concentrations of atrazine and dimethoate were one to three orders of magnitude
below unity. For chlorpyrifos and cypermethrin, the RQ values exceeded significantly the threshold of unity. Also, extremely high RQ values were found for
endosulfan, for based on the mean concentration, the RQ value was 20,000.
Similarly high RQ values were found for endosulfan, also within the framework
of a long-term monitoring campaign (data not shown). The situation for
chlorothalonil was not as clear as for the other pesticides, with a mean concentration yielding an RQ value distinctly below unity, whereas the RQ computed based
on the maximum concentration was twice the threshold. For all pesticides, RQ
values based on the maximum concentration were about one order of magnitude
higher than the RQ values calculated from the mean concentrations.
The results from the risk assessment show that among the seven investigated
pesticides, endosulfan posed by far the most serious environmental hazard.
Eliminating this pesticide from stream water would; therefore, drastically improve
water quality and strongly impair the pesticide-induced stress experienced by the
aquatic ecosystem in the Mae Sa River. Although endosulfan has been officially
banned under Thai law since 2004, our monitoring data, as well as the survey data
of Schreinemachers et al. (2011), prove that this pesticide is still in use. Therefore,
more efforts are urgently needed to put this law into action, and farmers must be
better educated and trained in the safe and environmentally-friendly handling of
pesticides. Beyond that, the presented data underline the importance of using data
with a high temporal resolution in risk assessments. Taking water samples over
longer periods, such as a day or a week, averages out short-term concentration
peaks, which typically show up during and shortly after heavy rain events (see
Chap. 3). If data with a lower temporal resolution are used, the extreme short-term
exposure of organisms might remain undetected, leading to an under-assessment of
the eco-toxicological risk associated with the input of pesticides into surface waters.
4.4.3 Loss of Pesticides from Paddy Rice Fields in Northern
Vietnam
According to the Vietnamese Ministry of Agriculture and Rural Development
(MARD 2003), only 20 % of the uplands population in northern Vietnam has access
to clean water, as local people rely on surface and groundwater for drinking
purposes. Especially in rural regions, water sources such as wells and irrigation
channels are susceptible to pesticide contamination, because they are either integral
parts of, or connected to the paddy rice irrigation systems.
Anyusheva et al. (2012) and Lamers et al. (2011) conducted a series of field and
watershed studies in Chieng Khoi in 2008, in order to quantify the loss of pesticides
from paddy rice fields into adjoining environmental compartments such as fish
ponds, wells or receiving streams. On the field scale, the experimental set-up
included the monitoring of the water balance and of pesticide concentrations
162
M. Lamers et al.
below unity. For chlorpyrifos and cypermethrin, the RQ values exceeded significantly the threshold of unity. Also, extremely high RQ values were found for
endosulfan, for based on the mean concentration, the RQ value was 20,000.
Similarly high RQ values were found for endosulfan, also within the framework
of a long-term monitoring campaign (data not shown). The situation for
chlorothalonil was not as clear as for the other pesticides, with a mean concentration yielding an RQ value distinctly below unity, whereas the RQ computed based
on the maximum concentration was twice the threshold. For all pesticides, RQ
values based on the maximum concentration were about one order of magnitude
higher than the RQ values calculated from the mean concentrations.
The results from the risk assessment show that among the seven investigated
pesticides, endosulfan posed by far the most serious environmental hazard.
Eliminating this pesticide from stream water would; therefore, drastically improve
water quality and strongly impair the pesticide-induced stress experienced by the
aquatic ecosystem in the Mae Sa River. Although endosulfan has been officially
banned under Thai law since 2004, our monitoring data, as well as the survey data
of Schreinemachers et al. (2011), prove that this pesticide is still in use. Therefore,
more efforts are urgently needed to put this law into action, and farmers must be
better educated and trained in the safe and environmentally-friendly handling of
pesticides. Beyond that, the presented data underline the importance of using data
with a high temporal resolution in risk assessments. Taking water samples over
longer periods, such as a day or a week, averages out short-term concentration
peaks, which typically show up during and shortly after heavy rain events (see
Chap. 3). If data with a lower temporal resolution are used, the extreme short-term
exposure of organisms might remain undetected, leading to an under-assessment of
the eco-toxicological risk associated with the input of pesticides into surface waters.
4.4.3 Loss of Pesticides from Paddy Rice Fields in Northern
Vietnam
According to the Vietnamese Ministry of Agriculture and Rural Development
(MARD 2003), only 20 % of the uplands population in northern Vietnam has access
to clean water, as local people rely on surface and groundwater for drinking
purposes. Especially in rural regions, water sources such as wells and irrigation
channels are susceptible to pesticide contamination, because they are either integral
parts of, or connected to the paddy rice irrigation systems.
Anyusheva et al. (2012) and Lamers et al. (2011) conducted a series of field and
watershed studies in Chieng Khoi in 2008, in order to quantify the loss of pesticides
from paddy rice fields into adjoining environmental compartments such as fish
ponds, wells or receiving streams. On the field scale, the experimental set-up
included the monitoring of the water balance and of pesticide concentrations
162
M. Lamers et al.