second such pond (B) was filled with paddy water and sampled periodically in a similar way.
LOSS OF CARBOFURAN FROM RICE PADDY WATER
135
All field samples were accompanied by pH and temperature measurements.
They were stored at 5^0 until analyzed.
Laboratory Samples
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One. liter of water whose pH was adjusted with phosphate buffer was placed in a 2.5 1 bottle, sterilized in an autoclave, cooled, and spiked asceptically with 10 mg of carbofuran from a 10 mg/ml stock solution in ethanol.
The solution was mixed,
purged with either N. or air for 1 hr, and closed.
Samples were
withdrawn at intervals by a technique which avoided biological contamination.
The solution was shaken and purged with No or
air periodically during the experiment. All such experiments were done in duplicate using either deionized water, water from the rice paddy before insecticide treatment or water from an adjacent irrigation drainage ditch. In ¡studying the effect of pH and air, sample solutions were kept in the dark at 27 + 2°C.
In studying the effect of sunlight,
samples in clear soft glass bottles were kept in a phytotron glass house maintained at 29°C for 16 hr during the day, and 21°C for 8 hr at night. Samples in brown bottles wrapped with carbon paper were similarly placed to serve as dark controls. Analysis For field collected water, 100 ml portions were extracted with 3 x 50 ml of 3:1 méthylène chloride-ether.
Combined
136
SEIBER, CATAHAN, AND BARRIL
extracts were dried with sodium sulfate and concentrated to 2 ml on a rotary evaporator.
Evaporation was completed under a
stream of N£ whereupon the sample was dissolved in 2 ml of
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acetone for GLC analysis. For laboratory samples a similar procedure was followed except that the water volume (20 ml) and volume of extracting solvent (25 ml) were reduced. Samples were analyzed by gas-liquid chromatography using a Hewlett-Packard Model 5711 A equipped with NP detector and a 1 m x 2 mm (id) glass column packed with 3 % Apeizon L on 80/100 mesh Chromosorb W HP.
Gas flows were: N- carrier, 20 ml/min;
H_, 3 ml/min; air, 70 ml/min.
Temperatures were: injector, 200°C;
column, 165°C; detector, 300°C. A linear response for standard carbofuran was obtained from 1 to 80 ng.
Recovery of carbofuran
from fortified water was quantitative, with no interferences encountered in any samples at amounts in excess of 0.01 ppm carbofuran equivalent. RESULTS AND DISCUSSION Loss of Carbofuran from Paddy and Pond Hater in the Field A maximum concentration of 2.3 ppm may be calculated for an application of 2 kg/ha of carbofuran at the water depth prevailing initially in the treated paddy.
The maximum observed
concentration, 2.0 ppm, occurred 1 day after treatment, the lag being attributed to release from the granular formulation employed.
Specifications for this formulation are for greater
LOSS OF CARBOFURAN FROM RICE PADDY WATER
137
than 90% release within 24 hr in periodically agitated water At 1 and 5 days after treatment paddy water was pumped from the field into stagnant ponds designed to simulate fish ponds
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which are often near or downstream from paddies. Raw data from analysis of paddy and pond water (Table 1) was used to construct logarithmic decline curves. From the best fit straight lines for data points from 1-8 days in the paddy water, 0-4 days in pond A, and 0-4 days in pond B the observed rate constants K_K and half lives (t
\>b
) were calculated from the
1/2
formulas:
where Co and C are the initial concentration and concentration at time t, respectively. The calculated half lives (Table 1) did not differ greatly for water from the three locations.
In all three temperature,
pH, and sunlight intensity varied within and between days of the sampling period so that such small differences are probably not meaningful.
For example, diurnal variations in pH for
paddies irrigated with slightly alkaline water, such as that used in these experiments, are from £a 7-8 at night to ca 9 during the day.
The maximum pH occurs at about 2:00 pm when
algal photosynthesis shifts the CO2 equilibrium away from
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TABLE 1 Loss o£ Carbofuran from Paddy and Pond Water in the Field Paddy Elapsed time (days)
Concn (ppm)
Pond A Elapsed time (days)
Concn (ppm)
Pond B Elapsed time (days)
Concn (ppm)
0.12
0.94
0
2.6
0
0.63
0.25
1.4
0.15
2.0
0.50
0.63
1
2.0
0.80
1.8
1
0.45
2
1.6
1.2
1.5
2
0.38
3
1.3
4
0.59
3
0.34
5
0.76
5
0.22
4
0.14
6
0.68
6
0.18
8
0.01
8 13
0.34 0.02
7 8
0.06 0.02
w w H
K v (hr"1) t 1 / 0 (hr)
1.0 x io" 67
1.4 x io" 48
M
n 1.2 x lo" 2 55
1
% tel
Avg pH's at 8:00 am and 5:00 pm were 7.8 and 8.5, respectively; Avg temperatures at 8:00 am and 5:00 pm were 26 and 30°C, respectively. b
i
Calculated for days 1-8 (paddy), days 0-4 (pond A ) , and days 0-4 (pond B ) .
^ g
LOSS OF CARBOFURAN FROM RICE PADDY WATER
139
"HCO3 to ~CO_; it is influenced by the degree of algal infestation and may reach 9.5 when heavy algal growth is encountered
'
. Average pH in paddy and pond water studied here
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was estimated from our measurements at 8:00 am and 5:00 pm, and diurnal fluctuation data
'
as 8 during the study period from
which rate data was obtained. In all three environments an increase in rate of carbofuran loss occurred toward the end of the study period.
This may have
been due to a slight increase in temperature and pH which took place then, or to concentration dependence of the rate. Adsorption to suspended or sedimented solids, for example, may take; on importance at low solute concentrations.
It has
been reported, however, that clays are weak adsorbents for 8 N-methylcarbamates and in our ponds we found negligable residues of carbofuran in the sediment during the study. Decomposition by microorganisms, another possible cause for a change in rate with time after treatment, was found to be insignificant in the loss of carbofuran from treated water in 17 a similar environment Loss of Carbofuran from Water under Controlled Conditions The effect of pH was studied in sterilized and buffered water at 27°C.
Solutions were purged with N„ and kept in closed
containers in the dark.
Loss of carbofuran was clearly pH
dependent, the rate at pH 10 being over 700 times that at pH 7
140
SEIBER, CATAHAN, AND BARRIL
in deionized water (Table 2 ) . The rates were comparable to those reported for carbaryl (t
= 252 and 0.25 hr at pH 7 and
o
10, respectively) , allowing for the lower temperature (20°C) in
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the carbaryl study.
Decomposition of carbofuran in soil has also
been reported to be pH dependent up to pH 7.8, the most alkaline 18
condition studied
Loss of carbofuran from water in the pH range 7-10 is consistent with a mechanism of simple hydrolysis mediated by hydroxyl ions :
[OH] H20 OC NHCH3 n 0 Carbofuran phenol was found during the early stages of our laboratory studies in approximately the amount expected from this equation. Decomposition of carbofuran was more rapid in paddy water than deionized water at pH 7 and 8.7 (Table 2 ) . Suspended 10 19 particles
and metallic ions
are known to accelerate
decomposition of some pesticides. An increase in ionic strength, however, retards hydrolysis of N-methylcarbamates, apparently by 20 decreasing the activity of either hydroxide ion or the carbatnate The interaction of these factors as mediators of hydrolysis was not studied directly here.
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TABLE 2 Effect of pll on Loss of Carbofuran from Deionized and Paddy Water under Controlled Conditions
pH 7.0 Concn (ppm) Elapsed Deionized Paddy time (days) water water '
pH 8.7 Concn (ppm) Elapsed Deionized Paddy time (days) water water
pH 10 .0 Concn (ppm) Elapsed Deionizec1 Paddy time (hr) water water
0
10.4
10.6
0
9.6
9.6
0
6.5
7.9
1
10.4
8.8
0.25
8.3
7.3
2
1.8
2.3
3
9.1
8.4
1
4.6
3.4
4
0.52
. 0.70
6
9.3
6.8
2
2.0
1.1
6
0.20
0.26
9
9.0
6.0
4
0.27
0.07
8
0.08
0.12
K„, (hr"1) OD t 1 / 2 (hr)
0.08 x 10" 2 0.29 x 10" 2
864
240
3.6 x 10" 2 19.4
5.0 x 10" 2 13.9
All solutions were sterilized, buffered, and kept in the dark under N_. 27" + 2°C.
-2 58 x 10
1.2
Avg temperature was
53 x 10-2
1.3
142
SEIBER, CATAHAN, AND BARRIL Carbofuran loss was more rapid in solutions exposed to
sunlight than in those kept in the dark (Table 3 ) . While the maximum absorption of UV light for carbofuran in water, 275 nm,
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is below the minimum sunlight-derived UV wavelengths encountered at the earth's surface, 292 nm, some end-adsorption at 290-300 nm occurs in the spectrum of carbofuran such that photoactivation may enhance hydrolysis.
The increase in water from the paddy
(130%) and an adjacent irrigation drainage ditch (1507«) on exposure to sunlight was greater than in deiortized water (110%). Photosensitization by other UV absorbing compounds
21
may explain
this observation. We found no evidence that oxidation plays a role in the loss of carbofuran from water.
Measured rates at pH 8.7 were identical
when solutions were maintained under N. or air. Furthermore, analysis of field and laboratory solutions did not reveal the presence of 3-hydroxy- or 3-ketocarbofuran, known oxidation 22 products of carbofuran . The presence of these products in water fron a model ecosystem
23
may have been due to their
formation by metabolism of carbofuran rather than by direct chemical oxidation. Evaporation is a major factor in the decline of some chemicals in water. The approximate half-life in water due to evaporation only can be calculated with the equation and estimate of water evaporation given by
Wolkoff and Mackay .
LOSS OF CARBOFURAN FROM RICE PADDY WATER
143
TABLE 3 Effect of Light on the Observed Rate Constants (K ) and Half-Lives ob (t . ) for Carbofuran Loss under Controlled Conditions
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K
ob O«' 1 )
t, ,„ (hr) 1/2
Deionized water
light
0.105 x 10" 2
660
Deionized water
dark
0.092
753
Paddy water
light
0.40
173
Paddy water
dark
0.31
224
Drainage water
light
0.37
187
Drainage water
dark
0.24
288
a
All solutions were sterilized, buffered to pH 7.0, and kept under N o . Temperature was maintained at 29° C for 16 hr (day and 21°t for 8 hr (night).
Using reported values for the water solubility (700 ppm at 25 e C) -5
and vapor pressure (2 x 10 t. .„ due to evaporation
24 mm Hg at 33'C) of carbofuran
,
from water of 10 cm depth was
calculated to be well in excess of 1 year.
Thus it seems
unlikely that evaporative losses played a significant role in carbofuran decline in our field experiments, although we did not attempt to evaluate this factor by direct controlled study. Temperature also was not studied directly as a mediator of loss rate. The effect of this variable, to increase rate 2-3
144
SEIBER, CATAHAN, AND BARRIL
times for each 10°C rise, has been amply documented elsewhere for hydrolysis of N-methylcarbamates
and organophosphates
'
.
Summary of Factors involved in the Loss of Carbofuran from Water
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A plot of log t
vs pH from Table 2 data for paddy water
gave a straight line from which t,,« pH (8) was calculated to be 40 hr. average t..
at
'he average field water
This agrees well with the
observed for paddy and pond water in the field,
57 hr, particularly so considering the variation in pH, temperature, sunlight, and other parameters that occurred in the field. We conclude that pH is the predominating factor in the decline of carbofuran from water in the pH range of this study. Among other factors which may influence the rate of loss of carbofuran are the following: Impurities —
The rate of loss was more rapid in paddy
water than deionized water when the two were maintained under the same controlled conditions. We did not differentiate between dissolved or suspended impurities as the cause nor attempt to identify the chemicals responsible for this apparent catalytic or adsorptive effect. The effect was noted only at the lower pH's (7 and 8.7) and apparently became more pronounced at carbofuran concentrations below ca 0.2 ppm. Sunlight —
Our controlled experiments indicated sunlight
could increase the rate of loss of carbofuran, but the increase
LOSS OF CARBOFURAN FROM RICE PADDY WATER
145
in solutions exposed to constant sunlight compared with dark controlo was not a large one. Foliage cover and attenuation of the light path in water by suspended solids would act to reduce
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the effect of sunlight in field water. There was some indication that photosensitization was involved in the sunlight acceleration of hydrolysis. Temperature -- Although not studied directly here, a rate increase of 2-3 times is expected for each 10°C rise in temperature. Among those chemical and physical factors which did not appear to influence carbofuran loss from water, or which would be expected to be minor ones under conditions such as those employed in this study, were decomposition by oxidation and evaporation. Implication Duration of efficacy of carbofuran applied to rice paddy water will decrease in paddies of high pH. This situation is most likely to occur when deep well water is used as the irrigation source, in paddies with heavy growth of algae, and when alkaline fertilizers or amendments are applied.
Since it
may not be possible to control paddy water pH, broadcast application of carbofuran under such conditions should be done when the water depth is as low as is consistent with plant 'maintenance.
Promoting rapid penetration of dissolved carbofuran
146
SEIBER, CATAHAN, AND BARRIL
to the top soil, by allowing water present at the time of treatment to percolate to the soil before additional irrigation is done, will in effect transfer the chemical from a more
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alkaline environment (the paddy water) to a less alkaline one (the top soil) and thus extend persistence. Treatments which place the insecticide below the soil surface initially, by incorporation or root-zone treatment
27
would be particularly
advantageous in alkaline paddies. Some reported LC^Q values for carbofuran in water to exposed fish are 0.21 (channel catfish, Ictalurus punctatus), 0.24 (bluegill, Lepomis macrochirus), 0.28 (rainbow trout, Salmo gairdnerii)
, and 0.53 ppm (Tilapia mossambique)
. Making
liberal allowance for complete fish survival a two-week waiting period between carbofuran broadcast application and placement of fish should be observed under paddy conditions (particularly pH) similar to those prevailing in this study.
In ponds or ditches
using paddy drainage or overflow as the water source dilution during water transfer may allow the waiting interval to be shortened somewhat.
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Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesb20
Loss of carbofuran from rice paddy water: Chemical and physical factors J. N. Seiber Barril
a b
c
, M. P. Catahan & C. R.
c
a
Pesticide Laboratory , The International Rice Research Institute , Los Baños, Laguna, Philippines b
Department of Environmental Toxicology , University of California , Davis, California, 95616 c
Department of Chemistry , The University of the Philippines , Los Baños, Laguna, Philippines Published online: 21 Nov 2008.
To cite this article: J. N. Seiber , M. P. Catahan & C. R. Barril (1978) Loss of carbofuran from rice paddy water: Chemical and physical factors, Journal of Environmental Science and Health, Part B: Pesticides, Food Contaminants, and Agricultural Wastes, 13:2, 131-148, DOI: 10.1080/03601237809372083
To link to this article: http://dx.doi.org/10.1080/03601237809372083
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PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/ terms-and-conditions
J. ENVIRON. SCI. HEALTH, B13(2), 131-148 (1978)
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LOSS OF CARBOFURAN FROM RICE PADDY WATER: CHEMICAL AND PHYSICAL FACTORS
KEY WORDS: Carbofuran, rice paddy water, pH-dependent hydrolysis J. N. Seiber1 Pesticide Laboratory The International Rice Research Institute Los Baños, Laguna, Philippines
M. P. Catahan and C. R. Barril Department of Chemistry The University of the Philippines Los Baños, Laguna, Philippines
ABSTRACT The loss of carbofuran was studied from rice paddy water treated with a granular formulation of the insecticide, and from ponds filled with drainage from the paddy.
The average
half-life (t 1/2 ) for carbofuran loss was 57 hr.
Controlled
experiments indicated that pH was the predominating factor governing carbofuran loss from water in the environment studied. The loss due to hydrolysis was over 700 times more rapid at 131 Copyright © 1978 by Marcel Dekker, Inc. All Rights Reserved. Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher.
132
SEIBER, CATAHAN, AND BARRIL
pH 10 (t 1 / 2 = 1.2 hr.) than at pH 7 (t1/2 = 864 hr.) in buffered deionized water. The average pH of the rice paddy was 8, but diurnal fluctuations of 7 to 9.5 are common in
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similar environments.
Impurities in the water, sunlight, and
temperature influence the rate of carbofuran loss but not nearly so much as pH.
There was no evidence for significant loss due
to evaporation or oxidation.
The results have important
implications for the duration of the insecticide's activity and the effect on fish within or downstream from treated paddies.
INTRODUCTION Hand broadcast of systemic insecticides to the paddy water has become quite popular in rice culture in Asian countries. The method gives uniform coverage and fair residual control without 2 the need for complicated application equipment . When applied as granules systemic insecticides are released to the water and penetrate to the root zone for subsequent uptake by the plant and translocation to the foliage. Fumigation, systemic uptake through the stems, and capillary movement en plant surfaces are alternate mechanisms for movement of broadcast insecticides from 3 water to target foliage . Duration of insecticide action and environmental side-effects depend partly on the lifetime of the chemical in the water; too rapid dissipation would lessen the
LOSS OF CARBOFURAN FROM RICE PADDY WATER
133
chance for systemic uptake, but too slow dissipation would increase the chance for run-off and damage to non-target biota in the paddy vicinity.
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The persistence of pesticides in water are influenced by physical, chemical, and biochemical processes, the relative importance of which depend on the chemical and environment in question.
For example, pesticides of high vapor pressure and
low water solubility, such as llndane, volatilize readily particularly when environmental conditions favor a high rate of water evaporation
5,6
. On the other hand N-methylcarbamates and
organophosphates are much more prone to undergo degradation in water than organochlorine insecticides ; hydrolysis, a principal o
degradation route, may be influenced by pH and temperature , 9 10 sunlight', and catalytic impurities in the water
. For some
insecticides, notably the organophosphates, degrading microorganisms can accelerate hydrolysis which otherwise would be 11 12 quite slow in sterile systems Carbofuran is one of several N-methylcarbamates currently marketed in the Philippines for control of insects in rice It may be applied as a foliage spray or by hand broadcast of granules, the latter being preferred for control of leafhoppers, stem borers, and whorl maggot. As many as 4-5 applications of 1-2 kg/ha each may be required with susceptible rice varieties when heavy insect infestations are encountered.
This work was
134
SEIBER, CATAHAN, AND BARRIL
done to determine the rate of carbofuran loss from paddy water, and the factors which most influence it. An understanding of such factors could suggest application methods which increase
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the efficiency of this relatively expensive insecticide, and minimize the chance of harmful side-effects to fish within or downstream from the paddies. MATERIALS AND METHODS Field Samples A 1 ha rice paddy located at the International Rice Research Institute, Los Baños, Laguna, Philippines, was treated with Furadan 2G at 2 kg/ha (a.i.) on February 24, 1977, approximately 60 days after transplanting.
Water was sampled at intervals
after treatment by combining 50 ml dip samples taken at random from 1/3 sections of the field. Two composite samples for each 1/3 ha section were analyzed, and the results for duplicates of all 3 sections were averaged to arrive at the concentration in the entire paddy. At 1 day after treatment water from the paddy was pumped into a drained 7 x 4 m pond (A) located adjacent to the paddy until a depth of 1 m was attained.
Duplicate composite water
samples were taken at intervals after treatment. At 5 days after treatment a second such pond (B) was filled with paddy water and sampled periodically in a similar way.
LOSS OF CARBOFURAN FROM RICE PADDY WATER
135
All field samples were accompanied by pH and temperature measurements.
They were stored at 5^0 until analyzed.
Laboratory Samples
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One. liter of water whose pH was adjusted with phosphate buffer was placed in a 2.5 1 bottle, sterilized in an autoclave, cooled, and spiked asceptically with 10 mg of carbofuran from a 10 mg/ml stock solution in ethanol.
The solution was mixed,
purged with either N. or air for 1 hr, and closed.
Samples were
withdrawn at intervals by a technique which avoided biological contamination.
The solution was shaken and purged with No or
air periodically during the experiment. All such experiments were done in duplicate using either deionized water, water from the rice paddy before insecticide treatment or water from an adjacent irrigation drainage ditch. In ¡studying the effect of pH and air, sample solutions were kept in the dark at 27 + 2°C.
In studying the effect of sunlight,
samples in clear soft glass bottles were kept in a phytotron glass house maintained at 29°C for 16 hr during the day, and 21°C for 8 hr at night. Samples in brown bottles wrapped with carbon paper were similarly placed to serve as dark controls. Analysis For field collected water, 100 ml portions were extracted with 3 x 50 ml of 3:1 méthylène chloride-ether.
Combined
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SEIBER, CATAHAN, AND BARRIL
extracts were dried with sodium sulfate and concentrated to 2 ml on a rotary evaporator.
Evaporation was completed under a
stream of N£ whereupon the sample was dissolved in 2 ml of
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acetone for GLC analysis. For laboratory samples a similar procedure was followed except that the water volume (20 ml) and volume of extracting solvent (25 ml) were reduced. Samples were analyzed by gas-liquid chromatography using a Hewlett-Packard Model 5711 A equipped with NP detector and a 1 m x 2 mm (id) glass column packed with 3 % Apeizon L on 80/100 mesh Chromosorb W HP.
Gas flows were: N- carrier, 20 ml/min;
H_, 3 ml/min; air, 70 ml/min.
Temperatures were: injector, 200°C;
column, 165°C; detector, 300°C. A linear response for standard carbofuran was obtained from 1 to 80 ng.
Recovery of carbofuran
from fortified water was quantitative, with no interferences encountered in any samples at amounts in excess of 0.01 ppm carbofuran equivalent. RESULTS AND DISCUSSION Loss of Carbofuran from Paddy and Pond Hater in the Field A maximum concentration of 2.3 ppm may be calculated for an application of 2 kg/ha of carbofuran at the water depth prevailing initially in the treated paddy.
The maximum observed
concentration, 2.0 ppm, occurred 1 day after treatment, the lag being attributed to release from the granular formulation employed.
Specifications for this formulation are for greater
LOSS OF CARBOFURAN FROM RICE PADDY WATER
137
than 90% release within 24 hr in periodically agitated water At 1 and 5 days after treatment paddy water was pumped from the field into stagnant ponds designed to simulate fish ponds
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which are often near or downstream from paddies. Raw data from analysis of paddy and pond water (Table 1) was used to construct logarithmic decline curves. From the best fit straight lines for data points from 1-8 days in the paddy water, 0-4 days in pond A, and 0-4 days in pond B the observed rate constants K_K and half lives (t
\>b
) were calculated from the
1/2
formulas:
where Co and C are the initial concentration and concentration at time t, respectively. The calculated half lives (Table 1) did not differ greatly for water from the three locations.
In all three temperature,
pH, and sunlight intensity varied within and between days of the sampling period so that such small differences are probably not meaningful.
For example, diurnal variations in pH for
paddies irrigated with slightly alkaline water, such as that used in these experiments, are from £a 7-8 at night to ca 9 during the day.
The maximum pH occurs at about 2:00 pm when
algal photosynthesis shifts the CO2 equilibrium away from
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TABLE 1 Loss o£ Carbofuran from Paddy and Pond Water in the Field Paddy Elapsed time (days)
Concn (ppm)
Pond A Elapsed time (days)
Concn (ppm)
Pond B Elapsed time (days)
Concn (ppm)
0.12
0.94
0
2.6
0
0.63
0.25
1.4
0.15
2.0
0.50
0.63
1
2.0
0.80
1.8
1
0.45
2
1.6
1.2
1.5
2
0.38
3
1.3
4
0.59
3
0.34
5
0.76
5
0.22
4
0.14
6
0.68
6
0.18
8
0.01
8 13
0.34 0.02
7 8
0.06 0.02
w w H
K v (hr"1) t 1 / 0 (hr)
1.0 x io" 67
1.4 x io" 48
M
n 1.2 x lo" 2 55
1
% tel
Avg pH's at 8:00 am and 5:00 pm were 7.8 and 8.5, respectively; Avg temperatures at 8:00 am and 5:00 pm were 26 and 30°C, respectively. b
i
Calculated for days 1-8 (paddy), days 0-4 (pond A ) , and days 0-4 (pond B ) .
^ g
LOSS OF CARBOFURAN FROM RICE PADDY WATER
139
"HCO3 to ~CO_; it is influenced by the degree of algal infestation and may reach 9.5 when heavy algal growth is encountered
'
. Average pH in paddy and pond water studied here
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was estimated from our measurements at 8:00 am and 5:00 pm, and diurnal fluctuation data
'
as 8 during the study period from
which rate data was obtained. In all three environments an increase in rate of carbofuran loss occurred toward the end of the study period.
This may have
been due to a slight increase in temperature and pH which took place then, or to concentration dependence of the rate. Adsorption to suspended or sedimented solids, for example, may take; on importance at low solute concentrations.
It has
been reported, however, that clays are weak adsorbents for 8 N-methylcarbamates and in our ponds we found negligable residues of carbofuran in the sediment during the study. Decomposition by microorganisms, another possible cause for a change in rate with time after treatment, was found to be insignificant in the loss of carbofuran from treated water in 17 a similar environment Loss of Carbofuran from Water under Controlled Conditions The effect of pH was studied in sterilized and buffered water at 27°C.
Solutions were purged with N„ and kept in closed
containers in the dark.
Loss of carbofuran was clearly pH
dependent, the rate at pH 10 being over 700 times that at pH 7
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SEIBER, CATAHAN, AND BARRIL
in deionized water (Table 2 ) . The rates were comparable to those reported for carbaryl (t
= 252 and 0.25 hr at pH 7 and
o
10, respectively) , allowing for the lower temperature (20°C) in
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the carbaryl study.
Decomposition of carbofuran in soil has also
been reported to be pH dependent up to pH 7.8, the most alkaline 18
condition studied
Loss of carbofuran from water in the pH range 7-10 is consistent with a mechanism of simple hydrolysis mediated by hydroxyl ions :
[OH] H20 OC NHCH3 n 0 Carbofuran phenol was found during the early stages of our laboratory studies in approximately the amount expected from this equation. Decomposition of carbofuran was more rapid in paddy water than deionized water at pH 7 and 8.7 (Table 2 ) . Suspended 10 19 particles
and metallic ions
are known to accelerate
decomposition of some pesticides. An increase in ionic strength, however, retards hydrolysis of N-methylcarbamates, apparently by 20 decreasing the activity of either hydroxide ion or the carbatnate The interaction of these factors as mediators of hydrolysis was not studied directly here.
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TABLE 2 Effect of pll on Loss of Carbofuran from Deionized and Paddy Water under Controlled Conditions
pH 7.0 Concn (ppm) Elapsed Deionized Paddy time (days) water water '
pH 8.7 Concn (ppm) Elapsed Deionized Paddy time (days) water water
pH 10 .0 Concn (ppm) Elapsed Deionizec1 Paddy time (hr) water water
0
10.4
10.6
0
9.6
9.6
0
6.5
7.9
1
10.4
8.8
0.25
8.3
7.3
2
1.8
2.3
3
9.1
8.4
1
4.6
3.4
4
0.52
. 0.70
6
9.3
6.8
2
2.0
1.1
6
0.20
0.26
9
9.0
6.0
4
0.27
0.07
8
0.08
0.12
K„, (hr"1) OD t 1 / 2 (hr)
0.08 x 10" 2 0.29 x 10" 2
864
240
3.6 x 10" 2 19.4
5.0 x 10" 2 13.9
All solutions were sterilized, buffered, and kept in the dark under N_. 27" + 2°C.
-2 58 x 10
1.2
Avg temperature was
53 x 10-2
1.3
142
SEIBER, CATAHAN, AND BARRIL Carbofuran loss was more rapid in solutions exposed to
sunlight than in those kept in the dark (Table 3 ) . While the maximum absorption of UV light for carbofuran in water, 275 nm,
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is below the minimum sunlight-derived UV wavelengths encountered at the earth's surface, 292 nm, some end-adsorption at 290-300 nm occurs in the spectrum of carbofuran such that photoactivation may enhance hydrolysis.
The increase in water from the paddy
(130%) and an adjacent irrigation drainage ditch (1507«) on exposure to sunlight was greater than in deiortized water (110%). Photosensitization by other UV absorbing compounds
21
may explain
this observation. We found no evidence that oxidation plays a role in the loss of carbofuran from water.
Measured rates at pH 8.7 were identical
when solutions were maintained under N. or air. Furthermore, analysis of field and laboratory solutions did not reveal the presence of 3-hydroxy- or 3-ketocarbofuran, known oxidation 22 products of carbofuran . The presence of these products in water fron a model ecosystem
23
may have been due to their
formation by metabolism of carbofuran rather than by direct chemical oxidation. Evaporation is a major factor in the decline of some chemicals in water. The approximate half-life in water due to evaporation only can be calculated with the equation and estimate of water evaporation given by
Wolkoff and Mackay .
LOSS OF CARBOFURAN FROM RICE PADDY WATER
143
TABLE 3 Effect of Light on the Observed Rate Constants (K ) and Half-Lives ob (t . ) for Carbofuran Loss under Controlled Conditions
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K
ob O«' 1 )
t, ,„ (hr) 1/2
Deionized water
light
0.105 x 10" 2
660
Deionized water
dark
0.092
753
Paddy water
light
0.40
173
Paddy water
dark
0.31
224
Drainage water
light
0.37
187
Drainage water
dark
0.24
288
a
All solutions were sterilized, buffered to pH 7.0, and kept under N o . Temperature was maintained at 29° C for 16 hr (day and 21°t for 8 hr (night).
Using reported values for the water solubility (700 ppm at 25 e C) -5
and vapor pressure (2 x 10 t. .„ due to evaporation
24 mm Hg at 33'C) of carbofuran
,
from water of 10 cm depth was
calculated to be well in excess of 1 year.
Thus it seems
unlikely that evaporative losses played a significant role in carbofuran decline in our field experiments, although we did not attempt to evaluate this factor by direct controlled study. Temperature also was not studied directly as a mediator of loss rate. The effect of this variable, to increase rate 2-3
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SEIBER, CATAHAN, AND BARRIL
times for each 10°C rise, has been amply documented elsewhere for hydrolysis of N-methylcarbamates
and organophosphates
'
.
Summary of Factors involved in the Loss of Carbofuran from Water
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A plot of log t
vs pH from Table 2 data for paddy water
gave a straight line from which t,,« pH (8) was calculated to be 40 hr. average t..
at
'he average field water
This agrees well with the
observed for paddy and pond water in the field,
57 hr, particularly so considering the variation in pH, temperature, sunlight, and other parameters that occurred in the field. We conclude that pH is the predominating factor in the decline of carbofuran from water in the pH range of this study. Among other factors which may influence the rate of loss of carbofuran are the following: Impurities —
The rate of loss was more rapid in paddy
water than deionized water when the two were maintained under the same controlled conditions. We did not differentiate between dissolved or suspended impurities as the cause nor attempt to identify the chemicals responsible for this apparent catalytic or adsorptive effect. The effect was noted only at the lower pH's (7 and 8.7) and apparently became more pronounced at carbofuran concentrations below ca 0.2 ppm. Sunlight —
Our controlled experiments indicated sunlight
could increase the rate of loss of carbofuran, but the increase
LOSS OF CARBOFURAN FROM RICE PADDY WATER
145
in solutions exposed to constant sunlight compared with dark controlo was not a large one. Foliage cover and attenuation of the light path in water by suspended solids would act to reduce
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the effect of sunlight in field water. There was some indication that photosensitization was involved in the sunlight acceleration of hydrolysis. Temperature -- Although not studied directly here, a rate increase of 2-3 times is expected for each 10°C rise in temperature. Among those chemical and physical factors which did not appear to influence carbofuran loss from water, or which would be expected to be minor ones under conditions such as those employed in this study, were decomposition by oxidation and evaporation. Implication Duration of efficacy of carbofuran applied to rice paddy water will decrease in paddies of high pH. This situation is most likely to occur when deep well water is used as the irrigation source, in paddies with heavy growth of algae, and when alkaline fertilizers or amendments are applied.
Since it
may not be possible to control paddy water pH, broadcast application of carbofuran under such conditions should be done when the water depth is as low as is consistent with plant 'maintenance.
Promoting rapid penetration of dissolved carbofuran
146
SEIBER, CATAHAN, AND BARRIL
to the top soil, by allowing water present at the time of treatment to percolate to the soil before additional irrigation is done, will in effect transfer the chemical from a more
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alkaline environment (the paddy water) to a less alkaline one (the top soil) and thus extend persistence. Treatments which place the insecticide below the soil surface initially, by incorporation or root-zone treatment
27
would be particularly
advantageous in alkaline paddies. Some reported LC^Q values for carbofuran in water to exposed fish are 0.21 (channel catfish, Ictalurus punctatus), 0.24 (bluegill, Lepomis macrochirus), 0.28 (rainbow trout, Salmo gairdnerii)
, and 0.53 ppm (Tilapia mossambique)
. Making
liberal allowance for complete fish survival a two-week waiting period between carbofuran broadcast application and placement of fish should be observed under paddy conditions (particularly pH) similar to those prevailing in this study.
In ponds or ditches
using paddy drainage or overflow as the water source dilution during water transfer may allow the waiting interval to be shortened somewhat.
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