Environmental Pollution 66 (1990) 253-262

Dissipation of s-triazines and Thioearbamates from Soilas Related to Soil Moisture Content*

G. Singh
Haryana Agricultural University, Hisar 125004, India

W. F. Spencer,§ M. M. Cliath
USDA-ARS, Department of Soil and Environmental Sciences, University of California,
Riverside, California 92521, USA&
M. Th. van Genuchten
USDA-ARS, US Salinity Laboratory, 4500 Glenwood Drive, Riverside,
California 9250, USA(Received 21 November 1989; accepted 4 April 1990)

ABSTRACTHalf-lives (tl/2) of two soil incorporated s-triazine ( atrazine and prometon )and two thiocarbamate ( E P T C and triallate ) herbicides were determined inrelation to soil moisture content in two California soils. Treated soils wereincubated at three moisture levels in aerated glass vials at 25 +_I°C and wereanalyzed at O, 7, 16, 28, 56 and l l2 day intervals. Loss of herbicides in alltreatments followed first-order kinetics. The t~/2-values o f all herbicides
decreased with increasing soil moisture and jbllowed an empirical equation,t~/2 = aM -b (where tl/2 is half-life; M the moisture content; and a and b are* Contribution from USDA-Agricultural Research Service and the University of California,
Riverside, California 92521, USA.
§To whom correspondence should be addressed.
253
Environ. Pollut. 0269-7491/90/$03'50 © 1990 Elsevier Science Publishers Ltd, England.
Printed in Great Britain

254

G. Singh et al.
constants). Soil moisture had a greater effect on carbamates than ons-triazines. Prometon exhibited the longest half-life in both soils, whereas
EPTC was least persistent in one soil and atrazine in another. The t 1/2-values
for atrazine, prometon, EPTC, and triallate with medium moisture levels andlO lag/g concentration were 34.6, 43"2, 25.4 and 38.1 days in sandy loam and26.5, 44"4, 44.1 and 25.9days in loamy sand, respectively. Disappearance of
50% of the applied concentrations of most of the herbicide-soil combinations(except EPTC and triailate in one soil) took longer for lower initialconcentrations (1 pg/g) than for higher concentrations ( lO pg/g).

INTRODUCTION
Soil moisture is an important variable determining the behavior of agrochemicals; for example, by controlling soil oxygen levels (Brock, 1970) andby providing microbial water requirements (Gray, 1970). Degradation ofmost organic chemicals intentionally or unintentionally applied to the soil
decreases as soil moisture content decreases (Lee et al., 1984; Yaron et aL,
1985; Ghani & Allbrook, 1986; Walker, 1987). In addition, their diffusionand volatilization rates are greatly affected by soil moisture (Spencer et al.,
1982; Glotfelty et al., 1984).
Degradation half-life (tl/2) is a major input parameter for models thatpredict the overall behavior of solutes in soil (Jury et al., 1983; Walker, 1987;
van Genuchten & Wagenet, 1989). Walker and co-workers (Walker, 1974,
1987; Walker & Barnes, 1981; Walker & Zimdahl, 1981) demonstrated thatthe correlation of pesticide half-life and soil moisture content follows theempirical relationship:tl/2 =

aM-b(1)

where tl/2 is the half-life; M the moisture content; and a and b are constants.
The present laboratory experiment was carried out to determine thedegradation life (t~/2) as a function of soil moisture content, for two
s-triazine (atrazine and prometon) and two thiocarbamate (EPTC andtriallate) herbicides using two soils varying in organic matter content andparticle-size distribution.

MATERIALS AND METHODS
Herbicides

Four herbicides viz., atrazine, [2-chloro-4-(ethylamino)-6-(isopropylamino)s-triazine]; EPTC, [s-Ethyl dipropylthiocarbamate]; prometon, [2,4-bis-

Dissipation of s-triazines and thiocarbamates from soil

255

TABLE 1
Selected Properties of Four Herbicidesa

Herbicide

Vapor
pressure
(Pa)

Vapor
density(mg/litre)

Aqueoussolubility(mg/litre)

Henry'slaw
constant

Koc(m3/mg)Atrazine
Prometon
EPTC
Triallate

9.0E-05 b
8.3 E - 0 4
2'8E+00
2.6E-028.0E-06
7.5E-05
2.2E-01
3.2E-03

33
750
370
4

2.5E-071-0E-07
5-9 E - 0 4
7.9 E - 0 4

54
76
196
2838

Singh et al. (1990).
h 1 . 0 E - 0 5 = 1.0 × 10 -5 .

(isopropylamino)-6-methoxy-s-triazine];andtriallate,[s-(2,3,3trichloroallyl) diisopropylthiocarbamate] were used in this study. Analytical standards of the herbicides were obtained from the US Environmental
Protection Agency, Research Triangle Park, NC. Relevant properties are
given in Table 1.
Soils
Hanford sandy loam (HSL), (a coarse-loamy, mixed non-acid, thermic typic
xerorthents) from the Moreno Experimental Farm of the University ofCalifornia, Riverside, and Tujunga loamy sand (TLS), (a mixed, thermic
typic xerosament) from Fontana, California, were selected as the test soils.
The soils had not previously received EPTC or triallate applications. Surface
bulk samples (0-15cm) were air dried and passed through a 2mm sieve.
Selected physical and chemical properties of the soils are listed in Table 2.

TABLE 2
Important Properties of the Soils

Soil type

Hanford Sandy
Loam (HSL)
Tujunga Loamy
Sand (TLS)

Organic
carboni%1

pH(1 : 1)

CEC(cmol/kg)Clay( < 2 #m)
(%)

Silt
Sand
(2-50 Fire) (50 ~m-2 mm)
(%)
(%)

0.43

6.05

5"95

7.05

25-84

67.11

0.33

6.30

0.45

4.47

13.52

82.01

256

G. Singh et al.

Treatments
The soils were prepared at three gravimetric moisture levels (M1, M2, andM3) viz., 1, 2 and 4% for TLS, and 3, 6 and 12% for HSL. Soil moisture
content batches were treated separately with the four herbicides at aconcentration of approximately 10/~g/g (C2). M2 soils were also preparedwith approximately 1 ~g/g herbicide concentration (C1). All treatments werereplicated twice and six vials (6 cm × 2.5 cm ID) for each replication werefilled with 20g soil subsamples. At 0, 7, 14, 28, 56 and 112 days one of the six
vials was sacrificed from each replication.

Soil preparation
Soils at the two lower moisture contents, M 1 and M2, were prepared in 2 kgbatches in 5 litre wide-mouth glass bottles. A known amount of deionizedwater was sprayed on the soil over a 1-h period with regular tumbling to
avoid sticking of the soil on the walls. Since it was not possible to prepare theM3 treatment by direct spraying, the water content was first adjusted to theM2 level, and then after applying the herbicides the additonal amount ofwater was added to individual treatment vials. After overnight equilibration,
the soil from each moisture treatment was sub-divided into four 500 g lots
and each lot sprayed with 25ml of herbicide solution in acetone ofappropriate concentration. For this purpose, the soils in 1 litre wide-mouth
glass jars were sprayed in small increments with an atomizer attached to a
dry N2 cylinder. The soil was mixed by tumbling until all the organic solvent
was evaporated.
The treated soils were further equilibrated for 12 h prior to the transfer of
20 g sub-samples to individual glass vials in duplicate. Each vial was covered
with aluminum foil, containing five pin holes, and incubated at 25 + I°C.
The incubation chamber was provided with three, 1-1itre capacity beakers
containing distilled water to generate approximately 50% relative humidity.
The zero-interval samples, drawn directly from the 1-1itre jar, were assumed
to contain the initial herbicide concentrations. The vials for the ll2-day
period from each treatment were weighed initially and at 3-4 day intervals,
and the lost water was replenished in all treatments by adding appropriate
amounts of deionized water.

Extraction
At each time interval, 15 g of soil from each replicate of the four herbicide
treatments was composited into one 60 g sample. This sample, containing
the four herbicides, was extracted with 150ml acetone:hexane (1:1) byDissipation of s-triazines and thiocarbamates.from soil

257

shaking for 1 h on an end-to-end electrical shaker. The supernatant was
filtered through a bed of anhydrous sodium sulfate. These steps wererepeated twice by adding 100ml solvent and shaking for 30min. The
extracted batches were combined and concentrated to 25 ml on a rotary flash
evaporator. Using this method, recovery of all the herbicides was found tovary between 85-95%.

Analysis
All the extracts were analyzed using a Varian 3700 Gas Chromatographequipped with NP and EC detectors. The column used for atrazine,
prometon and triallate was 0-9 m x 2 mm ID, Ultrabond 20M on Carbowax
100/200 mesh (column temperature for each herbicide was 150, 180, 155~C,
respectively) with NP detector (300°C). DB-Wax Megabore with the sameliquid phase was used for EPTC (temperature 110~'C) with EC detector(280°C).

RESULTS AND DISCUSSION
The data for percent residue recovered after various incubation times arepresented in Table 3. The initial herbicide concentration for each treatment
was considered to be 100%. The residue data were log~o transformed andsubjected to linear regression analysis to calculate the degradation constant,k. The q/a-values for different herbicides under different treatments
calculated by eqn (2):0.301
tl/2 -k(2)

are presented in Table 4. Except for three combinations (atrazine with M2C1
in HSL, M3C2 in TLS and triallate with M2C2 in TLS), the r2-values weregreater than 0"91. In these three cases also the values were above 0.75.
Further, eqn (1) was employed to correlate the t~/2-values with the soil
moisture contents of the two soils. The data fit the empirical equation andthe values of the constants a and b for each herbicide-soil combination are
given in Table 5.
The four chemicals exhibited different dissipation behaviors. Atrazine
showed an apparent lag period of 14-28 days depending upon treatment(Table 3). This was followed by a phase of rapid loss which resulted in acumulative dissipation being comparatively faster at higher moisture levels
in both soils. The t~/z of 26"6 days in the HSL soil was significantly greaterthan 22"9 days in TLS. Similar atrazine behavior was reported by Armstrong

258

G. Singh et al.

TABLE 3
Percentage of Each Herbicide Recovered after Various Incubation Intervals at 25 ___I°C
Interval(days)HSL

TLS

MlaC2 b

M2C2

M3C2

M2CI

MIC2

M2C2

M3C2

M2CI

0
7
14
28
56
112

100-0
99-2
94-6
82.8
63.9
11.I

100-0
91.3
79-1
79-8
55-3
9.9

100.0
107.1
102-4
104.0
18.9
7-6

Atrazine
100.0
100.0
68.7
84.5
53.4
73.1
40.2
72.6
31.6
33.4
24.9
8.8

100.0
94.7
84.7
71-3
15-5
6.9

100.0
96.6
95-2
78-4
4.7
6.2

100-0
90.5
88.2
61.7
54.5
21.7

0
7
14
28
56
112

100.0
68"8
67.4
53.3
40.3
25.3

100.0
84.9
82.4
76.1
35'4
17.3

100.0
97.7
95.2
82.5
35.7
16.5

Prometon
100"0
100.0
93-5
87.6
76"4
96"9
47.2
78.3
43.1
41.7
28'3
18-4

100.0
102'2
107"1
81"3
31"8
21"9

100.0
98-9
102'1
76'2
33'2
19'4

100.0
97.4
101.9
90'4
84'9
29'6

0
7
14
28
56
112

100-0
83"9
64-3
41'2
29.6
14.1

100.0
112-6
64-8
49-5
21-6
5"3

100.0
85.2
58.5
49-0
23-0
0-7

EPTC
100.0
100-0
75'2
109.9
63'8
102.8
36'4
86.7
34.8
75.1
14.5
33.0

100.0
80'2
53'6
49-5
46.0
13.8

100'0
60'4
56'0
42-7
23"5
7.8

100"0
75'5
77-0
41"1
30'3
2.4

0
7
14
28
56
112

100.0
78.0
62.4
70.4
40.4
18-0

100.0
92.3
94.6
93.8
69"5
12.4

100-0
80.6
85'9
75.3
15.8
6.4

Triallate
100"0
100.0
I00.0
75.6
66-4
70.0
63"1
64-9
21.3
55.9
16.7
14.8

100.0
74.9
82.1
72"9
19.7
8.7

100.0
80'0
75'0
66.4
5'7
7.5

100.0
82.6
99'7
68'6
19'0
7.6

MI, M2, and M3 equals 3, 6, and 12% water in HSL and 1, 2, and 4% in TLS.
h CI and C2 represent approximately 1 and 10/~g/g initial soil herbicide concentrations. The
actual concentrations were assumed to be those recovered on Day 0.
aand Chesters (1968) and Scott and Phillips (1972) who found increased
atrazine diffusion with increased soil moisture level. Atrazine half-life of 40
and 50days was reported earlier by Ghadiri et al. (1984) under tilled anduntilled field plots, whereas in the laboratory, they reported 50% dissipation
within 20 days. Half-lives of 23 and 20 days were also reported by Ivany et al.
(1985) and Buchanan and Hiltbold (1973). Our t~/z-values are consistent259

Dissipation o f s-triazines and thiocarbamates f r o m soil

TABLE 4Half-life Values of Herbicides as Related to Water Content and Herbicide Concentrations
Herbicidetl/2 (days)
HSL

TLS

MIC2 ~

M2C2

M3C2

M2C1

M1C2

M2C2

M3C2

M2C1

36.7
63' 1
40.7
48"2

34.6
43.2
25.4
38.1

26.6
40.0
16.2
26.3

64.0
62.6
43.6
41'6

32.1
44.2
66.9
44.9

26.5
44.4
44.0
30"5

22-9
42.6
32.7
25.9

51.9
64.2
21.6
27.9

Atrazine
Prometon
EPTC
Triallate

See footnotes Table 3.

with these reports. The sudden rise in degradation rates after the lag period
may be attributed to build up of herbicide-degrading microbial populations(Obien & Green, 1969).
Prometon exhibited a lag period of about 14 days in both soils. In HSL, it
persisted longer at low moisture levels. However, in TLS no moisture
content effects were observed, perhaps because of experimental errors.
Higher dissipation at higher moisture levels may be associated with higher
diffusion rates (7-fold) as reported by Scott and Phillips (1972).
EPTC dissipation rate tended to decrease with time in most treatments(Table 3). EPTC probably volatilized rapidly during the first few days after
application (Cliath et al., 1980; Lee et al., 1984; Ghani & Allbrook, 1986) andhence, volatilization could have overshadowed degradation losses during
any initial lag period. Degradation flux was higher at higher soil moisture
contents (Table 4). Higher moisture contents are reported to increase EPTCTABLE 5
Relationship Between Herbicide Half-life (/1/2) at 10/~g/g Soil Concentration and Soil Water
Content (M) as expressed by Eqn (1)aHerbicideAtrazine
Prometon
EPTC
Triallatea tl/2

~

aM-b.

HSL

TLS

rE

a

b

r2

a

b

0"909
0'934
0.999
0.992

46'82
86.17
94'35
79.82

0'212 30.32940"665 50-437 9

0"996
0.811
0'995
0.973

31.86
44.53365.54
43-2140-243 10"02590'515 80.396 2

260

G. Singh et al.

degradation because of increased microbial activity (F.ang et al., 1961;
Obrigawitch et al., 1982). Irrespective of moisture levels, EPTC persisted
longer in TLS than HSL, probably because of its higher organic carbon(Anderson, 1984; Sarathchandra et al., 1984; Ghani & Allbrook, 1986). Our
tl/2-values for EPTC are in agreement with those reported by Lee et al.
(1984) in stored soils without any previous EPTC exposure.
Triallate losses during the first 7 days were fairly high in all treatments.
Since triallate degrading microbes usually take some time (lag period) tobuild up their populations, the initial high losses may be due to volatilization
as triallate has a relatively high Henry's constant (7.9 x 10- 4). A similar early
trend of dissipation by volatilization was reported by Jury et al. (1980) and
Grover et aL (1988). Half-life for triallate decreased as soil moisture content
increased for both soils (Table 4).
The herbicides were more persistent when applied at lower concentrations
except the thiocarbamates in TLS soil. In other words, the tt/2-values for the
herbicides were generally greater when applied at 1 #g/g than when applied
at 10 #g/g concentration (Table 4). It had been earlier reported that it may
take longer for one-half of a chemical to disappear at lower concentrations(Hamaker, 1972).
The t~/2-values and soil moisture levels were found highly correlated(Table 5). The magnitude of the 'b' constant for each herbicide indicates the
moisture dependence of its dissipation rate. The carbamate herbicides(EPTC and triallate) were affected more by changing soil moisture content
than were the s-triazines. This correlation for a particular soil-herbicidecombination can be used to predict variations in persistence that may occur
in the field with changes in soil moisture content. Hence, half-life values
through eqn (1) should be generated for many herbicides to improvepredictions from pesticide behavior models. Similar to observations ofYaron et aL (1985) and Walker (1987), we did not find any association of t l/2values with physico-chemical properties of the two soils.
There exists a controversy over the assumption that degradation proceeds
only in the liquid phase because the adsorbed phase may not be available fordegradation (Moyer et al., 1972; Marshman & Marshall, 1981; Scott et aL,
1983; Ogram et al., 1985). Current evidence indicates that degradation rates
in the sorbed phase are generally lower than those in the liquid phase (vanGenuchten & Wagenet, 1989). Consequently, conditions that facilitate
increased adsorption should lead to overall lowered degradation rates. One
such condition is realized by lowering herbicide concentrations. Since theexponent of the Freundlich Isotherm is less than one for these herbicide-soil
combinations (Singh et al., 1990), lowering the concentration effectively
leads to stronger sorption which, in turn, could result in lowering the overall
degradation rate. Our data are consistent with this hypothesis.

Dissipation o[ s-triazines and thiocarbamatesfrom soil

261

In summary, degradation o f the herbicides followed first-order kinetics
a n d t h e i r disappearance was faster at higher soil moisture levels. Half-life of
carbamates in these soils was more affected by soil moisture content thanthat of the s-triazines. Prometon exhibited the longest half-life in the twosoils, whereas, E P T C in H S L and atrazine in TLS were least persistent. The
half-lives of the herbicides were greater at low initial application dose except
for E P T C and triallate in TLS.

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