Inflow and outflow of agricultural chemicals in Lake Biwa

The annual pesticide inputs and outputs crossing the boundary of Lake Biwa were estimated. The inputs were calculated from continuous observations in four rivers flowing into Akani Bay, and from simultaneous observations in 65 influent rivers. The annual losses of pesticide from the watershed to Lake Biwa were 1.7% for esprocarb, 4.8% for mefenacet, 2.3% for thiobencarb, 3.7% for molinate, 9.4% for simetryn and 13.0% for isoprothiolane. The outputs from the lake were estimated from weekly observations in the Seta River. The loss of pesticides discharged downstream through the Seta River, compared with the amount applied, were estimated to be 0.9% for esprocarb, 3.2% for mefenacet, 0.5% for thiobencarb, 1.3% for molinate, 10.0% for simetryn and 5.7% for isoprothiolane. Pesticide outputs generally decreased to 30–80% of the input amounts, although simetryn was retained in the lake without substantial elimination.