Paddy herbicide inputs in the entire river inflow reaching Lake Biwa, Japan

This study estimated the inputs of four paddy herbicides in the entire river inflow reaching Lake Biwa, the largest lake in Japan, which serves as a water resource for 14 million people. The Uso River and the Hino River, the main contaminated rivers among the inflow rivers, were selected as daily and hourly monitoring sites to provide data on the seasonal trends in the concentration and load of herbicides and to determine the effect of rainfall events on load. The monitoring was also performed four times in 15 inflow rivers. The total input to the lake was calculated from the loads during fine weather conditions and additional loads during rainfall events. The former based on the lumped load from the two rivers and by prorating for the 15 rivers, and the latter was estimated from the relation between precipitation and increased load rate. The annual losses of herbicide from the basin to Lake Biwa were estimated to be 14.5% for bromobutide, 3.0% for pretilachlor, 5.2% for molinate, and 8.8% for simetryn. The loads caused by rainfall events accounted for 9%–18% of the total annual loads.