Browsing by Subject "Tropical river"

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Transport of pesticides in a river of a tropical mountainous watershed in northern Thailand
(2013) Sangchan, Walaya; Streck, Thilo
In the northern region of Thailand, in the upland areas population growth and migration of people from the lowlands have rapidly driven land use changes. The expansion of cultivation to increasingly vulnerable areas such as the slopes of mountainous watersheds has led to increasingly adverse impacts on the environment. In particular, intensive application of pesticides poses a contamination risk for stream water and the aquatic ecosystem. This thesis identified the transport patterns of pesticides with different physico-chemical properties during single runoff events under farmer?s practice conditions on the catchment scale. Moreover, the exposure concentrations of frequently used pesticides in surface water and sediment in the watershed were measured in the frame of long-term monitoring. The data were used to calculate pesticide loads in the Mae Sa watershed (Chiang Mai, Thailand) and to assess the ecological risk of pesticides for the aquatic ecosystems. Prior to start of the monitoring program, methods to extract and analyze pesticides in the surface water and sediment samples were established. The pesticides in water samples were extracted by solid phase extraction with a graphitized carbon black sorbent. The recoveries of pesticides in a simultaneous analysis ranged from 58 % to 117 % for the seven pesticides (dichlorvos, atrazine, dimethoate, chlorothalonil, chlorpyrifos, (α, β) endosulfan, cypermethrin) with a high repeatability of the method (Relative Standard Deviation, (RSD)<20 %), except for chlorothalonil (RSD=27 %). For analysis of sediments, the QuEChERS method was adapted. Extraction conditions such as solvent, partitioning of pesticide due to salt effect and clean up step with dispersive solid phase extraction were optimized. Except for dichlorvos in the bed sediment sample and for dimethoate in bed and suspended sediments, recoveries were between 81 % and 116 %. The results show that the QuEChERS method is a valuable method for extracting pesticides from sediment samples. To identify the transport pathways contributing to pesticide losses from soil to the Mae Sa River, automatic gauging stations were installed at the headwater (HW) and outlet (OL) of the watershed to measure discharge and to collect water samples for pesticide analysis. During three runoff events in May, August and September 2008, water samples were collected in a high temporal resolution (1 hour). The potential transport pathways of pesticides were elucidated by time series analysis. Three different input patterns of pesticides were observed: (a) pesticide peaks during the rainfall events as discharge increased, (b) sporadic high concentrations of pesticides during the falling limb of the runoff peak, and (c) low concentrations but more or less continuous values on a baseline level. A chromatographic effect was observed for many pesticides, for example between dimethoate and chlorpyrifos. Highly mobile pesticides such as atrazine and dimethoate were likely to suffer loss at the beginning of the runoff event, while strongly sorbing pesticides such as chlorpyrifos were slightly delayed. This indicates an interaction with the soil matrix, during transport along a sub-surface pathway. The results obtained in the middle of the rainy season in August and September events showed that antecedent rainfall plays an important role in triggering pesticide transport by preferential interflow. In both events the sporadic appearances of strongly sorbing pesticides such as chlorothalonil and chlorpyrifos after peak flow suggest this transport type. For ecotoxicological risk assessment, the highly dynamic nature of pesticide input to surface waters must be considered in the design of representative monitoring schemes. Not only the periods during rain event and peak runoff, but also the following recession phase, during which short and pulsed concentration peaks might show up, must be captured by a representative sampling scheme. Therefore, a high temporal resolution is advisable. To study the long-term dynamics of seven selected pesticides in the Mae Sa River and to evaluate their environmental impacts to aquatic organisms, the exposure concentrations of the pesticides in water and sediment samples were monitored at three stations (HW, Mae Sa Noi flume (MSN), and OL) in the watershed over a period of one and half year (from July 2007 to November 2008). Aquatic risk assessment concerning the observed pesticide concentrations was performed by using the risk characterization ratio (RCR). Chlorpyrifos was the most frequently detected pesticide in surface water at the HW and OL. Cypermethrin was the most frequently detected pesticide in bed and suspended sediment samples along the Mae Sa Noi tributary and at the HW. Regarding the change of pesticide use in the area (compared with data recored in 2002), the measurements suggest that the use of endosulfan has been reduced in recent years, while the observed concentrations of chlorothalonil and chlorpyrifos were in the same concentration ranges as in 2002. The temporal distribution of pesticides shows that the concentrations are highest during the rainy season. Outstandingly high losses of dichlorvos and atrazine were found at Mae Sa Noi flume. Loads of chlorothalonil and chlorpyrifos in stream water were extremely high in the headwater area. Based on interview data of pesticide use in the Mae Sa watershed, in both years the losses of single pesticides to surface water ranged from 0.004 % (chlorothalonil) to 4.7 % (dimethoate) of the applied pesticide mass. The loss of atrazine could not be included because the data did not contain information on the application rate of atrazine. The risk assessment shows that particularly dichlorvos and endosulfan have a high potential to cause adverse effects to the aquatic ecosystem. The RCRs of endosulfan and cypermethrin show that they are the main stressors in the sediment phase. This reveals that aquatic ecosystem of the Mae Sa watershed is facing adverse effects by the contamination of surface water and sediment with pesticides. Hence, measures are urgently needed to reduce the loss of pesticides from soil to surface waters.