imidacloprid, fenitrothion and dichlorvos. Table 4.5 shows the total pesticide loss
from the watershed for the applied mass of imidacloprid, fenubocarb, fenitrothion
and dichlorvos at the midstream and outlet positions. These pesticide losses were
highly correlated with the physical-chemical properties of the octanol-water partition coefficient and the water solubility levels. Figure 4.6 clearly indicates that runoff losses significantly increased with increasing water solubility and with a
decreasing partition coefficient.
The monitoring of the groundwater wells indicates that the concentration of all
tested pesticides exceeded the detection limit in almost half of the groundwater
samples. Fenitrothion; however, was detected in every water sample. Mean
measured concentrations for all wells and sampling dates were 0.16, 0.10, 0.025
and 0.007 mg L
À1 for imidacloprid, fenitrothion, fenobucarb and dichlorvos respectively. It is noteworthy that the European threshold for pesticide residues in
drinking water of 0.1 mg L
À1 was exceeded in 46 %, 13 %, 8 % and 0 % of all
samples for fenitrothion, imidacloprid, fenubocarb and dichlorvos respectively.
Table 4.5 Mean and maximum pesticide concentration, number of detections and total pesticide
loss measured at the midstream and outlet positions (in brackets) of the receiving stream in the
Chieng Khoi catchment, Vietnam, during the spring cropping season 2008
Substance
Chemical
group
Mean
concentration
[mg L
À1
]
Maximum
concentration
[mg L
À1 ]
Number of
detections
a
[%]
Total
pesticide
loss [kg]
Total
pesticide
loss [%]
Imidacloprid Neonicotinoid 0.12 (0.19)
0.26 (0.48)
100 (100) 1.19 (0.39) 16.0 (13.4)
Fenubocarb Carbamate
0.36 (0.33)
1.25 (1.70)
83 (67)
1.80 (1.06) 6.4 (9.2)
Fenitrothion Organophosphate
0.06 (0.04)
0.15 (0.11)
83 (83)
0.21 (0.17) 0.6 (1.3)
Dichlorvos Organophosphate
0.02 (0.03)
0.10 (0.15)
33 (17)
0.14 (0.06) 0.4 (0.4)
Source: Modified after Lamers et al. (2011)
a Concentration values below the detection limit were set to zero
Fig. 4.6 Correlation between total cumulative pesticide loss and water solubility or octanol-water
partition coefficient, for the insecticides imidacloprid, fenitrothion, and fenobucarb measured at
the midstream (left) and outlet (right) gauging stations (Source: Lamers et al. (2011))
4 Agricultural Pesticide Use in Mountainous Areas of Thailand and Vietnam. . .
165
from the watershed for the applied mass of imidacloprid, fenubocarb, fenitrothion
and dichlorvos at the midstream and outlet positions. These pesticide losses were
highly correlated with the physical-chemical properties of the octanol-water partition coefficient and the water solubility levels. Figure 4.6 clearly indicates that runoff losses significantly increased with increasing water solubility and with a
decreasing partition coefficient.
The monitoring of the groundwater wells indicates that the concentration of all
tested pesticides exceeded the detection limit in almost half of the groundwater
samples. Fenitrothion; however, was detected in every water sample. Mean
measured concentrations for all wells and sampling dates were 0.16, 0.10, 0.025
and 0.007 mg L
À1 for imidacloprid, fenitrothion, fenobucarb and dichlorvos respectively. It is noteworthy that the European threshold for pesticide residues in
drinking water of 0.1 mg L
À1 was exceeded in 46 %, 13 %, 8 % and 0 % of all
samples for fenitrothion, imidacloprid, fenubocarb and dichlorvos respectively.
Table 4.5 Mean and maximum pesticide concentration, number of detections and total pesticide
loss measured at the midstream and outlet positions (in brackets) of the receiving stream in the
Chieng Khoi catchment, Vietnam, during the spring cropping season 2008
Substance
Chemical
group
Mean
concentration
[mg L
À1
]
Maximum
concentration
[mg L
À1 ]
Number of
detections
a
[%]
Total
pesticide
loss [kg]
Total
pesticide
loss [%]
Imidacloprid Neonicotinoid 0.12 (0.19)
0.26 (0.48)
100 (100) 1.19 (0.39) 16.0 (13.4)
Fenubocarb Carbamate
0.36 (0.33)
1.25 (1.70)
83 (67)
1.80 (1.06) 6.4 (9.2)
Fenitrothion Organophosphate
0.06 (0.04)
0.15 (0.11)
83 (83)
0.21 (0.17) 0.6 (1.3)
Dichlorvos Organophosphate
0.02 (0.03)
0.10 (0.15)
33 (17)
0.14 (0.06) 0.4 (0.4)
Source: Modified after Lamers et al. (2011)
a Concentration values below the detection limit were set to zero
Fig. 4.6 Correlation between total cumulative pesticide loss and water solubility or octanol-water
partition coefficient, for the insecticides imidacloprid, fenitrothion, and fenobucarb measured at
the midstream (left) and outlet (right) gauging stations (Source: Lamers et al. (2011))
4 Agricultural Pesticide Use in Mountainous Areas of Thailand and Vietnam. . .
165