Observed field drainage and concentration dynamics reveal the importance of onsite mitigation of pesticide pollution in a rice growing area in eastern China

Pesticide application is an indispensable part of crop production, but its negative impact on the agricultural eco-systems should be carefully examined. In this study, we investigated field drainage discharge and pesticide losses in a rice growing area between 2018 and 2020 in the lower Yangtze River plain, Eastern China. As the local pest control protocol adopts a pre-irrigation practice to pond the application fields for enhanced killing effect, we examined the potential pesticide losses with field drainage under the increased hydraulic gradient. Based on the observed drainage flow and concentrations of a popular pesticide -chlorpyrifos (CPF), we analyzed the concentration distribution in the paddy environment, especially in field drainage ditches (FDs), and subsequently developed an event-based analytical model to calculate concentration changes along the drainage pathway. The monitoring results in 2018 and 2019 showed that CPF concentration spikes appeared in a FD under low flow condition, the observed higher concentrations as compared to paddy field water and groundwater indicate direct deposits from spray drifts or over-application in the FD. A large storm after pesticide application in 2020 caused high flow event, but no apparent concentration spikes were observed in the FD. Of the 3 monitoring events, the pesticide chemographs did not synchronize with the flow hydrographs, the average pesticide losses were as low as 0.16–0.2 % of the applied amounts. Prediction with the event-based analytical model showed that pesticide concentrations may be lowered by 2–3 orders from the FD to downstream waters for the low flow and high concentration events. Further analysis for onsite mitigation of pesticides showed that, controlled drainage can be adopted to reduce pesticides in the field ditches through the dilution and degradation effects. Findings from this research highlight the importance of water quality buffers and the onsite mitigation measures for pesticide pollution control in agricultural watersheds.