Short-term dynamics of pesticide concentrations and loads in a river of an agricultural watershed in the outer tropics

The intensification of agriculture in the mountainous regions of northern Thailand has led to an increased input of agrochemicals, which may be lost to streams and contaminate the surface water of the lowlands. The present study quantifies the dynamics of pesticide loads in a tropical river during three runoff events. To elucidate the processes involved in pesticide transport from agricultural fields to the stream water we used a high temporal resolution of sampling (1 h) and applied a time series analysis. Water samples were analyzed for seven pesticides (atrazine, chlorothalonil, chlorpyrifos, cypermethrin, dichlorvos, α- and β-endosulfan). Six of the seven pesticides were detected in the river water. Only dichlorvos was below the detection limit in all samples. In particular, pesticides with low Koc value such as atrazine and dimethoate were transported during the runoff peaks. In case of chlorothalonil, chlorpyrifos, α- and β-endosulfan and cypermethrin, short concentration peaks lasting about 1 h were detected during the falling limbs of the runoff peaks, indicating that a fast and sporadic sub-surface flow component (e.g., preferential interflow) plays an important role as a transport pathway. Our study demonstrates that in tropical areas sampling schemes with a high temporal resolution are needed to adequately assess the pesticide contamination of rivers. Otherwise, extreme situations may remain unsampled.

► The study was performed in a tropical region with a pronounced dry and wet season.
► The study performed at the catchment scale under real agricultural practice conditions.
► Dynamics of wildly range of pesticide concentrations showed a characteristical input pattern.
► For risk assessment, sampling schemes with high temporal resolution are advisable.