Landschaftsökologie und Umweltforschung 38 p. 265-275 Braunschweig 2002
Analytical investigations on the fate of pesticides in soils of the Jhikku
Khola catchment
CLAUDIA VINKE, ROBERT KREUZIG & MÜFIT BAHADIR
Institute of Ecological Chemistry and Waste Analysis, Technical University of Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Abstract
Investigations on conventional and integrated pesticide applications and their impact on soil
quality were performed at Jhikku Khola catchment. The experimental design was based on
laboratory batch experiments for data gathering on degradation and sorption as concentration
determining processes for pesticides under defined laboratory conditions. The target
compounds malathion, dimethoate, fenvalerate and metalaxyl were applied to khet and bari
soil, and batches were incubated at 20°C and 30 C in the dark. Soil samples were adjusted to
15 % moisture corresponding to approximately 40% of maximum water capacity. Further
batches were additionally performed under ponding conditions. In most experiments carried
out, the disappearance of the pesticides were mainly depending on soil moisture and
incubation temperature. The fungicide metalaxyl was found to be the most persistent
compound while the organophosphorous insecticide malathion disappeared most rapidly.
Model field studies were carried out at khet and bari investigation sites in order to validate
results of the laboratory experiments, as well. On khet land, higher disappearance rates were
determined for the target compounds emphasizing the significance of leaching under ponding
irrigation. The transport of metalaxyl and dimethoate into deeper soil layers was directly
corresponding to their water solubility whereas a transport by preferential flow had to be
assumed for the low water soluble fenvalerate.
Introduction
In Nepal as in many other developing countries, the use of pesticides has increased during the
last decades to a worrying level leading yield reduction due to resistance, resurgence and
secondary pest outbreaks (Baker & Gyawali 1995). Therefore, the joint research project
“Environmental risks of pesticides and sustainable development of integrated pesticide
management for mountain areas of developing countries considering socio-economic
conditions and taking Middle Mountains, Central Nepal as an example” (IPMS Project) has
been established in 1999. One of the main objectives has been the data gathering on Nepalese
farming practices and the assessment of their impacts on the environment. The research
activities were carried out in the Kabhrepanalchok district approximately 50km east from
Kathmandu as an example for an agricultural area with one of the highest pesticide
application rates in Nepal (CEAPRED 2000).
Within this project, fate and behaviour of pesticides in soils were investigated by the Institute
of Ecological Chemistry and Waste Analysis. According to application amounts and
frequencies at bari and khet investigation sites, 4 target compounds were selected: the
organophosphorous insecticides malathion and dimethoate, the pyrethroid fenvalerate and the
fungicide metalaxyl. Their chemical structures are illustrated in Fig. 1. As to generate a
thorough data basis, laboratory batch experiments were performed particularly regarding the
dependence of degradation processes on soil inherent moisture and incubation temperature.
Consequently, the results of these experiments were validated in field studies with
supplementary consideration of leaching processes under ponding conditions.
O
O
S
O
P
O
O
S
O
S
O
P
O
N
S
O
N
O
O
O
malathion
metalaxyl
dimethoate
O
Cl
O
O
O
N
fenvalerate
Fig. 1 Chemical structures of the target compounds malathion, dimethoate, metalaxyl and
fenvalerate
Experimental design
Laboratory experiments
The laboratory batch experiments were conducted with bari and khet soils concerning rain fed
and irrigated land, respectively. Selected physico-chemical properties of both soils are listed
in Table 1 indicating only little differences. The first series of batch experiments were
performed according to standard degradation experiments at 20 °C and 15 % soil moisture
under aerobic conditions in the dark (BBA 1986). Application rates of the target compounds
were due to good agricultural practice. Incubation periods were 0, 3, 7, 14, 28, 56 and 102
days. Taking the particular Nepalese climate and cropping systems into account, higher
temperature (30°C) and ponding conditions were additionally realized. Supplementary,
microbial activity and redox potentials were determined for each batch series.
Table 1 Physico-chemical properties of bari and khet soil
soil
sand [%]
silt [%]
clay [%]
Corg [%]
PH
bari
54
32
13
1
5,0
khet
51
35
14
1
5,3
Field studies
From September 1999 until June 2001, conventional and integrated pesticide applications
were monitored at both investigation sites. In order to identify the impact on soil quality, plant
protection products containing the 4 target compounds were directly applied to fallow soil
surfaces in special model field studies. On bari land, one experiment was carried out after the
monsoon season to study the disappearance of the target compounds under rain fed irrigation,
only. Samples were taken from the superficial soil layer (0-5 cm) within 17 days after
application.
With special respect to the transport of pesticides into deeper soil layers under water ponding
conditions, 2 field trials were carried out on khet land, one after the dry season and another
after monsoon. Directly after pesticide applications, the field plots were ponded. Ponding
height was approximately 10 cm. In the first trial, samples were taken from the 0-5, 5-10,
10-20 and 20-30-cm soil layers. Sampling was conducted 0, 2, 4, 8, 14, 28 and 56 days after
the application. In the second trial, sampling activities were focused on 0-5, 5-10, 10-20, 2030, 30-40 and 40-50-cm soil layers and 0, 5, 7, 13 and 33 day intervals.
Target compound analysis
The analytical procedures followed multimethods usually applied in pesticide analysis (DFG
1991, Steinwandter 1989), which are described in detail by Kreuzig (1998). Field moist soil
samples were slurry extracted over night using an acetone/water mixture (2:1). The slurries
were subsequently submitted to a liquid/liquid partition with cyclohexane. After phase
separation and extract drying, gel permeation chromatographic clean up followed. Finally, the
target compounds were determined by gas chromatography using an electron capture and a
nitrogen/phosphorous specific detector. Limits of determination were 10 µg/kg dry soil.
Results and discussion
Degradation under standard laboratory conditions
The laboratory batch experiments under aerobic conditions which are predominantly
representative for terrestrical topsoils showed different disappearance times of the target
compounds applied. In both soils, malathion was found to be the least persistent pesticide
under investigation. Fig. 2 illustrates the results for bari soil. Within 14 days after application,
residues decreased from approximately 700 µg/kg to 10 µg/kg soil and DT50 values calculated
were ≤ 2 days. An intermediate persistence was revealed for dimethoate and fenvalerate. DT50
were 5 and 16 days. In contrast, the degradation of metalaxyl is characterized by a significant
lag phase within 14 days after application. Then, concentrations continuously dropped down
from 498 to 177 µg/kg bari soil.
Variation of test conditions
In supplementary batches, incubation temperature was adjusted to 30 °C and ponding
conditions were simulated. The disappearance of malathion was again too rapid to show any
dependence on the test conditions modified. Therefore, no accumulation under field
conditions could be derived from these laboratory tests
concentration [µg/kg]
800
700
600
500
400
300
200
100
0
0
20
40
malathion
60
days after application
dimethoate
fenvalerate
80
100
metalaxyl
Fig.2 Disappearance of the target compounds in bari soil under standard laboratory conditions
at 20°C and 15% soil moisture
In contrast, relevant differences were still determined for dimethoate (Fig. 3). On the one
hand, the temperature increase from 20 to 30°C corresponded with a faster degradation due to
the enhancement of soil inherent microbial activity. Whereas the residues in the experiment
with 15 % moisture and ponding at 20 C decreased from 486 µg/kg to 33 µg/kg and from 720
µg/kg to 48 µg/kg khet soil, respectively, dimethoate disappeared during 28 days in the
studies performed at 30°C. Thus, DT50 values dropped down from 8 to 2 days at 15%
moisture and from 14 to 4 days under ponding conditions. On the other hand, water ponding
led to a change from aerobic to anaerobic conditions during a few hours. According to this,
dimethoate disappeared slowly.
Particularly under ponding conditions at 20 C, the microbial degradability of fenvalerate is
definitely decelerated and a relevant persistence is revealed by the extended lag phase of 48
days (Fig 4). Then, residues disappeared until the end of the incubation period. According to
the other batches, differences are not ermerged very clearly and DT50 values ranged from 17
to 37 days.
concentration [µg/kg]
800
700
600
500
400
300
200
100
0
0
20
20 °C - 15 % water
40
60
days after application
30 °C - 15 % water
80
20 °C - ponding
100
30 °C - ponding
Fig.3 Disappearance of dimethoate in khet soil under laboratory conditions at 20 C and 30 C
with 15% soil moisture and water ponding
concentration [µg/kg]
800
700
600
500
400
300
200
100
0
0
20
20 °C - 15 % water
40
60
days after application
30 °C - 15 % water
80
20 °C - ponding
100
30 °C - ponding
Fig. 4 Disappearance of fenvalerate in khet soil under laboratory conditions at 20 °C and
30 °C with 15 % soil moisture and water ponding
Nearly the same tendency was found for metalaxyl. At 15% soil moisture, this fungicide
continuously disappeared and, within 102 days, the residues fell under the determination limit.
Under ponding conditions, metalaxyl was identified to be the most persistent compound under
investigation although shorter lag phases occurred. Already after 56 days, concentrations were
132 µg/kg at 20°C and 12 µg/kg dry soil at 30°C. Thus, DT50 values varied between 8 and
47 days. At the end of the incubation period, no residues were detetectable.
concentration [µg/kg]
6000
5000
4000
3000
2000
1000
0
0
2
4
6
malathion
8
10
days after application
dimethoate
12
fenvalerate
14
16
18
metalaxyl
Fig. 5 Disappearance of the target compounds in bari soil under rain fed conditions after
monsoon
Degradation under field conditions
Field studies performed at bari and khet investigation sites gave a similar impression on the
degradability of the target compounds. As it is examplified in Fig. 5, the concentrations of the
pesticides applied on bari dropped down rapidly within 17 days, e.g. the concentration of
dimethoate decreased from 5425 µg/kg to 23 µg/kg khet soil. The lag phases were reduced
and DT50 values listed in Table 2 were definitely below those of the laboratory trials. In total,
they met the lower range of the literature data (Perkow 1999, EXTOXNET 2000). Thus, no
long-term accumulation in soil is revealed under Nepalese field conditions. In order to
transfer results from laboratory to field approaches, varying soil temperatures and moisture
contents, differences in the microbial activity and the impact of photochemical processes have
to be additionally taken into account for an environmentally relevant risk assessment.
Table 2 DT50 values [days] of the target compounds in soils under standard laboratory and
field conditions compared to literature data
pesticide
1
DT50 lab
DT50 lab
DT50 field
DT50
khet soil
bari soil
bari soil
literature
malathion
0.2
2
1
1 – 251
dimethoate
8
5
2
4 – 201, 2
fenvalerate
17
16
7
15 – 902
metalaxyl
28
47
4
2 – 562
: Perkow, 1999; 2:EXTOXNET, 2000
concentration [µg/kg]
3000
2500
2000
1500
1000
500
0
0
2
0 - 5 cm
4
8
days after application
5 - 10 cm
10 - 20 cm
14
28
20 - 30 cm
Fig. 6 Degradation and transport of dimethoate in khet soil under ponding irrigation after dry
season
Leaching under ponding conditions
Particularly under ponding conditions at khet site, leaching has to be considered as a second
concentration determining process for pesticides in soils. The significance of this approach
was still confirmed for dimethoate although this insecticide underlay a rapid degradation from
2736 µg/kg to 1055 µg/kg within 2 days after application (Fig. 6). Due to its high polarity and
water solubility of 25 g/L, dimethoate was distributed already in the topsoil down to 30 cm.
The same situation was given 8 days after application. Subsequently, residues markedly
drecreased according to the compound specific degradability. Furthermore, leaching into
deeper soil layers could not be excluded.
concentration [µg/kg]
900
800
700
600
500
400
300
200
100
0
0
2
0 - 5 cm
4
8
days after application
5 - 10 cm
10 - 20 cm
14
28
20 - 30 cm
Fig. 7 Degradation and transport of metalaxyl in khet soil under ponding irrigation after dry
season
Due to the higher persistence of metalaxyl, leaching tendencies could even be investigated
after the dry period in more detail. Directly after the application, residues only were found in
the 0-5-cm soil layer (Fig. 7). Here, concentrations of 774 µg/kg continuously decreased to
191 µg/kg within 28 days. In the other soil layers, the distribution consecutively moved on.
Due to its water solubility (8 mg/L) and its persistence, highest concentrations occurred in the
20-30-cm layer after 14 days. Due to degradation and leaching into subsoil, residues finally
fell under the determination limits. In the next monitoring period after monsoon, sampling
was continued down to 50-cm soil depth in order to investigate the transport into deeper soil
layers. In this investigation period, however, the disappearance of metalaxyl was highly
enhanced (Fig. 8). Thus, only little concentrations of 90 µg/kg and 43 µg/kg dry soil,
respectively, were already found in the in the 0-5-cm and 5-10-cm soil layers after 5 days.
Contrary to this, the subsoil was not significantly polluted. Therefore, leaching tendencies of
the first period could not be confirmed. This special situation may be caused in a water regime
changed by the monsoon period from unsaturated to saturated water flow conditions or the
formation of preferential water pathways, which are possibly combined with an enhanced
microbial activity resulting in higher degradability.
concentration [µg/kg]
900
800
700
600
500
400
300
200
100
0
0
0 - 5 cm
5
5 - 10 cm
7
days after application
10 - 20 cm
20 - 30 cm
13
30 - 40 cm
33
40 - 50 cm
Fig. 8 Degradation and transport of metalaxyl in khet soil under ponding irrigation after
monsoon
Conclusion
These research activities on pesticide fate and behaviour in soils emphasized the necessity of
investigation site specific monitoring programmes with special respect on climate and
cropping system. According to this fact, a multistep control concept have to be applied on the
basis of standard and specialised laboratory test systems as well as on long-term field studies
in order to establish a pesticide registration procedure in Nepal.
References
Baker, S. L. & Gyawali, B. K. (1994) Promoting Proper Pesticide Use in Nepal. HMG
Ministry of Agriculture (Nepal)/Winrock Internat. Report Ser. 28, Kathmandu, Nepal
BBA (1986) Fate of pesticides in soil – degradation, decomposition, metabolism. Standard of
the official pesticides registration (in German). Part IV, 4-1, Federal Research Center
for Agriculture and Forestry, ACO Druck, Braunschweig
CEAPRED (2000) Socio-economic survey of the Jhikhu-Khola watershed. Center for
Environmental and Agricultural Policy Research, Extension and Development
(CEAPRED). Unpubl. IPMS Project Report, Kathmandu, 48pp.
DFG Pesticide Commision (1991) Multimethod S 19, manual of pesticide residue analysis (in
German). 1-11 Lfg., VCH, Weinheim
EXTOXNET (2000) Data bank (http://ace.orst.edu/info/extoxnet)
Kreuzig, R. (1998) Development of analytical methods for the differentiation of degradation
and sorption as concentration determining processes of pesticides in soils (in
German). Habilitation dissertation, Natural Sci. Faculty, Technical University of
Braunschweig (ISBN: 3-89720-291-3)
Perkow, W. (1999) Active ingredients of pesticides (in German). Paul Parey Verlag, Berlin
Steinwandter, H. (1989) Contributions to the on-line method for extraction and isolation of
pesticide residues and industrial chemicals. Fresenius Z. Anal. Chem. 332, 752-754