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Abstract. Land-use change in the mountainous parts of northern Thailand is reflected by an increased application of agrochemicals, which may be lost to surface and groundwater. The close relation between flow paths and contaminant transport within hydrological systems requires recognizing and understanding the dominant hydrological processes. To date, the vast majority of studies on runoff generation have been conducted in temperate regions. Tropical regions suffer from a general lack of data, and little is known about runoff generation processes. To fill this knowledge gap, a three-component hydrograph separation based on geochemical tracers was carried out in a steep, remote and monsoondominated study site (7 km 2 ) in northern Thailand. Silica and electrical conductivity (EC) were identified as useful tracers and were applied to calculate the fractions of groundwater (similar to pre-event water), shallow subsurface flow and surface runoff on stormflow. K + was a useful indicator for surface runoff dynamics, and Ca 2+ provided insights into groundwater behaviour. Nevertheless, neither measure was applicable for the quantification of runoff components. Cl − and further parameters (e.g. Na + , K + , and Mg 2+ ) were also not helpful for flow path identification, nor were their concentrations distinguishable among the components.Groundwater contributed the largest fractions to stormflow (62-80 %) throughout all events, followed by shallow subsurface flow (17-36 %) and surface runoff (2-13 %). Our results provide important insights into the dynamics of the runoff processes in the study area and may be used to assess the transport pattern of contaminants (i.e. agrochemicals) here.

Despite intensive efforts motivated by the European Water Framework Directive, many water bodies still suffer from poor water quality in Germany. Intensively drained agricultural areas are still a critical source for nitrate which is responsible for negative effects on aquatic ecosystems. Basic measures such as the fertiliser ordinance are expected to be not sufficient to completely eliminate nitrate exports via drainage tiles and ditches. Consequently, there is the demand to manage the reduction of nitrate concentrations with new additional end of pipe solutions. For this, the presented study focuses on a simply to implement reactive ditch for denitrification, which is installed into drainage ditches to reduce nitrate concentrations. An existing drainage ditch that is fed by tile drainage water was filled with wood chips to keep installation efforts as simple and cheap as possible. In situ parameters and nutrient concentrations were determined at the inflow and the outflow of the reactive ditch for 2 years.The results reveal a promising potential for wood chipbased filters to reduce nitrate concentrations by 28 % on average over all seasons. Within the filter, favourable conditions for denitrification were predominantly found without flooding of surrounding areas. Investigations revealed decreasing nitrate concentrations especially in cases of low flow rates and high temperatures. These encouraging results demonstrate the successful application of reactive ditches under German lowland conditions in general. Since tile drainages are installed for many agricultural areas in this region, there seems to be high potential for the application of this easily implementable type of bioreactor.

The increasing application of pesticides in the uplands of northern Thailand has increased the transfer of pesticides to surface water. To assess the risk of pesticide use for stream water quality, we monitored the concentrations of seven pesticides (atrazine, dichlorvos, chlorpyrifos, dimethoate, chlorothalonil, (α-, β-) endosulfan, cypermethrin) frequently used in the Mae Sa watershed (77 km(2)) in water and sediment samples over a period of one and a half years (2007-2008). All investigated pesticides were recorded in the river. Chlorpyrifos was detected most often in water samples (75% at the headwater station), while cypermethrin was most often found in riverbed (86%) and in all suspended sediment samples. The highest concentrations of the pesticides were detected during the rainy season. About 0.002 to 4.1% by mass of the applied pesticides was lost to surface water. The risk assessment was based on the risk characterization ratio (RCR). The RCRs of dichlorvos in water, (α-, β-) endosulfan, and cypermethrin in water and sediments were higher than unity indicating that they are likely to pose a threat to aquatic ecosystem. Finally, we discuss the role of sampling design on ecotoxicological risk assessment. Our study shows that pesticide contamination of surface waters is an environmental issue in the Mae Sa watershed and that measures need to be undertaken to reduce the loss of pesticides from soil to surface waters.

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