A study on pesticide runoff from paddy fields to a river in rural region—1: field survey of pesticide runoff in the Kozakura River, Japan

Runoff characteristics of nine kinds of herbicides from paddy fields were surveyed in the Kozakura River, that is one of the tributaries flowing into the Lake of Kasumigaura, over a period of 23 April to 30 June (before and after rice transplantation) of year. The flow rates of river water and the concentrations of herbicides in the river water were measured every day in May and every 2 days in April and June at six survey sites along the river. The runoff characteristics of herbicides were elucidated by taking account of the rainfall data, the detailed application data (application date and quantities of herbicides applied to each paddy field in a region), and their physico-chemical properties. The runoff rates (the runoff/application amounts ratio) were calculated for each herbicide, resulting in the range of 8.2–22.4%. The runoff rates were correlated fairly well with octanol–water partition coefficient, log Pow, rather than with water solubility of herbicides.