Pesticide Extraction from Soil into Runoff Under a Rainfall Simulator

R unoff is a poorly understood aspect of pesticide environmental behaviour. To improve understanding of pesticide runoff, data from three rainfall simulator studies was used. Fourteen pesticides were studied ranged from tightly sorbed (DDE, soil sorption coefficient (K D ) ~15,000 L kg -1 ) to weakly sorbed (dimethoate, K D <30). Event runoff pesticide concentrations were closely related to soil concentrations (0-25mm). The ratio of runoff to soil concentration (runoff extraction ratio, E RO ), was similar for pesticides with a wide range of sorption and across three soils: runoff concentration (μg L -1 ) = 28 x soil concentration (mg kg -1 ). Only three pesticides did not fit this relationship, and these were not consistent in being excluded. E RO decreased with time after spraying, presumably due to lower concentrations in the top few mm of soil. Similar E RO ’s between sites was probably due to similar hydrology (high rainfall intensity, surface sealing, moist subsoils) and erosion, and because the similar masses of soil and water are involved in mixing. Reduction in runoff concentrations by leaching was not influential , because infiltration was small and soil sorption too high. Conditions studied apply during summer storms on most cotton and grain land on clay soils in North-Eastern Australia