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Pesticides in water streams in northern thailand
Pesticides in water streams in Northern Thailand
Agrochemikalien in nord-thailändischen Wassereinzugsgebieten: Flusswasseruntersuchungen mit
Bewertung nach ökologischen Kriterien
Diploma thesis of Peter Ballarin
The intensification of agriculture in Thailand has increased the risk of surface water contamination from pesticides. The objective of this study was to quantify pesticide discharge with the main streams of three draining water catchments, used for cash crop production (flowers, vegetables, lychees) at Chiang Mai Province, Northern Thailand. To achieve this, I (a) monitored 24 pesticides in river water samples obtained from the base flow with a combined discharge measurement sampling device (Sutro and trapezoidal weirs) and additionally from two highly resolved discharge peaks, (b) monitored these pesticides in rainfall samples and (c) established a microcosm experiment to determine the persistence of a choice of eleven pesticides in the water phase, which are commonly applied during the rainy season (Chlorpyrifos, Endosulfan, Malathion, Mevinphos, Monocrotophos, Parathion-Methyl, Permethrin, Profenofos, Chlorothalonil, Metalaxyl and Atrazine).
In nearly all base flow samples, Chlorpyrifos (occurring in 92 - 100% of the samples), Dicofol (84 - 92%), Endosulfan isomers α and β (88 - 96%) and DDT (92 - 100%) were detected despite a prohibited use of DDT since 1999. Endosulfan exceeded the threshold for pesticides in drinking water of the European Union (0,1 µgL-1) in all three catchments. Chlorthalonil was particularily found in the catchments used mainly for flower production, Metalaxyl in that used for vegetable production.
The rainwater samples contained the same pesticides and in similar concentrations as the adjacent streams, suggesting that Chlorpyrifos, Endosulfan, DDT and Dicofol were present in high background concentrations in this region while the other pesticides contaminated the rivers more likely in short terms and are probably caused by point sources. At storm events, I found that some pesticides were fronting the main peak of river discharge, reflecting a first flush of pesticide runoff from surrounding streets and fields. In addition, the proportions of pesticides bound to particles were elevated, exceeding 60% of the total amount during the sampled peak flow events.
Mean sum concentrations in the two peak flow events reached 1,72 µgL-1 and 2,23 µgL-1, as sum of particular-bound and dissolved transport, and exceeded average mean concentrations of base flow (0,47 - 0,6 µgL-1, dissolved transport only) in the three catchments. Particularly important for pesticide transport therefore is the ratio of direct flow and base flow for the particular catchments.
Because of steep slopes and low cover by forests the catchments that are mainly used for vegetable production represent the highest ratio. Most pesticides exhibited a short half-life in the water phase (≤5 d), and might thus contribute little to downstream contamination. Only the fungicide Metalaxyl, mostly used in vegetable plantations, was persistent with a half-life of 86 days. Hence, vegetable production, more so than flower or lychee plantations seem to determine pesticide loads in the examined tropical ecosystem.
Supervisors: Prof. Dr. J. Zeddies, Institute for Farm Management and Prof. Dr. M. Kaupenjohann, Institute for Soil Science (TU Berlin)
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