Herbicide transport via surface runoff during intermittent artificial rainfall: A laboratory plot scale study

Surface runoff is highly relevant for herbicide transport into water bodies. Herbicide losses and the influence of physicochemical properties of flufenacet, metazachlor and terbuthylazine were studied on a 0.2 m2 microplot of bare soil and slight slope (6%). Rainfall events were simulated with 13 mm h− 1 (± 3 mm h− 1) precipitation intensity 4, 11, 18, 25, 32 and 39 days after herbicide application. The runoff was sampled at 10-minute intervals during the simulated events. Herbicide concentrations and sediment amounts in the water phase were measured. The maximum concentrations in the water phase of the runoff ranged from 3184 μg l− 1 for metazachlor, 661 μg l− 1 for terbuthylazine and 207 μg l− 1 for flufenacet. The highest concentrations were detected in the first runoff samples which decreased exponentially with further rainfall. Highest loads occurred during the second rainfall event and after the first time surface runoff was generated. The artificial setup of intermittent rainfall revealed that with every new rainfall event after a rainless period, a higher amount of herbicides was released from the plot than at the last interval of the previous rainfall event. This effect was caused by alterations to the soil surface that occurred during the rainless periods. Furthermore, the results show that the herbicide loss relates to herbicide adsorption to organic carbon (KOC) and herbicide solubility in water. Therefore, the highest losses for terbuthylazine were 6.6% of the pesticide application, followed by flufenacet (3.8%) and metazachlor (2.5%).